Rabbit haemorrhagic disease
Encyclopedia
Rabbit haemorrhagic disease (RHD), also known as rabbit calicivirus disease (RCD) or viral haemorrhagic disease (VHD), is a highly infectious and often fatal disease that affects wild and domestic rabbits of the species Oryctolagus cuniculus. The infectious agent responsible for the disease is rabbit haemorrhagic disease virus (RHDV), or rabbit calicivirus (RCV), genus Lagovirus
of the family Caliciviridae
. The virus infects only rabbits, and has been used in some countries to control rabbit populations.
There are vaccines available against VHD in the UK. One is named Cylap and made by Fort Dodge Animal Health. Another is Lapinject and is made by CEVA Animal Health Ltd. Both last 12 months and contain inactivated strains of VHD. A third called Anivac has recently become available.
RHDV and RCV are members of a larger group of Lagovirus
es within the Caliciviridae
. Another member of this group, the European brown hare syndrome virus (EBHSV) is 71% homologous in nucleotide sequence, but exposures to the two viruses do not cross-protect. RHDV is known only to infect the European Rabbit
, while EBHSV is limited to the range of the European Brown Hare but can infect Varying Hares and American Eastern cottontail
s within that range or in experimental tests, respectively.
. Scientists cannot pinpoint its exact origins; however, it is believed the disease emerged from a virulent calicivirus spreading asymptomatically in European rabbit populations, particularly in the German Democratic Republic
. The Chinese outbreak was spread by the angora rabbit
, which had originated in Europe
. Fourteen million domesticated rabbits died within nine months in the outbreak.
In 1984 the virus that caused the disease was identified. The virus spread westward and reached Europe in 1988. The virus has since appeared in Mexico
, Cuba
, Australia
, New Zealand
and the USA. In 1992, the United Kingdom
reported its first case of RHD in domestic show rabbits. By the late 1990s, RHD stretched to forty countries and had become endemic in wild rabbit populations in Europe, Australia
, New Zealand
and Cuba
. In Europe there was a rapid increase in research into RHD, due to the importance of the commercial breeding of rabbits for meat and fur production.
The first reported case in the United States was in Iowa on March 9, 2000. The affected population included Palominos and California Whites
. By April 6, 25 of the 27 affected rabbits had died as a result of RHD. In order to contain the disease, the remaining two rabbits were euthanized. No new introductions of rabbits were placed on the farm for two years after the discovery of RHD and August 1999 was the last time rabbits left and/or returned to the farm. The United States experienced other outbreaks of RHD in 2001 (Utah
, Illinois
, New York
) and 2005 (Indiana
).
. From 1988 to 1992 Mexico
dealt with an RHD endemic in domestic rabbits.
After outbreaks of RHD in 2000, 2001, and 2005 in domesticated rabbits, the United States has eradicated RHD from its rabbit populations. The native species, cottontails
(Sylvilagus floridanus), black-tailed jackrabbit
s (Lepus californicus) and volcano rabbit
s (Romerolagus diazzi) seem not to be susceptible to the virus.
conditions to research the safety and usefulness of the virus if it was used as a biological control agent against Australia and New Zealand's rabbit pest problem. Testing of the virus was undertaken on Wardang Island
in Spencer Gulf
off the coast of the Yorke Peninsula
, South Australia
. In 1995 the virus escaped quarantine and subsequently killed 10 million rabbits within 8 weeks of its release. The disease is currently epidemic throughout Australia and New Zealand.
decided not to allow RHDV to be imported into New Zealand to control rabbit populations. This was backed up in an early August review of the decision by the Director-General of Agriculture. By late August, it was confirmed that RHDV had been deliberately introduced to the Cromwell
area of the South Island
.
An unsuccessful attempt was made by New Zealand officials to control the spread of the disease. It was, however, being intentionally spread, and several farmers (notably in the Mackenzie Basin
area) admitted to processing rabbits that had died from the disease in kitchen blenders for further spreading.
Had the disease been introduced at a better time, there would have been a more effective control of the population. Unfortunately, it was released after breeding had commenced for the season, and rabbits under 2 weeks old at the time of the introduction were immune to the disease. These young rabbits were therefore able to continue to grow and breed back up. Ten years on, rabbit populations (in the Mackenzie Basin
in particular) are beginning to reach near pre-plague proportions once again though they have not yet returned to pre RCD levels.
