RHODOCOCCUS-EQUI INFECTION IN FOALS - A REPORT OF AN OUTBREAK ON A THOROUGHBRED STUD IN ZIMBABWE

Twenty-four foals were confirmed to be infected with Rhodococcus equi on a private stud in Zimbabwe over a two-year period. Six mares had foals which were affected in each of the two years. All the foals were febrile and early cases were detected by this pyrexia. Bronchopneumonia was only clinically detectable in advanced cases. In spite of energetic hygiene measures relating to pasture and housing management, the incidence was higher in the second year (23 per cent of foals born ) than in the first (15 per cent of foals born). The mean age of the foals was significantly greater in the second year. The immunological status of some of the foals was obtained from zinc sulphate turbidity tests performed at 24 hours old and all the affected foals so tested were considered to have had effective colostral transfer. Clinically, affected foals showed pyrexia, tachypnoea and tachycardia and many had a scanty purulent nasal discharge. Only one animal had diarrhoea which was attributed to enteric infection with R equi. Diagnosis was confirmed by clinical examination, culture of the organism from tracheal aspirates and thoracic radiography and was supported by significant elevations of plasma fibrinogen and platelet and neutrophil counts in all cases. Anaemia was not a major finding in any case. Rifampicin and erythromycin were administered per os and weight-related doses were continued until plasma fibrinogen and the absolute neutrophil and platelet counts had been persistently normal for two weeks. Supportive radiography was obtained in some cases. Treatment regimes were continued for between nine and 12 weeks in the first year and for between four and six weeks in the second year. This difference in the treatment course was attributed to earlier diagnosis in the second year, when some cases were identified without any overt clinical signs. All 24 foals survived and, although several had a persistent nasal discharge the following year, growth was considered normal.