An investigation of the role of immunologic factors in anemia associated with canine heartworm disease

Direct Coombs' antiglobulin tests were performed on 80 dogs with patent Dirofilaria immitis infection and 170 dogs negative for microfilaria of D. immitis. Presence or absence of anemia was determined by hematocrit in 55 of the heartworm negative dogs and 68 of the dogs with heartworm disease. Heartworm infected dogs showed a higher incidence (37%) of anemia than noninfected dogs (14.5%). Anemia was most prevalent in two groups of dogs with heartworm infection, one group showing vena caval syndrome (91%) and the other occult dirofilariasis (62.5%). These latter two groups of dogs also showed a significantly higher number of positive Coombs' reactions at 37 degrees C than other dogs with heartworm disease and the noninfected dogs. The number of positive Coombs' reactions at 4 degrees C among the total of 80 dogs with heartworm infection was significantly higher than that for dogs without heartworm disease. However, there was no positive correlation between anemia and the outcome of the Coombs' test at either temperature. These findings do not suggest that immunologic factors play a major role in the pathogenesis of anemia in dogs with heartworm disease.     

Comparison of four serotests for the detection of Dirofilaria immitis infection in dogs

Three hundred two dogs were tested with 4 serotests for heartworm antigen (AG) or antibody (AB) and with the Knott test. The 4 serotests evaluated were an enzyme-linked immunosorbent assay (ELISA) for adult heartworm-specific AB (AB-ELISA), a quantitative, indirect immunofluorescent assay (IFA) for adult heartworm-specific AB (AB-IFA), an IFA test for microfilaria (MF)-specific AB (MF-IFA), and an ELISA for adult heartworm AG (AG-ELISA). The presence of heartworms was ascertained in all dogs by necropsy examination. Of 302 dogs, 20 (6.6%) had heartworms in the heart at necropsy. Of infected dogs, 9 (45%) had occult infections. Test sensitivities were 75%, 95%, 70%, and 75% for the AB-ELISA, AB-IFA, MF-IFA, and AG-ELISA, respectively. Test specificities were 85% (AB-ELISA), 77% (AB-IFA), 87% (MF-IFA), and 99% (AG-ELISA). The best agreement between serotest results and necropsy findings was obtained with the AG-ELISA (97%). The 4 serotests detected 86% (AB-ELISA), 100% (AB-IFA), 67% (MF-IFA), and 78% (AG-ELISA) of the dogs with occult heartworm infection. A significant (P less than 0.05) association between intestinal parasitism and positive heartworm test results was found with only AB-IFA. Seemingly, the Knott test, or some other concentration method for detecting circulating MF should be the first heartworm test performed. If the examination for MF is negative, the dog has clinical signs, and radiographic findings are suggestive of occult heartworm infection, then a serotest for adult heartworm AG is recommended.     

Diagnostic criteria for an enzyme-linked immunosorbent assay for occult heartworm disease: standardization of the test system in naturally exposed dogs

The development of criteria for interpreting and reporting the results of an occult heartworm enzyme-linked immunosorbent assay to practitioners is described. The antigen is a saline extract of adult female Dirofilaria immitis. The cutoff absorbance A400 nm values were estimated, using 106 dogs free of infection. Any A400 nm value less than 0.526 is considered negative and values greater than or equal to 0.784 are positive. Intermediate A400 nm values are interpreted as suspect. Absorbance values for serum samples from 13 client-owned amicrofilaremic dogs revealed a bimodal distribution consistent with presumptive diagnosis based on clinical signs, which indicates that the test may be used to support a diagnosis of occult heartworm disease. The present serotest, however, is unable to distinguish microfilaremic dogs from noninfected dogs. Serum from dogs infected with other common helminths failed to crossreact with the D immitis antigen, with the exception of Dipetalonema reconditum.     

