Prevalence of naturally occurring Dirofilaria immitis infection among nondomestic cats housed in an area in which heartworms are endemic

OBJECTIVE: To determine prevalences of heartworm exposure (ie, positive heartworm antibody test results) and heartworm infection (ie, positive heartworm antigen test results or identification of mature heartworms at necropsy) among nondomestic cats housed in an area in rural North Carolina where Dirofilaria immitis is known to be endemic and among nondomestic cats housed in areas with a low prevalence of dirofilariasis or in an area considered to be free from heartworms. DESIGN: Cross-sectional prevalence survey. ANIMALS: 97 nondomestic cats in North Carolina (study population) and 29 nondomestic cats in Colorado; Queensland, Australia; or Auckland, New Zealand (control population). PROCEDURE: Results of serologic tests and postmortem examinations were reviewed. RESULTS: Results of heartworm antibody tests were positive for 57 of 75 (76%) study cats and 1 of 29 (3%) control cats. Male study cats had a significantly higher risk of heartworm exposure than did female study cats (relative risk, 1.3). Results of heartworm antigen tests were negative for all 47 study cats and 16 control cats that were tested. Postmortem examinations were performed on 21 study cats, and 1 (5%) was found to be infected with heartworms. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that nondomestic cats housed outdoors in the southeastern United States are at risk for heartworm exposure and infection, with male cats having a greater risk of exposure than female cats.     

Incidence of positive heartworm antibody and antigen tests at IDEXX Laboratories: trends and potential impact on feline heartworm awareness and prevention

Data from the IDEXX Laboratories Reference Laboratory Network were retrospectively examined for feline heartworm testing trends in testing frequency, geographic bias, and prevalence for the years 2000--2006. Examination of the data supports the commonly held view that veterinarians do not embrace heartworm disease testing or prevention in cats to the same degree they do in dogs. Despite significant awareness and adoption of heartworm testing and prevention in dogs, we hypothesized that heartworm testing rates are lower for cats than for dogs despite a significant prevalence of feline infection. This is important because a perceived low rate of infection in cats is likely to manifest in a low adoption of testing and prevention. In reality, the overall feline heartworm antigen-positive rate is significant--on average 0.9% over the period studied--and in some regions was estimated to be as high as 4.6%. This compares with an average canine heartworm prevalence rate of 1.2%, a feline leukemia virus prevalence of 1.9%, and a feline immunodeficiency prevalence of 1.0%. Based on the low rate of testing and these prevalence rates, practitioners are routinely missing cases of adult feline heartworm infections and the recently defined heartworm-associated respiratory disease (H.A.R.D). Increased antigen testing would result in detection of a significant number of positive cases. In addition, this population of infected cats would represent the "tip of the iceberg" relative to the greater number of cats that have early infection or are at risk for infection.     

Canine heartworm testing in Canada: are we being effective?

Mass testing of dogs in Canada for the presence of Dirofilaria immitis has been ongoing since 1977. Since that time, there have also been changes in the diagnostic tests available to detect the presence of heartworm and changes in the therapy for heartworm, which necessitate a reevaluation of heartworm screening as currently practiced in Canada. The principles of evidence-based medicine were used to determine the prevalence of heartworm infection in various dog populations, and the effectiveness of screening these populations. The annual surveys of heartworm testing have shown that Canada is a low prevalence area (0.16%), with most of the test-positive dogs located in southern Ontario (0.19%), southern Manitoba (0.18%), southern Quebec (0.09%), and the southern Okanagan Valley (0.04%). Foci of higher prevalence are found within these 4 main geographic areas. Furthermore, the prevalence of heartworm infection is higher in the population of dogs not on preventative medication (0.62%), when compared to the population of dogs on preventative medication (0.04%). The evidence indicates that a heartworm diagnostic test applied to an asymptomatic dog on preventative medication contributes little information regarding the heartworm infection status of that dog. However, testing of a dog characterized as being high risk will provide clinically useful information. Recommendations regarding the testing of dogs for heartworm in Canada are derived on the basis of available evidence.     

