Canine Filariasis in Northeastern Kansas

Abstract

Prevalence of Dirofilaria immitis and Dipetalonema reconditum in a sample of 288 dogs from Leavenworth County, Kansas, was determined during the summer, 1972. Microfilariae of D. immitis were found in 48 dogs (16.7%) and of D. reconditum in 22 dogs (7.6%) with no concurrent infections detected. D. reconditum infections were found throughout the county, but D. immitis was concentrated along the eastern border of the county where 45 of 48 infections were located within 4 miles of the Missouri River. Among surveyed dogs neither D. immitis nor D. reconditum occurred in one sex of host more frequently than in the other. Dogs maintained in homes and screened kennels were found to have significantly fewer infections of filariasis than dogs continually exposed to the insect vectors. In dogs less than 2 years old no microfilariae of D. immitis were found, but in dogs 3 to 5 years old prevalence of D. immitis reached its highest level (27.3%). In dogs 11 to 16 years old prevalence of D. immitis infections was less than half of the peak level (12.5%). A two-stage catalytic model agreed with age-frequency data for D. immitis. For many years microfilariae in canine blood were attributed only to Dirofilaria immitis (Leidy, 1856), which was considered endemic to the southeastern United States, but in 1956 Newton and Wright detected a second type of microfilariae which they tentatively assigned to Dipetalonema reconditum (Grassi, 1890). Subsequently mosquitoes were shown to serve as vectors for D. immitis and fleas for D. reconditum. Although both species of filariids are now known to have a wide geographic range, their highest reported prevalence is in the southeastern states (Lindsey, 1961; Thrasher et al., 1963). Reports from northeastern states indicate that both filariids occur at a low prevalence (Rothstein et al., 1961; Hirth et al., 1966). In the north central states reports suggest a general low prevalence for both filariids (Groves and Koutz, 1964; Zydeck et al., 1970). Both filariids have been reported in Hawaii at a prevalence comparable to the southeastern United States (Ash, 1962; Gubler, 1966). No filariasis data have been reported for western states prior to 1968. Keegan et al. (1968) examined direct blood smears from pound dogs in southern Texas and found microfilariae in 25% of 354 dogs from coastal localities from Houston through Brownsville. Inland at Laredo, microfilariae occurred in 9.1% of 121 dogs. Butts (1970) reported D. immitis in dogs received for military training Received for publication 20 August 1973. * Present address: Department of Zoology, University of Michigan, Ann Arbor, Michigan 48104. from several midwestern and western states. In northern California D. immitis was found in one of 800 beagles and D. reconditum in 5% of 515 pound dogs (McGreevy et al., 1970). In 1972, the present filariasis survey was undertaken in Leavenworth County in northeastern Kansas. Local veterinarians indicated the occurrence of D. immitis, but the prevalence of both filariids was unknown. MATERIALS AND METHODS Household pets, commercial kennel dogs, and private hunting packs residing within or less than 1 mile from Leavenworth County, Kansas, were surveyed for filariasis through cooperation of county veterinarians and dog owners. When each dog was examined, data on age, sex, breed, geographic location in the county, past history, use of animal, and type of shelter were recorded. Animals less than 10 months old were not examined; furthermore, no attempt was made to sample only dogs with overt signs of disease. Blood samples of 1 ml were drawn from the forelimb radial vein of 288 dogs, treated with citrate or EDTA, and refrigerated. County veterinarians submitted 20 samples for analysis; the remaining 268 samples were drawn from dogs at their residence. All blood samples were examined by the modified Knott's test using methylene blue stain, but fresh blood was also examined for live microfilariae. Differentiation of microfilariae was based on morphological criteria as discussed by Lindsey (1965). After centrifugation, stained sediment was transferred to a microscope slide, covered with a 2-cm-square cover slip, and scanned for microfilariae at 100X. All microfilariae on a slide were examined to check for concurrent infections, and when microfilarial density was low, one or more additional slides were examined. When both species of filariid were detected in a