Cats are at risk for infection with the agent of heartworm disease, Dirofilaria immitis, wherever infected dogs exist.1 Most cats are capable of eliminating heartworms before the parasites reach the mature adult stage detectable by antigen tests; however, they are still susceptible to chronic heartworm-associated respiratory disease induced by immature heartworms. Such cats may have positive results of heartworm antibody testing or may escape detection altogether.
The most common clinical signs of infection in cats involve the respiratory system, with coughing, tachypnea, dyspnea, and increased bronchovesicular sounds similar to those in cats with asthma.1 Infected cats may have vomiting or neurologic signs. In some situations, sudden death is the first indication of infection. In contrast to dogs, curative treatment is not safe or practical for cats, so treatment is aimed at palliation of clinical signs. Whereas diagnosis and treatment of heartworm infection in cats can be challenging, heartworm preventives are safe and highly effective.
A suggested method for estimating the local prevalence of heartworm infection in cats when feline-specific data are unavailable is to calculate 5% to 20% of the regional heartworm infection rate for unprotected dogs.2–4 Over the past 27 years, necropsy studies3,5–10 involving a total of 2,360 southeastern US shelter cats have collectively led to the identification of adult heartworms in 115 cats, for an overall prevalence of 4.9%, but less information is available regarding the prevalence of infection in owned cats, particularly in those with no clinical signs. The purpose of the study reported here was to estimate the seroprevalence of and risk factors for heartworm infection in owned cats in the United States and Canada and to determine the frequency with which heartworm preventives were prescribed for cats.
Materials and Methods
Participant selection
Veterinary clinics and animal shelters in the United States (excluding the territories) and Canada were invited to participate in the study through a letter addressed to purchasers of diagnostic test kits (IDEXX customers), animal shelters listed in web directories, members of the American Association of Feline Practitioners, and members of the Association of Shelter Veterinarians. Facilities were eligible to participate if they performed a monthly minimum of 25 point-of-care tests for heartworm, FeLV, and FIV infection. Facilities were provided with a data-reporting form and a copy of the American Heartworm Society guidelines for heartworm management in cats, which recommend testing to confirm a diagnosis on the basis of clinical suspicion, to monitor infection status in cats in which heartworm infection had already been diagnosed, and to determine infection status prior to starting heartworm preventive administration.1 For a companion study on FeLV and FIV seroprevalence, they also received a copy of the American Association of Feline Practitioners retrovirus management guidelines, which recommend testing ill cats, cats with a suspected or unknown exposure history, cats about to be vaccinated against FeLV or FIV, and new pets.11
Data collection
Over a 7-month period (March to September 2010), staff members at clinics and shelters that agreed to participate were asked to perform serologic testing and record the results, as well as data regarding cat signalment and health status, on a standardized data-reporting form. Only veterinary clinic personnel reported the reasons cats were selected for testing because animal shelters typically have blanket policies for infectious disease testing and not protocols based on risk assessment for individual animals. For ease of survey completion, participants were asked to report whether they had prescribed a heartworm preventive at the time of testing, but they were not required to report whether cats had previous prescriptions for a heartworm preventive. Completed forms were sent to study investigators by facsimile transmission.
Testing protocol
All tests were performed with a commercially available point-of-care ELISA test kita for heartworm antigen, FeLV antigen, and FIV antibody detection in blood, serum, or plasma samples. Reported sensitivity and specificity of the test were 86.7% and 100%, respectively, for detection of heartworm antigen; 100% and 98.6%, respectively, for detection of FeLV antigen; and 99.2% and 100%, respectively, for detection of FIV antibody.b Confirmatory testing was not included in the study.