Diagnostic, treatment, and prevention protocols for feline heartworm infection in animal sheltering agencies

Cats are at risk for heartworm infection (Dirofilaria immitis) wherever the disease is endemic in dogs. Diagnosis is more difficult in cats, and little information is available regarding effective palliative and curative treatments for infected cats. In contrast to the challenges of diagnosis and treatment, chemoprophylaxis is highly effective, and current guidelines call for preventive medications to be administered to all cats in endemic areas. The purpose of this study was to survey feline heartworm management protocols used by 400 animal shelters and foster programs in the endemic states of Alabama, Florida, Georgia, and Mississippi. Only 23% of shelters performed feline heartworm testing. The most common reasons for not testing were expense (36%), lack of treatment options for infected cats (18%), and because the agency considers heartworm infections in cats to be less important than in dogs (12%). Most agencies (69%) did not provide preventive medication to cats. Reasons included because testing was not performed (36%), expense (35%), and the perception that local heartworm risk was low (10%). When preventive was provided, feline-labeled broad-spectrum products were used more commonly (81%) than livestock products (14%). The survey also indicated that many policy decisions were based on inaccurate knowledge of feline heartworm prevalence and pathogenesis. Issues of cost, feasibility, and education prevent most Southeastern sheltering agencies from adequately protecting cats against heartworm disease. Practical guidelines tailored to the needs of these agencies should be developed. Subsidized testing and preventive products may facilitate implementation of feline heartworm management protocols in sheltering agencies.     

Epidemiology of heartworm: what is happening in South America and Mexico?

Canine heartworm has been known to exist in the Americas for more than 150 years (1847), and the first South American report was published approximately 30 years later (1875). The first human case was reported in 1887 and the first feline case in 1921. Coincidently, these two reports were from Brazil, where most surveys of the disease during the past two decades have been conducted in dogs. Feline cases are seldom found in the South American literature, although feline heartworm disease has been reported in Venezuela and Brazil. The prevalence in Mexico seems to be stable over the past 10 years (7.3-7.5%). Only 5 of the 13 South American countries have reported on canine heartworm infection prevalence. In Argentina, the national prevalence ranges from 3.5 to 12.7%; however, there was a much higher prevalence (17.7 and 23.5%) reported for two sections of Buenos Aires. In Lima, Peru, the prevalence is 4.35%, and Colombia reports 3.8-4.8% of dogs are infected. No infections were detected in 1056 dogs sampled in Chile. In Brazil, the national prevalence has declined from 7.9% in 1988 to 2% in 2001. The downward trend observed for canine heartworm in Brazil suggests that multiple pressures may be affecting the nematode transmission, including a possible decrease in mosquito vector population due to better control programs, reduction of transmission due to effective chemoprophylaxis, reduction of microfilaremic dog populations with the off-label use of injectable ivermectin, or reduction of the reproductive capacity of worms from tetracyclines used to control ehrlichiosis. Survey of a large population of captured mosquitoes indicated the most frequent species captured were vectors of heartworms (Aedes taeniorhynchus (73.9%), Aedes scapularis (20%) and Culex quinquefasciatus (2.5%), suggesting that the composition of the mosquito population is not a critical factor in the decrease in heartworm infections in dogs in South America.     

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.     

Can heartworm prevalence in dogs be used as provisional data for assessing the prevalence of the infection in cats?

Cats are considered a susceptible host for Dirofilaria immitis; however, increased host resistance is reflected by relatively low adult worm burdens in natural and experimental infections; the prolonged prepatent period (8 months); the low level and short duration of microfilaremia; and the short life span of adult worms (2-3 years). From April to September 2006, 212 cats and 608 dogs, all exposed for at least one transmission season, were screened for D. immitis infection in a multi-center study in the Po River Valley in northern Italy. Cats were initially evaluated by antibody testing; positive subjects were followed up by antigen testing and echocardiography (and necropsy if death occurred). The prevalence in dogs was 29% by a modified Knott test and antigen testing compared with a prevalence of 4.7% in cats by an antibody test; six of these infections (2.8%) were confirmed by the follow-up evaluations. This field study demonstrated that the prevalence of heartworm infection in cats in this area is within the expected limits of 9-18% of the prevalence in dogs. Antibody testing likely underestimates the real prevalence of D. immitis infection in cats. These results also emphasize the importance of preventive treatment in cats.     