Resistance to RHD in New Zealand rabbits has led to the widespread use of 1080 poison. Farmers and land managers are having to increase their use of 1080 to protect pastoral land from rabbits and preserve the gains made in recent years through the use of RHD.
's first case of RHD was recorded in 1986 and Spain
's in 1988. France
, Belgium
(June) and Scandinavia
followed in 1990. Within a few years of RHD's first appearance in Europe it had caused the largest mortality in domestic and wild rabbits in Germany
, Austria
, Spain and Italy. Spain was the worst affected by RHD.
When the United Kingdom's first case of RHD in 1992 was discovered, the disease was transmitted into the wild by domesticated pet rabbits. Sources vary in the number of confirmed cases of RHD; there were 9 known outbreaks in 1994, 32 cases but some sources believe there were as many as 512 cases of RHD in 1995, and around 30 RHD cases in 1996 throughout Scotland
, England
and Wales
.
s. Rabbits acquire RHD through oral, nasal or conjunctival pathways. Urine, faeces and respiratory secretions may also shed the virus. The virus may also be carried by the wind. Carriers of the virus may remain infectious for up to a month depending on climate conditions; however, the virus has been known to persist for as little as 2 days and as long as 215 days. An infected carcass or hairs from an infected animal may also transmit RHD. Fomites such as clothing, contaminated food, cages, bedding, feeders and water will also harbour the virus. Even though the virus cannot reproduce in other mammals, predators and scavengers such as foxes, ferrets and some birds can excrete the virus through their faeces after ingesting an infected rabbit carcass. Flies, rabbit fleas, and mosquitoes can also spread the virus between rabbits.
Climate appears to play a crucial role in the transmission of RHD. In normal conditions, most outbreaks of RHD occur in winter or spring. High temperatures in late spring and summer will considerably reduce the spread of the virus. RHD will also be more prevalent in dry and semi-dry areas than in areas that are relatively cool and humid.
Symptomatic cases of RHD will display fever, squeals, and often coma leading to death within 12 to 36 hours. In less severe cases, rabbits may display uneasiness, excitement, anorexia
, swollen eyelids, paralysis
, ocular haemorrhages, and paddling
. Convulsions may be seen as well. A fatal bloody discharge from the nose has been exhibited along with blood-stained cage floors, though these symptoms may have occurred after death. Rabbits who have recovered from the less severe symptoms usually develop severe jaundice
with weight loss and lethargy. Diarrhoea, constipation and abdominal cramping are then exhibited right before death a few weeks later.
Research conducted at the CSIRO Australian Animal Health Laboratory claimed that rabbits did not show any outward signs of pain or suffering as a result of having RHD. The researchers further claimed that most infected rabbits became quieter and died quietly with no pain or suffering. However, the research did not in anyway prove or disprove that the rabbits did not experience severe and prolonged pain, only, that based on the subjective observation of their researchers, that the rabbits CSIRO infected grew quieter before death. It should be noted that CSIRO was actively researching RHD to use as an agent to help exterminate selected rabbit populations in Australia.
RHD causes rapid development of blood clot formation in major organs such as the heart, lungs and kidneys. The clots block blood vessels causing heart and respiratory failure. An infected rabbit that has died from RHD will often have its legs straight out and head over its neck as if trying to catch its breath.
, myxomatosis
, poisoning, heat exhaustion, and E. coli or Clostridium perfringens
type E enterotoxemia
.
Rabbits that die from RHD are usually in good outward state. However, the most frequent post-mortem lesions are hepatic necrosis and splenomegaly
. The liver of RHD rabbits may have a fine reticular pattern of necrosis outline each lobule and maybe yellow, gray or pale in colour. The liver is also usually friable and swollen. The spleen will be black in colour and also swollen with rounded edges, while the kidneys are dark brown in colour. Haemorrhages will also be seen many other organs and tissues. The trachea may present a foamy, bloody mucous. Enteritis
of the small intestine and swollen meninges
may also occur.