Risk factor identification
Information collected on the data-reporting form included age (juvenile [< 6 months] or adult [> 6 months]; only cats > 6 months were included in the study), sex and neuter status, test results, whether a heartworm preventive was prescribed at the time of testing (yes or no), outdoor access (yes or no), ownership duration (≤ 30 days or > 30 days) or unowned status (stray, feral, or owner relinquished), reason for testing (new pet, at risk for infection, illness, or recheck), and current health status. Health status options included healthy, respiratory disease, oral disease, abscess or bite wound, and a free-text entry space for other conditions. Geographic region of testing was determined by the location of the veterinary clinic or animal shelter where testing was performed. These locations were grouped by US Census Bureau divisions (geodivisions) as follows: mid-Atlantic (New Jersey, New York, and Pennsylvania), mountain (Arizona, Colorado, Idaho, Montana, New Mexico, Nevada, Utah, and Wyoming), New England (Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island, and Vermont), east north central (Illinois, Indiana, Ohio, Michigan, and Wisconsin), Pacific (Alaska, California, Hawaii, Oregon, and Washington), east south central (Alabama, Kentucky, Mississippi, and Tennessee), west north central (Iowa, Kansas, Minnesota, Missouri, North Dakota, Nebraska, and South Dakota), south Atlantic (Delaware; Florida; Georgia; Maryland; North Carolina; South Carolina; Virginia; Washington, DC; and West Virginia), and west south central (Arkansas, Louisiana, Oklahoma, and Texas). Canada was treated as a separate geodivision.
Statistical analysis
Because detectable heartworm antigen only develops in mature worms that are at least 6 months of age, results from cats ≤ 6 months of age were excluded from the analysis. Seroprevalence was defined as the percentage of cats with positive ELISA results for heartworm antigen. Unadjusted estimates of heartworm seroprevalence and their 95% CIs were calculated by use of the modified Wald method. Asymptotic χ2 tests were used to test for bivariate associations between each of the putative risk factors and seropositivity. Crude (unadjusted) ORs and their 95% CIs were calculated. All statistical analyses were performed with standard software.c-e Values of P < 0.05 were considered significant.
Results
A total of 1,333 veterinary clinics (26,441 cats) and 121 animal shelters (8,125 cats) in the United States participated in the study and submitted complete information regarding 34,566 adult cats from all 50 states. A total of 20 veterinary clinics (266 cats) and 4 animal shelters (143 cats) in Canada submitted test results from 409 adult cats from 7 of 10 Canadian provinces (British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, and Nova Scotia). Overall, 26,707 cats were tested at veterinary clinics, and 8,268 cats were tested at animal shelters.
Of the 26,707 cats tested in veterinary clinics, 7,570 (28.3%) were tested because of current illness; 6,285 (23.5%) were tested as part of a new pet examination; 6,066 (22.7%) were tested because they were perceived to be at risk for heartworm, FeLV, or FIV infection; and 3,367 (12.6%) were having a recheck test.
One hundred forty-one (0.4%) cats were seropositive for heartworm antigen. No cats were seropositive in Canada. A high proportion of seropositive cats (n = 120 [85.1%]) had outdoor exposure. In addition, more than half of seropositive cats were reported to have illness (84 [59.6%]) or retroviral infection (41 [29.1%]).
Risk factors identified through bivariate analysis as significantly associated with seropositivity for heartworm antigen included outdoor access (vs no outdoor access), sexually intact males (vs castrated males), and clinical disease or retroviral infection (vs no disease or infection; Table 1). Without controlling for other factors, unhealthy cats had a 2.5-fold increased risk of seropositivity, compared with the risk for healthy cats (OR, 2.5; 95% CI, 1.8 to 3.5; P < 0.001). Specific health conditions associated with an increased risk of seropositivity relative to that for healthy cats included abscess or bite wound, oral disease, and respiratory disease. The greatest risk factor for seropositivity for heartworm antigen was simultaneous seropositivity for retroviral antibody or antigen. Seropositivity for antigen or antibody against 1 retrovirus increased the risk of seropositivity for heartworm antigen 2- to 4-fold, whereas seropositivity for antigen or antibody against both retroviruses increased the risk of heartworm seropositivity approximately 25-fold.