Evaluation of heartworm immunodiagnostic tests

In this report, the use of appropriate statistical methods for the evaluation of heartworm immunodiagnostic tests is discussed. The evaluation of these tests is complicated by factors causing variation in sensitivity, specificity, accuracy, and predictive values of positive and negative test results. The primary sources of inconsistency are variation in the prevalence of heartworm infection among populations of dogs and the sensitivity of immunodiagnostic tests to various categories of heartworm infections (ie, patent, immune-mediated occult, unisex occult, and immature occult). Sample size (ie, number of dogs tested) affects the confidence limit values of sensitivity and specificity. At least 100 dogs should be used in each testing group (infected and uninfected) to generate values of sensitivity or specificity within reasonably narrow confidence limits. Use of more than 200 dogs in each testing group contributes little to further narrowing of confidence limits. The selection of appropriate statistical tests for comparison of tests or comparison of the sensitivity or specificity of a single diagnostic test to various categories of heartworm infections is critical. The McNemar paired chi 2 test is appropriate for comparison of diagnostic tests, but it must be done by use of duplicate sera from each animal. A chi 2 test of independence, or, in the case of a small sample size, the Fisher exact test, is appropriate for comparing the sensitivity or specificity of a single diagnostic test to various categories of heartworm infection.     

Canine Dirofilaria immitis infection in a hyperenzootic area: examination by parasitologic findings at necropsy and by two serodiagnostic methods

A total of 174 dogs from an area hyperenzootic for Dirofilaria immitis were grouped into 4 age categories and necropsied; information was obtained on adult D immitis infections and on the presence of microfilariae. Serum samples from these dogs were examined by an enzyme-linked immunosorbent assay (ELISA) for antibody to adult D immitis and by an indirect fluorescent antibody test (IFAT) for antibody to microfilarial surface antigens. In dogs less than or equal to 5 months of age, necropsy demonstrated no evidence of infection; however, positive serologic results indicated that some of these dogs had prepatent infections. The percentage of dogs with ELISA titers (positive) increased with age, as did the percentage of dogs with adult D immitis infections. The IFAT results were positive in some dogs in each age category. Sera from all 29 dogs with occult infections were positive by ELISA. Sera from 6 of 20 dogs with occult dual-sex heartworm infections and 1 of 9 dogs with occult single-sex heartworm infections were positive by IFAT. For diagnosing occult dirofilariasis, the ELISA had a positive predictive value which increased with age of the dog to a maximum of 65.0% in dogs greater than or equal to 12 months of age; ELISA had a negative predictive value of 100% in all age groups. In contrast, positive and negative predictive values for the IFAT decreased with age of the dog to 60% and 37.5%, respectively, in dogs greater than or equal to 12 months of age.     

Immune response to and tissue localization of the Wolbachia surface protein (WSP) in dogs with natural heartworm (Dirofilaria immitis) infection

Human and animal parasitic filarial nematodes, including the agent of canine and feline heartworm disease Dirofilaria immitis, harbour intracellular bacteria of the genus Wolbachia (Rickettsiaies). It is thought that these bacteria play an important role in the pathogenesis and immune response to filarial infection. Immunoglobulin G (total IgG, IgG1, IgG2) production against and immunohistochemical staining of tissues for the Wolbachia surface protein (WSP) from dogs with natural heartworm infection were evaluated. All infected dogs had significant total anti-WSP IgG levels compared to healthy controls. Interestingly, WSP was recognized by the IgG2 subclass in both microfilariemic dogs and in dogs with no circulating microfilariae (occult infection). However, microfilariemic dogs also produced gG1 antibodies. Positive staining for WSP was observed in lungs, liver and kidneys, in particular in glomerular capillaries of naturally infected dogs who had died from heartworm disease. Our results show for the first time that Wolbachia is recognized specifically by D. immitis--infected dogs and that the bacteria is released into host tissue. Furthermore, microfilariemic status appears to effect immune responses to this endosymbiont.     

Indirect fluorescent antibody test in occult dirofilariasis.

