Animal shelters: managing heartworms in resource-scarce environments

Further variation of the adulticide protocol for the treatment of canine heartworm infection: can it be even shorter and cost less?

BACKGROUND

This retrospective study evaluated modified three-dose melarsomine treatment protocols in a shelter setting and compared them to the American Heartworm Society (AHS)-recommended protocol.

METHODS

As compared with the AHS protocol, the shelter protocols utilized doxycycline 10 mg/kg once daily (SID) or twice daily (BID), and varied the time from initiation of doxycycline (day 1) to the first melarsomine injection (M1). Dogs were retrospectively grouped based on the shelter's current protocol (M1 on day 14; Group A) and the AHS protocol (M1 on day 60; Group C), allowing a week on either side of the target M1 day. Treatments that fell outside these ranges formed two additional treatment groups (Groups B and D). Respiratory complications were defined as respiratory signs requiring additional treatment, and were statistically compared for Groups A and C. New respiratory signs and gastrointestinal (GI) signs were compared between dogs receiving SID or BID doxycycline.

RESULTS

One hundred fifty-seven dogs with asymptomatic or mild heartworm disease at presentation were included. All dogs survived to discharge. There was no statistically significant difference between Groups A (n = 79) and C (n = 27) for new respiratory signs post-melarsomine (P = 0.73). The time to M1 for 14 dogs that developed new respiratory signs was a median of 19 days, compared with 22 days for 143 dogs without new respiratory signs (P = 0.2). Respiratory complications post-melarsomine were uncommon. New respiratory signs post-melarsomine occurred in 10/109 (9.2%) dogs receiving SID doxycycline and 4/48 (8.3%) dogs receiving BID doxycycline (P > 0.999). GI signs prior to M1 were recorded for 40/109 (36.7%) dogs receiving SID doxycycline and 25/48 (52.1%) receiving BID doxycycline (P = 0.08). Forty-four follow-up antigen test results were available; all tests performed > 3 months after the third melarsomine injection were negative.

CONCLUSIONS

This study provided support for initiating melarsomine after 14 days of doxycycline and for a lower doxycycline dose. Shorter and less expensive treatment protocols can increase lifesaving capacity and improve quality of life for shelter dogs by reducing the duration of exercise restriction and length of stay.

Optimizing heartworm diagnosis in dogs using multiple test combinations

Background

Various heartworm (HW) diagnostic testing modalities detect products of, or reactions to, different life cycle stages of Dirofilaria immitis. Microfilariae (Mf) can be directly visualized in blood, antigen (Ag) from immature and adult heartworms may be detected on commercial assays, and antibody (Ab) tests detect the host immune response to larval stages. Ag and Mf tests are commonly used in dogs, which frequently carry adult HW infections, but Ab tests have only been validated for use in cats. In some HW-infected dogs, Ag is blocked by immune complexing leading to false-negative results. Heat-treatment (HT) to disrupt these complexes can increase the sensitivity of HW Ag tests. The aim of this study was to compare different methods for diagnosing HW infection in dogs at high risk using individual and paired diagnostic tests, including an exploration of using Ab tests designed for cats to test canine samples.

Methods

One hundred stray adult (≥ 2-year-old) dogs in Florida shelters were tested using Mf, HW Ag, and HW Ab tests (feline HW Ab tests currently not commercially validated/approved for use in dogs); two versions of each test platform were used.

Results

Fourteen dogs tested positive using point-of-care (POC) Ag tests; an additional 2 dogs tested positive with microtiter well assay, and an additional 12 dogs tested positive using HT Ag testing. For individual tests, Ag test sensitivity/specificity compared to HT Ag was 50–57%/100%, and Ab tests were 46–64%/82–94%. Sensitivity estimates for individual tests were higher when comparing to non-HT Ag. Pairing POC Ag tests with Mf tests improved sensitivity without loss of specificity, while pairing POC Ag and Ab tests modestly increased sensitivity at the expense of specificity.

