Jul 15, 2000 - seized by an animal shelter in St Clair County, Michigan, were included in the study. Cats were relinquished by own- ers, brought in as strays ...
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Michael Hermesmeyer, DVM; Robyn K. Limberg-Child, DVM; Alice J. Murphy, BS; Linda S. Mansfield, VMD, PhD
Objective—To determine prevalence of Dirofilaria immitis infection among shelter cats. Design—Cross-sectional study. Animals—239 cats euthanatized at an animal shelter in southeastern Michigan. Procedure—A gross necropsy focusing on the thoracic cavity, heart, lungs, and pulmonary vessels was performed within 5 hours after cats were euthanatized. Blood was collected directly from the heart of cats found to be infected and tested, using a filter test for microfilariae. Serum was tested for D immitis antigens by use of 2 antigen detection kits and for D immitis-specific antibodies by use of 2 antibody detection kits. Results—Cats ranged from approximately 4 months to 15 years old. Adult D immitis were found in 6 (2.5%) cats. Blood could not be recovered from 1 of the cats with heartworm infection. For the 5 other cats, results of the filter test were negative, and results of both antigen and both antibody tests were positive. Conclusions and Clinical Relevance—Results suggest that cats living in an urban area in the northern part of the United States have a low prevalence of adult D immitis infection. However, this is likely to be an underestimate of the true prevalence of infection, because no attempts were made to identify cats infected with larval or juvenile stages of D immitis. (J Am Vet Med Assoc 2000;217:211–212)
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eartworm infection involving cats has been reported from 38 states,1,2 and in 11 surveys of necropsy results for 796 cats, prevalence of infection ranged from 0 to 8.5%.3 However, these studies included mainly states in the southeastern United States, and few studies of the prevalence of heartworm infection among cats in the northern United States have been published. Heartworm infection can be prevented in cats by oral administration of prophylactic drugs. However, owners need information about the prevalence of heartworm infection in their area to make a rational decision about the need for prophylactic treatment. Therefore, the purpose of the study reported here was to determine prevalence of Dirofilaria immitis infection among shelter cats in southeastern Michigan. We believe that such cats are representative of cats living in urban areas in the northern part of the United States
From the Michael Hermesmeyer, PC Veterinary Practice, 5252 Yankee Rd, St Clair, MI 48079 (Hermesmeyer, Limberg-Child); and the Animal Health Diagnostic Laboratory, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824 (Murphy, Mansfield). Address correspondence to Dr. Mansfield. JAVMA, Vol 217, No. 2, July 15, 2000
with similar climates, and prevalence of infection among these cats would be similar to prevalence among cats in similar urban areas. Materials and Methods Cats—Two hundred thirty-nine cats donated to or seized by an animal shelter in St Clair County, Michigan, were included in the study. Cats were relinquished by owners, brought in as strays by the general public or animal control officials, or captured as feral cats. All healthy cats were held for 1 week for possible adoption before euthanasia. All sick cats were euthanatized when first admitted to the animal shelter, and none of these were part of this study. Cats were euthanatized with an overdose of a barbiturate and necropsied. All necropsies were performed within 5 hours after cats were euthanatized. The study was performed between March 1997 and late December 1997. Experimental procedure—A gross necropsy focusing on the thoracic cavity, heart, lungs, and pulmonary vessels was performed. The abdominal cavity was also examined for migrating larvae and ectopic adult worms. Cadavers were not refrigerated or frozen prior to necropsy. If D immitis were found, physical condition, other health problems, and sex of the cat were recorded, and age was estimated by examining dentition, body size, and physical condition. In addition, blood was collected directly from the heart. Unclotted blood (1 to 2 ml) was tested using a filter test for microfilaria. The rest of the blood, including any clotted blood, was centrifuged at 2,500 X g. Serum was removed and submitted for use in antigen and antibody tests. Species, sex, and stage of any worms found were identified by the Parasitology Section of the Michigan State University Animal Health Diagnostic Laboratory. For the filter test, a commercially available test kit approved for detection of D immitis microfilaria in dogsa was used. Serum samples were tested for D immitis antigens by use of 2 commercially available antigen detection kitsb,c and for D immitis-specific antibodies by use of 2 commercially available antibody detection kits.d,e The microfilaria filter testa and one antigen testb were performed at the local clinic. The other antigen testc and the 2 antibody testsd,e were performed at Michigan State University.
