Diseases caused by Salmonella bacteria are some of the most important diseases found in cattle. Not because they are very common or because infection cause high disease and death rates, but because all salmonellae found in cattle can potentially spread to humans, so a considerable amount of government money is spent on investigating Salmonella outbreaks in order to protect public health, particularly on dairy farms. This has been very effective in the UK with very few cases of Salmonellosis in humans linked to cattle.
A wide range of salmonellae have been isolated from cattle in the UK, most of them only occasionally. The most common type of Salmonella affecting cattle in the UK is currently Salmonella Dublin, although the number of cases seen in the last twelve months are significantly down on the peak of cases seen in 2002. S. Dublin is most common in the wetter areas of the UK especially Wales, SW and NW England and SW Scotland. It is also very seasonal with most cases occurring in the autumn, particularly around October. However cases can be seen at any time of the year and in any part of the country. Although it’s the commonest Salmonella causing disease in cattle, because, unlike other salmonellae, it’s specifically adapted to cattle, cases in humans are very unlikely and thus very rare.
Clinical signs
S. Dublin causes a wide range of diseases in cattle, not just diarrhoea. In adult cattle, acute and subacute forms of disease are seen. Acute disease:
- Fever, dullness, decreased appetite and milk drop
- Severe bloody (and often watery) diarrhoea with blood, mucus and casts
- Death occurs in around 75% of affected animals if they are not treated.
The subacute form varies from a milder form of the acute disease to infection without obvious disease. Abortion can occur in severely ill animals, but more often it occurs in a cow with no other signs of disease. Abortion due to S. Dublin is the most commonly diagnosed cause of abortion in UK laboratories.
Disease in calves:
This is much more variable. It is usually seen in calves between two and six weeks of age. However, because the disease can be slow to resolve older infected calves can be seen. Clinical signs include:
- Pasty diarrhoea which becomes bloody and watery with an offensive odour
- Calves become dehydrated, collapse and die.
- Calves may also die suddenly with no previous diarrhoea
- Pneumonia, stiffness, joint-ill and meningitis are also seen.
Diagnosis
Diagnosis in affected adults is relatively easy as large numbers of bacteria are found in faeces (or in aborted calves). However, in calves excretion in the faeces is much less reliable, so not finding any bacteria does not mean that a calf does not have S. Dublin. A proper post mortem examination is much more useful in such cases, as it is much better at uncovering evidence of infection.
Treatment
Antibiotics and supportive treatment, particularly fluids either orally or in the vein, increase survival rates in calves and adults. S. Dublin, unlike some other salmonellas is usually sensitive to most antibiotics. However, it is still important to check which antibiotics are effective as soon as possible after diagnosis.
An additional problem with treating adults is that some, but not all, treated animals will become carriers, that is excrete S. Dublin in their faeces for prolonged periods without ever showing signs of disease. When treating S. Dublin get advice from your vet on which animals to treat and what to treat them with.
Control and Prevention
In infected herds, infected animals must be separated and isolated away from the rest of the animals. Ensure you have proper barrier nursing and handle, feed and treat ill animals after you have fed and handled the others. Ensure that isolation is effective – too many farms have isolation facilities that are near (or actually are) calving boxes. This is a highly effective means of spread of disease as calving cows are at their peak risk of infection.
Also ensure that milk from ill cows (or cows that have been in contact with such cows) is not fed to calves. Milk is a very good source of bacteria and disease is very common in calves fed infected milk.
Hygiene is essential. Clear out and disinfect all calving boxes thoroughly and if you have infected calves clean and disinfect calf pens. If possible use temporary facilities to allow bacterial numbers to decline even further.
Vaccines are available, however once S. Dublin has entered a herd, vaccination alone will not control the spread of infection. Good husbandry and hygiene is essential if control is to be achieved.