While in veterinary school, we were taught about many different diseases and the signs associated with those diseases. Some were common and others were very uncommon or exotic. While it is important to always think about the uncommon diseases, in our early days of vet school we were often reminded that if you hear hoof beats, it’s probably a horse and not a zebra coming down the road. This analogy is akin to us keeping the most common causes of a disease on the top of our list, but not forgetting about the others.
A good example of this would be if we saw a bred heifer with a fever and lesions in her mouth. In the USA, we thankfully do not have foot-and-mouth disease, but we do have another disease called vesicular stomatitis. So, although we need to be aware of the uncommon disease, it will likely not be our final diagnosis. We found the horse, not the zebra. However, occasionally we do find a zebra.
If we have a group of four- to six-week-old calves with fever and signs of respiratory disease (coughing, labored breathing), we will most likely consider the most common bacterial and viral causes of bovine respiratory disease (BRD) in calves and treat with one of the many long-acting antibiotics approved for this purpose. Would many of us even think that the cause of pneumonia was Salmonella?
Up until three to five years ago, the presence of a specific species of Salmonella, Salmonella dublin, was fairly uncommon. When we think of Salmonella infections, diarrhea and fever are likely the first signs that come to mind. But with Salmonella dublin, the most common signs are that of respiratory disease.
When I saw my first case of Salmonella dublin in a group of calves, Salmonella was not even on my list as a possible cause. These calves did not respond to the conventional treatment of antibiotics. Following a further workup of the outbreak, samples from a necropsy of one calf that died identified Salmonella dublin as the likely cause. I needed to be looking for the zebra.
Over the past years, Salmonella dublin has become a more recognized cause of BRD, especially in some regions of the country. This type of Salmonella rarely causes diarrhea in calves and almost always presents as respiratory disease. This is one reason why it may often be underdiagnosed. It can cause disease in adults, but most commonly affects young cattle. In adults, the infection can cause diarrhea, fever and abortions.
Salmonella dublin has also become resistant to many commonly used antibiotics, making this disease very difficult to treat. It is also a very contagious bacteria, so strict biosecurity is necessary to contain and prevent further spread of this pathogen. Overall attention to colostrum management, nutrition and hygiene will also help maintain calves that have a strong immune system and greater resistance to disease.
All Salmonella species are zoonotic, which means they can be transmitted from animal to man (and vice versa). It is not only important for the calves, but also human health, to maintain strict biosecurity while working with any calves suspect of being infected with Salmonella. Infected animals can shed the bacteria in all of their bodily fluids (feces, urine, saliva, milk).
Given we often do stumble upon a zebra, it is important to keep open communication with your herd veterinarian, especially when dealing with animals that are unresponsive to conventional treatments. It is important to recognize these new and emerging pathogens on a dairy in a timely manner to prevent further spread and develop a treatment and management plan.
Mark J. Thomas is a veterinarian and partner in Countryside Veterinary Clinic, LLP in Lowville, N.Y.