Indirect fluorescent antibody titers to Dirofilaria immitis microfilaria (IFA-mf) and peripheral eosinophilia were recorded from 15 to 52 months in ten experimentally infected dogs with occult dirofilariasis (heartworm infection without microfilaremia). Five dogs which were experimentally sensitized with D immitis microfilaria did not exhibit microfilaremia after inoculation with infective-stage larvae. In three other dogs, microfilaremia suddenly ceased after 4 to 7 months. In these three dogs, antimicrofilarial antibodies were detectable by IFA-mf test as soon as microfilaremia ended. In the remaining two dogs, which exhibited spontaneous occult dirofilariasis, antibodies were detected at the end of the prepatent period of 6 months. The presence of adult worms was confirmed by angiocardiography. Significant IFA=mf titers (greater than or equal to 1:8) persisted after successful treatment with an adulticide. Reinfection of treated dogs reestablished occult dirofilarasis. Eosinophilia was present in all dogs and peaked at about 3, 6, and 9 months after they were inoculated with infective-stage larvae. At necropsy, the ten dogs harbored gravid, reproducing adult worms in the heart and pulmonary arteries.     

Two-dimensional echocardiography for diagnosis of dirofilariasis in nine dogs

Two-dimensional echocardiography (real-time ultrasonography) of the heart was used to detect adult Dirofilaria immitis in the right ventricle and atrium of a 2-year-old Labrador Retriever with occult heartworm disease and in 8 microfilaremic dogs. Adult worms were easily identified as parallel echogenic lines separated by a hypoechoic region. Images of worms fixed in formalin after recovery from dogs with experimentally induced heartworm infection had an identical appearance. The sensitivity of this imaging procedure was tested on the 8 dogs with naturally acquired infection, and worms were observed in the right ventricle of 7 of the dogs. Two-dimensional echocardiography appears to be a sensitive noninvasive procedure, which can be used in combination with thoracic radiography and serotests to improve accuracy of diagnosis of canine occult heartworm disease.     

The occurrence of Dirofilaria immitis in dogs in South Australia

Heart, lung and samples of blood from 230 dogs were examined for infections of filarial parasites. Dirofilaria immitis worms and microfilariae were detected in one dog. Blood samples from a further 1428 dogs were examined for microfilariae and 22 were found to be infected. Eighteen dogs were infected with D immitis microfilariae and four with Dipetolonema reconditum microfilariae. The histories were available for 18 of the dogs infected with heartworm and only seven dogs had not travelled outside South Australia. It was concluded that heartworm infection was endemic in South Australia but the apparent prevalence was only about 1%.     

Interaction between canine platelets and adult heartworms: platelet recognition of heartworm surfaces

An interaction between blood platelets and adult heartworms was examined in vitro. Surfaces of glutaraldehyde-fixed heartworms, which were taken from infected dogs washed, and incubated in platelet-rich plasma (PRP), were evaluated by scanning electron microscopy. Adherence of platelets to heartworms occurred only with PRP from infected dogs. Aggregation to epinephrine and adenosine diphosphate of PRP incubated with heartworms was monitored. Seemingly, platelet activation to heartworm membranes occurs in dogs with heartworm disease. The increased platelet reactivity was also observed in dogs with occult heartworm disease, indicating that the presence of circulating microfilaria was not important for this process. The ability to transfer the reactivity to heartworm-negative platelets by suspending them in heartworm-positive plasma indicated that this reactivity resided in the plasma. The processes leading to platelet activation may be responsible for the platelet-associated vascular disorders of canine heartworm disease.     

