AN APPROACH TO THE DIAGNOSIS OF CALF DIARRHOEA

Diarrhoea in young dairy and beef calves causes substantial economic losses mainly due to morbidity, cost of treatment and to a lesser degree, mortality. It has a multifactorial aetiology, resulting from an interaction between the calf, its environment, nutrition and infectious agents. Several such agents may cause diarrhoea, but it should be emphasised that solving these problems on a herd basis, requires the identification of poor management practices rather than focusing solely on the causative agents involved. Pursuing a specific aetiological diagnosis is, however, necessary in order to: determine the antibiotic sensitivity of the bacteria involved; allow for the implementation of specific therapeutic and prophylactic measures; and if feasible, institute a vaccination program.

An aetiological diagnosis may be missed due to the incorrect and inadequate collection, storage and/or transportation of specimens to the laboratory for examination. We advise the following samples to be collected to allow identification of the infectious agents involved in calf diarrhoea. Faecal samples, not rectal swabs, and specimens collected at necropsy (if applicable), should be submitted. At least 5 g of faeces each from 5 affected, untreated calves and from 5 clinically normal, age-matched animals is required. The faeces of the diarrhoeic calves must be collected as soon as possible after the onset of diarrhoea. It is usually pointless to even attempt a diagnosis on faecal samples from only one or 2 calves. Samples must be stored at 4ºC on ice, not frozen, and submitted in separate sterile containers.

If an affected, untreated calf is available for necropsy, specimens required for bacteriological and pathological examination should include: small and large intestines (preferably ileum and colon), mesenteric lymph node, spleen, liver and brain. Faeces must be collected from the rectum as outlined above. A post mortem examination on one or more affected, preferably untreated calves submitted live to the laboratory, can be a valuable aid in arriving at a diagnosis. This is often not acceptable to the farmer, particularly when valuable animals are involved. Information gained by doing a thorough necropsy may be beneficial to: detect agents not found by faecal examination; assist in identifying the portal of entry of pathogens (e.g. umbilical); evaluate the extent to which bacteria have disseminated; and rule out concomitant disease.

At the laboratory, faecal samples are routinely examined for: pathogenic bacteria by aerobic culture; viruses by negative staining electron microscopy; Coccidia and worm eggs by faecal flotation; and Cryptosporidium oocysts by fluorescent staining of faecal smears.

Serotyping of Salmonella and E. coli species and identification of Coccidia species are routinely performed. Serotyping of Salmonella and E. coli species is especially important in advising on vaccination as a prophylactic measure. The diagnostic value of faeces from normal calves is often underestimated. Up to 30% of clinically healthy calves can harbour one or more organisms capable of causing diarrhoea. The diagnosis of a viral diarrhoea in South Africa for example is based on the detection of significantly higher concentrations of the agent in the faeces of diarrhoeic calves compared with healthy calves on the same farm. In addition, examination of specimens from several calves is also necessary, as mixed infections can occur in a herd and may also be present in the same animal. Viral isolation in the diagnosis of calf diarrhoea is not routinely done, as enteric viruses are difficult to grow. On the other hand, the advantages of electron microscopic examination of feacal specimens include a rapid response and the possibility of identifying more than one virus. There is an added advantage that viruses which have been implicated a causes of diarrhoea, but whose importance in field situations is unknown e.g Bredavirus and Astrovirus, can be evaluated.

To a large extent, it is the standard of management that controls the extent, severity, and duration of disease in a herd. On the other hand, identification of pathogenic agents is important to control an outbreak of calf diarrhoea. Correct sampling and submission of specimens significantly improves the chance of making a specific diagnosis.