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.     

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

The high prevalence of heartworm infection in shelter dogs creates a dilemma for shelter managers, who frequently operate with insufficient funding, staffing, and expertise to comply with heartworm guidelines developed for owned pet dogs. The purpose of this study was to survey canine heartworm management protocols used by 504 animal sheltering agencies in the endemic states of Alabama, Florida, Georgia, and Mississippi. Open-admission shelters, which tended to be larger and more likely to perform animal control functions, were less likely (41%) to test all adult dogs than were limited-admission shelters (80%), which tended to be smaller non-profit humane agencies, and foster programs (98%) based out of private residences. Open-admission shelters were more likely to euthanize infected dogs (27%) or to release them without treatment (39%), whereas limited-admission shelters and foster programs were more likely to provide adulticide therapy (82% and 89%, respectively). Of the 319 agencies that treated infections, 44% primarily used a standard two-dose melarsomine protocol, and 35% primarily used a three-dose split-treatment melarsomine protocol. Long-term low-dose ivermectin was the most common treatment used in 22% of agencies. Open-admission shelters were less likely (35%) to provide preventive medications for all dogs than were limited-admission shelters (82%) and foster programs (97%). More agencies used preventives labeled for monthly use in dogs (60%) than ivermectin products labeled for livestock (38%). The most common reason diagnostic testing and preventive medication was not provided was cost. These results indicate a lack of protocol uniformity among agencies and insufficient resources to identify, treat, and prevent infection. Sheltering agencies and companion animal health industries should develop guidelines that are feasible for use in sheltering agencies and provide improved access to preventive and treatment strategies for management of Dirofilaria immitis.     

Understanding feline heartworm infection: disease, diagnosis, and treatment

Feline heartworm disease is a very different clinical entity from canine heartworm disease. In cats, the arrival and death of immature heartworms in the pulmonary arteries can cause coughing and dyspnea as early as 3 months postinfection. Adult heartworms suppress the function of pulmonary intravascular macrophages and thus reduce clinical disease in chronic feline heartworm infection. Approximately 80% of asymptomatic cats self-cure. Median survival time for symptomatic cats is 1.5 years, or 4 years if only cats living beyond the day of presentation are considered. Aberrant worm migration is more frequent than it is in dogs, and sudden death can occur with no prior clinical signs. The bacterial endosymbiont Wolbachia likely contributes to the inflammatory pathology of heartworm disease, but its role is not yet fully clear. Unfortunately, the diagnosis, treatment, and management of feline heartworm disease are far from simple. Antemortem diagnosis is hampered by low worm burdens, the frequency of all-male infections, and nonspecific radiographic lesions. It is up to the veterinarian to determine the correct index of suspicion and choose the right combination of diagnostic tests to achieve an answer. Treatment is symptomatic because adulticide therapy is risky and does not increase survival time. Despite the dangers of feline heartworm disease, less than 5% of cats in the United States are on chemoprophylaxis. It is important for veterinarians to take a proactive preventive stance because heartworm infection in cats is a multisystemic disease that has no easy cure.     

四川省工作犬心丝虫感染率的调查

本文调查了四川省工作犬心丝虫的感染状况,从四川省成都、德阳、绵阳、乐山、宜宾、阿坝六地的128头工作犬（包括警犬和护卫犬）身上采集128份血样,用改良Knott’s法和ELISA快速检测试剂盒分别进行犬心丝虫微丝蚴和雌性成虫的检测,结果得其感染率分别为5．47％和20．31％,证明四川是心丝虫病的流行区域。     

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.     

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.     

Pulmonary vascular response of dogs with heartworm disease.

Heartworm diseases in dogs is an infectious disease that produces pulmonary hypertension. Dogs with the early vascular changes of heartworm disease, but without the clinical cardiopulmonary signs and pulmonary hypertension, were studied. Dogs with early heartworm were identified that had an exaggerated hypertensive response to hypoxia and to postaglandin F2alpha as compared to those of normal dogs. The pulmonary hypertensive response of dogs with spontaneous heartworm disease varied widely between individuals.     

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.     