Results of bivariate analyses of putative risk factors for heartworm seropositivity in 34,975 adult cats tested at 1,353 veterinary clinics (n = 26,707) and 125 animal shelters (8,268) in the United States (excluding the territories) and Canada from March to September 2010.
| Factor | Category | No. of cats tested | No. of cats with positive results | Seroprevalence (%) | OR | 95% CI | P value |
|---|---|---|---|---|---|---|---|
| Test site | Animal shelter | 8,268 | 31 | 0.4 | Referent | — | — |
| Veterinary clinic | 26,707 | 110 | 0.4 | 1.1 | 0.7–1.6 | 0.60 | |
| Geodivision | Canada | 409 | 0 | 0.0 | NA | NA | NA |
| Mid-Atlantic | 5,299 | 5 | 0.1 | Referent | — | — | |
| Mountain | 1,011 | 1 | 0.1 | 1.1 | 0.1–9.0 | 1.00 | |
| New England | 2,917 | 7 | 0.2 | 2.6 | 0.8–8.0 | 0.10 | |
| East north central | 6,506 | 21 | 0.3 | 3.4 | 1.3–9.1 | 0.01 | |
| Pacific | 3,330 | 11 | 0.3 | 3.5 | 1.2–10.1 | 0.02 | |
| East south central | 1,807 | 8 | 0.4 | 4.7 | 1.5–14.4 | 0.007 | |
| West north central | 3,268 | 15 | 0.5 | 4.9 | 1.8–13.4 | 0.002 | |
| South Atlantic | 7,564 | 51 | 0.7 | 7.2 | 2.9–18.0 | < 0.001 | |
| West south central | 2,864 | 22 | 0.8 | 8.2 | 3.1–21.7 | < 0.001 | |
| Sex | Female | 17,333 | 62 | 0.4 | Referent | — | — |
| Male | 17,642 | 79 | 0.4 | 1.3 | 0.9–1.8 | 0.19 | |
| Neuter status | Castrated male | 12,323 | 39 | 0.3 | Referent | — | — |
| (excluding unknowns) | Spayed female | 10,558 | 35 | 0.3 | 1.1 | 0.7–1.7 | 0.84 |
| Sexually intact female | 6,213 | 24 | 0.4 | 1.2 | 0.7–2.0 | 0.44 | |
| Sexually intact male | 5,110 | 40 | 0.8 | 2.5 | 1.6–3.9 | < 0.001 | |
| Outdoor access | No | 12,613 | 21 | 0.2 | Referent | — | — |
| Yes | 22,362 | 120 | 0.5 | 3.2 | 2.0–5.2 | < 0.001 | |
| Owned | Yes | 23,288 | 85 | 0.4 | Referent | — | — |
| No | 11,687 | 56 | 0.5 | 1.3 | 0.9–1.8 | 0.10 | |
| Duration of ownership | > 30 d | 17,869 | 63 | 0.4 | Referent | — | — |
| (owned cats only) | ≤ 30 d | 5,419 | 22 | 0.4 | 1.2 | 0.7–1.9 | 0.60 |
| Unowned status | Relinquished | 3,303 | 9 | 0.3 | Referent | — | — |
| Feral | 1,372 | 7 | 0.5 | 1.9 | 0.7–5.1 | 0.20 | |
| Stray | 7,012 | 40 | 0.6 | 2.1 | 1.0–4.3 | 0.04 | |
| Test reason (clinic only) | Recheck | 3,367 | 7 | 0.2 | Referent | — | — |
| New pet | 6,285 | 18 | 0.3 | 1.4 | 0.6–3.3 | 0.50 | |
| At risk | 6,066 | 19 | 0.3 | 1.5 | 0.6–3.6 | 0.90 | |
| Sick | 7,570 | 41 | 0.5 | 2.6 | 1.2–5.8 | 0.02 | |
| Health status | Healthy | 21,991 | 57 | 0.3 | Referent | — | — |
| Other | 6,824 | 28 | 0.4 | 1.6 | 1.0–2.5 | 0.05 | |
| Oral disease | 1,507 | 11 | 0.7 | 2.8 | 1.5–5.4 | 0.002 | |
| Abscess or bite wound | 1,742 | 15 | 0.9 | 3.3 | 1.9–5.9 | < 0.001 | |
| Respiratory | 2,911 | 30 | 1.0 | 4.0 | 2.6–6.3 | < 0.001 | |
| Coinfection | None | 32,065 | 100 | 0.3 | Referent | — | — |
| FeLV | 1,102 | 7 | 0.6 | 2.0 | 1.0–4.4 | 0.07 | |
| FIV | 1,632 | 21 | 1.3 | 4.2 | 2.6–6.7 | < 0.001 | |
| FeLV and FIV | 176 | 13 | 7.4 | 25.5 | 14.0–46.4 | < 0.001 | |
| Heartworm preventive | No | 30,567 | 117 | 0.4 | Referent | — | — |
| prescribed | Yes | 4,408 | 24 | 0.5 | 1.4 | 0.9–2.2 | 0.10 |
— = Not calculated. NA = Not applicable (Canada had no seropositive cats).
