Comparison of three diagnostic tests for Dirofilaria immitis in a low-incidence area.

Abstract

was obtained, tests were repeated by a different technologist. Microfilaria identification using the alkaline phosphatase technique was done to differentiate between D. immitis and Dipetalonema reconditum on all samples in which microfilariae were observed. Clinical follow-up was obtained for 11 of the 12 dogs that had any positive test result by reviewing the medical record or discussing the case with the clinician. The results of the laboratory tests are summarized in Table 1. Samples from 1,347/1,359 dogs (99.12% of samples submitted) were negative for microfilariae and adult D. immitis antigen using both commercial kits. Microfilariae, subsequently identified as D. immitis, were observed in blood samples from 7 dogs. Four of these dogs also had adult D. immitis antigen detected by both kits. Two microfilaremic dogs were negative for D. immitis adult antigen with both kits. Blood from 1 dog that was positive for microfilariae was serologically positive for adult antigen with only 1 of the commercial kits. Five dogs were serologically positive for adult D. immitis antigen but negative for microfilariae. The 5 dogs that had positive tests for adult D. immitis antigen but negative filter tests were considered otherwise healthy. They had no evidence of circulatory or respiratory compromise. Adulticide treatment was recommended for each of these dogs, but owners chose to administer ivermectin or milbemycin oxime monthly to their pets. Owners were advised to observe their dogs carefully and to restrict their pets’ exercise for at least 2 days following the administration of the monthly heartworm preventative. Adverse reactions were not reported in any of these dogs. Six of the 7 dogs that tested positive for D. immitis microfilariae underwent treatment for the elimination of adult D. immitis and microfilariae, regardless of the results of the adult antigen test. Additional information could not be obtained for 1 dog that was positive on all 3 tests. Treatment in 1 dog that had positive results with all 3 tests was initiated as an emergency procedure when the dog developed acute respiratory distress. Two dogs that were microfilaremic were adult antigen negative using both kits, and 1 microfilaremic dog was antigen negative with 1 kit but antigen positive with the other. Such results are likely to occur if the adult worm burden is low. In these cases, the sensitivity of the tests for detection of adult antigen is decreased. 2,3,6,8,9 Based on physical examination and radiographs, the clinicians strongly suspected that 2 of these 3 dogs had few adult D. immitis in their right ventricles, and the dogs were treated with adulticides before beginning preventative therapy. The presence of adult D. immitis infection in the right ventricle in the third dog was questionable, yet the owner preferred to have the dog treated with adulticide before preventative therapy was begun. Although the number of infected dogs detected in this study was small, infection in 5 of 12 animals would have gone undetected if only a filter test, accompanied by acid phos