Salmonella

Salmonellosis is the most common disease associated with acute diarrhoea in adult dairy cows. Salmonella bacteria from cattle can also be transferred to humans, causing illness and disease.

What causes Salmonellosis

Salmonellosis is caused primarily by ingestion of Salmonella bacteria, which can live for long periods of time in the environment (~28 weeks).

There are a variety of types or serovars of Salmonella bacteria that can affect dairy cattle. Salmonella typhimurium and Salmonella bovismorbificans are the most common in New Zealand.

The occurrence of Salmonella is increasing in New Zealand, in parallel with a world-wide trend, largely as a result of intensification, higher stocking rates and/or higher use of supplementary feed.

The most common sources of Salmonella include:

• Other clinically affected cows or carrier animals
• Contaminated environments
• Water or feed that has been contaminated with faeces

Signs in animals

Different types of Salmonella lead to a variety of clinical signs.

The most common signs to look for include:

• Sudden drop in milk production
• Loss of appetite
• Diarrhoea and/or dysentery
• Animals presenting as anorexic, with a fever
• Dehydration
• Abortions can happen with some types of salmonella
• Death can occur in severe cases or if left untreated

How is it spread?

Salmonella can be spread by several different means, including:

• Direct animal-animal contact within the herd
• Other animals or birds
• Dirty clothing/objects
• Contamination of feed or water supply
• Airborne bacteria
• Application of untreated effluent to pastures

The long period over which Salmonella can persist in the environment (up to 28 weeks) makes elimination of the organism difficult.

In New Zealand, carrier animals are the leading means of spreading infection, especially as re-shedding of infection can be brought on by stressors such as calving, transportation, bad weather or deprivation of food or water.

Clinical experience indicates that the risk of Salmonellosis increases when supplementation with magnesium e.g. Mag oxide, Mag chloride, occurs at above recommended dose rates.

Management, prevention and control of Salmonella

Prevention should focus on reducing the risk of infection, minimising the spread of infection, and enhancing the immunity of animals within the herd.

Such measures include:

• Vaccinating against Salmonella – vaccination programmes are an effective way of protecting the herd against Salmonellosis, and can be helpful in reducing the number of sick cows even once an outbreak of Salmonellosis occurs in a previously unvaccinated herd
• Good biosecurity procedures for any new animals to the herd
• Appropriate storage and application of effluent, with a minimum stand-down period between application and grazing
• Effective rodent and bird control, particularly around feed bunkers and grain silos
• Good hygiene practices at milking time, and procedures on-farm to help protect staff
• Wearing gloves while working with cattle, particularly around calving
• Cleaning and drying gear regularly to reduce the risk of contamination

If cases do occur, it is important to seek veterinary advice promptly. Early treatment of cases with broad spectrum antibiotics will usually result in the survival of the animal. Delayed treatment (~48 hours) will result in severe dehydration and irreversible damage to the gut. Treating these cases will require supportive therapy, which may improve the chances of recovery, however this is likely to be a lengthy process.