Microfilaria
Encyclopedia
The microfilaria is a stage in the life cycle of certain parasitic
nematode
s in the family Onchocercidae
. In these species, the adults live in the circulatory system of vertebrates (the "definitive hosts"
) while the early larval stages develop in blood-feeding arthropod
vectors (the "intermediate hosts
"). The microfilariae are released into the bloodstream of the vertebrate host, and are taken up by the arthropod vector. In the arthropod vector, the microfilariae develop into infective larvae.
The presence of microfilariae in the host bloodstream is called "microfilaraemia". The success of filariasis
eradication programs is typically gauged by the reduction in numbers of circulating microfilariae in infested individuals within a geographic area.
(cause of trichinosis
), rely on new hosts eating the tissues of their current host. For members of the family Onchocercidae whose adults live in the "closed" vertebrate circulatory system
, transmission to a new host is achieved by the microfilaria stage, with the help of blood-feeding arthropod
vectors.
This system is seen in the life cycle of Elaeophora schneideri
. The adults of E. schneideri typically reside in the carotid artery
of its definitive host
, the Mule deer
. The female may be up to 12 cm (almost 5 inches) long, and releases microfilariae which measure 207 by 13 µm (or 0.008 by 0.00051 inches!) into the bloodstream of the host. The blood flow carries the microfilariae away from the female in the carotid artery, and directly into the branching arteries of the head and face. Because of their size, the microfilariae pass easily through successively smaller vessles, becoming physically lodged in the small capillaries near the skin surface of the face and head.
Attracted by the carbon dioxide exhaled by the mule deer,http://www.ca.uky.edu/entomology/entfacts/ef511.asp the blood-feeding female horse fly often lands on the head or face to feed. The horse fly uses its scissor-like mouthparts to cut the surface of the skin, creating a pool of blood which it takes in through its sucking mouthparts. The microfilariae, which were just under the surface of the skin, are small enough to be ingested whole by the horse fly. Once inside the horse fly, the microfilariae bore through the stomach wall, and mature into infective larvae about two weeks later. These larvae migrate to the head and mouthparts of the horse fly, and enter the bloodstream of another vertebrate host when the horse fly feeds again.
In either case, the microfilaria is the stage which develops from the egg. In most tissue-dwelling species the eggs hatch in the uterus
of the female, and the unsheathed microfilariae are released. In most blood-dwelling species, embryonated eggs (or, microfilariae which are said to be sheathed in the envelope of the egg) are released; and they will only exsheath ("hatch") after being ingested by the arthropod
intermediate host
. All microfilariae burrow through the stomach wall after being eaten by the arthropod host, and develop into infective third stage (L3) larvae.
Many of the organs of microfilariae are in a very early stage of development. For some species, the developmental fates
of individual cells have been followed from the microfilaria stage to the adult worm. The microfilariae of many species undergo a development phase called the "sausage stage", becoming temporarily shorter and thicker, while the first-stage (L1) larval organs develop.
In some species of Onchocercidae, the release of microfilariae by the adult female is periodic - occurring daily at a particular time of the day or night. This timing increases the chance that they will be picked up by a blood-feeding arthropod vector, which are often more active at certain times of the day.
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s in the family Onchocercidae
Onchocercidae
Onchocercidae Chabaud & Anderson, 1959 is a family of nematodes in the superfamily Filarioidea. This family includes some of the most devastating human parasitic diseases, such as Lymphatic filariasis, Onchocerciasis, Loiasis, and other filariases....
. In these species, the adults live in the circulatory system of vertebrates (the "definitive hosts"
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
) while the early larval stages develop in blood-feeding arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
vectors (the "intermediate hosts
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
"). The microfilariae are released into the bloodstream of the vertebrate host, and are taken up by the arthropod vector. In the arthropod vector, the microfilariae develop into infective larvae.
The presence of microfilariae in the host bloodstream is called "microfilaraemia". The success of filariasis
Filariasis
Filariasis is a parasitic disease and is considered an infectious tropical disease, that is caused by thread-like nematodes belonging to the superfamily Filarioidea, also known as "filariae"....
eradication programs is typically gauged by the reduction in numbers of circulating microfilariae in infested individuals within a geographic area.