Macrocyclic lactones and Dirofilaria immitis microfilariae

Although there has been extensive veterinary focus on both the effectiveness of macrocyclic lactones for heartworm prevention in dogs and their adulticidal effects, little attention has been directed to their effects on heartworm microfilariae. With routine use of macrocyclic lactones, in some cases knowingly, in dogs with existing heartworm infections, veterinarians should recognize the benefits, and possible complications, arising from this behavior. Macrocyclic lactones remain our only class of heartworm prevention available, and preserving their effectiveness is critical. Drugs in this class share common traits: there are currently no Food and Drug Administration-approved microfilaricides in the US marketplace, but because all macrocyclic lactones have microfilaricidal properties (to varying degrees), they are widely used by veterinarians for this purpose. Originally formulated to be used in dogs without patent heartworm infections, all have been demonstrated as safe to use at label doses, and higher, in microfilaremic dogs. All of the product labels indicate that dogs should be tested for heartworm infection before starting preventive therapy. Although microfilaricidal, microfilariae reduction may take many months to occur, and some dogs may never clear. The effects of macrocyclic lactones on the numbers of circulating microfilariae may be due to several different underlying causes (i.e., direct effect on the nervous system, affecting stages found in the uterus of the female worms), but the details of all mechanisms by which microfilariae are killed and/or cleared in dogs treated with macrocyclic lactones have not yet been fully elucidated. Some 10% to 20% of heartworm-infected dogs that begin monthly heartworm preventive treatment without adulticide therapy will have the continued and persistent presence of circulating microfilariae, and the concern is that this may be selecting for resistance to these molecules. The veterinary literature now includes evidence of increased genotypic homozygosity in specific dogs in one area of the country for a marker gene associated with macrocyclic lactone resistance in nematodes of ruminants. This article will review the biology of microfilariae, as well as the evolution of diagnostic testing for heartworm infection. The effects of macrocyclic lactones on microfilaria behavior and survival will be discussed, as well as the use and effects of macrocyclic lactones in microfilaremic dogs, with or without adulticide treatment. The effect of doxycycline on heartworm microfilariae, optimal testing methodologies, and verification of effective clearance of microfilariae after adulticide treatment and microfilaricidal therapy so that dogs do not remain a potential source of infection for other dogs are all covered.     

Survey of 11 western states for heartworm (Dirofilaria immitis) infection, heartworm diagnostic and prevention protocols, and fecal examination protocols for gastrointestinal parasites

Heartworm infection in dogs and cats in the western United States is a fairly new phenomenon, and for this reason it is often considered to be of minimal significance.The purpose of this survey was to collect data from 11 western states (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming) in an effort to assess awareness of heartworm disease and identify areas in which improvement in understanding is needed. To accomplish this goal, veterinary clinics and hospitals in these states were sent a one-page survey in early 2006. The results of the survey demonstrate that cases of heartworm disease have been reported in all 11 states, illustrating the importance of annual testing and the routine use of preventives.     

Canine heartworm (Dirofilaria immitis) in Fresno and Madera Counties, California: prevalence differences between foothill and valley habitats

The aim of this study was to assess the prevalence of heartworm in domestic dogs in Madera and Fresno Counties, California, dependent on habitat and other host and environmental factors. Dogs were screened for presence of heartworm antigen using the PetChek^® ELISA on blood samples (N = 519) collected at seven sites during April-July 2009. Eighteen dogs were heartworm antigen positive. Pearson Chi-square analyses were conducted testing the presence of heartworm antigen against the following independent variables: elevation range, percentage of time spent outdoors during the day, percentage of time spent outdoors during the night, pet coat length, weight class, prevention status, and sex. Dogs that spent at least 50% of their time outdoors during the day were significantly more likely to have heartworm that those who spent less time outside (N = 519, df = 1, p = 0.031). Overall prevalence (3.47%) was lower than expected, with Madera County having 3.8% positive samples and Fresno County 3.5%; this prevalence is lower than in many previous studies. The effect of time spent outdoors on heartworm prevalence was similar to previous studies. The impact of elevation on infection, though not significant, requires further investigation, as does the prevalence and viability of larval stages in mosquitoes.     

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.     

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.     

