Outbreaks of Salmonella in dairy calves are one of the most difficult and frustrating issues that the veterinarian has to deal with. In most cases, the mortality rate is high and the dairyman is desperate to get the outbreak under control as soon as possible because of the significant economic loss he is experiencing as a result. In addition to the high mortality rate, the morbidity rate is also high. The animals that are fortunate enough to recover are often severely debilitated and are less productive in the future.
Besides the initiation of treatment protocols, the veterinarian has to do a thorough investigation of what the underlying factors are that precipitated the outbreak in the first place, and initiate management changes that will hopefully rectify the situation as soon as possible. However, it would be much more advantageous to the client if they would permit their veterinarian to do a thorough analysis of the current management practices and nutrition of the dairy calves during the raising process. Unfortunately, it often takes something like a Salmonella outbreak to occur before the owner allows or requests their veterinarian to do this evaluation.
Mohler et al., Vet Clin Food Anim 25 (2009) 37-54, published an excellent chapter on Salmonella in Calves. In this chapter, they make the following statement: “Three principle variables determine the outcome of host-salmonella interactions: host immunity, pathogen dose, and pathogen virulence. Environmental conditions have the potential to influence outcomes by impacting each of these variables.” Evaluating these environmental conditions, and their interactions with each other, is not an easy task. In most cases, there are multiple factors that are involved in precipitating the Salmonella outbreak and correcting one or two factors may not result in the resolution of the problem. For this reason, a very indepth investigation has to be done in order to correct all the underlying causes and obtain a rapid resolution to the problem.
Most dairy owners do not understand the fact that Salmonella is present at all times on their farms. However, when one or more of the three principles previously mentioned are adversely affected, then a significant outbreak may occur. Regardless of how well the maternity area and the area where the calves are raised is managed, there will always be the presence of Salmonella. Our goal is to reduce the numbers as much as possible and maximize the immune response of the calf. Obviously, the pathogenicity of the strain present on the farm also plays a major role, but that cannot always be controlled as well as the first two factors mentioned.
It is common to look for a potential source of the Salmonella when first investigating an outbreak. In previous investigations, I have found Salmonella in the calf feed, and also in feed on the truck before it is even unloaded on the farm. Feeds are often stored in areas where rodents and birds have access to them before being delivered to the farm. Calf housing systems where the feed bucket is outside can be easily contaminated by rodents or birds. Other locations where Salmonella can be cultured from are water troughs, calf hutches, milk, colostrum, feeding utensils, esophageal feeders, bolus guns, syringes and needles used for treatment, boots and clothing from employees, bedding material, etc. It is not uncommon to find the presence of Salmonella from multiple sources on one farm. There may also be some cows in the herd that are chronic asymptomatic carriers and are shedding significant numbers of Salmonella in their feces. This is especially important when these animals are in the maternity area, contaminating the environment for all calves born in that area.
A study reported by D. L. Hanson et al., Epidemiol. Infect. (2016), 144, 962-967, provides substantial evidence for vertical transmission of Salmonella in utero. Calves in this study were humanely euthanized immediately after birth and samples taken for Salmonella culture. These calves were from a commercial herd without any signs of Salmonella, and the dams were healthy. 12.7% of all samples taken from calves 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. 94.7% of the dam fecal samples were positive. These samples were taken from a dairy herd in Texas during August and September. This is obviously a significant time of heat stress. Even though the cows were asymptomatic, their immune systems were somewhat compromised by the effects of heat stress.
This study makes it evident that 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 may have already become infected in utero. It is also a very significant finding that the cows were healthy even though a high percentage of them were actively shedding Salmonella at calving.
I have not seen an outbreak of Salmonella that was not 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 the nutritional stress factors may need a more thorough investigation to determine the negative impact of a poor nutrition program on the normal function of the immune system. Because of its ubiquitous nature, Salmonella appears to be an opportunistic organism, that has the ability to 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. However, in most cases, the Salmonella has already been on the premises for some time, and some factors such as poor hygiene, environmental stress, or poor nutrition have made the animal unable to keep the Salmonella infection under control, resulting in severe clinical disease.
It is important to investigate all the potential areas of contamination previously mentioned and implement management changes to reduce the exposure of the calf from these sources. It is difficult to control the serogroup of Salmonella on the farm and it resulting pathogenicity, but it is possible to reduce the exposure as well as reducing the stress factors on the calf that negatively impacts their immune system.
If you couple a poor nutrition program along with excessive amounts of environmental stress, the likelihood of having an outbreak of Salmonella 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 that 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 to 2.0 lbs. in Holstein calves. Calves that are 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. M. A. Ballou et al., published a paper in the Journal of Dairy Science, J. Dairy Sci, 98:1972-1982, addressing the effects of a higher plane of nutrition on the innate leukocyte response of calves exposed to an oral Salmonella enterica serotype Typhimurium. They found an improved leukocyte response as well as more resistance to Salmonella infection when on a higher plane of nutrition.
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 that is focused more on disease prevention. The major hurdle with these types of programs in calves is that the owner is often reluctant to pay their veterinarian for these types of services. Convincing the client to allow their veterinarian to be actively involved in this aspect of preventive medicine will result in substantial dividends, and perhaps prevent the unpleasant experience of having a major outbreak of Salmonella on their farm.