Effects of diagnostic test accuracy and treatment efficacy on the occurrence of suspected failure of heartworm prophylaxis in dogs

[Overview on utility of in-house tests for detection of systemic infectious diseases in dogs]

For detection of infectious diseases, several point-of-care (POC) tests are on the market in addition to methods performed in commercial laboratories. These POC tests are based on enzyme-linked immunosorbent assay (ELISA) or other immunochromatographic technologies and present results within few minutes in veterinary practice. This article gives an overview of the utility of numerous POC tests of different manufacturers for detection of parvovirus antigen in feces, Dirofilaria (D.) immitis antigen in blood as well as antibodies against Borrelia (B.) burgdorferi, Anaplasma (A.) spp., Ehrlichia (E.) spp., Leptospira (L.) spp. and Leishmania (L.) infantum in blood (single or in different combinations). Sensitivity and specificity of these tests are important for their usefulness in veterinary practice. Furthermore, presence of antibodies or detection of antigen has to correlate with the presence of clinical signs. POC tests for detection of canine parvovirus antigen have a very high specificity, the sensitivity of all evaluated POC tests, however, is very low. POC tests for detection of D. immitis antigen have a very high sensitivity and specificity. As they detect antigen from the uterus of female adult parasites, test results are negative when only very few female or only male adults are present. POC tests for detection of antibodies against B. burgdorferi only indicate contact with Borrelia spp. and do not prove clinical Lyme disease, as the infection only extremely rarely causes clinical signs. POC tests for detection of antibodies against A. phagocytophilum are also not suitable for diagnosis of clinical anaplasmosis. Infections with A. phagocytophilum only lead to clinical disease in very rare cases and in these, clinical signs occur before the development of antibodies. POC tests for detection of antibodies against E. canis have a very high sensitivity as well as specificity. POC tests for detection of antibodies against L. infantum and Leptospira species (spp.) show a very high specificity and a high sensitivity. However, Leptospira spp. antibody-positive results may occur following vaccination, as the POC tests cannot distinguish between field and vaccination strains.

Relative diagnostic accuracy of point-of-care tests to rule-in Dirofilaria immitis infection in clinically suspect dogs: A systematic review and meta-analysis

Canine heartworm, Dirofilaria immitis, can cause severe disease and sometimes death of the host. Associated clinical signs, lack of preventative usage and regional endemicity are unlikely sufficient by themselves to reach a definitive diagnosis. Several point-of-care (POC) diagnostic tests are commercially available to aid in-clinic diagnosis, however, there is variable diagnostic accuracy reported and no synthesis of published evidence. This systematic review aims at meta-analysing the likelihood ratio of a positive result (LR⁺) to inform the selection and interpretation of POC tests in practice to rule-in heartworm infection when there is clinical suspicion. Three literature index interfaces (Web of Science, PubMed, Scopus) were searched on November 11th, 2022, for diagnostic test evaluation (DTE) articles assessing at least one currently commercialised POC test. Risk of bias was assessed adapting the QUADAS-2 protocol and articles with no evidence of high risk of bias were meta-analysed if deemed applicable to our review objective. Substantial between DTE heterogeneity was investigated including potential threshold or covariate effects. A total of 324 primary articles were sourced and 18 were retained for full text review of which only three had low risk of bias in all four QUADAS-2 domains. Of the nine heartworm POC tests evaluated, only three, IDEXX SNAP (n DTEs = 6), Zoetis WITNESS (n DTEs = 3) and Zoetis VETSCAN (n DTEs = 5) could be analysed. Both WITNESS and VETSCAN DTEs showed substantial heterogeneity due to a putative threshold effect and no summary point estimates could be reported. SNAP DTEs showed acceptable heterogeneity, and a summary LR⁺ was estimated at 559.0 (95%CI: 24.3-12,847.4). The quality and heterogeneity of heartworm POC test DTEs was highly variable which restricted our summary of the diagnostic accuracy to only the SNAP test. A positive result from the SNAP test provides strong evidence of the presence of an infection with adult heartworm(s) in a dog patient and this test is warranted to rule-in clinical suspicion(s) in clinics. However, our review did not appraise the literature to assess the fitness of SNAP test, or any other POC tests, to rule-out heartworm infection in dogs without clinical suspicion or following heartworm therapy.

Multiple diagnostic tests demonstrate an increased risk of canine heartworm disease in northern Queensland, Australia

Background

Canine heartworm (Dirofilaria immitis) is a life-threatening infection of dogs with a global distribution. Information on the prevalence of D. immitis and associated risk factors for canine heartworm antigen positivity—and thus disease—in Australia is scarce or outdated. The current reference method for D. immitis diagnosis in dogs is via the detection of heartworm antigen in blood using commercially available microwell-based enzyme-linked immunosorbent assays (ELISAs). Heat treatment of canine plasma prior to testing has been suggested to increase test sensitivity. The aim of the current study was to estimate the prevalence of D. immitis in dogs confined to shelters in Queensland, Australia. The impact of heat treatment on antigen test results was also assessed.