Laboratory tests such as reverse transcription polymerase chain reaction
(RT-PCR), Western blotting, negative-staining immunoelectron microscopy and ELISA
s may be performed on samples from the liver, blood, spleen or other organs.
estimates for RHD have a broad range. The morbidity rate ranges from 30% to 100% and the mortality rate from 40% to 100%; however, the typical mortality rate is usually around 90%.{
In the wild, outbreaks in rabbits vary depending on the season, breeding cycles and geographical location. Some areas will see a high morbidity and mortality among its rabbit populations followed by calmer periods.
Maternal antibodies such as immunoglobulin G
(IgG), which are readily transmitted to the young across the placenta, may explain why very young rabbits are resistant to RHD. Some scientists also believe that the immature immune system of a young rabbit cannot produce the number of chemicals needed to initiate clotting in order to kill. Rabbits may develop immunity against other strains of the RHD virus, while others may endure persistent infections. The immunity does not survive through the next generation, leaving open the possibility of further outbreaks in the population.
A population of rabbits on the San Juan Islands
, Washington, USA appears to be immune to RHD. The cottontail rabbit
and jackrabbit are not affected by the disease; nor are humans or other mammals.
/Merial
Manual For Pet Health, Home Edition, 2007, RHD is a reportable disease in the United States. If a diagnosis is made by a veterinarian, a notification to the "appropriate government authorities" must be made.
Because of the highly infectious nature of the disease, strict quarantine is necessary when outbreaks occur. Depopulation, disinfection, surveillance and quarantines are the only way to properly and effectively eradicate the disease. Good disinfectants include 10% sodium hydroxide, 1-2% formalin, 2% One-Stroke Environ, and 10% household bleach
. The RHD virus is resistant to ethers or chloroform
. Deceased rabbits must be removed immediately and discarded in a safe manner. Surviving rabbits should be quarantined or humanely euthanized. Test rabbits may be used to monitor the virus on vaccinated farms.
An effective, safe "killed" vaccine (ATCvet code: ) has been developed in various countries, particularly Spain, and is believed to give protection for 5 to 15 months; however, vaccine manufacturers recommend a booster be administered annually. The vaccine is also recommended in areas where the disease is widespread and eradication efforts have been made.
is the second most serious pest in New Zealand. Rabbits compete with livestock for grazing pasture, kill trees, shrubs, and have contributed to the extinction of some native plants. Consequently, rabbits contribute to soil erosion by eliminating the protective vegetation and disturb the soil by burrowing.
The estimated combined cost of control and production losses in New Zealand as a result of rabbits is about $23 million annually. This figure is only a small portion of the damage caused by rabbits.
Control measures used against rabbits in New Zealand include poisoning, shooting, ripping, blasting, releasing predators, and fencing.
After the safety of RHD was confirmed by laboratory research, RHD was approved for release as a biological control agent in New Zealand. RHD is safe because it infects rabbits, but not other animals or humans. It is also safe to eat the meat of infected RHD rabbits. Virus mutation is not a concern; many years of research show no evidence that the virus has changed to affect any other species other than European rabbit.
However, it is possible that widespread rabbit deaths might cause predators to prey upon other food sources, such as endangered or rare native species. With proper vaccination plans, the safety of domesticated rabbits should not be a concern.
Lagovirus
-Morphology:Virions consist of a capsid. Virus capsid is not enveloped, round with icosahedral symmetry. The isometric capsid has a diameter of 35-39 nm. Capsids appear round to hexagonal in outline. The capsid surface structure reveals a regular pattern with distinctive features. The capsomer...
of the family Caliciviridae
Caliciviridae
The Caliciviridae family are a family of viruses, members of Class IV of the Baltimore scheme. They are positive-sense, single stranded RNA which is non-segmented. The caliciviruses have been found in a number of organisms such as humans, cattle, pigs, cats, chickens, reptiles, dolphins and...
. The virus infects only rabbits, and has been used in some countries to control rabbit populations.
There are vaccines available against VHD in the UK. One is named Cylap and made by Fort Dodge Animal Health. Another is Lapinject and is made by CEVA Animal Health Ltd. Both last 12 months and contain inactivated strains of VHD. A third called Anivac has recently become available.
