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Salmonella can have a deeper impact than clinical cases

Gary Neubauer Published on 16 September 2011

We all know the signs of a clinical salmonella case – sudden weight loss, weakness, fever, explosive diarrhea and dehydration. If a cow produced 50 pounds of milk last night and none this morning, you know that salmonella could be the cause.

Although clinical cases are easy to spot, they can be only the tip of the salmonella iceberg. Other animals may harbor subclinical salmonellosis that can be just as damaging to your bottom line.



Salmonella bacteria can be found on virtually all dairies, no matter where they are located. Whether salmonella starts as an undetected presence or causes a major disease outbreak depends greatly upon the exposure level.

Salmonella can be symptomless
One of the most dangerous and frustrating aspects of salmonella is its ability to exist as a subclinical infection in an otherwise healthy-appearing animal. While clinical cases are obvious, it is more difficult to identify subclinically infected individuals that are carriers.

Salmonella carriers are animals that had been infected at some point in their life but have cleared the active infection. However, the bacteria may remain at a low level throughout the animal’s lifetime. This often happens pre-weaning, before the calf is able to fully develop its immune system.

Making matters worse, subclinically infected cattle will not always shed salmonella on a consistent basis. There can be several different categories of subclinical salmonellosis:

• Active carriers that shed salmonella in manure and/or milk
• Symptom-free carriers that infrequently shed organisms
• Latent carriers that harbor salmonella but do not shed the bacteria


This can make it very difficult and expensive to identify salmonella carriers in your herd. However, it is important to identify these carriers because they can infect the rest of the herd by shedding bacteria through their manure during periods of stress.

Salmonella is most often transmitted to other cattle through fecal-oral contamination. The number of cattle shedding bacteria and the rate of shedding is a main contributor to the length and impact of salmonella on a herd.

A widespread problem
While carriers can be difficult to identify, studies have demonstrated that they exist on many dairy operations, often in high numbers.

In one five-herd study in Ohio, researchers determined that subclinical salmonella shedding can persist in dairy herds for up to 18 months with no measurable effects on health or production of individual cows. In that study, prevalence of salmonella shedding at individual collection times ranged from 0 percent to 99 percent for cows and 0 percent to 67 percent for unweaned calves, even though none of the adult animals in the study had clinical signs of disease.

In a 2007 U.S. Department of Agriculture study of dairies across the country, 48 percent of all dairies were infected with salmonella, nearly double the percentage of herds infected in a similar study 10 years earlier. In addition, the percentage of cows infected with salmonella has more than doubled to 13.7 percent since 1996.

Disease control starts with sanitation
Dairy producers can help control salmonella through sanitation and paying close attention to high-risk animals. For most dairies, it becomes a numbers game. Cattle may become sick if salmonella numbers climb high enough and/or cows become less able to resist the bacteria.


High-risk cattle on a dairy are most likely to break with clinical disease. Fresh cows and young calves are most susceptible to salmonella because they may have compromised immune systems and often face other disease challenges.

Sanitation is an important defense in the salmonella numbers game. Salmonella bacteria can be spread by many methods, and these tips can help protect vulnerable cattle and help reduce disease exposure across the dairy.

• Make sure loaders and other feeding equipment are not also used to handle manure

• Pasteurize waste milk and colostrum fed to calves

• Maintain sanitary calving facilities to avoid infecting newborns

• Keep populations of rodents, feral cats and birds low in feed storage and animal housing areas

• Control flies throughout the dairy by using common methods

• Restrict visitors and insist on biosecurity measures (such as clean boots and clothing) by all who enter the facility, including the herd veterinarian

• Clean calf-feeding utensils and oral treatment equipment with chlorhexidine (three ounces per gallon)

• Wash boots regularly with orthophenylphenol and change and launder work clothes daily. Ideally, boots and work clothing should be left at the dairy

• Thoroughly sanitize transport trailers, particularly when hauling young calves

Vaccination is part of disease control
A key component to a successful salmonella disease control program is vaccination. Vaccines have been developed that help provide protection against salmonella.

Just like the cattle they infect, salmonella bacteria require iron for growth and survival. Salmonella bacteria use transport proteins called sideorphores to capture iron and deliver it to the bacteria through sideorphore receptors and porins (SRPs). This allows the bacteria to thrive and grow.

Researchers used this knowledge to develop a vaccine technology called SRP technology. The antibodies generated by vaccination with the salmonella Newport Bacterial Extract vaccine* with SRP technology specifically target and block this iron-gathering mechanism, resulting in the death of the salmonella Newport bacteria.

The vaccine effectively helps control disease and fecal shedding of salmonella Newport, resulting in reduced disease incidence and improved herd performance. Antibodies generated through vaccination also have been shown to recognize the SRPs of several other common salmonella serotypes.

Research shows that helping control the disease through vaccination can help improve milk production and performance of cattle with subclinical salmonellosis. A published study conducted in a Colorado dairy herd with no clinical signs of salmonella showed that vaccinating with the salmonella Newport Bacterial Extract vaccine resulted in 2.5 pounds more milk per cow per day. Reduced somatic cell count was also associated with vaccination.

Whole-herd vaccination utilizing the salmonella Newport Bacterial Extract vaccine allows producers to create whole-herd immunity, helping protect all cattle on the operation against salmonella Newport.

The right thing to do
Dairy producers have a significant role to play in the safety of all food products originating from their operations. Salmonella is a serious foodborne pathogen and the leading cause of foodborne disease hospitalizations and deaths in the country.

Don’t wait until you’ve seen a clinical outbreak in your herd to address this important issue. Start by working with your veterinarian to develop and implement a salmonella control program.

Sanitation and biosecurity protocols, supported by annual vaccination, can be an effective strategy to help control salmonella on the dairy. PD

*Efficacy and potency test studies are in progress.

References omitted due to space but are available upon request to .


Gary Neubauer
Senior Manager
Dairy Veterinary Operations
Pfizer Animal Health