Methods

Blood samples (n = 166) were collected directly from dogs in seven shelters across Queensland (latitudinal span of approx. 1700 km) into EDTA blood collection tubes. A commercially available ELISA (DiroCHEK®) was used to detect canine heartworm antigen in untreated and heat-treated plasma. Whole blood was concurrently tested for the presence of microfilariae and D. immitis DNA using a modified Knott’s test and real-time PCR, respectively. Risk factors (age, gender, source, location) associated with the odds of positivity for canine heartworm were assessed using binary logistic regression models.

Results

A total of 16 dogs (9.6%; 95% confidence interval [CI]: 5.9–15.2%) were positive for canine heartworm based on combined test results. Heat treatment did not impact on the positivity of D. immitis antigen within samples (Cohen’s kappa = 0.98), but the optical density was significantly increased in paired plasma samples for D. immitis antigen-positive samples (Wilcoxon matched-pairs signed rank test, two-tailed P < 0.01). Location of the dog in a shelter in northern Queensland was the only risk factor significantly associated with the odds of a dog being more likely to be D. immitis antigen positive (odds ratio: 4.39; 95% CI: 1.26–13.51). All samples positive for the modified Knott’s test were also positive for D. immitis DNA by PCR.

Conclusions

This study demonstrated the presence of heartworm-positive dogs in shelters in Queensland, with positive animals significantly more likely to occur in northern Queensland than southern Queensland. Sustained testing for the presence of D. immitis microfilariae and antigen remain important diagnostic tools in areas with known and re-emerging canine heartworm activity.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04896-y.

An Accessible Alternative to Melarsomine: "Moxi-Doxy" for Treatment of Adult Heartworm Infection in Dogs

Canine heartworm infection, caused by the filarial parasite Dirofilaria immitis, represents a serious and expanding animal welfare concern that is expected to increase due to the effects of climate change and the COVID-19 pandemic. A body of evidence has emerged to support the use of a non-arsenical adulticide treatment protocol, using moxidectin and doxycycline to kill adult heartworms over a prolonged period. While a three-dose protocol using the arsenical drug melarsomine is currently the safest and most effective treatment for heartworm infection, this drug is not available in some countries and is inaccessible for many owners and animal shelters. Moxidectin-doxycycline (moxi-doxy) provides a viable alternative to no treatment at all, in cases where arsenical treatment is not possible. Based on current evidence, the most effective non-arsenical treatment regimen is doxycycline 10 mg/kg PO q 12 or 24 h for 28 days, combined with topical moxidectin at label dose. Moxidectin is repeated monthly until no antigen detected (NAD) status is confirmed. Sustained release injectable moxidectin, in combination with doxycycline, may provide an alternative in remote regions or in settings where significant compliance or accessibility concerns exist, but more studies are needed. In moxi-doxy protocols, doxycycline should be repeated annually until NAD. This review summarizes the safety and efficacy of moxi-doxy, addresses controversies surrounding this treatment approach, and provides detailed recommendations for treatment regimens and post-treatment testing.

The Incidence of Dirofilaria immitis in Shelter Dogs and Mosquitoes in Austria

To estimate the incidence of Dirofilaria immitis in Austrian shelter dogs and mosquitoes trapped in their proximity, 115 shelter dogs from fourteen animal shelters located in five different Austrian states were examined. Blood samples were screened for D. immitis using ELISA antigen-testing, PCR and microscopical examination for microfilariae. In total, 91% of the dogs originated from countries endemic for dirofilariosis. Eleven dogs (9.6%), all originating from Hungary, tested positive for D. immitis. None of the dogs examined showed microfilaremia. Eight dogs showed no or only mild clinical signs (e.g., infrequent coughing), and three dogs showed frequent coughing, dyspnea, exercise intolerance, blunt fur or weight loss. In total, 205 Mosquitoes of ten different species were caught at five different shelter sites in four different Austrian states, using CO₂-baited mosquito traps set once a month (June–September 2019) for 24 h. All 205 mosquitoes tested negative for Dirofilaria spp. via PCR. The risk of endemisation of D. immitis in Austria (and other non-endemic countries in a similar situation) is very serious and its zoonotic potential should be communicated more strongly. To monitor a possible transmission of microfilariae from untreated or even untested positive dogs, e.g., in animal shelters, to mosquitoes in the near surroundings, frequent screening for Dirofilaria in mosquitoes should be used more intensively. Current knowledge on D. immitis should be integrated into daily veterinary practice and dog owners should be proactively educated, especially before traveling to endemic areas or adopting dogs from endemic countries. Animal shelters and animal welfare organizations should be provided with appropriate education and veterinary guidance regarding the testing and treatment of dogs imported from high-risk areas.

