With all that is known about heartworm disease in cats and dogs, many questions are yet unanswered. Perhaps the most intriguing questions remaining revolve around the relationship between the worms and their ever-present symbionts, the bacteria Wolbachia pipientis. Wolbachia organisms are present in all stages of the parasite, and when they are removed by treatment with tetracycline antibiotics, adult worms fail to mature sexually and have a reduced lifespan. In humans, pretreatment with tetracycline to remove Wolbachia before administration of ivermectin results in a reduced host inflammatory response following the death of the adult worms. This observation, combined with a recent report that the combination of ivermectin and doxycycline is adulticidal when given to naturally infected dogs, spurred these researchers to examine the relationship between Wolbachia levels and disease severity in cats and dogs. Blood and lung tissue samples were collected from 180 animals negative for, exposed to, or infected with heartworm. Pathologic features of the pulmonary specimens were examined in each sample, and the samples were screened for the presence of Wolbachia DNA, antigen, and antibodies. A weak positive correlation was found between the level of heartworm antibodies and Wolbachia antibodies, but otherwise no clear relationship between Wolbachia status and pathologic characteristics of pulmonary lesions emerged. However, it was noted that pulmonary arteriolar occlusion was significantly more common in infected cats than dogs (57% and 17%, respectively).

Commentary: Unfortunately, this report did not provide any new data about the relationship between Wolbachia and heartworm disease severity, but contributed to the growing understanding that heartworm disease is very different in dogs than in cats. Whereas infected dogs usually have a high prevalence of adult worms, most larvae die before reaching adulthood in the pulmonary arteries in cats. However, cats may be more sensitive to infection—just a single adult worm can be fatal, and even a transient infection can result in long-lasting pulmonary disease (heartworm-associated respiratory disease). This study reinforces that heartworm prevention is equally important for cats as for dogs.

Association of Wolbachia with heartworm disease in cats and dogs. Dingman P, Levy JK, Kramer LH, et al. VET PARASITOL 170:50-60, 2010.