Origins
The rabbit hemorrhagic disease virus (RHDV) appears to have evolved from an avirulent (non-pathogenic) precursor virus, rabbit calicivirus (RCV), which was detected in rabbit sera from years before the outbreak of the disease. Infection with RCV is protective against RHDV, sparing some rabbit colonies and many wild rabbits in cool, wet areas of Australia and New Zealand from the disease. The Australian government refers to both viruses as RCV.RHDV and RCV are members of a larger group of Lagovirus
Lagovirus
-Morphology:Virions consist of a capsid. Virus capsid is not enveloped, round with icosahedral symmetry. The isometric capsid has a diameter of 35-39 nm. Capsids appear round to hexagonal in outline. The capsid surface structure reveals a regular pattern with distinctive features. The capsomer...
es within the Caliciviridae
Caliciviridae
The Caliciviridae family are a family of viruses, members of Class IV of the Baltimore scheme. They are positive-sense, single stranded RNA which is non-segmented. The caliciviruses have been found in a number of organisms such as humans, cattle, pigs, cats, chickens, reptiles, dolphins and...
. Another member of this group, the European brown hare syndrome virus (EBHSV) is 71% homologous in nucleotide sequence, but exposures to the two viruses do not cross-protect. RHDV is known only to infect the European Rabbit
European Rabbit
The European Rabbit or Common Rabbit is a species of rabbit native to south west Europe and north west Africa . It has been widely introduced elsewhere often with devastating effects on local biodiversity...
, while EBHSV is limited to the range of the European Brown Hare but can infect Varying Hares and American Eastern cottontail
Eastern Cottontail
The eastern cottontail is a New World cottontail rabbit, a member of the family Leporidae. It is one of the most common rabbit species in North America.-Distribution:...
s within that range or in experimental tests, respectively.
History
RHD first appeared in 1984 in the People's Republic of ChinaPeople's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
. Scientists cannot pinpoint its exact origins; however, it is believed the disease emerged from a virulent calicivirus spreading asymptomatically in European rabbit populations, particularly in the German Democratic Republic
German Democratic Republic
The German Democratic Republic , informally called East Germany by West Germany and other countries, was a socialist state established in 1949 in the Soviet zone of occupied Germany, including East Berlin of the Allied-occupied capital city...
. The Chinese outbreak was spread by the angora rabbit
Angora rabbit
The Angora rabbit is a variety of domestic rabbit bred for its long, soft wool. The Angora is one of the oldest types of domestic rabbit, originating in Ankara , Turkey, along with the Angora cat and Angora goat. The rabbits were popular pets with French royalty in the mid 18th century, and spread...
, which had originated in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. Fourteen million domesticated rabbits died within nine months in the outbreak.
In 1984 the virus that caused the disease was identified. The virus spread westward and reached Europe in 1988. The virus has since appeared in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
, Cuba
Cuba
The Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
and the USA. In 1992, the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
reported its first case of RHD in domestic show rabbits. By the late 1990s, RHD stretched to forty countries and had become endemic in wild rabbit populations in Europe, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
and Cuba
Cuba
The Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...
. In Europe there was a rapid increase in research into RHD, due to the importance of the commercial breeding of rabbits for meat and fur production.
The first reported case in the United States was in Iowa on March 9, 2000. The affected population included Palominos and California Whites
Californian rabbit
The Californian breed of domestic rabbit was developed in the early 1920s by George West. Rabbit breeders in the United States crossed Himalayan breeds and the Standard Chinchilla rabbit breed and New Zealand Whites. The purpose of this breed was to have a good meat breed that also had good fur....
. By April 6, 25 of the 27 affected rabbits had died as a result of RHD. In order to contain the disease, the remaining two rabbits were euthanized. No new introductions of rabbits were placed on the farm for two years after the discovery of RHD and August 1999 was the last time rabbits left and/or returned to the farm. The United States experienced other outbreaks of RHD in 2001 (Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...
, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
) and 2005 (Indiana
Indiana
Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...
).
Asia
Within a few months of RHD being reported in China in 1984, the disease was widely seen in many commercial rabbitries and had reached the Republic of Korea. RHD has also been reported in India and the Middle East.Americas (North and South)
Since 1993, RHD has been endemic in Cuba; it is also believed to be thriving in BoliviaBolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
. From 1988 to 1992 Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
dealt with an RHD endemic in domestic rabbits.