Testing was performed by use of a point-of-care ELISA designed to detect heartworm antigen, FeLV antigen, and FIV antibody in blood, plasma, or serum samples. Values of P < 0.05 were considered significant.
Although cats were seropositive in 35 of 50 US states, seroprevalence had a highly regional distribution (Figure 1; Table 2). Seroprevalence was highest in several southern (west south central, east south central, and south Atlantic divisions) and Midwestern (west north central division) regions and lowest in several northeastern (middle Atlantic and New England divisions) and western (mountain division) regions.
Zip-code locations ofveterinary clinics or animal shelters at which 141 cats were seropositive for heartworm infection from March to September 2010. A total of 34,566 cats were tested by use of a point-of-care ELISA at 1,333 veterinary clinics (n = 26,441) and 121 animal shelters (8,125) in the United States, for an overall seroprevalence of 0.4%.
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.873
Seroprevalence of heartworm infection by state in 34,566 cats tested by use of a point-of-care ELISA at 1,333 veterinary clinics (n = 26,441) and 121 animal shelters (8,125) across the United States from March to September 2010.
| State | No. of cats tested | No. of cats with positive results | Seroprevalence (%) | 95% CI (%) |
|---|---|---|---|---|
| Kansas | 439 | 7 | 1.6 | 0.7–3.3 |
| Georgia | 1,343 | 16 | 1.2 | 0.7–1.9 |
| Hawaii | 85 | 1 | 1.2 | 0.0–7.0 |
| Louisiana | 345 | 4 | 1.2 | 0.3–3.1 |
| Alabama | 96 | 1 | 1.0 | 0.0–6.2 |
| South Carolina | 390 | 4 | 1.0 | 0.3–2.7 |
| Texas | 1,449 | 12 | 0.8 | 0.5–1.5 |
| Arkansas | 503 | 4 | 0.8 | 0.2–2.1 |
| Florida | 2,016 | 16 | 0.8 | 0.5–1.3 |
| Oregon | 701 | 5 | 0.7 | 0.3–1.7 |
| North Carolina | 1,422 | 10 | 0.7 | 0.4–1.3 |
| Missouri | 1,021 | 6 | 0.6 | 0.2–1.3 |
| Mississippi | 344 | 2 | 0.6 | 0.0–2.2 |
| Vermont | 176 | 1 | 0.6 | 0.0–3.5 |
| Michigan | 1,709 | 7 | 0.4 | 0.2–0.9 |
| Tennessee | 759 | 3 | 0.4 | 0.1–1.2 |
| Ohio | 1,579 | 6 | 0.4 | 0.2–0.9 |
| Colorado | 270 | 1 | 0.4 | 0.0–2.3 |
| Illinois | 558 | 2 | 0.4 | 0.0–1.4 |
| Washington | 282 | 1 | 0.4 | 0.0–2.2 |
| Oklahoma | 567 | 2 | 0.4 | 0.0–1.4 |
| Kentucky | 608 | 2 | 0.3 | 0.0–1.3 |
| Virginia | 1,220 | 4 | 0.3 | 0.1–0.9 |
| Maine | 957 | 3 | 0.3 | 0.1–1.4 |
| Indiana | 1,444 | 4 | 0.3 | 0.1–0.7 |
| Connecticut | 526 | 1 | 0.2 | 0.0–1.2 |
| California | 2,244 | 4 | 0.2 | 0.1–0.5 |
| New Hampshire | 567 | 1 | 0.2 | 0.0–1.1 |
| Minnesota | 1,166 | 2 | 0.2 | 0.0–0.7 |
| Wisconsin | 1,216 | 2 | 0.2 | 0.0–0.6 |
| Massachusetts | 625 | 1 | 0.2 | 0.0–1.0 |
| New Jersey | 894 | 1 | 0.1 | 0.0–0.7 |
| New York | 2,013 | 2 | 0.1 | 0.0–0.4 |
