No abstract

A selective medium was used to isolate Yersinia sp from the intestinal tract of 222 scouring cattle in Gippsland during 1985 and 1986. Intestinal infection with Y. pseudotuberculosis, particularly of serotype III, was found to be especially prevalent in weaned calves, yearlings and young adult cattle. Clinically affected cattle had a profuse liquid diarrhoea and many were systemically ill. Haematological changes suggestive of infection were present in 38 of 49 of these cattle. At least 35 cattle died and characteristic microabscesses were demonstrated in the intestinal mucosa of 20 of 26 examined histologically. Y. pseudotuberculosis was sensitive to tetracyclines in vitro and this drug produced a rapid bacteriological cure. Yersiniosis occurred during the winter, spring and early summer. Challenge of adult cattle with Y. pseudotuberculosis serotype III did not result in intestinal colonisation or clinical disease. Intestinal infection was, however, established in 4 weaned calves and haematological changes and antibody production were demonstrated in them. Intestinal microabscesses were seen in three calves killed on days 8, 14 and 18 after challenge. The fourth calf eliminated infection by day 18 and no lesions were demonstrated when it was killed on day 72. There is a very high prevalence of antibodies reacting with Y. pseudotuberculosis serotype III in adult cattle. It is concluded that cattle are a common host for this bacterium, infection being frequent, with clinical and fatal disease occurring occasionally. The factors leading to clinical disease are unknown.

No abstract

The mathematical relationships between the sensitivity and specificity of screening tests and the proportion of animals which are positive to the test, but which have no visible lesions (NVL) at subsequent examination or necropsy, are outlined briefly. NVL reactors may be defined as a proportion of the total number of reactors to the test (reactor NVL per cent) or of the total number of animals tested (herd NVL per cent). Neither the herd nor reactor NVL per cent can provide valid estimates of the efficacy of any particular test, unless the precise prevalence of disease in the tested herd is determined. However under conditions of low or moderate prevalence specificity is approximately equal in magnitude to 100-herd NVL per cent but sensitivity cannot be reliably estimated from either the herd or reactor NVL per cent.

No abstract