After outbreaks of RHD in 2000, 2001, and 2005 in domesticated rabbits, the United States has eradicated RHD from its rabbit populations. The native species, cottontails
Eastern Cottontail
The eastern cottontail is a New World cottontail rabbit, a member of the family Leporidae. It is one of the most common rabbit species in North America.-Distribution:...
(Sylvilagus floridanus), black-tailed jackrabbit
Black-tailed Jackrabbit
The black-tailed jackrabbit , also known as the american desert hare, is a common hare of the western United States and Mexico, where it is found at elevations from sea level to up to...
s (Lepus californicus) and volcano rabbit
Volcano Rabbit
The Volcano Rabbit also known as teporingo or zacatuche is a small rabbit that resides in the mountains of Mexico. It is the world's second smallest rabbit, second only to the Pygmy Rabbit. It has small rounded ears, short legs, and short, thick fur. The Volcano Rabbit lives in groups of 2 to 5...
s (Romerolagus diazzi) seem not to be susceptible to the virus.
Australia
In 1991 the virus that caused RHD was imported to Australia under strict quarantineQuarantine
Quarantine is compulsory isolation, typically to contain the spread of something considered dangerous, often but not always disease. The word comes from the Italian quarantena, meaning forty-day period....
conditions to research the safety and usefulness of the virus if it was used as a biological control agent against Australia and New Zealand's rabbit pest problem. Testing of the virus was undertaken on Wardang Island
Wardang Island
Wardang Island, also known as Wauraltee Island, is a low-lying 20 km2 island in the Spencer Gulf close to the western coast of the Yorke Peninsula, South Australia. It acts as a natural breakwater, protecting the former grain port of Port Victoria and providing a sheltered anchorage. ...
in Spencer Gulf
Spencer Gulf
The Spencer Gulf is the westernmost of two large inlets on the southern coast of Australia, in the state of South Australia, facing the Great Australian Bight. The Gulf is 322 km long and 129 km wide at its mouth. The western shore of the Gulf is the Eyre Peninsula, while the eastern side is the...
off the coast of the Yorke Peninsula
Yorke Peninsula
The Yorke Peninsula is a peninsula located north-west and west of Adelaide in South Australia, Australia, between Spencer Gulf on the west and Gulf St Vincent on the east. It has geographic coordinates of...
, South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
. In 1995 the virus escaped quarantine and subsequently killed 10 million rabbits within 8 weeks of its release. The disease is currently epidemic throughout Australia and New Zealand.
New Zealand
In July 1997, after considering over 800 public submissions, the New Zealand Ministry of HealthNew Zealand Ministry of Health
The Ministry of Health , formerly the Department of Health from 1903 to 1993, is a department of the New Zealand government. This is the channel through which the government channels its funding for health services, VOTE: Health.The ministry is overseen by the Minister of Health in the New Zealand...
decided not to allow RHDV to be imported into New Zealand to control rabbit populations. This was backed up in an early August review of the decision by the Director-General of Agriculture. By late August, it was confirmed that RHDV had been deliberately introduced to the Cromwell
Cromwell, New Zealand
Cromwell is a town in Central Otago in the Otago region of New Zealand.It is situated between State Highway 6 and State Highway 8 leading to the Lindis Pass, 75 km northeast, and Alexandra, 33 km south. The road to Alexandra winds through the Cromwell Gorge...
area of the South Island
South Island
The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...
.
An unsuccessful attempt was made by New Zealand officials to control the spread of the disease. It was, however, being intentionally spread, and several farmers (notably in the Mackenzie Basin
Mackenzie Basin
The Mackenzie Basin , is an elliptical intermontane basin, located in the Mackenzie and Waitaki Districts, near the centre of the South Island of New Zealand. It is the largest such basin in New Zealand...
area) admitted to processing rabbits that had died from the disease in kitchen blenders for further spreading.