Results Cats ranged from approximately 4 months to approximately 15 years old (mean, 2.2 years). One hundred one cats were male, and 138 were female. At the time of euthanasia, 18 males were neutered, and 7 were declawed. Twenty female cats were spayed, 6 were declawed, 2 were pregnant, and 6 were nursing. Adult D immitis were found in 6 of the 239 (2.5%) cats, but larvae were not found in any cats. The 6 cats consisted of 3 sexually intact females, 2 sexually intact males, and 1 castrated male. Three of the cats appeared to be in poor health or had multiple scars, suggesting that they had lived outdoors or had routine access to the outdoors. The remaining 3 cats appeared to have Scientific Reports: Original Study
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Prevalence of Dirofilaria immitis infection among shelter cats
SMALL ANIMALS
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been in good health and well cared for; 1 was nursing. All were adult cats ranging from approximately 2 to approximately 6 years old. Between 1 and 7 adult D immitis worms were identified in each cat; the 3 younger cats (approx 2 to 3 years old) had 2 to 7 worms each, and the 3 older cats (approx 5 to 6 years old) had only 1 or 2 worms each. At the time of necropsy, no more than 5 hours after euthanasia, all heartworms were found in the right ventricle with extension of some of the larger worms into the pulmonary vessels. Two adult Toxocara cati (1 female, 1 male) were found in the thoracic cavity of 1 cat. Blood could not be recovered from 1 of the cats with heartworm infection. For the 5 other cats, results of the filter test were negative, and results of both antigen and both antibody tests were positive, indicating that all 5 cats had occult infections. Discussion Prevalence of heartworm infection in this population of cats was 2.5% (6/239). However, for several reasons, this likely underestimates the true prevalence of heartworm infection in these cats. First, no immature or larval worms were found in any of these cats. This is not surprising, as larval stages are hard to detect and recover; however, prepatent infections in some cats in the present study may have been missed, because cats were only examined at necropsy. Second, infection with latestage larvae or adults in ectopic sites may not have been identified. Despite this, a substantial number of cats with adult heartworms were identified. Additionally, we believe that our findings represent prevalence for cats resident in this region, because many cats included in the study were feral or stray cats, and it is unlikely that many of these cats had traveled long distances or been outside the state during their lifetimes. The younger 3 of the 6 cats with heartworm infection frequently had more heartworms than the older 3. In addition, the 3 younger cats were in better body condition, suggesting that they had been better cared for and, likely, lived indoors. A previous study4 found that indoor cats had an equal chance of D immitis infection as outdoor cats in the same geographic area, and our results seem to agree with this. However, we did not have complete information for cats included in the present study and could only draw conclusions about lifestyles of these cats on the basis of body condition at the time of euthanasia, and the number of cats with heartworm infection was relatively low. In addition, in some areas of the United States, cats housed indoors were found to have approximately half the risk of exposure to heartworm infection as outdoor cats.2 Cats are considered to have higher immune
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responsiveness to D immitis than dogs, which may account for the lower numbers of adult worms, shorter longevity of adult worms, and low prevalence of microfilaremia in cats, compared with dogs.3 Results of the present study seem to agree with this, as none of the cats with heartworm infection had microfilaremia, and older cats typically had fewer worms than younger ones. For all 5 cats with heartworm infection, results of both antigen and both antibody tests were positive. However, it is unknown whether collection of blood from the heart after euthanasia affected the samples. It is possible that antigen titers may have been somewhat high because of dead or dying worms. Results of the present study suggest that heartworm prophylaxis should be considered for cats living in urban areas in the northern part of the United States. Because domestic and wild animals can act as reservoirs of the organism, cats should be considered at risk for developing heartworm disease, and even cats housed strictly indoors are not completely protected from infection.5 However, cats probably are not an important source of microfilariae for other animals. Although clinically inapparent heartworm infections occur in cats, infection with this parasite often causes serious consequences.3,6 a
Difil-Test, Evsco Pharmaceuticals, Buena, NJ. CITE Canine Heartworm Antigen Test Kit, Agritech Systems Inc, Portland, Me. c Dirochek Canine Heartworm Antigen Test Kit, Synbiotics Inc, San Diego, Calif. d Assure/FH Feline Heartworm Antibody Test Kit, Synbiotics Inc, San Diego, Calif. e Solostep FH, Heska Corp, Fort Collins, Colo. b
References 1. Ryan WG, Newcomb KM. Prevalence of feline heartworm disease: a global review. In: Soll MD, Knight DH, eds. Proceedings of the Heartworm Symposium, 1995. Batavia, Ill: American Heartworm Society, 1995;79–86. 2. Miller MW, Atkins CE, Stemme K, et al. Prevalence of exposure to Dirofilaria immitis in cats from multiple areas of the United States. In: Soll MD, Knight DH, eds. Proceedings of the American Heartworm Symposium, 1998. Batavia, Ill: American Heartworm Society, 1998;161–166. 3. McCall JW, Calvert CA, Rawlings CA. Heartworm infection in cats: a life-threatening disease. Vet Med 1994;89:639–647. 4. Roncalli RA, Yamane Y, Nagata T. Prevalence of Dirofilaria immitis in cats in Japan. Vet Parasitol 1998;75:81–89. 5. Atkins CE, DeFrancesco TD, Miller MW, et al. Prevalence of heartworm infection in cats with signs of cardiorespiratory abnormalities. J Am Vet Med Assoc 1997;212:517–520. 6. Selcer BA, Newell SM, Mansour MS, et al. Radiographic and 2-D echocardiographic findings in eighteen cats experimentally exposed to D immitis via mosquito bites. Vet Radiol Ultrasound 1996;37:37–44.
JAVMA, Vol 217, No. 2, July 15, 2000
