There’s a growing menace to U.S. dairy herds that once gains a foothold become very difficult, if not impossible, to contain. Its name is Salmonella Dublin, a bacterial disease that can cause pneumonia, diarrhea and reproductive losses.
Once infected, those cattle that don’t succumb to the disease can become persistently infected and intermittent shedders, says Simon Peek, a University of Wisconsin dairy veterinarian. Antibiotic treatments can knock back acute infections, but might not rid the animal of the disease. And while a vaccine is available for use in calves, later tests can’t differentiate between a vaccinated and an infected animal.
Salmonella Dublin is a fairly recent new comer to U.S. dairy herds. “The prevalence of Salmonella infections in general and Salmonella Dublin specifically have been steadily rising over the past 20 years,” says Peek.
In 1996, USDA’s National Animal Health Monitoring System (NAHMS) survey of dairy herds found Salmonella in about 20% of U.S. herds and just 5% of cows, and at that time most of Salmonella Dublin infections were being found in California. In the 2002 NAHMS survey, the number of Salmonella infected herds had climbed to 31%, and by 2007, it had risen to 40%. The number of cows found infected in 2007 had grown to 14%, but Peek believes the number of cows actually infected is more than that because of the intermittent shedding of the disease.
Additionally, Dublin is also now the most common serotype of Salmonella isolated from samples submitted to a number of diagnostic labs from dairy herds. “Twenty years ago, it would have been rare,” he says. “Unfortunately, it has slowly crept into every major dairy area in the U.S. since then.”
The problem with Salmonella Dublin is that it is host adapted to cattle. “Those calves that don’t die from it, a proportion (5 to 20%) will become carriers for life and shed the bacteria intermittently in feces, colostrum, milk or semen,” Peek says.
In addition to diarrhea, the most common manifestation of Salmonella Dublin is pneumonia, but it also can migrate to joints and cause swelling. Calves can also have fevers of apparently unknown origin and be unthrifty.
Fecal culture tests can be done, but a single negative test won’t rule out infection because of the intermittent shedding pattern of the disease. A PCR test can tell you whether an animal is positive or negative for Salmonella, but few labs have PCR tests available specific to Salmonella Dublin. An ELISA test is significantly more sensitive than fecal cultures to identify carriers, but it is most accurate when used on calves 3 to 10 months of age. It can also be used on adults and milk.
“The current definition of a carrier animal is three strongly positive ELISA tests done over an 8-month period beginning no earlier than when the animal is three months of age,” Peek says. On a herd basis, it requires an awful lot of testing, so culling of carriers is likely not a practical option. The only real option is intense management.
For calves, that means very rapid removal and separation from their dams at calving. If you suspect Salmonella Dublin is present in your herd, test colostrum sources, consider pasteurization of all colostrum or use colostrum replacements.
“No waste milk should be fed to calves unless its pasteurized,” says Peek. Control of the disease is very hard in group feeding of pre-weaned calves, he adds. And if possible, have dedicated personnel for calf care that do not work with adult cattle.
For heifer raisers, require screening of your source farms, and require they do at least bulk tank testing for Salmonella Dublin.
For adult cows, the best way to prevent infection is to maintain a closed herd. If purchasing animals, require a bulk tank ELISA test or blood test individual animals that you are purchasing. Also be sure to house sick cows and transition cows separately, and minimize over-crowding.
Finally, remember that Salmonella Dublin is a zoonotic disease, meaning it can be transferred to humans. It is killed by pasteurization, so never drink raw milk or consume uncooked dairy beef.