Had the disease been introduced at a better time, there would have been a more effective control of the population. Unfortunately, it was released after breeding had commenced for the season, and rabbits under 2 weeks old at the time of the introduction were immune to the disease. These young rabbits were therefore able to continue to grow and breed back up. Ten years on, rabbit populations (in the Mackenzie Basin
Mackenzie Basin
The Mackenzie Basin , is an elliptical intermontane basin, located in the Mackenzie and Waitaki Districts, near the centre of the South Island of New Zealand. It is the largest such basin in New Zealand...
in particular) are beginning to reach near pre-plague proportions once again though they have not yet returned to pre RCD levels.
Resistance to RHD in New Zealand rabbits has led to the widespread use of 1080 poison. Farmers and land managers are having to increase their use of 1080 to protect pastoral land from rabbits and preserve the gains made in recent years through the use of RHD.
Europe
RHD is endemic throughout most of Europe. ItalyItaly
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
's first case of RHD was recorded in 1986 and Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
's in 1988. France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
(June) and Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
followed in 1990. Within a few years of RHD's first appearance in Europe it had caused the largest mortality in domestic and wild rabbits in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
, Spain and Italy. Spain was the worst affected by RHD.
When the United Kingdom's first case of RHD in 1992 was discovered, the disease was transmitted into the wild by domesticated pet rabbits. Sources vary in the number of confirmed cases of RHD; there were 9 known outbreaks in 1994, 32 cases but some sources believe there were as many as 512 cases of RHD in 1995, and around 30 RHD cases in 1996 throughout Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
and Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
.
Transmission
Transmission of RHD occurs by direct contact with an infected animal and fomiteFomite
A fomite is any inanimate object or substance capable of carrying infectious organisms and hence transferring them from one individual to another. A fomite can be anything...
s. Rabbits acquire RHD through oral, nasal or conjunctival pathways. Urine, faeces and respiratory secretions may also shed the virus. The virus may also be carried by the wind. Carriers of the virus may remain infectious for up to a month depending on climate conditions; however, the virus has been known to persist for as little as 2 days and as long as 215 days. An infected carcass or hairs from an infected animal may also transmit RHD. Fomites such as clothing, contaminated food, cages, bedding, feeders and water will also harbour the virus. Even though the virus cannot reproduce in other mammals, predators and scavengers such as foxes, ferrets and some birds can excrete the virus through their faeces after ingesting an infected rabbit carcass. Flies, rabbit fleas, and mosquitoes can also spread the virus between rabbits.
Climate appears to play a crucial role in the transmission of RHD. In normal conditions, most outbreaks of RHD occur in winter or spring. High temperatures in late spring and summer will considerably reduce the spread of the virus. RHD will also be more prevalent in dry and semi-dry areas than in areas that are relatively cool and humid.
Signs
RHD primarily infects only adult rabbits. In fact, research has shown that rabbits younger than 8 weeks of age are resistant to the virus. The incubation period for the RHD virus is between 1 to 3 days, with death following 1 to 2 days after the infection. There is a wide range of RHD symptoms. Most rabbits will show no signs of external symptoms of RHD.Symptomatic cases of RHD will display fever, squeals, and often coma leading to death within 12 to 36 hours. In less severe cases, rabbits may display uneasiness, excitement, anorexia
Anorexia (symptom)
Anorexia is the decreased sensation of appetite...
, swollen eyelids, paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...
, ocular haemorrhages, and paddling
Flaying
Flaying is the removal of skin from the body. Generally, an attempt is made to keep the removed portion of skin intact.-Scope:An animal may be flayed in preparation for human consumption, or for its hide or fur; this is more commonly called skinning....
. Convulsions may be seen as well. A fatal bloody discharge from the nose has been exhibited along with blood-stained cage floors, though these symptoms may have occurred after death. Rabbits who have recovered from the less severe symptoms usually develop severe jaundice
Jaundice
Jaundice is a yellowish pigmentation of the skin, the conjunctival membranes over the sclerae , and other mucous membranes caused by hyperbilirubinemia . This hyperbilirubinemia subsequently causes increased levels of bilirubin in the extracellular fluid...
with weight loss and lethargy. Diarrhoea, constipation and abdominal cramping are then exhibited right before death a few weeks later.