Prevalence and epidemiological aspects of Dirofilaria immitis in dogs from Kayseri Province, Turkey

This study was conducted to determine the prevalence of Dirofilaria immitis infection and to investigate the risk factors related to heartworm disease in dogs from Kayseri, Turkey. Blood samples were collected from 280 dogs from May 2005 to March 2006 and were examined by membrane filtration-acid phosphatase histochemical staining and antigen Elisa techniques to detect circulating microfilariae and antigens of D. immitis, respectively. Of the total of 280 dogs, 27 were positive for D. immitis with a prevalence value of 9.6%. In addition 29.6% of positive dogs determined to have occult D. immitis infections. D. immitis was the only canine filarial parasite present in the study area. The mean number of microfilariae in infected dogs was 4730+/-5479 per ml of blood. The highest heartworm prevalence were observed in 7-10 age group (28.6%) followed by 4-6 (17.1%) and 0.5-3 (4.8%) age groups. The differences between 0.5-3 and other age groups were found significant, whereas no statistically significant difference was observed between 4-6 and 7-10 age groups. The infection was more prevalent in males, larger breeds and the dogs not on prophylaxis. No statistically significant difference was observed between stray and owned dogs. Our results suggest that heartworm treatment and prophylaxis should be considered in Kayseri Province.     

Dogs with patent Dirofilaria immitis infection have higher expression of circulating IL-4, IL-10 and iNOS mRNA than those with occult infection

Dirofilaria immitis is the agent of canine heartworm disease, in which adult worms reside in the pulmonary arteries, producing first stage larvae (microfilariae) that are released into the bloodstream. The present work describes the cytokine and iNOS mRNA expression in the peripheral blood of naturally infected dogs classified as either microfilariemic or amicrofilariemic. Results show that microfilariemic dogs had higher expression of IL-4 and iNOS mRNA than amicrofilariemic dogs. Furthermore, IL-10 mRNA expression was strongly expressed in dogs with circulating microfilariae, compared to only negligible expression in amicrofilariemic dogs. Finally, mf+ status was associated with a predominance in IgG1 production against worm antigens. These results would suggest that circulating mf may stimulate, like in other filarial infections, an immune bias towards unresponsiveness in D. immitis-infected dogs, consenting long-term adult worm survival.     

Epidemiological aspects of filariosis in dogs on the coast of Paraná state, Brazil: with emphasis on Dirofilaria immitis

The present study determined the prevalence and geographical distribution of Dirofilaria immitis and other filariae, from dogs in littoral areas of Paraná state, in Brazil. This survey spanned eight months, between 1998 and 1999, and was also designed to compare the efficacy of different tests for diagnosis of heartworm infection in that area. Blood samples were collected from 256 native-owned dogs distributed along the Paraná coastal area. Five diagnostic procedures were used: direct smear examination, the Knott's modified test, filtration assay, and two heartworm antigen detection kits. A follow-up imaging exam was performed to support the heartworm diagnosis. The imaging diagnosis included radiographic and ultrasonographic exams of six dogs that had positive results for the heartworm antigen detection kits, but showed different microfilarial burdens. The presence and severity of radiographic and ultrasonographic signs were compared with the results obtained in microfilariae detection and antigen tests. Diagnostic parasitology results indicated that 31.25% of the dogs were microfilaremic. Three different microfilariae were recovered: D. immitis, Dipetalonema reconditum, and the third (mf3) was not identified. D. reconditum was the species with the highest prevalence: 22.6%. In general, D. immitis prevalence was 5.47% (28.57% occult infections), but it varied along the coast and the range was from 0 to 20%. No correlation could be established between the overall scores for microfilarial counts (small or large numbers) and the severity of radiographic results or the likelihood of detecting filariae in the pulmonary artery using echocardiography. The finding of a different type of microfilaria (mf) suggested the existence of a third species in Paraná state, whose prevalence was 4.68%. These results show that to obtain a reliable diagnosis of heartworm infection, antigen detection kits are indicated. Knott's test or filtration should be performed to confirm microfilaremia and not for diagnosis of heartworm infection. Imaging tests support parasitology exams and add more about severity of infection. The northern areas, specially Guaraque?aba and Ilha das Pe?as, presented the highest number of heartworm-infected dogs.     