Selected advanced imaging techniques were unable to quantify in vivo parasitic burden in heartworm-infested dogs

This prospective exploratory study aimed to determine whether certain noninvasive advanced imaging techniques could estimate parasitic burden in heartworm-infested dogs; a noninvasive method is needed for ethical considerations and permitting longitudinal drug studies. Three cardiac-gated and respiratory-gated 3T MRI techniques and CT pulmonary angiography (CTPA) were performed in three healthy beagles to optimize imaging techniques. Once the imaging techniques were established, a pilot study was performed to determine which one of the MRI techniques would be used in an observer comparison study. Ultimately, spoiled gradient recalled (SPGR)-cine-MRI and CTPA were performed in four and five heartworm-infested dogs, respectively. Heartworms were detected in the pulmonary arteries in all dogs during SPGR-cine-MRI and in no dog during CTPA. However, counting the number of worms was unsuccessful. In conclusion, CTPA and SPGR-cine-MRI were unable to replace necropsy for quantifying parasitic burden in heartworm-infested dogs.

Evaluation of the SNAP® 4Dx® plus test for the detection of Dirofilaria immitis antigen and characterization of exposure to tick-borne pathogens in wild canids in southern Ontario

Population-level surveys for Dirofilaria immitis in wild canids typically rely on identification of the parasite at necropsy. More recently, some studies have employed the use of the SNAP® 4Dx® Plus test. However, since the assay was designed for use with domestic dogs it needs to be validated for use with wild canids for accurate interpretation of results. We therefore evaluated the performance of the SNAP® 4Dx® Plus test for detection of D. immitis in wild canids in southern Ontario. Overall, 199 wild canid carcasses were collected from across the region and assessed for the presence of D. immitis parasites at necropsy; ten were infected. Lung tissue extract (LE) and thoracic fluid filter paper extract (TFE) prepared from each wild canid were tested via the SNAP® 4Dx® Plus test, which simultaneously tests for the presence of D. immitis antigen and antibodies to Borrelia burgdorferi, Anaplasma spp., and Ehrlichia spp. The prevalence adjusted bias adjusted kappa (PABAK) and Gwet's first-order-agreement coefficient (AC1) were used to assess the level of agreement between sample pairs. The PABAK and AC1 between LF and TFE applied to the SNAP® 4Dx® Plus test and the necropsy-confirmed D. immitis status indicated a very high level of agreement for all sample pairs. Compared to necropsy-confirmed D. immitis status, the estimated sensitivity and specificity of the SNAP® 4Dx® Plus test for D. immitis antigen in LE was 80 % (95 % CI 44.4-97.5%) and 98.9 % (95 % CI 96.2-99.9%), respectively. For the TFE, the sensitivity was 70 % (95 % CI 34.8-93.3%) and the specificity was 97.9 % (95 % CI 94.7-99.4%). With respect to the tick-borne pathogen components, 1.5 % (3/199; 95 % CI 0.3-4.5%) of wild canids tested positive for B. burgdorferi antibody; 1.0 % (2/199; 95 % CI 0-3.8%) of LE samples were positive and 0.5 % (1/199; 95 % CI 0-3.1%) of TFE samples were positive. No samples tested positive for antibody to Anaplasma spp. (95 % CI 0-2.3%) or Ehrlichia spp. (95 % CI 0-2.3%). Collectively, the results suggest that the SNAP® 4Dx® Plus test may be a suitable test for use with LE and TFE for the detection of D. immitis antigen in wild canids from southern Ontario.

Canine heartworm and heat treatment: An evaluation using a well based enzyme-linked immunosorbent assay (ELISA) and canine sera with confirmed heartworm infection status