Escaping the circulatory system
All parasites need a mechanism for spreading to new individual hosts. Parasites in the lower gastrointestinal tract usually shed eggs in the host feces. Tissue-dwelling parasites, such as Trichinella spiralisTrichinella spiralis
Trichinella spiralis is a nematode parasite, occurring in rats, pigs, bears and humans, and is responsible for the disease trichinosis. It is sometimes referred to as the "pork worm" due to it being found commonly in undercooked pork products...
(cause of trichinosis
Trichinosis
Trichinosis, also called trichinellosis, or trichiniasis, is a parasitic disease caused by eating raw or undercooked pork or wild game infected with the larvae of a species of roundworm Trichinella spiralis, commonly called the trichina worm. There are eight Trichinella species; five are...
), rely on new hosts eating the tissues of their current host. For members of the family Onchocercidae whose adults live in the "closed" vertebrate circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
, transmission to a new host is achieved by the microfilaria stage, with the help of blood-feeding arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
vectors.
This system is seen in the life cycle of Elaeophora schneideri
Elaeophora schneideri
Elaeophora schneideri is a nematode which infests several mammalian hosts in North America. It is transmitted by horse-flies. Infection in the normal definitive hosts, Mule deer or Black-tailed deer, seldom produces clinical symptoms...
. The adults of E. schneideri typically reside in the carotid artery
Carotid artery
Carotid artery can refer to:* Common carotid artery* External carotid artery* Internal carotid artery...
of its definitive host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
, the Mule deer
Mule Deer
The mule deer is a deer indigenous to western North America. The Mule Deer gets its name from its large mule-like ears. There are believed to be several subspecies, including the black-tailed deer...
. The female may be up to 12 cm (almost 5 inches) long, and releases microfilariae which measure 207 by 13 µm (or 0.008 by 0.00051 inches!) into the bloodstream of the host. The blood flow carries the microfilariae away from the female in the carotid artery, and directly into the branching arteries of the head and face. Because of their size, the microfilariae pass easily through successively smaller vessles, becoming physically lodged in the small capillaries near the skin surface of the face and head.
Attracted by the carbon dioxide exhaled by the mule deer,http://www.ca.uky.edu/entomology/entfacts/ef511.asp the blood-feeding female horse fly often lands on the head or face to feed. The horse fly uses its scissor-like mouthparts to cut the surface of the skin, creating a pool of blood which it takes in through its sucking mouthparts. The microfilariae, which were just under the surface of the skin, are small enough to be ingested whole by the horse fly. Once inside the horse fly, the microfilariae bore through the stomach wall, and mature into infective larvae about two weeks later. These larvae migrate to the head and mouthparts of the horse fly, and enter the bloodstream of another vertebrate host when the horse fly feeds again.
Microfilaria as a developmental stage
Most recent parasitology textbooks consider the microfilariae to be "pre-larvae or advanced embryos" which will develop into the first stage larvae (L1) in the arthropod vector (p. 364). Some consider them to be the first larval stage, such as "microfilariae; i.e. first larva (= L1)" (p. 361).In either case, the microfilaria is the stage which develops from the egg. In most tissue-dwelling species the eggs hatch in the uterus
In utero
In utero is a Latin term literally meaning "in the womb". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to refer to unborn children. Under common law, unborn children are still considered to exist for property transfer purposes.-See also:*...
of the female, and the unsheathed microfilariae are released. In most blood-dwelling species, embryonated eggs (or, microfilariae which are said to be sheathed in the envelope of the egg) are released; and they will only exsheath ("hatch") after being ingested by the arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
intermediate host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
. All microfilariae burrow through the stomach wall after being eaten by the arthropod host, and develop into infective third stage (L3) larvae.
Many of the organs of microfilariae are in a very early stage of development. For some species, the developmental fates
Cell fate determination
Within the field of developmental biology one goal is to understand how a particular cell develops into the final cell type , essentially how a cell’s fate is determined. Within an embryo, 4 processes play out at the cellular and tissue level to essentially create the final organism...
of individual cells have been followed from the microfilaria stage to the adult worm. The microfilariae of many species undergo a development phase called the "sausage stage", becoming temporarily shorter and thicker, while the first-stage (L1) larval organs develop.
In some species of Onchocercidae, the release of microfilariae by the adult female is periodic - occurring daily at a particular time of the day or night. This timing increases the chance that they will be picked up by a blood-feeding arthropod vector, which are often more active at certain times of the day.