Epidemiology of canine heartworm infection.

Factors affecting transmission of D. immitis between dogs are summarized in table 4. Although this list may not be exhaustive, it illustrates the complexity of heartworm epidemiology. There are interesting and provocative similarities in the epidemiology of canine and human filariasis. Ottesen (148) has described the spectrum of disease observed in regions endemic for lymphatic filariasis. It includes patients with 1) microfilariae, but no symptoms; 2) disease, but no microfilariae; 3) tropical pulmonary eosinophilia of filarial etiology; and 4) no disease and no evidence of infection. A dog population in areas endemic for D. immitis may show a similar range of clinical manifestations. For example, dogs with microfilariae may not demonstrate clinical disease; dogs with occult infection may show signs ranging from none to severe, including signs similar to the human tropical pulmonary eosinophilia syndrome. Although some dogs live in endemic areas and, apparently, are not infected, it is difficult to determine whether those dogs are resistant to infection or if they have not had adequate exposure. One study has explored the relationship between human W. bancrofti infection and genetic factors (149). Although familial clustering of cases was apparent, the authors were unable to distinguish with certainty whether this association was due to environmental or genetic influences. Carefully designed seroepidemiologic studies of canine heartworm infection may shed some light on genetic or other factors which confer resistance or susceptibility to filarial infections. Natural strain variation has been recognized in several filariae of medical importance including O. volvulus (150), B. malayi (151), and L. loa (2). Strain variation may be manifested as differences in behavior patterns such as microfilarial periodicity, vector and host affinities, and pathogenicity. A laboratory study (88) demonstrated how such strains could arise. In this study, a strain of Brugia patei was adapted to a new, normally refractory host, by artificial selection. The possible existence of different physiologic races of D. immitis should be investigated. Experimental infections with D. immitis have been achieved under laboratory conditions in numerous mosquito species (77).(ABSTRACT TRUNCATED AT 400 WORDS)