Feline heartworm disease: a clinical review

Feline heartworm disease is caused by the filarial nematode Dirofilaria immitis, and is transmitted by mosquitoes in heartworm-endemic areas worldwide. While dogs are the definitive hosts for this parasite, cats can also be infected, and the overall prevalence in cats is between 5% and 10% of that in dogs in any given area. The spectrum of feline presentations varies from asymptomatic infections to chronic respiratory signs, sometimes accompanied by chronic vomiting to acute death with no premonitory signs. Ante-mortem diagnosis can be challenging and relies on a combination of tests, including antigen and antibody serology, thoracic radiography and echocardiography. As treatment with heartworm adulticidal drugs can be life-threatening and heartworm infection in cats is often self-limiting, infected cats are frequently managed with supportive treatment (corticosteroids, bronchodilators, and anti-emetics). Surgical removal of filariae using extraction devices may be considered in some acute cases where immediate curative treatment is necessary, but filarial breakage during the procedure may result in an acute fatal shock-like reaction. Necropsy findings are mainly pulmonary and include muscular hypertrophy of the pulmonary arteries and arterioles on histopathology. A number of safe and effective macrocytic lactone drugs are available for prophylaxis in cats. These drugs can kill a range of larval and adult life-cycle stage heartworms, which may be advantageous in cases of owner compliance failure or when heartworm infection status is undetermined at the time prophylaxis is commenced. An index of suspicion for feline heartworm disease is warranted in unprotected cats with respiratory signs, and perhaps chronic vomiting, in areas where canine heartworm disease is endemic. Many cats, once diagnosed and with appropriate supportive care and monitoring, will resolve their infection and be free of clinical signs.     

Prevalence and risk factors for heartworm infection in cats from northern Florida

Necropsies were performed on 630 adult cats in northern Florida to determine the prevalence and risk factors for heartworm infection in cats of this region. Heartworms were identified in 4.9% of cats, and serological evidence of heartworm exposure was present in 17% of cats. Not all cats from which heartworms were recovered were seropositive for heartworm antigen or antibody. There was no association between heartworm infection and co-infection with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV). Male cats were at higher risk of infection with heartworm, FeLV, or FIV than were females. Because even a single heartworm can cause clinical disease or death in cats, the authors conclude that cats in this region should receive heartworm prophylaxis to prevent heartworm infection.     

Attraction of mosquitoes to domestic cats in a heartworm enzootic region

Heartworm disease is caused by a mosquito-borne parasite that can affect many different mammalian species and has worldwide distribution. The agent, Dirofilaria immitis (Leidy 1856), infect mainly dogs but feline infection have been frequently reported in the last decade. Feline heartworm infection is difficult to detect, therefore, low reported prevalence could reflect true low prevalence or poor diagnostic efficiency. As mosquitoes are known to be attracted differently by different mammalian species, mosquitoes were collected from both a cattery and a contiguous home located in a canine heartworm enzootic area in Niterói, state of Rio de Janeiro, Brazil. For 14 months, mosquitoes were collected weekly for genus identification, speciation when possible, and for individual blood meal identification. Culex species mosquitoes were the most captured and those most frequently found with feline blood meal, followed by Aedes species that, although captured in lower numbers, also fed on feline blood. While Culex species mosquitoes have been reported as potential secondary heartworm vectors for dogs and primary vectors for cats, the present results suggest that Aedes species mosquitoes may also be involved in feline heartworm transmission in a larger proportion than previously thought.     

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.     

Efficacy of heartworm preventatives against ascarids and hookworms in client-owned dogs: a retrospective case control study

Gates, M.C., Nolan, T.J. Efficacy of heartworm preventatives against ascarids and hookworms in client‐owned dogs: a retrospective case control study. J. vet. Pharmacol. Therap. 34 , 116–119. The efficacy of heartworm preventatives against ascarids and hookworms was assessed in a retrospective analysis of 1329 dogs that received a fecal examination and were surveyed about heartworm preventative use upon presentation to the veterinary teaching hospital at the University of Pennsylvania. To remove confounding due to age, patients under 6 months old were analyzed separately from the remaining population. Although there were no reported cases of ascarids or hookworms in patients under 6 months old receiving monthly heartworm prevention, the prevalence reached 5.2% and 11.7%, respectively, in patients that were not using any products. For patients over 6 months old, there were no apparent associations between parasites and heartworm preventative use. Of the 75.5% of dogs that were administered heartworm preventatives, 16.1% reported seasonal use and 83.9% reported using the products year round. Patients using heartworm preventatives seasonally were no more likely to be harboring nematode parasites than patients using preventatives year round (OR = 0.70, 95% CI: 0.19–2.55). Overall efficacy rates were consistent with prior studies on the active ingredients. Heartworm preventative have the greatest value for controlling nematode endoparasites in patients under 6 months old.     