Epidemiology of canine heartworm in its southern distribution limit in South America: Risk factors, inter-annual trend and spatial patterns

This study was aimed at understanding some aspects of the canine heartworm epidemiology in the southern distribution limit of the parasite in South America. With this objective, 19,298 blood samples of owned dogs from 65 localities of 13 municipalities of Buenos Aires Province were tested for Dirofilaria immitis circulating microfilariae and/or female antigens. The overall heartworm prevalence was 1.63% by microhematocrit tube technique (n=19,136), 3.65% by modified Knott (n=713), and 14.41% by antigen test kit (n=118). Microfilaremic dogs showed a median of 1933 microfilariae per millilitre (q1=375, q3=5625, n=100). Male dogs belonging to breeds of short hair and large size recorded significantly higher prevalences than the other categories. Also, the prevalence increased significantly with the age and only dogs younger than 12 months were not found infected. A clear decreasing trend of the annual prevalence was observed during the whole study period, from 3.91% in 2001 to 1.17% in 2006. D. immitis-infected dogs were detected in 32 localities of 9 municipalities (prevalence range: 0.2-6.7%). Generalized linear models were used to assess associations between heartworm prevalence and environmental variables. The resulting significant models were univariate and included variables related with soil cover and human population density. The best model predicted maximum heartworm prevalences around middle values of bare soil cover, and lower at high and low covers. According to our analyses, canine heartworm infection in urban temperate Argentina could be described as relatively low, endemic, and spatially heterogeneous. Host and environmental factors affecting heartworm transmission at local level were identified and discussed.     

Efficacy and safety of selamectin against fleas and heartworms in dogs and cats presented as veterinary patients in North America

A series of randomized, controlled, masked field studies was conducted to assess the efficacy and safety of selamectin in the treatment of flea infestations on dogs and cats, and in the prevention of heartworm infection in dogs. In addition, observations were made on the beneficial effect of selamectin treatment on dogs and cats showing signs of flea allergy dermatitis (FAD). In all studies selamectin was applied topically, once per month, in unit doses providing a minimum dosage of 6mgkg(-1). Dogs and cats with naturally occurring flea infestations, some of which also had signs associated with FAD, were assigned randomly to receive three months of topical treatment with selamectin (220 dogs, 189 cats) or a positive-control product (dogs: fenthion, n=81; cats: pyrethrins, n=66). Selamectin was administered on days 0, 30, and 60. Day 0 was defined as the day that the animal first received treatment. Flea burdens were assessed by flea comb counts and clinical evaluations of FAD were performed before treatment, and on days 14, 30, 60, and 90. On days 30, 60, and 90, mean flea counts in selamectin-treated dogs were reduced by 92.1, 99.0, and 99.8%, and mean flea counts in fenthion-treated dogs were reduced by 81.5, 86.8, and 86.1%, respectively, compared with day 0 counts. Also, on days 30, 60, and 90, mean flea counts in selamectin-treated cats were reduced by 92.5, 98.3, and 99.3%, and mean flea counts in pyrethrin-treated cats were reduced by 66.4, 73.9, and 81.3%, respectively, compared with day 0 counts. Selamectin also was beneficial in alleviating signs in dogs and cats diagnosed clinically with FAD. A total of 397 dogs free of adult heartworm infection from four heartworm-endemic areas of the USA were allocated randomly to six months of treatment with selamectin (n=298) or ivermectin (n=99). Selamectin achieved a heartworm prevention rate of 100%, with all dogs testing negative for microfilariae and adult heartworm antigen on days 180 and 300. Selamectin was administered to a total of 673 dogs and 347 cats having an age range of 6 weeks to 19 years (3954 doses). The animals included 19 purebred or crossbred Collies (Bearded, Border, and unspecified). There were no serious adverse events. Results of these studies indicated that selamectin was highly effective in the control of flea infestations in dogs and cats without the need for simultaneous treatment of the environment or of in-contact animals and also was beneficial in alleviating signs associated with FAD. Selamectin also was 100% effective in preventing the development of canine heartworms and was safe for topical use in dogs and cats.     