Conclusions

Screening dogs for HW infection using both POC Ag and Mf detection, which is recommended by the American Heartworm Society, improved diagnostic performance in this study compared to single Ag test use, but may have missed more than one in four infected dogs. The need to improve access to highly accurate, rapid, and inexpensive large-scale HW testing for dogs in animal shelters remains largely unmet by current testing availability. The development of practical and validated protocols that incorporate heat or chemical treatment to disrupt Ag-Ab complexes in POC testing or decreasing the cost and time required for such testing in reference laboratories might provide solutions to this unmet need. Similar studies performed in countries where the prevalence of parasites such as D. repens or A. vasorum is different to the USA could potentially yield very different positive predictive values for both HT and non-HT Ag tests.

Graphic abstract

Dirofilarioses in cats: European guidelines from the ABCD on prevention and management

OVERVIEW

Dirofilaria immitis and Dirofilaria repens are the most important filarial worms, causing heartworm disease and subcutaneous dirofilariosis, respectively. D repens is currently considered an emerging zoonotic agent in Europe.

LIFE CYCLE AND INFECTION

Filarial worms infect mainly dogs, but also cats, ferrets, wild carnivores and humans. The life cycle involves an intermediate mosquito host. Compared with dogs, cats are imperfect hosts for dirofilarial worms. After inoculation, only a low number of L3 larvae develop to the adult stage in a small percentage of cats. Heartworm disease in cats may be associated with severe pulmonary thromboembolism and an eosinophilic inflammatory response in the lungs, potentially leading to sudden death. Otherwise self-cure occurs in most cases after 18-48 months. Subcutaneous dirofilariosis may present as subcutaneous nodules or dermatitis.

DIAGNOSIS AND TREATMENT

Diagnosis in cats is more difficult compared with dogs and needs a multistep approach (antigen and antibody tests, as well as diagnostic imaging). Cats with acute heartworm disease require stabilisation within an intensive care unit. Cats with respiratory signs or suggestive radiographic changes should receive prednisolone and follow-up with a similar multistep approach. Adulticidal therapy is not safe in cats.

PREVENTION

In endemic areas cats should receive year-round chemoprophylaxis from 2 months of age.

Prevalence of Dirofilaria immitis in shelter dogs in Bucaramanga metropolitan area, Colombia

The heartworm Dirofilaria immitis is the causative agent of cardiopulmonary dirofilariosis in dogs and cats and also infects humans. The current knowledge about this parasite in South America is scarce compared to North America and Europe. Nevertheless, this parasite has been reported in some regions of Colombia. The objective of this study was to determine the prevalence and risk factors associated with D. immitis infection in shelter dogs from Bucaramanga metropolitan area. We performed a cross-sectional study using blood samples from shelter dogs. Microfilariae testing of whole blood included a blood smear and modified Knott's test. Antigen testing of serum was performed using the Anigen Rapid HW Ag Test Kit 2.0 ®. A descriptive analysis and univariable logistic regression analysis were applied. A total of 207 whole blood samples, from 7 shelters dogs in the Bucaramanga metropolitan area, were utilized. The overall prevalence of D. immitis observed in shelter dogs in the Bucaramanga metropolitan area by blood smears and modified Knott's test was 6.3% (13/ 207) and 0.5% (1/207) by immunochromatography test kit. Regarding the prevalence by municipalities, there was a statistical association (P < .05), indicating that the prevalence was higher in dogs in Girón municipality. Likewise, there was a statistical association (P < .05) between the prevalence with respect to heartworm knowledge and mosquito control. For prevalence by gender, age group, dog breed, and hair length, there was no statistical association (P > .05). For Girón municipality there was a 7.1 times (OR = 7.1; CI95% 0.8-59.2; P < .05) increased infection risk. This study provides current data and the first known report of the prevalence of D. immitis microfilaremia and antigenemia in shelter dogs in the Bucaramanga metropolitan area, and these results could be useful for designing new control measures for this infection. We observed that the prevalence of both microfilaremia and antigenemia was significantly lower in these sampled dogs compared to previous reports.