Prevalence of equine ulcerative keratomycosis in Colorado and association of environmental factors: A retrospective and descriptive study (2002–2017)

Equine ulcerative keratomycosis is relatively commonly presented to teaching hospitals in North America, with a prevalence from 24 to 86%. Horses in Colorado may have lower risk due to the dry mountain climate, but data are lacking. This study aimed to determine the prevalence of ulcerative keratomycosis amongst horses with ulcerative keratitis presented to Colorado State University Veterinary Teaching Hospital (CSU-VTH) ophthalmology service and to evaluate environmental factors (season, temperature, humidity, wind speed and elevation) associated with ulcerative keratomycosis in Colorado. A database search identified horses with ulcerative keratitis presented to the ophthalmology service at CSU-VTH from January 2002 to August 2017. Sixty-one horses met the inclusion criteria of a corneal cytology and/or culture or histopathology at the time of diagnosis; cases lacking that were excluded. Environmental factors at the boarding sites, including season, temperature, humidity, wind speed and elevation and clinical outcomes for fungal cases, were recorded. Prevalence of fungal infection amongst equine ulcerative keratitis was 16.4% (10/61), suggesting that CSU-VTH has considerably fewer ulcerative keratomycosis cases than other veterinary teaching hospitals in North America. Spring (50%, 5/10) and fall (40%, 4/10) had the highest prevalence of fungal ulcerative keratitis in Colorado. Only one case was reported in summer (10%, 1/10); no horses were positive (0%, 0/10) in winter. Only wind speed seemed to influence the development of ulcerative keratomycosis with higher wind speeds associated with greater rates of fungal involvement (P = 0.047). Other environmental factors did not show a detectable association (all P-values >0.05). Outcomes were variable. It was concluded that horses from the area of CSU-VTH appear to be at lower risk for ulcerative keratomycosis than from the areas around other North American veterinary teaching hospitals that have reported data. Most horses with keratomycosis in this area present in the spring and fall.     

Use of echocardiography for the diagnosis of heartworm disease in cats: 43 cases (1985-1997)

OBJECTIVE: To determine the usefulness of echocardiography in the diagnosis of heartworm disease in cats and to compare this modality with other tests. DESIGN: Retrospective study. ANIMALS: 43 cats with heartworm infection that had echocardiographic examinations at 2 veterinary teaching hospitals between 1985 and 1997. Twenty-two of these 43 cats also underwent radiography of the thorax and heartworm antibody and heartworm antigen testing. PROCEDURE: Cats were determined to be infected with Dirofilaria immitis infection on the basis of 1 or more of the following findings: positive modified Knott or antigen test result, echocardiographic evidence of heartworm disease, or confirmation of the disease on postmortem examination. The percentage of echocardiographs in which heartworms were evident was compared with the percentage of radiographs in which pulmonary artery enlargement was evident and results of antigen or antibody tests in cats in which all tests were performed. RESULTS: Overall, heartworms were detectable by use of echocardiography in 17 of 43 cats, most often in the pulmonary arteries. In the 22 cats in which all tests were performed, antibody test results were positive in 18, antigen test results were positive in 12, and pulmonary artery enlargement was evident radiographically and heartworms were identifiable echocardiographically in 14. Heartworm infection was diagnosed exclusively by use of echocardiography in 5 cats in which the antigen test result was negative. CONCLUSIONS AND CLINICAL RELEVANCE: Although echocardiography was less sensitive than antigen testing, it was a useful adjunctive test in cats that had negative antigen test results in which there was a suspicion of heartworm disease. The pulmonary arteries should be evaluated carefully to increase the likelihood of detection of heartworms echocardiographically.     

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.