Research conducted at the CSIRO Australian Animal Health Laboratory claimed that rabbits did not show any outward signs of pain or suffering as a result of having RHD. The researchers further claimed that most infected rabbits became quieter and died quietly with no pain or suffering. However, the research did not in anyway prove or disprove that the rabbits did not experience severe and prolonged pain, only, that based on the subjective observation of their researchers, that the rabbits CSIRO infected grew quieter before death. It should be noted that CSIRO was actively researching RHD to use as an agent to help exterminate selected rabbit populations in Australia.
RHD causes rapid development of blood clot formation in major organs such as the heart, lungs and kidneys. The clots block blood vessels causing heart and respiratory failure. An infected rabbit that has died from RHD will often have its legs straight out and head over its neck as if trying to catch its breath.
Diagnosis
RHD may be indicated when several animals in the herd die after experiencing a fever and lethargy. Differential diagnosis includes pasteurellosisPasteurellosis
Pasteurellosis is an infection with a species of the bacteria genus Pasteurella, which is found in humans and animals.Pasteurella multocida is carried in mouth and respiratory tract of several animals, notably cats. It is a small Gram negative bacillus with bipolar staining by Wayson stain...
, myxomatosis
Myxomatosis
Myxomatosis is a disease that affects rabbits and is caused by the Myxoma virus. It was first observed in Uruguay in laboratory rabbits in the late 19th century. It was introduced into Australia in 1950 in an attempt to control the rabbit population...
, poisoning, heat exhaustion, and E. coli or Clostridium perfringens
Clostridium perfringens
Clostridium perfringens is a Gram-positive, rod-shaped, anaerobic, spore-forming bacterium of the genus Clostridium. C. perfringens is ever present in nature and can be found as a normal component of decaying vegetation, marine sediment, the intestinal tract of humans and other vertebrates,...
type E enterotoxemia
Enterotoxemia
Enterotoxemia is an infection by Clostridium perfringens which affects several types of domesticated animals, but is not known to affect humans. It is also known as overeating disease and can kill the animals within 2 hours.-External links:...
.
Rabbits that die from RHD are usually in good outward state. However, the most frequent post-mortem lesions are hepatic necrosis and splenomegaly
Splenomegaly
Splenomegaly is an enlargement of the spleen. The spleen usually lies in the left upper quadrant of the human abdomen. It is one of the four cardinal signs of hypersplenism, some reduction in the number of circulating blood cells affecting granulocytes, erythrocytes or platelets in any...
. The liver of RHD rabbits may have a fine reticular pattern of necrosis outline each lobule and maybe yellow, gray or pale in colour. The liver is also usually friable and swollen. The spleen will be black in colour and also swollen with rounded edges, while the kidneys are dark brown in colour. Haemorrhages will also be seen many other organs and tissues. The trachea may present a foamy, bloody mucous. Enteritis
Enteritis
In medicine, enteritis, from Greek words enteron and suffix -itis , refers to inflammation of the small intestine. It is most commonly caused by the ingestion of substances contaminated with pathogenic microorganisms. Symptoms include abdominal pain, cramping, diarrhea, dehydration and fever...
of the small intestine and swollen meninges
Meninges
The meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...
may also occur.
Laboratory tests such as reverse transcription polymerase chain reaction
Reverse transcription polymerase chain reaction
Reverse transcription polymerase chain reaction is a variant of polymerase chain reaction , a laboratory technique commonly used in molecular biology to generate many copies of a DNA sequence, a process termed "amplification"...
(RT-PCR), Western blotting, negative-staining immunoelectron microscopy and ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
s may be performed on samples from the liver, blood, spleen or other organs.
Morbidity, mortality, and immunity
RHD is extremely hard to locate in the wild since about 75% of rabbits with RHD will die in their burrows underground. Due to this difficulty, the morbidity and mortalityMortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
estimates for RHD have a broad range. The morbidity rate ranges from 30% to 100% and the mortality rate from 40% to 100%; however, the typical mortality rate is usually around 90%.{
In the wild, outbreaks in rabbits vary depending on the season, breeding cycles and geographical location. Some areas will see a high morbidity and mortality among its rabbit populations followed by calmer periods.
