By Sophia Kenney and Erika Ganda
Salmonella are bacteria commonly associated with food-borne illness and diarrhea in humans and animals. Salmonella Dublin (S. Dublin) is a specific type of Salmonella, known as a serotype, that is well adapted to infecting cattle and has become one of the most common serotypes isolated from dairies across the country. Because of this, understanding 1) the symptoms of S. Dublin infection, 2) the specific hygiene challenges, and 3) what can be done to minimize emergence and persistence in a herd is critical for maintaining the health of dairy herds.
Prevention and Risk Management
As the prevalence of S. Dublin among dairies across the United States increases, S. Dublin has the potential to become endemic in dairy herds. Because S. Dublin is spread almost exclusively between livestock, humans, and other animals (transmission via birds is uncommon), outbreak prevention largely revolves around cattle management practices. Risk factors for outbreaks and endemicity include larger herd sizes and high stocking density as these increase chances of exposure to contaminated manure. Purchase of animals is an additional risk of S. Dublin introduction into an operation. To manage these risks, regular cleaning and disinfection is pivotal to prevent S. Dublin spread between livestock and humans. The primary focus should be in areas where high risk groups are housed and other possible risk areas such as livestock trailers where animals from outside of the existing herd may have been transported, feeding equipment, and areas where infected animals are being isolated. Regarding calf health specifically, maternity area hygiene is very important to reduce transmission risk to neonatal calves and minimizing postpartum exposure to the dam can also reduce exposure risk.
Disease and Treatment
Like other Salmonella, S. Dublin infection can lead to diarrheal disease. In addition, however, S. Dublin can cause more invasive and severe illness compared to other serotypes. Calves are especially susceptible to respiratory infections, bloodstream infections, and often death, with respiratory illness being most common in the late nursing and post-weaning age groups. Adult cattle infected with S. Dublin may or may not have symptoms. Adults are also less likely to develop the severe disease. Nonetheless, infected adults can experience decreases in milk production and pregnant heifers are at risk for abortion.
While ill, cattle shed S. Dublin bacteria into the environment most often via feces. Even after disease symptoms have resolved, they can still shed. Additionally, cows may become infected via fecal-oral transmission but remain entirely asymptomatic. Whether or not an animal will develop symptoms depends on a number of environmental factors like stress and individual factors like animal health. While cattle with symptomatic infections will shed more, these asymptomatic cattle can still shed S. Dublin. Typically, treatment of Salmonella infections involves fluid restoration. Antibiotics are more often used in animals at high risk of bloodstream infections like calves. Relative to other Salmonella serotypes, S. Dublin is naturally more resistant to antibiotics, complicating the ability to treat infections.
Managing an Outbreak
Managing outbreaks is critical as S. Dublin infections lead to animal loss, milk production loss, and overall economic loss. A major challenge to managing these outbreaks in a herd is the shedding of bacteria that occurs by both symptomatic and asymptomatic infected animals. While shedding is increased in symptomatic animals, asymptomatic shedders often shed more when under increased stress such as when calving. When bacteria are shed via feces, saliva, and/or milk, they can linger in the environment for months to years. This provides ample opportunity for infection of otherwise healthy animals and a prolonged outbreak.
Rapid diagnosis, isolation, and intervention are very important to prevent the spread to healthy animals. Upon diagnosis, isolation and strict hygiene should be maintained for up to weeks after the infection appears to have resolved as the isolated animal may still be shedding. For larger herds, this can be difficult to implement particularly if multiple animals are infected and would need to be taken through higher traffic areas for milking where they could contaminate those environments. Fecal-oral transmission is a concern at all ages, but transmission via colostrum and unpasteurized milk is another possible infection source for calves. Fecal contamination of milk, colostrum, and feeding equipment is, therefore, important to monitor. If resources are limited, focus on hygiene of this equipment, high traffic areas, and in areas where high risk groups are housed (transition cows, maternity pens, calf housing) should be prioritized.
For the duration of an outbreak, farm management and individuals in contact with infected animals should be aware of the transmission risk since S. Dublin can infect humans and other animals. While transmission from direct contact can occur, the primary way humans can become infected is through contaminated milk. In fact, most S. Dublin outbreaks in humans have been attributed to raw milk consumption. To address this, conventional pasteurization is sufficient.
S. Dublin is an emerging infectious disease that is particularly significant for dairy producers nationwide. While known that it is becoming increasingly prevalent, it is largely underreported and the true prevalence, therefore, is likely underestimated. In Pennsylvania specifically, the true herd-level prevalence is not known. To address this, the Ganda Lab here at Penn State is actively recruiting farms to participate in a study investigating S. Dublin prevalence.
Source : psu.edu