The Prevalence of Dog Heartworm (Dirofilaria immitis) Infection in Stray Dogs in Okayama, Japan
The prevalence of dog heartworm, Dirofilaria immitis, was examined in stray dogs collected from 3 local public health centers in southwestern region of Okayama Prefecture, Japan during the period between October 1990 to December 1991. A total of 286 dogs (113 males and 173 females, 4.0 to 14.9 kg in body weight) were examined, and D. immitis infection was found in 154 (53.8%) dogs (62 males, 54.9% and 92 females, 53.2%). The monthly infection rates of D. immitis were 31.6 to 72.2%, and apparently lower rates (31.6 to 44.0%) were recognized between June and September. The prevalence of D. immitis infection in dogs and the number of heartworms per infected dog increased along with body weight of the hosts. Altogether 1,722 worms were detected from 154 dogs, and the number of worms per infected dog ranged from 1 to 57 (av. 11.2). The worms were found more in females (52.3%) than in males (47.7%). The sites of worm infestation in 154 dogs were found most in right ventricle (52.3%), followed by pulmonary artery (24.2%) and right atrium and superior vena cava (23.5%). The detected worms measured 47 to 210 mm (av. 164 mm) long in 830 males and 5 to 342 mm (av. 250 mm) long in 609 females. Unisexual worms were found in 36 (23.4%) dogs and the number of males and females per infected dog ranged from 1 to 4 (av. 1.4), and 1 to 10 (av. 2.6) respectively. The diagonal ventral ridges peculiar to D. immitis were recognized on the cuticular surface of the male worms by scanning electron microscope (SEM) observation.