Factors influencing heartworm,flea, and tick preventative use in patients presenting to a veterinary teaching hospital

The introduction of modern heartworm, flea, and tick preventatives has provided a safe and effective means of controlling companion animal endoparasites, but achieving good owner compliance remains an ongoing challenge for the veterinary profession. Based on a sample of patients from the veterinary teaching hospital at the University of Pennsylvania, this study retrospectively examined factors associated with preventative use and areas of potential weakness in client communication. Between 1999 and 2006, records of 5276 canine and 1226 feline patients were searched for signalment, survey results for heartworm, flea, and tick preventative use, date of visit, presenting complaint, vaccination history, and owner zip code. Data were analyzed using bivariate and multivariate techniques. Overall, only 13–23% of patients were questioned about heartworm, flea, or tick preventative use during routine medical history taking. Patients with a prior history of parasites, younger patients, or those presenting with signs of cardiac disease were no more likely to be questioned about preventative use than healthy animals. Patients presenting to a specialty service were also less likely to be questioned. Approximately 74–79% of dogs and 12–38% of cats in the sample were on preventative products at any given time. There was a distinct seasonality to preventative use corresponding to the heartworm transmission season from June through November in the northeastern United States. Only 50% of patients seen for a yearly physical examination in winter were reported to be using preventative products when surveyed later in the year, compared to the roughly 85% on patients in heartworm preventatives when they received their routine physical examination in spring. Month of presentation and neuter status were the only signalment factors significantly (P < 0.05) associated with preventative use in the multivariate analysis. Findings from this study emphasize target areas for increasing owner compliance.     

Performance of serologic tests used to detect heartworm infection in cats

OBJECTIVE: To compare heartworm serum antibody (Ab) and antigen (Ag) test results, using commercial laboratories and in-house heartworm test kits, with necropsy findings in a population of shelter cats. DESIGN: Prospective study. ANIMALS: 330 cats at an animal shelter. PROCEDURE: Between March and June 1998, 30 ml of blood was collected from the cranial and caudal venae cavae of 330 cats that were euthanatized at a local animal shelter. Results of heartworm Ab and Ag serologic tests for heartworm infection were compared with necropsy findings in this population of cats, using commercial laboratories and in-house test kits to measure serum Ab and Ag concentrations. RESULTS: On necropsy, adult Dirofilaria immitis were found in 19 of 330 (5.8%) cats. Combining results from serum Ab and Ag tests achieved higher sensitivities than using serum Ab and Ag test results alone (i.e., maximum sensitivities of 100% vs 89.5%, respectively, whereas use of serum Ag and Ab test results alone achieved higher specificities compared with the use of a combination of serum Ab and Ag results (i.e., maximum specificities of 99.4% vs 92.9%, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: On the basis of our findings, if a cat has clinical signs that suggest heartworm disease despite a negative heartworm serum Ab test result, an alternative heartworm Ab test, a heartworm Ag test, thoracic radiography, or two-dimensional echocardiography should be performed.     

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.     

Renal haemosiderosis in association with canine heartworm disease

Twenty-five cases of renal haemosiderosis associated with canine heartworm disease are reported. Severe intravascular haemolysis was a feature of six dogs with filariae in the right atrium and venae cavae (caval syndrome). An association between haemosiderin deposition and filarial burden was apparent in nineteen dogs not showing clinical signs of caval syndrome. The possible reasons for the sub-clinical haemolysis in canine heartworm disease are discussed with regard to current literature.     

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.     

Comparison of serological tests for the detection of natural heartworm infection in cats

Serological tests were performed on 380 cats with necropsy-confirmed heartworm disease to compare the performance of currently available commercial laboratory and point-of-care heart-worm serological tests in a heartworm-endemic area. Overall, antigen tests detected 79.3% to 86.2% of heartworm infections and were highly specific. Most cats with false-negative antigen tests had a single male worm. Antibody tests detected 62.1% to 72.4% of heartworm infections and had a wider range of false-positive results (1.4% to 19.1%) than antigen tests (0.3% to 2.0%). Serological tests for feline heartworm infection varied in diagnostic performance. Combining results from antigen and antibody tests achieved greater sensitivity than using either test alone.     