Maternal antibodies such as immunoglobulin G
Immunoglobulin G
Immunoglobulin G are antibody molecules. Each IgG is composed of four peptide chains — two heavy chains γ and two light chains. Each IgG has two antigen binding sites. Other immunoglobulins may be described in terms of polymers with the IgG structure considered the monomer.IgG constitutes 75%...
(IgG), which are readily transmitted to the young across the placenta, may explain why very young rabbits are resistant to RHD. Some scientists also believe that the immature immune system of a young rabbit cannot produce the number of chemicals needed to initiate clotting in order to kill. Rabbits may develop immunity against other strains of the RHD virus, while others may endure persistent infections. The immunity does not survive through the next generation, leaving open the possibility of further outbreaks in the population.
A population of rabbits on the San Juan Islands
San Juan Islands
The San Juan Islands are an archipelago in the northwest corner of the contiguous United States between the US mainland and Vancouver Island, British Columbia, Canada. The San Juan Islands are part of the U.S...
, Washington, USA appears to be immune to RHD. The cottontail rabbit
Cottontail rabbit
The cottontail rabbits are among the 16 lagomorph species in the genus Sylvilagus, found in the Americas.In appearance, most cottontail rabbits closely resemble the wild European Rabbit...
and jackrabbit are not affected by the disease; nor are humans or other mammals.
Control
Countries that are uninfected by RHD may place restrictions on importation from endemic countries. According to the MerckMerck & Co.
Merck & Co., Inc. , also known as Merck Sharp & Dohme or MSD outside the United States and Canada, is one of the largest pharmaceutical companies in the world. The Merck headquarters is located in Whitehouse Station, New Jersey, an unincorporated area in Readington Township...
/Merial
Merial
Merial is a multinational animal health company. It was formed as a joint venture between Merck & Co. and Sanofi-Aventis in 1997. Merial is now the animal health subsidiary of Sanofi-Aventis. Merial is a company leader in innovation, and provides an ample range with products to improve the...
Manual For Pet Health, Home Edition, 2007, RHD is a reportable disease in the United States. If a diagnosis is made by a veterinarian, a notification to the "appropriate government authorities" must be made.
Because of the highly infectious nature of the disease, strict quarantine is necessary when outbreaks occur. Depopulation, disinfection, surveillance and quarantines are the only way to properly and effectively eradicate the disease. Good disinfectants include 10% sodium hydroxide, 1-2% formalin, 2% One-Stroke Environ, and 10% household bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...
. The RHD virus is resistant to ethers or chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
. Deceased rabbits must be removed immediately and discarded in a safe manner. Surviving rabbits should be quarantined or humanely euthanized. Test rabbits may be used to monitor the virus on vaccinated farms.
An effective, safe "killed" vaccine (ATCvet code: ) has been developed in various countries, particularly Spain, and is believed to give protection for 5 to 15 months; however, vaccine manufacturers recommend a booster be administered annually. The vaccine is also recommended in areas where the disease is widespread and eradication efforts have been made.
Use as biological control agent
The European rabbitEuropean Rabbit
The European Rabbit or Common Rabbit is a species of rabbit native to south west Europe and north west Africa . It has been widely introduced elsewhere often with devastating effects on local biodiversity...
is the second most serious pest in New Zealand. Rabbits compete with livestock for grazing pasture, kill trees, shrubs, and have contributed to the extinction of some native plants. Consequently, rabbits contribute to soil erosion by eliminating the protective vegetation and disturb the soil by burrowing.
The estimated combined cost of control and production losses in New Zealand as a result of rabbits is about $23 million annually. This figure is only a small portion of the damage caused by rabbits.
Control measures used against rabbits in New Zealand include poisoning, shooting, ripping, blasting, releasing predators, and fencing.
After the safety of RHD was confirmed by laboratory research, RHD was approved for release as a biological control agent in New Zealand. RHD is safe because it infects rabbits, but not other animals or humans. It is also safe to eat the meat of infected RHD rabbits. Virus mutation is not a concern; many years of research show no evidence that the virus has changed to affect any other species other than European rabbit.
However, it is possible that widespread rabbit deaths might cause predators to prey upon other food sources, such as endangered or rare native species. With proper vaccination plans, the safety of domesticated rabbits should not be a concern.