Heartworm and Wolbachia: therapeutic implications

A safer, more effective adulticidal treatment and a safe method for reducing microfilaremia and breaking transmission of heartworm disease early in the treatment are needed. The present study evaluated efficacy of ivermectin (IVM) and doxycycline (DOXY) alone or together (with or without melarsomine [MEL]) in dogs with induced adult heartworm infection and assessed the ability of microfilariae from DOXY-treated dogs to develop to L3 in Aedes aegypti mosquitoes and subsequently to become reproductive adults in dogs. Thirty beagles were each infected with 16 adult heartworms by intravenous transplantation. Six weeks later, dogs were ranked by microfilarial count and randomly allocated to 6 groups of 5 dogs each. Beginning on Day 0, Group 1 received IVM (6 mcg/kg) weekly for 36 weeks. Group 2 received DOXY (10 mcg/(kgday)) orally Weeks 1-6, 10-11, 16-17, 22-25, and 28-33. Groups 3 and 5 received IVM and DOXY according to doses and schedules used for Groups 1 and 2. At Week 24, Groups 3 and 4 received an intramuscular injection of MEL (2.5 mg/kg), followed 1 month later by two injections 24h apart. Group 6 was not treated. Blood samples were collected for periodic microfilaria counts and antigen (Ag) testing (and later immunologic evaluation and molecular biology procedures). Radiographic and physical examinations, hematology/clinical chemistry testing, and urinalysis were done before infection, before Day 0, and periodically during the treatment period. At 36 weeks, the dogs were euthanized and necropsied for worm recovery, collection of lung, liver, kidney, and spleen samples for examination by immunohistochemistry and conventional histological methods. All dogs treated with IVM + DOXY (with or without MEL) were amicrofilaremic after Week 9. Microfilarial counts gradually decreased in dogs treated with IVM or DOXY, but most had a few microfilariae at necropsy. Microfilarial counts for dogs treated only with MEL were similar to those for controls. Antigen test scores gradually decreased with IVM + DOXY (with or without MEL) and after MEL. Antigen scores for IVM or DOXY alone were similar to controls throughout the study. Reduction of adult worms was 20.3% for IVM, 8.7% for DOXY, 92.8% for IVM + DOXY + MEL, 100% for MEL, and 78.3% for IVM + DOXY. Mosquitoes that fed on blood from DOXY-treated dogs had L3 normal in appearance but were not infective for dogs. Preliminary observations suggest that administration of DOXY+IVM for several months prior to (or without) MEL will eliminate adult HW with less potential for severe thromboembolism than MEL alone.     

Comparison of results of three commercial heartworm antigen test kits in dogs with low heartworm burdens

OBJECTIVE: To compare results of 3 commercial heartworm antigen test kits performed on serum samples from dogs infected with low numbers of adult female heartworms. DESIGN: Blinded laboratory evaluation. Sample Population-Serum samples from dogs (n = 208) proven at necropsy to be infected with 1 to 4 adult female heartworms and from dogs (32) without heartworms. PROCEDURE: Samples were sequentially tested with each test kit, following the manufacturers' instructions, by licensed veterinary technicians in private practice who were not aware of infection status of the dogs. The order of test kit evaluations was randomly chosen. For each test kit, sensitivity, specificity, accuracy, positive predictive value, and negative predictive value were evaluated. RESULTS: All tests yielded some false-negative results, and there were significant differences among tests in regard to ability to detect low heartworm burdens. Sensitivity of the test kits ranged from 78 to 84%. For all test kits, sensitivity increased as number of female heartworms increased. All 3 test kits had high specificity (97%). CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that sensitivity of the 3 commercially available heartworm antigen test kits ranged from 78 to 84% when used to test serum samples from dogs with low heartworm burdens, and that sensitivity varied among test kits. For all 3 test kits, specificity was 97%. All 3 test kits yielded false-positive and false-negative results for some dogs with low heartworm burdens.     