Clinical evolution and radiographic findings of feline heartworm infection in asymptomatic cats

Clinical manifestations of heartworm disease in cats are variable; most cats seem to tolerate the infection well for extended periods. Heartworm-infected cats may undergo spontaneous self-cure due to the natural death of parasites without any symptomatology, or they may suddenly show dramatic and acute symptoms. Sudden death in apparently healthy cats is not a rare event. Thoracic radiographs are important tool for the diagnosis of cardiopulmonary disease. However, thoracic abnormalities are often absent or transient and highly variable in heartworm-infected cats. Findings, such as enlargement of the peripheral branches of the pulmonary arteries, with a varying degree of pulmonary parenchymal disease and hyperinflation, are the most typical features consistent with infection. A field study was performed for cats referred to the Veterinary Hospital Città di Pavia from January 1998 to December 2001 for routine health examinations and procedures to evaluate the clinical evolution and radiographic findings of feline heartworm infection. Thirty-four asymptomatic cats diagnosed with feline heartworm infection by antibody and antigen tests together with an echocardiogram that allowed worm visualization were included in the follow-up study. Cats were routinely examined every 3 months from the time of heartworm diagnosis until the outcome (self-cure or death). Self-cure was defined as no positive serology for heartworm antigens and no visualization of worms by echocardiography. A final examination for antibodies was carried after 12 months as a final confirmation of self-cure. Twenty-eight cats (82.4%) self-cured; including 21 that showed no clinical signs of infection throughout the study. Six cats died. The most common clinical features observed were acute respiratory symptoms and sudden death. Infection lasted over 3 years in the majority of the cats enrolled in the study. Thoracic radiograph appearance was variable, and the most commonly observed findings were focal and diffuse pulmonary parenchymal abnormalities.     

Pulmonary manifestations of heartworm disease

The clinical signs associated with heartworm disease are the result of changes in the pulmonary arterial system. These clinical signs are the result of either pulmonary hypertension or lung parenchymal disease associated with vascular changes. An increase in pulmonary arterial pressure produces an increase in right ventricular afterload, which may lead to exercise intolerance, syncope, and right-sided congestive heart failure. Coughing, dyspnea, and hemoptysis are the results of pulmonary parenchymal disease.     

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.     

Survey of heartworm prevention practices among members of a national hunting dog club

Surveillance data indicate that failures have been reported for virtually all heartworm prevention product categories. Resistance of third and fourth stage larvae of Dirofilaria immitis to macrocyclic lactones, lack of compliance, other unknown factors, or a combination of these reasons may be the cause of failure. A survey of members of a national hunting dog club was conducted to identify practices used to prevent canine heartworm infections. Questionnaires were completed by 708 dog owners. Year-round administration of heartworm preventive medication was reported by 208 (88%) respondents residing north of the 37th parallel. Dosing was based on the estimated weight of the dog by 54 (7%) respondents, 389 (55%) did not record the date prophylaxis was administered, and 89 (13%) observed the dogs spit out pills. Heartworm testing at least once per year was done by 556 (79%) respondents and test dates were spread throughout the year. Only 448 (64%) respondents tested newly acquired dogs for heartworm. These findings suggest that veterinarians should place a greater emphasis on the frequency and timing of heartworm diagnostic tests, the importance of weighing dogs, duration of administration, recording the date monthly heartworm prophylaxis is given, and observing dogs to ensure that oral medication is retained.     

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.     

Mosquito vectors of dog heartworm in the United States: vector status and factors influencing transmission efficiency

Dog heartworm (Dirofilaria immitis) is dependent on mosquito vectors for its maintenance and transmission among vertebrate hosts. Consequently, D. immitis abundance and distribution are closely linked with mosquito vector biology and ecology. Information on the important dog heartworm vectors in the United States is limited and no comprehensive surveillance of dog heartworm in US mosquitoes has been undertaken to date. Here, we review information gleaned from a number of field surveys documenting heartworm presence in wild mosquito populations as well as laboratory assessments of mosquito vector capacity. Various biological and ecological factors likely contribute to the relative importance of different vector species. We describe some of these factors, rank the leading criteria for efficient vectors, and present the most likely vector species found across the United States. Considering the recent emergence of drug resistance among D. immitis strains, practical knowledge of heartworm vector biology and control should be incorporated into heartworm disease management programs. We conclude by proposing that heartworm control would benefit by targeting mosquito vectors, and we suggest ways in which veterinarians can incorporate the recognition of vector importance into heartworm prevention recommendations imparted to clients.