Unexpected field observations and transmission dynamics of contagious caprine pleuropneumonia in a sand gazelle herd
Résumé
Contagious caprine pleuropneumonia (CCPP), caused by Mycoplasma capricolum subsp. capripneumoniae, has long been considered a goat-specific disease. Since 2007 there has been growing evidence that this disease can affect wild ungulates either kept in captivity or in the wild. In 2013, a large collection of sand gazelles (Gazella marica) held in the United Arab Emirates suffered heavy losses due to a CCPP epizootic confirmed by PCR and isolation. Animals displayed typical lesions, with unilateral pneumonia and profuse pleurisy. An initial antibiotic treatment consisting of tylosin administered in drinking water did not improve the animals' condition and vaccination failed to stop the spread to contiguous pens. A treatment with tetracycline mixed in feed pellets finally succeeded to stop the evolution of the disease. A subsequent vaccine trial, performed on naive animals, showed that only a reference CCPP vaccine produced according to OIE standards induced a sero-conversion by CCPP competition ELISA, while the commercially available vaccines did not. A SEIRD compartment transmission model was developed to better understand the dynamics of the disease. The parameters were initially set as per expert opinion and then adjusted to fit the observed mortality data. The basic reproductive number R-0 was estimated to be between 2.3-2.7, while the final mortality rate reached up to 70% in some pens. Transmission of infectious droplets from an external source, through a distance of at least the 50 m separating the pens from the perimeter fence, remains the most plausible explanation for the contamination of this stock of gazelles.