Prevalence of heartworm in relocated, local and outreach clinic dogs: A Canadian sheltering perspective

This study reports heartworm (Dirofilaria immitis) prevalence in dogs tested by an animal shelter located in a low-prevalence region of Ontario, Canada. From 2015-2018, 4567 unique dogs were tested. The prevalence of heartworm was 3.9 %, with sub-prevalence of 0.3 % (2/662) for dogs surrendered within the Greater Toronto Area (both dogs were originally imported from the US); 6.6 % (130/1,981) for dogs relocated from beyond the Greater Toronto Area; 0% (0/1,668) for dogs tested at the shelter's public veterinary services clinic and 18.4 % (47/256) for owned dogs tested at outreach clinics at a First Nations community in south-eastern Ontario. More than half (54.7 %) of the heartworm-positive dogs originated from Canada. Most heartworm-positive dogs from the US (72/80; 90 %) were transferred from Texas and Georgia. Ninety-three heartworm-positive Canadian dogs were from Ontario, 4 from Manitoba and 1 from Quebec. Most (83/98, 84.7 %) were from four First Nations communities. The prevalence in homeless dogs from one First Nations community in south-western Ontario was 36.5 % (31/85). For 140 shelter dogs with at least one positive test result, there was 91 % concordance between shelter and reference laboratory antigen tests and poor agreement between antigen tests and microfilarial tests (approximately 50 %). Results of historical tests and post-relocation antigen tests were discordant in 28.2 % (shelter point-of-care tests) and 36.2 % (reference laboratory tests) of cases. This was most commonly due to negative historical results followed by positive results after relocation. Microfilarial filtration tests were positive for 77.1 % antigen-positive dogs from First Nations communities, compared with 37.1 % of dogs from other sources. Microfilarial counts were significantly higher for dogs from First Nations communities. This study demonstrated endemic and hyperendemic foci of heartworm within Canada, which were presumed to be associated with limited access to veterinary care. The results support recommendations to retest previously negative animals after relocation.

Pig-hunting dogs are an at-risk population for canine heartworm (Dirofilaria immitis) infection in eastern Australia

BACKGROUND

Canine heartworm disease, caused by Dirofilaria immitis, has global veterinary importance. In Australia, the prevalence of canine heartworm infection decreased markedly following the introduction of over-the-counter macrocyclic lactones. We aimed to estimate the prevalence of canine heartworm infection in at-risk populations of dogs in eastern Australia and analyse published prevalence data from Australia.

METHODS

In total, 566 dogs from eastern Australia were tested for the presence of D. immitis antigen. Four cohorts were studied: pig-hunting dogs from Queensland (Cohort 1, n = 104), dogs from remote New South Wales (NSW) (Cohort 2, n = 332), urban pets from rural NSW (Cohort 3, n = 45) and ex-racing Greyhounds from Sydney, NSW (Cohort 4, n = 85). Serum samples were screened for D. immitis antigen using a reference laboratory microwell-based assay (DiroChek^®) or a point-of-care immunochromatography test kit (Anigen Rapid^®). Risk factors associated with the odds of D. immitis antigen seropositivity were identified using binary logistic regression models. Seropositive blood samples were tested for the presence and quantity of D. immitis DNA using a species specific real-time (q)PCR assay. A metanalysis of the Australian canine heartworm literature was conducted.

RESULTS

The prevalence of dirofilariasis in pig-hunting dogs from Queensland (Cohort 1) was 12.5% (95% CI: 6.5-18.9%), with a subpopulation of dogs from Central Queensland having a prevalence of 21% (95% CI: 12.3-33.4%). Age was significantly associated with D. immitis antigen seropositivity (increased risk with increased age). The odds of being > 5 years versus ≤ 5 years was 3.7-times (95% CI: 1.1-12.5) greater in antigen positive versus antigen negative dogs. No D. immitis antigen positive dogs were detected in dogs from NSW (Cohorts 2-4). The Australian canine heartworm disease literature includes 98 peer-reviewed publications (1901-2019) with 30 studies reporting on D. immitis prevalence in dogs. Throughout the publication peak period (1980s), the primary antemortem diagnostic test was detection of microfilariae.

