''Dirofilaria tenuis'' is a species of

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

, a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

roundworm that infects the

subcutaneous tissue The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, or superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and ma ...

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. ''D. tenuis'' most commonly infects

raccoon The raccoon ( or , ''Procyon lotor''), sometimes called the North American, northern or common raccoon (also spelled racoon) to distinguish it from Procyonina, other species of raccoon, is a mammal native to North America. It is the largest ...

s,Studies on the Development of Dirofilaria tenuis Chandler 1942. Warren R. Pistey, The Journal of Parasitology, Vol. 44, No. 6 (Dec., 1958), pp. 613–626
/ref> but some human cases have been reported. They are vectored by

mosquitoes Mosquitoes, the Culicidae, are a family of small flies consisting of 3,600 species. The word ''mosquito'' (formed by '' mosca'' and diminutive ''-ito'') is Spanish and Portuguese for ''little fly''. Mosquitoes have a slender segmented body, ...

and follow similar development and transmission patterns as other ''

Dirofilaria ''Dirofilaria'' is a filarial (arthropod-borne) nematode (roundworm), in the family Onchocercidae. Some species cause dirofilariasis, a state of parasitic infection, in humans and other animals. There are about 27 species in the genus.Michalsk ...

''.

Morphology

Adults of ''D. tenuis'' are long and thin with a pointed anterior end. The posterior end is long, drawn out, and typically remains coiled. It possesses a hook and may have up to fifteen pairs of sensory organs known as papillae covering the posterior region of the body. Females vary in length, from 8–13 cm, averaging about 9 cm, while males are 4–4.8 cm in length, averaging 4.5 cm. ''D. tenuis'' possess eight papillae in addition to two amphids around the mouth opening. The mouth leads to a short esophagus, which is connected to the intestine and leads to the excretory opening. ''D. tenuis'' also possesses a nerve cord at the anterior end of the body cavity and differentiated reproductive organs. The outermost surface of the

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

, or outer covering of the worm, is covered in small lines running transversely and more prominent ridges arranged longitudinally. These ridges are less prominent at the head and anal ends. The combination of the transverse lines and longitudinal ridges can give ''D. tenuis'' a beaded appearance.

Life cycle

''Dirofilaria tenuis'' is introduced to the host (either a raccoon or, as in some rare cases, a human) as a larva when the vector, most commonly an ''

Aedes ''Aedes'' (also known as the tiger mosquito) is a genus of mosquitoes originally found in tropical and subtropical zones, but now found on all continents except Antarctica. Some species have been spread by human activity: ''Aedes albopictus'', ...

'' or ''

Anopheles ''Anopheles'' () is a genus of mosquito first described by the German entomologist Johann Wilhelm Meigen, J. W. Meigen in 1818, and are known as nail mosquitoes and marsh mosquitoes. Many such mosquitoes are Disease vector, vectors of the paras ...

'' mosquito, takes a blood meal and the parasite enters the host through the bite wound. Once inside the host, the larva molts into its final larval stage and then into an adult. As an adult it resides in the

of the host. These adults are able to live up to ten years within the tissue of the host. The female is able to reproduce throughout her entire adult life and her eggs and microfilariae enter the peripheral blood stream of the host. They are then ingested by the vector mosquitoes during a blood meal and travel to the malphigian tubules of the mosquito. There, they undergo development, from microfilaria, to first, second, and finally third stage larvae. This typically spans a length of 10–12 days. Following the third larval stage, at which point they have become infective, they travel back to the proboscis of the mosquito via the

haemocoel In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart an ...

and enter another definitive host. If the mosquito is highly infected, multiple larvae can be transmitted in one blood meal. Humans are a dead-end host for ''D. tenuis''; after the larva molts into an adult, it cannot reproduce. The worm may live subcutaneously for several months before it dies.

Pathology and treatment

''Dirofilaria tenuis'' is frequently found in the southeastern part of the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and is most commonly a parasite of raccoons, but some human cases have been reported. Although there are multiple species of mosquitoes that can become infected with and possibly vector the nematode, ''Anopheles quadrimaculatus'' and ''Aedes taeniohynchus'' have proven to be the most effective and most common host species. There does not seem to be an overwhelming effect on the definitive host, the raccoon, that would cause significant health issues. The only apparent disturbances to the host are the small nodules that form as an inflammatory response at the surface of the skin when one of the worms dies. Most research suggests that the parasite and the host are compatible and able to coexist over a time period of many years. ''Dirofilaria tenuis'' is not compatible with a human host, so it dies much more quickly, living only a few months. During this time, a lump or nodule will form where the worm is located. This most commonly occurs near the site of the mosquito bite, but can also be much farther away. ''D. tenuis'' may migrate using the host's blood, leaving the worm anywhere in the body, even the conjunctiva of the eye. As the body attempts to clear the worm, the nodule may become painful. If the patient continues to experience discomfort, the worm may be surgically excised. Excision is a curative treatment and the worm is diagnosed as ''D. tenuis'' easily using its unique morphology.

References

{{Taxonbar, from=Q5280911 Parasitic nematodes of humans Parasitic nematodes of mammals Spirurida Nematodes described in 1942