Susceptibility of ponies to infection with Streptococcus pneumoniae (capsular type 3)

Welsh Mountain ponies were inoculated with an isolate of Streptococcus pneumoniae, SPE 1618 (capsular type 3) recovered from the equine respiratory tract: 10 ml of a suspension of 10(8) or 10(9) cfu/ml were instilled intratracheally. Fever was observed after either dose but the greater concentration also produced coughing, ocular and nasal discharge, depression and enlargement of submandibular lymph nodes. Cytological evidence of infection was also observed in tracheal washings during the first week after inoculation and corresponded with isolation of S. pneumoniae from the washes. Morbid anatomical and histopathological examinations of selected animals revealed focal pneumonia affecting the ventral lung, especially the cardiac area and accessory lobe, with a propensity to affect the right lung. S. pneumoniae was isolated directly in pure culture from these lesions or was demonstrable by immunostaining of macrophages bearing specific capsular type 3 antigen. By 10 days after inoculation, the ponies were healthy and had developed antibodies to S. pneumoniae. S. pneumoniae was therefore a primary pathogen in the horse under the conditions of the challenge.