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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,^[1] it is seldom used today.

Microfilaria found in blood slides LACEN State Laboratory of Amazonas Brazil

The microfilaria (plural microfilariae, sometimes abbreviated mf) is an early stage in the life cycle of certain parasitic nematodes in the family Onchocercidae.^[2] 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 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 eradication programs is typically gauged by the reduction in numbers of circulating microfilariae in infested individuals within a geographic area.^[3]

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 spiralis (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.^[4] 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,[1] 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^[5]). Some consider them to be the first larval stage, such as "microfilariae; i.e. first larva (= L1)" (p. 361^[6]).

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.

References

^ Purnomo; Bangs, M.J. (1995) "Microfilaria sundaicus sp.n., a Chabfilaria-like parasite (Filarioidea: Onchocerciae) from the blood of the horseshoe bat (Rhinolophus affinis) in Flores, Indonesia". Journal of the Helminthological Society of Washington 62(1):32-34.
^ Anderson, R.C. (2000) Nematode Parasites of Vertebrates: Their Development and Transmission. Second Edition. CABI Publishing: Wallingford, England, pp. 472-475.
^ Farid, H.A., Z.S. Morsy, H. Helmy, R.M.R. Ramzy, M. El Setouhy, and G.J. Weil (2007) "A critical appraisal of molecular xenomonitoring as a tool for assessing progress toward elimination of lymphatic filariasis." American Journal of Tropical Medicine and Hygiene 77(4):593-600.
^ Hibler, C.P. and C.J. Metzger (1974) Morphology of the larval stages of Elaeophora schneideri in the intermediate and definitive hosts with some observations on their pathogenesis in abnormal definitive hosts. Journal of Wildlife Diseases 10(4):361-369.
^ Bogitsh, B.J., C.E. Carter and T.N. Oeltmann (2005) Human Parasitology. 3rd Edition. Elsevier: Burlington, MA. ISBN 0-12-088468-2.
^ Mehlhorn, H. (ed) (2001) Encyclopedic Reference of Parasitology: Biology, Structure and Function. 2nd Edition. Springer: Berlin

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 [serial online] 2010 [cited 2010 Nov 22];27:78-80. Available from: http://www.jcytol.org/text.asp?2010/27/2/78/70759

UpToDate Contents

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1. リンパ管フィラリア症の診断、治療、および予防 diagnosis treatment and prevention of lymphatic filariasis
2. ロア糸状虫症（ロア糸状虫感染） loiasis loa loa infection
3. リンパ管フィラリア症の疫学、病因、および臨床症状 epidemiology pathogenesis and clinical manifestations of lymphatic filariasis
4. マンソネラ感染症 mansonella infections
5. オンコセルカ症 onchocerciasis

English Journal

Buffy coat smear or Knott's test: which to choose for canine microfilaria screening in field studies?

Marcos R1, Pereira C1, Santos M1, Luzzago C2, Lauzi S2, Maia JP3,4, Faustino A1, Puente-Payo P1.
Veterinary clinical pathology / American Society for Veterinary Clinical Pathology.Vet Clin Pathol.2016 Mar;45(1):201-5. doi: 10.1111/vcp.12321. Epub 2016 Jan 14.
BACKGROUND: In recent years, an increasing number of cases of canine dirofilariasis have been reported worldwide. However, the rate of infection in dogs is largely unknown in many remote areas, and the importance of field studies for determination of the prevalence of canine dirofilariasis in such a
PMID 26765989

Identification and Validation of Loa loa Microfilaria-Specific Biomarkers: a Rational Design Approach Using Proteomics and Novel Immunoassays.

Drame PM1, Meng Z2, Bennuru S1, Herrick JA1, Veenstra TD2, Nutman TB3.
mBio.MBio.2016 Feb 16;7(1). pii: e02132-15. doi: 10.1128/mBio.02132-15.
Immunoassays are currently needed to quantify Loa loa microfilariae (mf). To address this need, we have conducted proteomic and bioinformatic analyses of proteins present in the urine of a Loa mf-infected patient and used this information to identify putative biomarkers produced by L. loa mf. In to
PMID 26884435

Efficacy, Safety, and Pharmacokinetics of Coadministered Diethylcarbamazine, Albendazole, and Ivermectin for Treatment of Bancroftian Filariasis.

Thomsen EK1, Sanuku N2, Baea M2, Satofan S2, Maki E2, Lombore B2, Schmidt MS3, Siba PM2, Weil GJ4, Kazura JW5, Fleckenstein LL3, King CL6.
Clinical infectious diseases : an official publication of the Infectious Diseases Society of America.Clin Infect Dis.2016 Feb 1;62(3):334-41. doi: 10.1093/cid/civ882. Epub 2015 Oct 20.
BACKGROUND: Available treatments for lymphatic filariasis (LF) are limited in their longterm clearance of microfilaria from the blood. The safety and efficacy of a single-dose triple-drug therapy of the antifilarial drugs diethylcarbamazine (DEC), ivermectin (IVM), and albendazole (ALB) for LF are u
PMID 26486704

Japanese Journal

The Global Programme to Eliminate Lymphatic Filariasis : History and achievements with special reference to annual single-dose treatment with diethylcarbamazine in Samoa and Fiji