| Maryland | 1,033 | 1 | 0.1 | 0.0–0.6 |
| Pennsylvania | 2,392 | 2 | 0.1 | 0.0–0.3 |
| Alaska | 18 | 0 | 0.0 | 0.0–20.7 |
| Arizona | 285 | 0 | 0.0 | 0.0–1.6 |
| Delaware | 27 | 0 | 0.0 | 0.0–14.8 |
| Iowa | 385 | 0 | 0.0 | 0.0–1.2 |
| Idaho | 74 | 0 | 0.0 | 0.0–5.9 |
| Montana | 18 | 0 | 0.0 | 0.0–20.7 |
| North Dakota | 90 | 0 | 0.0 | 0.0–4.9 |
| Nebraska | 90 | 0 | 0.0 | 0.0–4.9 |
| New Mexico | 86 | 0 | 0.0 | 0.0–5.1 |
| Nevada | 25 | 0 | 0.0 | 0.0–15.8 |
| Rhode Island | 66 | 0 | 0.0 | 0.0–6.6 |
| South Dakota | 77 | 0 | 0.0 | 0.0–5.7 |
| Utah | 126 | 0 | 0.0 | 0.0–3.6 |
| West Virginia | 113 | 0 | 0.0 | 0.0–4.0 |
| Wyoming | 127 | 0 | 0.0 | 0.0–3.5 |
Overall, heartworm preventives were prescribed for only 12.6% of cats at the time of testing (Table 3). Although the proportion of cats for which heartworm preventive was prescribed was low in all regions, a significant (P = 0.03), moderate correlation (correlation coefficient, 0.7) was identified between these proportions and the regional seroprevalence (Figure 2). Heartworm preventive was prescribed least frequently in Canada (2.9% of cats tested) and most frequently in the east north central geodivision (19.2% of cats tested).
Correlation between regional (geodivision) heartworm seroprevalence and proportions of cats for which heartworm preventive was prescribed at the time of heartworm testing (correlation coefficient, 0.7; P = 0.03). A total of 34,975 cats were tested by use of a point-of-care ELISA at 1,353 veterinary clinics (n = 26,707) and 125 animal shelters (8,268) in the United States (excluding the territories) and Canada.
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.873
Proportions of cats in Table 1 for which heartworm preventives were prescribed at the time of heartworm testing.
| Geodivision | No. of cats tested | Heartworm seroprevalence (%) | No. of cats with preventive prescribed | Percentage of cats with preventive prescribed | OR for preventive prescribed | 95% CI | P value |
|---|---|---|---|---|---|---|---|
| Canada | 409 | 0.0 | 12 | 2.9 | Referent | — | — |
| Pacific | 3,330 | 0.3 | 156 | 4.7 | 1.6 | 0.9–3.0 | 0.11 |
| Mountain | 1,011 | 0.1 | 62 | 6.1 | 2.2 | 1.2–4.1 | 0.02 |
| Mid-Atlantic | 5,299 | 0.1 | 400 | 7.5 | 2.7 | 1.5–1.8 | < 0.001 |
| New England | 2,917 | 0.2 | 233 | 8.0 | 2.9 | 1.6–5.2 | < 0.001 |
| West north central | 3,268 | 0.5 | 363 | 1 1.1 | 4.1 | 2.3–7.4 | < 0.001 |
| East south central | 1,807 | 0.4 | 219 | 12.1 | 4.6 | 2.5–8.2 | < 0.001 |
| West south central | 2,864 | 0.8 | 412 | 14.4 | 5.6 | 3.1–10.0 | < 0.001 |
| South Atlantic | 7,564 | 0.7 | 1,301 | 17.2 | 6.9 | 3.9–12.2 | < 0.001 |
| East north central | 6,506 | 0.3 | 1,250 | 19.2 | 7.9 | 4.4–14.0 | < 0.001 |
See Table 1 for key.
