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Salmonella and Stress

Bob Corbett

October 20, 2020

Outbreaks of Salmonella in dairy calves are one of the most difficult and frustrating issues to deal with. In most cases, the mortality rate is high, and the economic loss is significant. The calves fortunate enough to recover are often severely debilitated and are less productive in the future.

Regardless of how well the maternity and calf areas are managed, Salmonella is always present. The goal is to reduce the numbers as much as possible and maximize the immune response of the calf.

When investigating an outbreak, it’s necessary to identify the source, or sources. I have found Salmonella in calf feed — even feed on the truck before it was unloaded on the farm. Salmonella can be cultured from water troughs, calf hutches, milk, colostrum, feeding utensils, esophageal feeders, bolus guns, syringes and needles, boots and clothing and bedding material.

There might also be some cows that are chronic asymptomatic carriers and shedding significant numbers of Salmonella in their feces. This is especially important when these animals are in the maternity area, contaminating the environment for every calf born in that area. 

A 2016 study by D.L. Hanson and others on vertical transmission of Salmonella in dairy cattle provides substantial evidence for transmission of Salmonella in utero. Calves in this study were humanely euthanized immediately after birth and samples were taken for Salmonella culture. These calves were from a commercial herd without any signs of Salmonella, and the dams were healthy. Of all samples taken from calves, 12.7% were positive, and 30% of the samples taken from the caecum were positive. Samples taken from the spleen and liver showed a 15% infection rate. There was at least one positive sample from 50% of the calves sampled. Almost 95% of the dam fecal samples were positive. These samples were taken from a dairy herd in Texas during August and September, a time of significant heat stress. Even though the cows were asymptomatic, their immune systems were somewhat compromised by the effects of heat stress. 

As evident by this study, it is not possible to eliminate the possibility of the calf becoming infected with Salmonella by good management and hygiene during and after the calving process, since they might have already become infected in utero. It is also a significant finding the cows were healthy even though a high percentage of them were actively shedding Salmonella at calving. 

Every Salmonella outbreak I’ve studied has been associated with some sort of environmental and/or nutritional stress that was compromising the immune system of the calf. It is quite easy to determine the environmental stress factors, but a thorough investigation is necessary to determine the negative impact of poor nutrition. Because of its ubiquitous nature, Salmonella appears to be an opportunistic organism that can co-exist in the animal without causing disease, until the immune system becomes compromised. The amount of exposure and the pathogenicity of the serogroup obviously plays an important role as well.

If you couple a poor nutrition program with excessive amounts of environmental stress, the likelihood of having a Salmonella outbreak significantly increases. If the amount of exposure is high at the same time because of poor management and hygiene, the likelihood increases even more. The possibility of a Salmonella outbreak exists on all farms, even those we believe have excellent management. As veterinarians, we are trained to look at possible sources of exposure, amount of exposure and management factors that can have a positive effect on reducing these. Vaccines are often brought into the picture during a Salmonella outbreak as well.  However, these vaccines are often quite stressful on the calf, and in most cases, the immune system of the calf is already compromised. The two most important factors to assure the optimum function of the immune system is to reduce environmental stress and provide a high level of nutrition.

Unfortunately, it is not a common practice on most dairies to get individual weights on calves and calculate the average daily gain. A good nutrition program should yield an average daily gain of at least 1.5 lb. to 2 lb. in Holstein calves. Calves gaining less than this will likely have an immune system that is not functioning as well as possible and be more susceptible to clinical disease caused by Salmonella.  

Veterinary medicine has changed significantly over the years to focus more on prevention than treatment. There is a major effort now to reduce the use of antibiotics in food producing animals. This can be done by implementing a production medicine program focused more on disease prevention. Ask your veterinarian to help.

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