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The prevalence of heartworm infection in cats | VETgirl Veterinary Continuing Education Podcasts

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In today’s VETgirl online veterinary continuing education podcast, we review the prevalence of heartworm disease in cats in the United States and Canada. Now, you may think that heartworm disease is pretty rare, depending on where you live (or practice), but you need to know about this disease. What clinical signs are seen with it? How do you diagnose it?

Cats can manifest infection by Dirofilaria immitis (heartworm) in two possible ways:

1) adult heartworm infection (less common due to cats’ ability to destroy immature larvae prior to adulthood)

2) heartworm-associated respiratory disease induced by immature stages of Dirofilaria immitis.

Clinical signs of heartworm infection in cats are generally respiratory in origin and mimic feline asthma, but may also include vomiting, neurologic disease, or sudden death. Traditional therapy for adult heartworm infection in canines (immiticide) is not safe in cats, whereas heartworm preventative in cats can be highly effective. In other words, please try to prevent it instead of trying to treat it! Confirmation of heartworm infection in cats can be a diagnostic challenge. Local heartworm prevalence has previously been estimated as a percentage (5-20%) of canine infection rate. The purpose of this study was to evaluate the seroprevalence of heartworm antigen, as well as risk factors, for infection of client-owned (e.g., non-feral) cats in the United States and Canada.

So, Levy et al out of Maddie’s Shelter Medicine Program at University of Florida wanted to evaluate this in a study called “Seroprevalence of heartworm infection, risk factors for seropositivity, and frequency of prescribing heartworm preventatives for cats in the United States and Canada.” In this study, they evaluated data collected over a period of 7 months from March-September 2010. Veterinary clinics and shelters within the United States and Canada were solicited for participation in this study. A total of 34,975 cats from 1,353 veterinary clinics and 125 animal shelters in the United States and Canada were analyzed. A total of 26,707 cats from veterinary clinics and 8,268 cats from shelters were included in the final analysis. The decision to test cats was based on shelter protocols or by veterinary personnel based on either suspicion of heartworm infection based on clinical signs, serial monitoring of previously infected cats, or screening for infection prior to initiating preventative. For testing, the point-of-care ELISA kit was used, which looks at 3 components: heartworm antigen, FeLV antigen, FIV antibody (This could be performed on whole blood, serum, or plasma). Other data collected in this study included the cats’ age, sex, neuter status, whether heartworm preventative was prescribed at time of testing, indoor/outdoor status, duration of pet ownership (> or < 30 days), ownership status, reason for testing, current health status [healthy versus non-healthy (e.g., categorized into diseases like respiratory disease, oral disease, abscess/wounds, or other)], and geographic region. Cats < 6 months of age were excluded, since it takes 6 months for maturation of adult heartworms.

Overall, what did Levy et al find? A total of 141 (0.4%) tested heartworm antigen seropositive – all of these cats were from the US, with none being reported in Canada. 85.1% (n=120/141) of these cats had outdoor exposure, and 59% (n=84/141) were considered non-healthy at the time of testing (in other words, they had some type of illness concurrently at the time of testing). 29.1% (n=41) also had concurrent retroviral infection at the time of testing. Statistically, cats were at higher risk for heartworm seropositivity if they had outdoor access (3x risk), were intact males, were non-healthy (clinically ill) cats (2.5x risk), and had abscesses, bite wounds, or oral disease. Cats were concurrent retroviral infection also had a greatest risk factor for testing positive for heartworm, with cats with FeLV (+) or FIV (+) having a 2-4x risk. In cats that tested positive for both FeLV (+) and FIV (+), there was a 25x risk of testing positive for heartworm disease. As for regional distribution, the highest seropositivity was seen in the southern and midwestern regions of the United States, with 35/50 states reporting seropositivity of cats with heartworm disease. Sadly, only 12.6% of cats were prescribed heartworm preventatives at the time of testing. There was correlation between seropositivity regions and the frequency of prescription of heartworm preventatives, which makes sense (meaning those veterinarians knew that their region was at higher risk and were more likely to prescribe preventatives!).

Geographic distribution of heartworm antigen seropositivity in cats in this study was similar to that which has previously been established for dogs. Although a significant protective effect for lack of outdoor access (e.g., indoor only cats) was identified, 15% of these cats still tested seropositive which is similar to prior studies (19-32%), thus the protective effect is not complete. In other words, mosquitoes can make it into your houses and attack your indoor cats, folks! No specific explanation for the increased risk of intact male cats was surmised, but it was clear from the data that reproductive status was the risk factor, not sex alone. This study identified a significantly increased risk of heartworm seropositivity in cats with concurrent retroviral infection, which is in contradiction to prior reports. It is highly possible this association has more to do with common risk factors (e.g., being an outdoor cat) than a direct correlation between the FeLV/FIV and heartworm infection. Regarding the identified risk factor for seropositivity of clinically ill cats (particularly abscesses, bite wounds, and oral disease), the authors suggest that these specific conditions should be considered as indications for heartworm testing in cats along with the most common indication of respiratory clinical signs. Prior surveys evaluating views or practices on heartworm testing and frequency of prescription of preventatives by veterinarians has indicated a significantly reduced perception of importance of testing, and frequency of administration of preventative in cats compared with dogs. The results of this study supported these prior surveys.

One limitation of this study is the potential false negatives that can occur with heartworm antigen testing in cats which can lead to underestimation of seropositivity (e.g., in other words, due to small heartworm burdens, male-only infections, antibody-antigen complex test interference). Another limitation is that seropositivity testing in this study was only evaluated using antigen testing, which only tests for adult heartworms. Antibody testing was not performed, which would allow for identification of cats with larval stages, which represent a significant population of cats with heartworm infection due to the development of heartworm-associated respiratory disease from these larval stages.

The major point I would add, that I found interestingly not mentioned, is the hypothesis that perhaps the identification of FeLV/FIV seropositivity and clinical illness (particularly abscesses, bite wounds, and oral disease) as risk factors for heartworm antigen seropositivity in cats suggests that the correlation may be due to an increased susceptibility to progression of immature larval stages to mature adult heartworms in these cats due to suppression of the immune system by the above factors/illnesses (as the immune system is the agent which destroys these larval stages in most cats). While this wasn’t “tested” for in this study, is this a potential cause?

So, what do we take from this VETgirl podcast?

Seroprevalence of heartworm antigen (adult heartworm infection) across the United States was 0.4% in this study, with significant regional variation mimicking the known geographic distribution of heartworm infection in dogs. A significant, but not complete, protective effect was identified with indoor-only status. FeLV/FIV seropositivity and presence of clinical illness (specifically cats with abscesses, bite wounds, and oral disease) were identified as risk factors for heartworm antigen seropositivity. This study did not evaluate heartworm antibody testing in cats. Most importantly, we should be better at recommending heartworm preventative in cats – even those that are indoor – especially in the southern regions and midwest regions of the United States. As there is no treatment for feline heartworm disease, let’s get better about prevention! More importantly, consider testing your male intact, outdoor, sick cats more!

References:
Levy JK, Burling AN, Crandall MM, et al. Seroprevalence of heartworm infection, risk factors for seropositivity, and frequency of prescribing heartworm preventatives for cats in the United States and Canada. J Am Vet Med Assoc 2017;250:873-880.

Abbreviations:
FeLV: Feline leukemia virus
FIV: Feline immunodeficiency virus

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