Rattles epidemiology: the role of the foal

Abstract

Rhodococcus equi causes pyogranulomatous bronchopneumonia (rattles) and gastrointestinal abscessation of foals. Early diagnosis is essential for minimising the impact of this disease on the horse industry. Current best clinical practice involves ultrasonographic (U/S) thoracic screening to detect small abscesses and initiate treatment before serious clinical abnormalities are obvious. Ecological and epidemiological studies in Australia have demonstrated that foals shed virulent R. equi from the respiratory tract and that high airborne levels of virulent R. equi are associated with high disease prevalence. The role of the foal as a contributor to environmental contamination remains an important practical area of R. equi research. Two studies were conducted. Samples from the breathing zone were collected from 53 foals as neonates and again at 6 weeks of age. These samples were cultured on selective bacteriological media. Samples were also collected from 80 mares and foals. Serum samples were collected from foals at 12 hours, 3 weeks and 6 weeks of age. Faecal and breath samples were collected at 3 weeks and 6 weeks of age. Colostrum samples were collected from mares at parturition. Serum and colostrum samples were tested using a VapA ELISA that detects antibody to the major R. equi virulence factor (the VapA protein). Faecal samples were cultured on selective media. There was strong correlation between anti-VapA IgG level in serum samples from newborn foals and maternal colostrum. However, foal VapA antibody concentration at 12 hours was not related to the likelihood of subsequent R. equi abscessation. Virulent R. equi was detected in breathing zone samples from 19% of neonatal foals and from 45% of 6 week old foals. The concentration of virulent R. equi in these samples was greater in the older foals. Antibody to VapA declined from 12 hours to 3 weeks of age as maternal antibody waned, but increased significantly between 3 and 6 weeks of age. Foals with ultrasonographic evidence of rattles had higher levels of antibody than non-diseased foals. Although >90% of 6 week old foals had virulent R. equi in faeces, foals with U/S evidence of rattles had a greater proportion. However, the concentration of virulent R. equi in faeces was not statistically greater than in foals with no U/S lesions. Six week old foals are a significant source of environmental contamination with virulent R. equi. These foals shed higher amounts of virulent R. equi from both faeces and breath compared to younger foals. This has significant implications for management and reinforces the current practice of separation of mares and foals based on age. These results also support collection of faeces from high traffic areas such as yards. Importantly, neither samples from the breathing zone, nor maternally derived antibody levels, were useful predictors of subsequent development of rattles. Perinatal airborne exposure to Rhodococcus equi is associated with increased risk of pneumonia caused by R equi in foals