::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown, it is seldom used today.'' Microfilaria manaus br

The microfilaria (plural microfilariae, sometimes abbreviated mf) is an early stage in the life cycle of certain

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

nematodes in the family

Onchocercidae The Onchocercidae are 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. Represent ...

. In these species, the adults live in a tissue or the circulatory system of vertebrates (the "definitive hosts"). They release microfilariae into the bloodstream of the vertebrate host. The microfilariae are taken up by blood-feeding

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

vectors (the " intermediate hosts"). In the intermediate host the microfilariae develop into infective larvae that can be transmitted to a new vertebrate host. The presence of microfilariae in the host bloodstream is called "microfilaraemia". The success of

filariasis Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding insects such as black flies and mosquitoes. They belong to the group of diseases called helminthiases. These ...

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 The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

usually shed eggs in the host feces. Tissue-dwelling parasites, such as ''

Trichinella spiralis ''Trichinella spiralis'' is a viviparous nematode parasite, occurring in rodents, pigs, bears, hyenas and humans, and is responsible for the disease trichinosis. It is sometimes referred to as the "pork worm" due to it being typically encount ...

'' (cause of

trichinosis Trichinosis, also known as trichinellosis, is a parasitic disease caused by roundworms of the '' Trichinella'' type. During the initial infection, invasion of the intestines can result in diarrhea, abdominal pain, and vomiting. Migration of ...

), 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 The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, transmission to a new host is achieved by the microfilaria stage, with the help of blood-feeding

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 In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...

, the

Mule deer The mule deer (''Odocoileus hemionus'') is a deer indigenous to western North America; it is named for its ears, which are large like those of the mule. Two subspecies of mule deer are grouped into the black-tailed deer. Unlike the related whi ...

. 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 vessels, 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 dee

the blood-feeding female

horse fly Horse-flies or horseflies are true flies in the family Tabanidae in the insect order Diptera. They are often large and agile in flight, and only the female horseflies bite animals, including humans, to obtain blood. They prefer to fly in sun ...

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. 361Mehlhorn, H. (ed) (2001) ''Encyclopedic Reference of Parasitology: Biology, Structure and Function''. 2nd Edition. Springer: Berlin). 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

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

, 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.

References

{{reflist Kolte SS, Satarkar RN, Mane PM. Microfilaria concomitant with metastatic deposits of adenocarcinoma in lymph node fine needle aspiration cytology: A chance finding. J Cytol erial online2010 ited 2010 Nov 2227:78-80. Available from: http://www.jcytol.org/text.asp?2010/27/2/78/70759 Spirurida