Heat treatment of serum has demonstrated improved detection of Dirofilaria immitis antigen in sera of sheltered dogs without knowing the true infection status of the animals and in dogs confirmed experimentally to be infected with heartworm. Utilizing archived sera with necropsy confirmed heartworm infection status (n = 665) and a micro-titer well based ELISA antigen assay, this study evaluated how the composition of heartworm infections affects antigen test results pre- and post-heat treatment, determined subsequent changes to the antigen test sensitivity and specificity, and application of optical density values. The composition of heartworm infections present in dogs with sera initially testing antigen negative consisted of infections by dead 1/34 (2.9 %), immature 10/34 (29.4 %), male only 7/34 (20.6 %), female only 5/34 (14.7 %), and mixed sex infections 11/34 (32.4 %) with 2-62 heartworms of which 6 were microfilaremic. The composition of heartworm infections remaining antigen negative post-heat treatment consisted of infections by dead 1/14 (7.1 %), immature 9/14 (64.3 %), male only 2/14 (14.3 %), and mixed sex infections 2/14 (14.3 %) with 6 and 62 heartworms of which 1 was microfilaremic. The overall sensitivity for all infections, mature heartworms, and mature females before heat treatment were 86.9 %, 90.7 %, and 93.3 % and after heat treatment sensitivity increased to 94.6 %, 98.4 %, and 99.2 % respectively. A decrease in specificity from 97.8%-96.1% was observed following heat treatment of heartworm negative sera. Optical density values for the varying infection intensities present in this study clearly indicate that result intensity is not reflective of the number of heartworms present. This study provides additional context for interpreting post-heat antigen results for dogs originating from animal shelters, demonstrates diagnostic utility of optical density, and highlights the need for improved heartworm diagnostics.

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.

Comparison of six commercial antigen kits for detection of Dirofilaria immitis infections in canines with necropsy-confirmed heartworm status

Patient-side test kits for detecting antigenemia in dogs associated with sexually mature female heartworms (Dirofilaria immitis) have been available for three decades, and these tests are continually updated and improved. To define the sensitivity (Se) and specificity (Sp) of contemporary antigen detection tests against cardiopulmonary D. immitis burden, we evaluated five patient-side kits-Anigen Rapid One Step^® (Bio note), SNAP^® 4Dx Plus Test Kit (IDEXX), WITNESS^® Heartworm Canine Heartworm Antigen Test Kit (Zoetis), VetScan^® Canine Heartworm Rapid Test (Abaxis), and Solo Step^® CH Canine Heartworm Antigen Test (Heska), and one microplate ELISA (DiroCHEK^®; Zoetis), using archived canine sera divided into five subclasses of female worms (0, 1-5, 6-20, 21-40, and >40). The patient-side tests were performed once, side-by-side according to each manufacturer's protocol by personnel blinded to the D. immitis status of each dog. The overall Se and Sp of the patient-side kits was ≥97.5 and =94.0%, respectively. For samples from dogs with 1-5, 6-20, and 21-40 D. immitis, the Se was between 96 and 100%, with a slight increase in Se in dogs with ≥41 worms. The agreement between tests for all subclasses of D. immitis burden was between 99 and 100%. The Se and Sp for the ELISA compared with the necropsy results of dogs was 99 and 96%, respectively. Agreement between each patient-side test and the ELISA was between 97 and 100%. All commercially available tests can give practitioners excellent patient-side information, allowing them to make informed decisions on the need for additional diagnostic work-up before instituting new or continuing D. immitis prophylaxis.

Heat treatment and false-positive heartworm antigen testing in ex vivo parasites and dogs naturally infected by Dirofilaria repens and Angiostrongylus vasorum

Background

Heartworm antigen testing is considered sensitive and specific. Currently available tests are reported as detecting a glycoprotein found predominantly in the reproductive tract of the female worm and can reach specificity close to 100%. Main concerns regard sensitivity in the case of light infections, the presence of immature females or cases of all-male infections. Research and development have been aimed at increasing sensitivity. Recently, heat treatment of serum prior to antigen testing has been shown to result in an increase in positive antigen test results, presumably due to disruption of natural antigen–antibody complexes. Cross-reactions in dogs with both natural and experimental infections with Angiostrongylus vasorum and Spirocerca lupi have been reported, but cross-reactions with other helminths have not been extensively studied. In order to evaluate potential cross-reactivity with other canine and feline parasites, two studies were performed. Study 1: Live adults of Dirofilaria immitis, Dirofilaria repens, Toxocara canis, Toxocara cati, Dipylidium caninum, Taenia taeniaeformis and Mesocestoides spp. larvae were washed and incubated in tubes with saline solution. All worms were alive at the time of removal from the saline. Saline solutions containing excretory/secretory antigens were then tested for heartworm with six different, commercially available antigen tests. All results were evaluated blind by three of the authors. Study 2: Sera from dogs with natural infections by A. vasorum or D. repens, living in areas free of heartworm disease, were tested with the same tests before and after heat treatment (103 °C for 10 min).

Results

Results suggest that antigens detected by currently available tests are not specific for D. immitis. They may give positive results through detection of different parasites’ antigens that are normally not released into the bloodstream or released in a low amount and/or bound to antibodies. Tests may even detect antigens released by male D. immitis adult worms. D. repens appears to release more detectable antigens than the other worms studied.

Conclusions

Cross-reaction with A. vasorum and D. repens does occur in the field and could potentially occur with other helminths. Heat treatment decreases specificity by enhancing cross-reactivity.