CONCLUSIONS

Canine heartworm infection in dogs used for pig hunting is a previously unexplored topic in Australia. Pig-hunting dogs are infected with canine heartworm in Queensland, Australia, placing pet dogs and cats at increased risk of infection.

Prevalence of canine heartworm infection in Mississippi animal shelters

Understanding diagnosis of heartworm disease in the context of animal shelters' needs and expectations is crucial for developing guidelines that specifically address the unique shelter population. Accurate and economical heartworm testing strategies are essential. However, current comprehensive guidelines for the management of heartworm disease in dogs are directed toward client-owned animals and do not address needs of animal shelters and other resource-scarce facilities. Additionally, testing recommendations do not take into account regional and local differences in heartworm prevalence across the United States that occur due to abiotic and biotic factors. The objective of this study was to determine the apparent prevalence of Dirofilaria immitis microfilaremia and antigenemia in dogs from Mississippi animal shelters. Further, we compare agreement between microfilaria and antigen testing in this population. We performed a cross-sectional study using canine serum and blood bank samples representative of the Mississippi shelter population. Microfilaria testing of whole blood included a blood smear and modified Knott test. Antigen testing of serum was performed using the DiroCHEK^® antigen ELISA test. Analyses included descriptive and analytic statistics as well as Cohen's kappa for test agreement. A total of 283 whole blood samples and 363 serum samples, representing 363 dogs from 18 shelters in 17 Mississippi counties, were utilized in this study. Sixty-four (22.6%) whole blood samples demonstrated D. immitis microfilariae on the modified Knott test and 125 (34.4%) serum samples had detectable D. immitis antigen. Increasing age and low body condition were associated with antigen-positive test results. Only age was associated with microfilaria-positive test results. There was moderate agreement between the antigen ELISA test and the modified Knott microfilaria test and poor agreement between the antigen ELISA and the blood smear. This study provides the first known report of the prevalence of D. immitis microfilaremia and antigenemia in Mississippi shelter dogs. We observed that prevalence of both microfilaremia and antigenemia was significantly higher in these sampled dogs compared to previous reports for the owned canine population in Mississippi. Heartworm infection presents unique management challenges for animal shelters. Knowledge of the expected prevalence in the area can be utilized for management decisions related to prevention, testing, and treatment of dogs in shelter populations.

Examination of the "susceptibility gap" in the treatment of canine heartworm infection

BACKGROUND

The "susceptibility gap" in a dog diagnosed with adult heartworms has been defined as the period of time in which some Dirofilaria immitis stages are not susceptible to treatment with either macrocyclic lactones or melarsomine dihydrochloride. This was previously defined within the American Heartworm Society guidelines as a period of about 3 months "as per product labels." It can be postulated, however, that a susceptibility gap does not exist with the combination of continued macrocyclic lactone therapy coupled with a three-dose melarsomine dihydrochloride protocol where the first intramuscular treatment is near the time of first diagnosis.

DISCUSSION

Melarsomine dihydrochloride was originally also investigated as a "preventive" as well as a treatment for adult heartworm infection where it would be given to dogs by intramuscular injection every 4 months; therefore, there was early interest in its ability to kill younger worms. A single intramuscular injection of 2.5 mg melarsomine dihydrochloride/kg has an efficacy of 82.1% against 4-month-old worms. When it was given to dogs with older heartworms, 7 and 12 months of age, a single injection was only 55.6% and 51.7% effective, respectively. Thiacetarsamide has been shown to be 99.7% effective against 2-month-old heartworms and other work has shown that melarsomine dihydrochloride is 100% efficacious against these younger forms. With the development and US Food and Drug Administration (FDA) approval of spinosad + milbemycin oxime (Trifexis®, Elanco), milbemycin oxime + praziquantel (Interceptor® Plus, Novartis, now Elanco), and milbemycin oxime + lufenuron + praziquantel (Sentinel® Spectrum®, Novartis, now Virbac), it was shown that repeated treatments of dogs with milbemycin oxime also has efficacy against 3-month-old heartworms. Thus, no improvement in efficacy is expected with a delay in initiating therapy with both melarsomine dihydrochloride and macrocyclic lactones, even with the presence of younger heartworms. Starting treatment at diagnosis appears to be acceptable for maximal heartworm clearance based on published data. Delaying treatment has the disadvantage of allowing disease progression and continued heartworm growth.

