Beware the spectre of salmonella

One of the difficulties in fighting the spread of infection is that cattle might appear healthy even while they are spreading disease.

Beware the spectre of salmonella
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Salmonella is a leading cause of gastro-enteritis in the world, with about 94 million people falling ill and more than 150 000 people dying each year.

In South Africa, there are specific slaughter regulations aimed at preventing salmonella from infecting humans. Yet, as was reported recently in Farmer’s Weekly, Dr Evelyn Madoroba of the Agricultural Research Centre (ARC) has warned that the absence of regular inspections in rural areas could result in some abattoirs taking short cuts that might increase the risk of salmonella spreading.

Overview of salmonella
In the case of cattle, Salmonella spp. infection usually occurs via water or feed contaminated with faeces from infected animals, such as other cattle, rodents, birds, flies and feral cats.

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In the case of cattle-to-cattle transmission, it can be difficult to tell which cows are infected because cows showing no signs of the disease can produce as many bacteria in their manure as can clinically affected (obviously sick) livestock.
Salmonella outbreaks typically last several months.

Worse, they often do not run their course. On some dairy farms, particularly those with large numbers of cattle, the disease may become endemic, with animals constantly being reinfected, thanks to the bacterium’s reproduction and survival strategy.

Under the right conditions, the bacteria can replicate every 30 minutes, and they can survive for four to five years in water, soil, dust, moist areas out of direct sunlight and on or within foods.

On the other hand, very low temperatures of -20°C kill 85% of Salmonella spp. in two days. Rendering – processing animal products using heat – also kills the bacteria. But post-processing adulteration accounts for 50% of contamination of rendered feed products worldwide.

Risk factors

  • Salmonella tends to be more common in dairy herds than in beef herds, mixed dairy and beef herds, and calf herds;
  • Salmonella outbreaks are more common in calving season; and in large herds;
  • Buying cattle from untrustworthy sources can introduce the disease;
  • Salmonella can spread through the air and via bacteria-laden dust, so close confinement can trigger an outbreak;
  • Mixing sick and calving cows can spread the disease;
  • Wild birds that have access to feed storage facilities, and feral cats, can cause an outbreak;
  • Salmonella can be introduced via flush water systems – that is, removing manure from an area with large volumes of fresh water, or recycling grey water from a manure pit or lagoon;
  • Feeding brewery products, animal by-pass protein, vegetable or other fat sources to lactating cows can cause problems.

Clinical signs to watch out for
Clinical symptoms of salmonella infection include the following: fever (pyrexia); lethargy and depression; lower milk production; anorexia; dehydration; increased salivation; and diarrhoea, progressing to dysentery (an inflammation of the intestine causing diarrhoea with blood).

Sources: ‘Salmonellosis in Cattle: A Review’ by Dr Sheila McGuirk and Dr Simon Peek, University of Wisconsin, School of Veterinary Medicine;