Discussion
Seropositivity for heartworm antigen was identified in cats from 35 states in the present study, and the seroprevalence of heartworm infection was highest in regions where the disease is reportedly most prevalent in the primary reservoir host, dogs.12 Overall prevalence was 0.4%, and individual state prevalences varied from 0% to 1.6%. This overall prevalence was similar to the prevalence of 0.5% reported for 2007 in a national survey involving cats.13
For cats in the present study, exposure (vs no exposure) to the outdoors increased the risk of seropositivity 3-fold. This was not surprising given that mosquitos are required for transmission. However, indoor containment was not fully protective, and 15% of seropositive cats were reported to live solely indoors. Heartworm infection in indoor cats has been reported previously, including 23% of infected cats in Oklahoma,14 19% of infected cats in the South,15 and 32% of infected cats in a national survey.16
The sex of cats (male vs female) in the present study was not a significant risk factor for heartworm seropositivity, but reproductive status was a significant risk factor, with sexually intact males having 2.5 times the odds of castrated males. Male cats have been occasionally reported to be at higher risk of natural heartworm infection5,15,17,18 than female cats and are more likely to develop mature infections following experimental exposure.19,20
Most seropositive cats in the present study had clinical signs of disease at the time of testing. As expected, respiratory signs were most common. However, the increased risk of seropositivity in cats with abscesses or bites wounds or oral disease was unexpected. Also unexpected was the marked increase in the odds of seropositivity for cats with retroviral infections, particularly those with FIV infection. In a previous study5 involving shelter cats, cats with retroviral infection had no increased risk of heartworm infection. It is possible that the aforementioned health conditions share a common predisposing factor, such as outdoor roaming, rather than a causal relationship with heartworm infection.
Although heartworm preventives are highly effective and are recommended for all cats by the American Heartworm Society1 and the Companion Animal Parasite Council,21 prescribing of heartworm preventives at the time of testing in the present study was uncommon (Table 3). Cats living in regions with a higher seroprevalence were more likely to have heartworm preventives prescribed than cats in regions with a lower seroprevalence. It is possible that heartworm preventives were prescribed at a different visit for some cats, leading to an underestimation of the proportion of cats protected against infection.
In a survey of 400 shelters and foster agencies in heartworm-endemic southern states, only 23% reported testing any cats for heartworm infection, and only 31% administered heartworm preventives to at least some of their cats.22 Reasons given for not testing or providing preventives included cost, lack of options for treating heartworm-infected cats, and the opinion that heartworm infection is not as important in cats as in dogs. In a survey of 357 veterinarians, feline heartworm preventives were stocked by 85% of respondents but recommended by only 61%. Actual compliance was estimated to be a mere 25%.23 In a university teaching hospital, 80% of dogs were reportedly receiving a heartworm preventive at the time of examination, compared with only 12% of cats.22
Sales patterns of heartworm preventives also support the supposition that cats are much less likely to receive protection against heartworms than dogs. One brand of heartworm preventive in 2014 had sales of $300 million for the canine-labeled product and only $5 million for the feline-labeled product.23 A downward trend in cat visits to veterinarians further undermines the opportunity to protect cats against heartworm infection. In a 2011 survey,24 owners reported that only 60% of cats had been taken to the veterinarian in the past year, compared with 85% of dogs. Of those taken to the veterinarian, cats had a mean of 1.7 visits/y, compared with 2.3 visits/y for dogs.24 These findings support the findings of the present study that most cats, even those in areas in which heartworm infection is highly endemic, do not receive heartworm preventives.
In cats, the pathological process of heartworm disease is more complex than in dogs.1 As in dogs, adult heartworms in cats can cause arteritis, pulmonary inflammation, and thromboembolism. However, in cats, the migration of larvae and arrival of immature adult worms in the pulmonary vasculature during the first 4 months of infection is followed by the death of most of the worms. Even though larval development is commonly aborted before the worms mature, cats can be left with persistent heartworm-associated respiratory disease, involving the development of asthma-like clinical signs caused by pulmonary vascular and parenchymal inflammation. In the study reported here, cats seropositive for heartworm antigen were identified in all regions where heartworm infection is reportedly common in dogs. As expected, respiratory disease was the most common clinical sign in these cats. However, the finding that oral disease, abscesses or bite wounds, and retroviral infection also increased the odds of seropositivity suggested that these conditions should be added to the list of indications for heartworm testing.