CONCLUSIONS

The collective data that has been reviewed indicates that continued macrocyclic lactone administration with two additional injections of melarsomine dihydrochloride a month later will protect dogs against all heartworm stages, including those heartworms 2 months of age or younger at diagnosis, when both treatments are started upon diagnosis.

Impact of heat treatment on Dirofilaria immitis antigen detection in shelter dogs

Background

The diagnosis and management of canine heartworm disease is a growing concern for shelter veterinarians. Although the accuracy of commercial antigen test kits has been widely studied, recent reports have renewed interest in antigen blocking as a causative factor for false “no antigen detected” results. The objectives of this study were to determine the prevalence of false “no antigen detected” results in adult dogs entering shelters in northern, southern, and western regions of the country and to identify historical and clinical risk factors for such results.

Methods

Serum samples were evaluated for Dirofilaria immitis antigen using a commercially available point-of-care ELISA; samples in which no antigen was detected underwent a heat treatment protocol and repeat antigen testing. Whole blood samples underwent Knott testing to identify the presence of microfilariae. Historical and clinical findings were analyzed using exact logistic regression.

Results

A total of 616 samples were analyzed. Overall prevalence of positive antigen test results (prior to heat treatment) was 7.3% and frequency of false “no antigen detected” results due to antigen blocking (ie, samples with no antigen detected prior to heat treatment and positive after heat treatment) was 5.2%. Among dogs that had no detectable antigen on the initial tests, dogs that had microfilariae detected via modified Knott testing (OR = 32.30, p-value = 0.013) and dogs that previously received a heartworm preventive (OR = 3.81, p-value = 0.016) had greater odds of antigen blocking than dogs without these factors. Among dogs that were heartworm positive, those without microfilariae detected had greater odds of antigen blocking than dogs with this factor (OR = 11.84, p-value = 0.0005). Geographic region of origin was significantly associated with occurrence of antigen blocking (p = 0.0036); however, blocking occurred in all regions sizably contributing to heartworm diagnoses. Of the 74 dogs found to be infected with heartworms in this study, 39.2% (29) had no detectable antigen prior to heat treatment.

Conclusions

Heat treatment of serum samples should be considered to improve diagnostic test accuracy, particularly in dogs that reportedly received a heartworm preventive prior to antigen testing regardless of region of origin.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2443-7) contains supplementary material, which is available to authorized users.

Forecasting United States heartworm Dirofilaria immitis prevalence in dogs

Background

This paper forecasts next year’s canine heartworm prevalence in the United States from 16 climate, geographic and societal factors. The forecast’s construction and an assessment of its performance are described.

Methods

The forecast is based on a spatial-temporal conditional autoregressive model fitted to over 31 million antigen heartworm tests conducted in the 48 contiguous United States during 2011–2015. The forecast uses county-level data on 16 predictive factors, including temperature, precipitation, median household income, local forest and surface water coverage, and presence/absence of eight mosquito species. Non-static factors are extrapolated into the forthcoming year with various statistical methods. The fitted model and factor extrapolations are used to estimate next year’s regional prevalence.

Results

The correlation between the observed and model-estimated county-by-county heartworm prevalence for the 5-year period 2011–2015 is 0.727, demonstrating reasonable model accuracy. The correlation between 2015 observed and forecasted county-by-county heartworm prevalence is 0.940, demonstrating significant skill and showing that heartworm prevalence can be forecasted reasonably accurately.

Conclusions

The forecast presented herein can a priori alert veterinarians to areas expected to see higher than normal heartworm activity. The proposed methods may prove useful for forecasting other diseases.