Radiographic findings in 16 dogs infected with Angiostrongylus vasorum

Thoracic radiographs of 16 dogs infected naturally with Angiostrongylus vasorum showed signs of bronchial thickening, an interstitial pattern and a multifocal and/or peripheral alveolar pattern. In dogs treated with fenbendazole, follow-up radiographs showed that the alveolar pattern had resolved and a mild, hazy interstitial pattern had developed. In contrast with dogs with heartworm (Dirofilaria immitis), no pulmonary vascular lesions were identified.     

Echocardiographic quantification of Dirofilaria immitis in experimentally infected cats

The safety of heartworm preventives in heartworm-positive cats has traditionally been evaluated using adult Dirofilaria immitis removed from infected dogs and surgically implanted into the cats. An alternate study model uses infective larvae to establish adult infections in cats. Unfortunately, the number of adult worms resulting from the latter method varies widely from none to more than 30, both unacceptable for studies of natural heartworm infection and for studies evaluating product safety in heartworm-infected cats. We sought to determine infection severity in experimental infections via echocardiography to reduce the chances of enrolling uninfected and heavily infected cats into a study. Eighty adult cats were each inoculated with 60 infective D. immitis larvae and maintained for 8 months to allow for the development of adult worms. Antigen and antibody testing, as well as echocardiographic imaging, were performed to confirm and estimate adult worm burdens. Approximately 8 and 12 months post-infection, echocardiographic examination was performed to confirm and enumerate adult D. immitis populations in the cardiovascular system. Worm burdens were stratified as 0, 1-3, 4-11, and > 11 adults, with 0 being considered uninfected and more than 11 considered too heavily infected to be relevant for anthelmintic studies. Cats with clinically relevant infections (1-10 adults) subsequently received multiple treatments with the investigational drug, and worm burdens were confirmed by necropsy 30 days following the final treatment. Worm burden estimated with echocardiography correlated well, but not precisely, with post-mortem counts (p < 0.001, r2 = 0.67). Echocardiography under-, over-, and exactly estimated heartworm burden 53%, 27%, and 22% of the time, respectively. Although the correct category (0-4) was determined by echocardiography in only 54% of cats, positive cats were distinguished from negative cats 88% of the time and the heaviest infections (> 11) were correctly categorized 95% of the time. Both false negative and false positive results were observed. We conclude that echocardiography is useful for detecting mature experimental heartworm infections, identifying cats that have rejected mature infection, and detecting very heavy heartworm burdens, but it is only moderately accurate in classifying lesser burdens. While echocardiography cannot be relied upon to consistently determine the exact heartworm burden in experimentally infected cats, it is useful in stratifying worm burdens for anthelmintic safety studies.     

Prevalence of heartworm infection in healthy cats in the lower peninsula of Michigan

OBJECTIVE: To determine prevalence of heartworm infection among healthy, client-owned cats in the lower peninsula of Michigan. DESIGN: Cross-sectional prevalence study. ANIMALS: 1,348 healthy cats examined at private veterinary practices throughout the lower peninsula of Michigan. PROCEDURE: Sera were tested by use of an ELISA-based antigen test kit to determine infection and 2 commercially available antibody detection kits to determine exposure. A questionnaire was used to collect data to assess risk factors associated with infection. RESULTS: 25 cats had positive results for heartworm antigen, yielding an observed prevalence of 1.9%. Neither antibody test was reliable or provided reproducible results, and neither yielded positive results for more than 20% of the antigen-positive heartworm-infected cats. Multivariate regression indicated that cats from southeastern Michigan and cats > or = 2 years old had a higher risk of infection. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that most (80%) heartworm-infected cats in the lower peninsula of Michigan were from the southeastern part of the state, a pattern that closely paralleled the prevalence of heartworm infection in dogs. Therefore, knowledge of the regional prevalence of heartworm infection in dogs may be useful in assessing the risk of infection in cats. Results also suggested that currently available in-clinic heartworm antibody detection kits have limited utility in the diagnosis of heartworm infection in cats.