In the present study, tests were performed at veterinary clinics and animal shelters in accordance with the instructions provided with the point-of-care ELISA that was used. Antigen-antibody complexes may reduce the sensitivity of heartworm antigen testing in both dogs and cats.25–27 In a study26 of 6 cats experimentally infected with heartworms via SC inoculation of third-stage larvae, results of antigen testing were positive for only 1 cat, whereas results of antibody testing were positive for all 6 cats. After serum samples were heated to disrupt antigen-antibody complexes, results of antigen testing were positive for 5 of the 6 cats. In a study27 of serum or plasma samples from 385 free-roaming and shelter cats, antigen detection increased 5-fold from 1.3% to 6.8% after heat treatment.
In studies28,29 involving comparison of serologic test results to necropsy findings, antigen test results were positive for 68% to 86% of cats in which adult worms were identified at necropsy, and antibody test results were positive for 32% to 90% of cats in which adult worms were identified. Together, these findings suggest that serosurveys such as that in the present study likely lead to underestimation of the true prevalence of heartworm infection in cats.
Although cats with mature adult heartworm infections may have positive results of heartworm antigen testing, cats with infections aborted in the early adult stage of worm development do not. They may, however, have positive results of anti-heartworm antibody testing. In a study15 of 215 cats from the South with respiratory and gastrointestinal signs, 6% were seropositive for heartworm antigen, and 44% were seropositive for antibody against heartworm. In other studies, results of antigen and antibody testing in a national survey were positive in 8% and 12%, respectively, of cats with no clinical signs of infection,30 and results were positive in 9% and 26%, respectively, of cats with cardiopulmonary abnormalities.31 In a necropsy study5 of 630 cats at a Florida shelter, 5% had adult heartworms, 7% had positive results of heartworm antigen testing, and 15% had positive results of anti-heartworm antibody testing. In total, 17% of cats had evidence of heartworm infection. The worm burden in these naturally infected cats was low, with 55% having just a single worm and no cat having > 4 worms. Low worm burden and infections with allmale worms (which, unlike female heartworms, are not detected by existing antigen tests) impede the ability of antigen tests to detect infected cats, resulting in an underestimation of true prevalence.
Extrapolation of the estimated seroprevalence of 0.4% to the estimated 85.8 million cats owned in the United States32 indicates that > 300,000 cats could have a heartworm infection. However, seroprevalence estimates based on heartworm antigen testing, which identifies only adult heartworms and not pathological aborted larval stages, likely represent the tip of the iceberg in terms of the number of cats with heartworm infection and the resulting clinical damage.1 Given the difficulty in diagnosing heartworm infection during all clinically relevant developmental stages of the parasite and lack of curative treatment options, veterinarians should prioritize increased compliance with national guidelines to protect cats from heartworm infection.1
Acknowledgments
Supported in part by a grant from Maddie's Fund. Participating clinics and shelters were awarded credits toward purchase of products from IDEXX Laboratories Inc ($1 credit/each test result reported). Dr. Levy has received support from IDEXX Laboratories Inc for previous research, consultation, and continuing education presentations.
ABBREVIATIONS
| CI | Confidence interval |
Footnotes
SNAP feline triple test, IDEXX Laboratories Inc, Westbrook, Me.
Licensing data provided to USDA for SNAP Feline Triple Test, October 2, 2007, Product code 502C.00, IDEXX Laboratories Inc, Westbrook, Me.
Odds ratio calculator, MedCalc, Ostend, Belgium. Available at: www.medcalc.net/tests/odds_ratio.php. Accessed Nov 25, 2015.
SigmaStat for Windows, version 3.5, Systat Software Inc, San Jose, Calif.
QuickCalcs CI calculator, GraphPad Software, San Diego, Calif. Available at: graphpad.com/quickcalcs/confInterval2/. Accessed Nov 25, 2015.
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