Lungworm infection and ovine visna-maedi: Real risk factor or a confounding variable?

Serum and fecal samples (n = 2081) were taken from 74 commercial meat ovine flocks in Galicia (NW Spain) in order to establish the influence of lungworm infection with protostrongylids and/or Dictyocaulus filaria on visna-maedi (VM) seropositivity. VM was tested by an indirect ELISA test and lungworm larvae were detected by the modified Baermann-Wetzel technique. In addition to lungworm infection (protostrongylids and D. filaria), flock, management and individual factors were included in univariate and multivariate statistical tests.Overall VM seroprevalence was 18.9% and the prevalence by protostrongylids and D. filaria was 11.6% and 10.7%, respectively. Protostrongylid infection was considered as a risk factor for VM with univariate statistical analysis (1.49 significant OR), but neglected in a logistic multivariate regression analysis when the age of the animals was introduced as variable. As VM, protostrongylid infection is a chronic disease with a cumulative pattern over time, so that these two infections are closely related to the age of the animal. A Mantel-Haenszel procedure was applied and lungworm infection was detected as a confounding variable with no real effect on visna-maedi infection.