Corynebacterium pseudotuberculosis biovar equi infection in horses: transmission and diagnosis

Abstract

A recent re-emergence of equine disease caused by C. pseudotuberculosis has been observed including spread to non-endemic regions. Corynebacterium pseudotuberculosis is a Gram-positive bacterium with an equine (equi) and a small ruminant (ovis) biovar. The general purpose of our studies was to investigate the mode of transmission and diagnosis of C. pseudotuberculosis infection in horses in order to institute measures to control further disease spread. Our specific objectives were to determine the role of house flies (Musca domestica L.) as a potential vector of this bacterium, and to provide further information to support the development of a more accurate diagnostic serological assay. Firstly, we demonstrated that house flies harbor live bacteria up to 24h following a 30min experimental exposure, providing support that this insect could be a plausible vector with the ability to spread the disease over a wide geographic area within a 24h period. Secondly, we demonstrated that house flies were mechanical vectors of C. pseudotuberculosis in an in vivo exposure study based on the development of clinical signs, positive culture results, increase in blood inflammatory markers, and increase in serological titers in ponies exposed to inoculated flies. For the second objective, the current serological assay (synergistic hemolysis inhibition (SHI)) was evaluated in a non-endemic population (Alabama) and its performance was compared to an ELISA test. Results showed that the seroprevalence of detectable antibodies determined by SHI testing in Alabama was high and exposure to ruminants was associated with increasing SHI titers. These findings could indicate possible false positive results caused by cross-reaction with antibodies against phospholipases from C. pseudotuberculosis biovar ovis. An ELISA test, based on C. pseudotuberculosis biovar ovis exotoxin and cell wall antigens, was inferior to the SHI test for the diagnosis of C. pseudotuberculosis biovar equi in horses. Analysis of the immune-dominant proteins from C. pseudotuberculosis in horses showed that bacterial antigens recognized by antibodies from sera of experimentally infected horses differed from sera before inoculation and differed from reaction against biovar ovis antigens. Our results provide evidence that house flies play a role in transmission of this disease in horses, which justifies further emphasizing insect control on equine farms to control its spread. In addition, the presence of detectable SHI titers in non-endemic populations, which was linked to ruminant exposure, and the preliminary analysis of immune-dominant proteins reiterates the need for further investigation to develop a more accurate diagnostic test.