Serological cross-reactivity of three commercial in-house immunoassays for detection of Dirofilaria immitis antigens with Spirocerca lupi in dogs with benign esophageal spirocercosis

The nematode Spirocerca lupi infects dogs and is endemic in Israel. It leads to formation of esophageal nodules and neoplasia. Infection is diagnosed by coproscopy, endoscopy and radiography. Dirofilaria immitis causes heartworm disease in dogs, and has a world-wide distribution, but autochthonous infection has never been detected in Israel. Infection is confirmed based on identifying D. immitis microfilariae, in concentrated blood specimens by microscopy (Knott's test or other tests) and serological tests specifically detecting circulating adult worm antigens. In the first part of this study, sera from dogs definitively diagnosed with esophageal spirocercosis by endoscopy were examined using three in-house immunoassays for detecting D. immitis antigen, and were positive in 2/19 (10.5%), 5/35 (14.3%) and 14/48 (29.2%) dogs, in assays 1 to 3, respectively, with no statistical difference between assays (P=0.08). Next, sera from 32 additional dogs with confirmed esophageal spirocercosis, which were confirmed to be negative for D. immitis and Dirofilaria repens DNA using a sensitive high-resolution melt PCR were tested using assay 3 and 8/32 (25%) were positive. These results demonstrate serological cross-reactivity between D. immitis and S. lupi in blood samples of dogs. In areas where the distributions of both nematodes overlap, this cross-reactivity should be considered when dog are screened for heartworm disease.