Salmonella, the bacteria that scares us all

Chris Gregory BVSc MRCVS, Molecare Veterinary Services

If there is one bacterium sure to incite mass panic and hysteria it is most likely Salmonella, no thanks to a certain ex-health minister. However, it’s not just the poultry industry where this bug likes to lurk.

The Salmonellae family are diverse and are all potentially capable of causing disease in cattle and humans alike; hence the relative importance of controlling them in farmed animals.

There is a tendency for certain Salmonellae to adapt to a preferred host, in the case of cows it is Salmonella Dublin. Adapting to a host allows the bacteria to survive and reproduce in the host without rapidly killing it. This tactic can allow an insidious accumulation of the disease within a herd, and be spread between herds when apparently healthy animals are bought-in unawares. Other potential points of entry to a herd could be wildlife (e.g. birds & rodents), flies and contaminated slurry.

Salmonella Dublin is spread via ingestion of contaminated faeces. Calves born to an infected dam will inevitably become infected at/around birth, and once recovered will become permanent carriers of the disease. Carrier animals will go on to periodically shed the bacteria, even when apparently healthy, particularly around stressful events (e.g. calving).

Clinical Signs:

Although adult cows are less likely to become obviously sick, it can result in a range of clinical signs from the subtle to the extreme:

• Fever, followed by low temperature
• Dull/depressed behaviour
• Milk drops
• Mid-late term abortions
• Watery/bloody/mucus diarrhoea
• Death if not treated promptly

Salmonella Dublin is more commonly diagnosed off the back of mid-late term abortion investigations and in young calves ~2-6 weeks of age; though due to the slow resolution of some of the symptoms older cattle can still appear affected. Clinical signs in calves can be varied, and often confused with other causes of calf disease:

• Fever
• Decreased feed intakes
• Watery/bloody/mucus diarrhoea
• Joint ill
• Nervous signs
• Respiratory signs
• Dry gangrene of extremities (mainly ear-tips)

Diagnosis

Diagnosis of S.dublin relies first and foremost on recognising an emerging clinical picture from some of the clinical signs. As alluded to above, the signs can be relatively non-specific and cross over with other diseases.

1. Dung: S.dublin can be cultured from dung from an infected animal, however it can be hit and miss as the cow may only shed the bacteria intermittently and/or in low numbers. Where there is any evidence of watery/bloody/mucus diarrhoea it would be worth ruling out S.dublin with a dung sample.
2. Foetal Stomach Contents: Not for the squeamish, but one of the more accurate methods is to get your vet to investigate any sudden abortions in the context of other suspicious signs e.g. milk drops, scours and calf disease. Any fresh aborted foetus is potentially very useful in reaching a diagnosis.
3. Blood Samples: Salmonella antibodies will prove exposure and may be a good way of screening potential carriers.
4. Bulk Tank Screen: A useful first step looking at herd level. Routine monitoring could show an emerging problem.

Treatment

Treatment of acute cases relies on the usual antibiotic, anti-inflammatory and fluid protocols combined with solid nursing and isolation of cases to prevent contagious spread. The emphasis should then focus on prevention.

How do I prevent it?

Establish the herd status by routine diagnostics e.g. bulk tank sample/sick animals/abortions.

All control measures revolve around preventing the initial infections, mainly in the calving pen and subsequent accumulation in the rearing pens. The advice is akin to the Johnes control measures, preventing a single source of infection spreading to multiple calves.

• Avoid pooled colostrum feeding. Additionally, pasteurisation would be preferable.
• Segregate obviously sick animals, especially if near to calving. Be aware that Salmonella can survive in faeces for long periods of time (up to 300 days in slurry) when planning where to muck spread.
• Quarantine and blood sample any incoming stock. Once a cow’s status is known she can be managed appropriately, again much the same as red-tagging a Johnes cow.
• Vaccination can act as a ‘firebreak’ as the other measures are adopted to prevent spread, though shouldn’t be relied upon to solve the problem.
• Establish a control plan with your vet. It’s very likely that a solid Johnes control plan will cover the same risk factors for Salmonella anyway.

For more information phone Molecare Veterinary Services on 01392 872934.