Tackling Salmonella hidden in dairy herds

The practice where the author works offers free quarterly bulk milk antibody screening to all of its dairy clients for Salmonella groups B and D, among other diseases. This is a useful tool to help understand a herd’s disease status, often prompting further investigation.

As a practice, it wanted to proactively monitor the disease status of all of its herds to see where opportunities to improve herd health existed.

Bulk milk samples are simple to obtain, repeatable and give a good initial overview of a dairy herd’s exposure to disease. Working with National Milk Laboratories, the practice utilises existing payment samples so the scheme is hassle-free to the farmer.

The results are interpreted by the farm’s vet, alongside vaccination records and knowledge of their general herd health and biosecurity. This service is provided free of charge to the farmer and the practice has found it a really useful tool.

Of the 190 farms enrolled in the screening – which covers infectious bovine rhinotracheitis, bovine viral diarrhoea, leptospirosis, liver fluke, Salmonella and, for some herds, Schmallenberg virus – 70% of herds had a positive result for Salmonella in the past two years. Only 51% of unvaccinated negative herds have maintained their negative status over this time.

Serologic prevalence

Official figures show that 7.5% of UK dairy farms sampled tested positive for Salmonella Dublin; however, it is widely thought to be more prevalent, due to diagnostic challenges¹.

The author believes the results from the practice’s screening indicate that Salmonella exposure is fairly widespread.

However, the presentation of the disease can vary. The most well-recognised signs of Salmonella are abortion, diarrhoea and septicaemic calves, but in endemically infected herds, it can be more of a grumbling underlying problem. Before starting the screening, the practice where the author works was only really diagnosing herds with visible symptoms, in the outbreak scenario, which can be catastrophic.

In those cases, it could see anything from a couple of abortions to a full-blown abortion storm. This depends on the herd’s immunity and dynamics. For example, a naive block calving farm may be particularly badly affected by an outbreak, as a high proportion of the cows are at risk at the same time.

As Salmonella can manifest differently between herds and is often difficult to isolate, it can be hard to identify.

Salmonella can be overlooked when the classic signs that people associate with the disease are not present. More subtle signs include a drop in milk production and poor growth rates in calves. Many potential causes may exist, but these impacts will be costly to the farmer and it is important to consider whether it could be Salmonella.

When the practice has suspicion of a disease in an unvaccinated herd, repeat positive bulk milk results or an abrupt increase in positive results would prompt further investigation. Follow-up testing could include a youngstock antibody screen to check for recent exposure, postmortem sampling from any calf deaths or cultures from suspected cases.

The impact of Salmonella

More than 100 strains of Salmonella exist, but the two most commonly seen in UK dairy herds are S Dublin, which has a high carrier status, and Salmonella typhimurium, which is less common, but associated with abortion following septicaemia and enteritis in freshly calved cows².

As well as the welfare and economic impact of the disease, Salmonella is also zoonotic and poses a risk to the health of farmers, staff and vets. Awareness of the disease’s presence and appropriate biosecurity precautions are, therefore, crucial to avoid illness.

After initial infection, more commonly with S Dublin than other strains, cows can adopt a carrier status, remaining infected and intermittently shedding the dormant bacteria to naive animals².

A carrier cow may look like a “normal” cow. At times of stress when the immune system is suppressed, such as calving, a carrier cow might start shedding. It’s likely that the farmer will notice signs like scouring. When this happens, Salmonella might be a differential, but it wouldn’t be the only risk, or top of the priority list to investigate – freshly calved cows are susceptible to various problems.

Carrier cows can act as a source for continual exposure, maintaining disease spread in the herd².

It can take many years to “clear” Salmonella from a herd, and unless you are actively hunting out carrier cows, excellent biosecurity and vaccination programmes are needed in a farm-specific action plan.

Managing and preventing Salmonella

S Dublin is spread mostly via the faeco-oral route and can be maintained in herds through transmission from cows to calves at birth. Management and hygiene around calving is, therefore, crucial to avoid perpetuating the disease.

Calving hygiene is really important. If Salmonella is a known risk on a farm, calves shouldn’t be left on cows for too long, and pens should be cleaned out and disinfected regularly. It is important to isolate any suspected cases, keeping them away from dry cows and youngstock in particular.

In the author’s opinion, while closed herds can work to remove carriers and cull out the disease, open and flying herds always have the risk of “buying in” Salmonella infection, as it can be difficult for farmers to determine incoming herd status.

It is so important that farmers find out if any routine disease screening has been performed in herds they are buying from and ascertain which vaccinations have been given.

Herd vaccination provides a useful tool to help prevent infection, as well as minimising the impact of Salmonella present within a herd.

When a Salmonella diagnosis is confirmed, the practice where the author works starts the herd on a primary vaccination regime – two injections for all at-risk adult cattle, separated by an interval of 21 days.

On some farms, an annual booster is put into place, done every 12 months after the primary course.

On others, a booster at dry off is looked into, where herds have a tight calving interval. Vaccinating at dry off boosts the cow’s immunity at the greatest risk period and helps colostrum quality. It protects both the cow and calf.

Collaboration with farmers

Offering free bulk milk screening to dairy clients has been a success for the practice where the author works, as it now has a better understanding of disease prevalence in its client base. From here, it will work closely with individual farmers to improve and maintain herd health.

Farmers have really appreciated the screening, which is free of charge and minimal effort for them.

Infectious diseases such as Salmonella can have a huge impact on productivity, so farmers will clearly be interested in knowing the status of their herd. Having a proactive approach to disease control on-farm is vital to limit the potentially catastrophic impact of a disease outbreak.