You have undoubtedly heard it before, but transporting our pets around the world is one of the most significant factors in the emergence of diseases in new geographic areas. This article discusses three primarily canine parasites: Angiostrongylus vasorum, Heterobilharzia americana, and Trypanosoma cruzi. Two of these parasites are already spreading within the United States, while the third is not yet present but is knocking at the door.
Canine pulmonary angiostrongylosis is caused by the nematode A. vasorum.1-3 The first anecdotal reports of canine pulmonary angiostrongylosis were in France in 1813 and 1833. Subsequent research on this nematode in France, coupled with endemic foci identified in southwestern France, led to this parasite's common name—the French heartworm. No longer limited to France, the distribution of this parasite is worldwide. Recent reports of canine pulmonary angiostrongylosis in Newfoundland and Canada,4,5 as well as in Scotland6 and elsewhere indicate this parasite continues to spread (see the sidebar titled "Angiostrongylus vasorum: Spreading in North America").
Angiostrongylus vasorum: Spreading in North America
Compared with Dirofilaria immitis, which are 120 to 310 mm, A. vasorum adults are small (14 to 20.5 mm).1 Like that of D. immitis, the life cycle of A. vasorum is indirect, but snails and slugs are the intermediate hosts rather than mosquitoes.1-3 Both male and female A. vasorum live in the right side of the canine heart and pulmonary arteries (Figure 1), although with aberrant migration, they can end up in other areas (eyes, kidneys, brain, pancreas, femoral artery).1-3
1. A histologic section of canine lung with adult Angiostrongylus vasorum present (arrow) (hematoxyllin and eosin stain; 10X). (Photo courtesy of Dr. Gary Conboy, University of Prince Edward Island.)
The females lay eggs that lodge in smaller capillaries where they develop and hatch (Figure 2). The hatched first-stage larvae (L1) penetrate capillaries and alveoli and migrate into larger airways where they are eventually coughed up, swallowed, and passed in the feces. They then infect a suitable gastropod intermediate host where development to the infective third-stage larvae (L3) occurs in as little as 16 to 18 days.
More than 25 species of slugs and terrestrial and aquatic snails have been shown to be suitable intermediate hosts. Dogs become infected by ingesting the infected gastropod intermediate hosts. Spontaneous expulsion of viable L3 from gastropods also occurs, indicating dogs may become infected by direct ingestion of the L3. Experimentally, Nile rats can develop patent infections, and frogs have been shown to be both intermediate and paratenic hosts. However, their relative importance to the maintenance of the life cycle is unclear.
2. A histologic section of canine lung showing numerous eggs (several indicated by arrows) and L1 of Angiostrongylus vasorum (hematoxyllin and eosin stain; 10X). (Photo courtesy of Dr. Gary Conboy, University of Prince Edward Island.)
Once ingested by the dog, the L3 penetrate the gastrointestinal tract wall, migrate to visceral lymph nodes, and develop into immature adult nematodes. They then migrate to the right ventricle and pulmonary arteries via the portal circulation. The prepatent period is around 38 to 57 days, although longer prepatent periods may also occur. The adult nematodes are long-lived (at least five years), and the dog may remain infected for life.
Reservoir hosts for A. vasorum include other canids, such as red foxes, wolves, coyotes, and jackals. This parasite has also been reported in lynx, Eurasian badgers, and red pandas.1-3,7