Prevalence and risk factors of Dirofilaria immitis infection in dogs from Mexico, including serum pre-heat treatment for the dissociation of immune complexes

<i>Dirofilaria immitis</i> is a nematode found worldwide and capable of infecting several animal species, including dogs. In Mexico, <i>D. immitis</i> infection in dogs has been detected in 22 of the 32 states, with a wide range of prevalence. Blood samples of 535 dogs from four ecological zones and 11 localities in Mexico were processed with ELISA (IDEXX SNAP 4Dx® Plus) with non-treated blood samples (NTB) and pre-heat-treated sera for immune complex dissociation (pre-ICD). Thick drop tests were also performed. The prevalence of <i>D. immitis</i> in each locality was estimated. The positive results were analyzed using <i>χ</i> ²-test to identify the associated factors, and the variables with <i>P</i> < 0.2 were analyzed using a logistic regression. The overall prevalence found using the ELISA test with NTB was 15.5%, while it was significantly higher (22.6%) using the same test with pre-ICD. With the thick drop test, a lower prevalence (7.2%) was observed. The detection of positive cases of <i>D. immitis</i> infection was significantly different (<i>P</i> < 0.05) between the three tests. Of the studied dogs, 15.5% (83/535) were positive to both ELISA using NTB and pre-ICD; however, 7.1% (38/535) were only positive after pre-ICD. The higher prevalence was found in Ciudad del Carmen (48.3%), Tampico (48.3%), and Puerto Morelos (64.0%). The risk factors associated with <i>D. immitis</i> infection were the ecological zone (warm subhumid with summer rains: <i>Aw1</i>, OR 2.8, <i>P</i> = 0.003 and <i>Aw0</i>, OR 20.4, <i>P</i> = 0.003); and age of the animals (dogs ≥ 6 years, OR 2.8, <i>P</i> < 0.001). In Mexico, dogs from four ecological zones were found highly infected with <i>D. immitis</i>, the associated risk factors were the ecological zone and age of the animals. Pre-ICD revealed that 7.1% of ELISA antigen-tested samples were false negatives with NTB.