Canine heartworm

Dirofilaria immitis

ORDER

Spirurida

FAMILY

Onchocercidae

TAXONOMY

Filaria immitis (Leidy, 1856), Canis familiaris, United States.

OTHER COMMON NAMES English: Dog heartworm.

PHYSICAL CHARACTERISTICS

Females are 9-12 in (25-30 cm) in length by only about 0.13 in (5 mm) in width, while males are about half the size of females with a length of 5-6 in (12-16 cm). They have somatic (coelomyarian) muscles. (Somatic muscles provide the shape to individual muscle cells thus, supplying a portion of the hydrostatic skeleton in nematodes). Specifically, coelomyarian mus cles (a type of muscle cell arrangement) make a protoplasmic zone that bulges into the pseudocel and fibrillar zone, which extends up the sides of the cell.) The average diameter of the filaments are 0.007-0.017 pm. The cuticle, which is the elastic covering of the body and all body openings, consists of a thin polysaccharide-rich layer. It does not possess longitudinal ridges.

DISTRIBUTION

Widespread throughout the world and exists anywhere mosquitoes live. (Specific distribution map not available.)

HABITAT

Primarily live in the tropics, subtropics, and some temperate areas. They are found in dogs, cats, foxes, wolves, and other wild carnivores as well as in sea lions and humans. Within the host, adults live in the right ventricle and the adjacent blood vessels from the posterior vena cava, hepatic vein, and anterior vena cava to the pulmonary artery.

BEHAVIOR

Usually infest the heart of its hosts through about 30 species of mosquitoes. Adults live in the peripheral branches of the pulmonary arteries and produce large numbers of microfilaria that circulate throughout the bloodstream. They usually infect dogs, but also cats, ferrets, and seals. These hosts are the definitive hosts, while mosquitoes are the intermediate hosts. Lifecycle begins when a dog with circulating microfilaria is bitten by a mosquito. They are passed into the bloodstream where they remain active for up to one year or more, but are incapable of further development until ingested by a mosquito. Microfilaria matures into infective larvae inside the mosquito within about 14 days. When the infective mosquito bites a host, larvae are injected into the host's skin and begin to mature into adults in the subcutaneous tissues, muscles, and fatty tissues; they develop to 0.98-4.33 in (25-110 mm) in length. They arrive in the right lower chamber of the heart at 2-4 months. After infection, an additional four months are required for the worms to reach maturity; microfilariae first appear in the peripheral blood circulation about eight months after infection. Adults may live and continue to produce microfilariae for several years. Adults live in the right ventricle and the adjacent blood vessels from the posterior vena cava, hepatic vein, and anterior vena cava to the pulmonary artery.

FEEDING ECOLOGY AND DIET

Live off nutrients of their hosts, primarily through the blood (mostly pulmonary arteries) in and around the heart and lungs.

REPRODUCTIVE BIOLOGY

Females produce large numbers of microfilaria after about six months, which circulate throughout the bloodstream. Up to 5,000 microfilariae are shed into the host's bloodstream each day, and can remain alive and infective in a host's bloodstream for up to three years.

CONSERVATION STATUS

Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

One dog may be infected with 25-50 canine heartworms. In heavy infestations, with 50-100 worms, a dilation of the heart is apparent, as well as pathological problems with lungs, liver, and kidney. Pharmacueticals or surgery are used to remove the worms from the infected host. ♦

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