What You Need to Know About Salmonella 

It’s probably been around dairies for as long as researchers have had microscopes to identify it. The number of dairies it affects is growing. Even if you’ve never had a severe clinical outbreak in your herd, it still could be robbing performance from your cows, and dollars from your bottom line. And, its effects can span beyond the farm gate, affecting dairy workers’ health and, potentially the human food supply.
 
What is this sometimes elusive, yet always damaging herd health scourge? It’s Salmonella. Bradford Smith, DVM, world-renowned Salmonella researcher at the University of California-Davis, says both the subclinical and human health aspects of the disease make Salmonella an organism that deserves extra attention on the dairy. “We need to fight Salmonella at the ground level,” says Smith, “because, as it gets out of hand, the stakes grow increasingly higher.”
 
The many faces of Salmonella infections are caused by invasion of the gastrointestinal tract by one of more than 2,000 different serotypes of the Salmonella bacteria. Therefore, it is nearly impossible to protect your whole herd with traditional, whole-cell bacterins. Among the most common pathogenic serotypes encountered in veterinary medicine are Salmonella Dublin, Salmonella Typhimirium, and Salmonella Newport, according to Simon Peek, DVM, associate professor at the University of Wisconsin-Madison’s School of Veterinary Medicine.
 
Even though Salmonella organisms, like cryptosporidia in calves, can be found on virtually all dairies today, Peek cautions that the organism is not a normal part of cows’ gut flora. “It is a disease pathogen, to be sure, and thus a concerted effort must be made to keep the exposure to Salmonella on the dairy as low as possible,” he advises. Purdue University veterinary school clinical assistant professor W. MarkHilton, DVM, describes Salmonella as a “multi-factorial” disease, meaning that a multitude of outside factors – sometimes working in combination – can trigger an outbreak.
 
“Anything that causes stress on cows can open the window for clinical Salmonella infections to take hold,” says Hilton. That may include a spell of extremely hot weather, a ration change that causes cows to go off feed, a displaced abomasum, or the stress of calving. In fact, many producers and their veterinarians first realize a herd has a Salmonella problem because a high proportion of fresh cows start getting sick. Once cows are infected with Salmonella, they can shed the organism in milk and feces intermittently for weeks. In fact, Salmonella Dublin will live indefinitely in some cows, which become carrier animals and may periodically shed the organism for the rest of their lives. Peek estimates that, in the entire U.S. dairy population, one in 20 cows is shedding Salmonella at any given time. What’s more, many other species can harbor the organism, including rodents, birds, cats, dogs and flies.
 
Smith reasons that the growing incidence of Salmonella on dairies is likely due to the trend toward larger, more densely populated herds, where animals can become stressed and have more direct contact with one another. Also, the exposure of animals to feces on some large dairies is greater than in pasture-management settings. Additionally feed storage and bunk management may be more complex on larger dairies, allowing contaminated feed to deliver Salmonella to cows in every bite of their rations. Because the bacteria thrive in warm, moist environments, salmonella is a summertime problem nationwide, with a greater likelihood of year-around infestation on dairies in the South and the West.
Is Salmonella lurking? If you have the classical clinical outbreak of Salmonella, “you’ll know it,” says Peek. The classic signs are cows with severe, foul-smelling diarrhea that may be bloody; fevers (106-108° F); depression; reproductive losses; vaginal discharge and decreased milk production. Deaths are common, and may occur in 10 percent or more of the milking herd.
 
But what about the subclinical cases that don’t surface as full-blown outbreaks? What damage do they do?  Subclinical Salmonella takes a little bit away from every infected cow,” answers Hilton. “So, milk production is not up to its full potential. Fertility is not at its best. Occasional abortions take place. Calves break with scours periodically.” On their own, each of these factors might appear as somewhat isolated incidents. But, when the puzzle pieces are fit together, the full toll of subclinical Salmonella starts to add up. Plus, “a subclinically infected herd is just one management hiccup away from a major clinical outbreak,” suggests Peek.
 
While it doesn’t make every infected cow sick, Peek says the subtle signs that can point to the presence of subclinical Salmonella include: 
  • Fevers of unknown origin in fresh cows.
  • Occasional loose stools in cows at any stage of lactation.
  • Calves that break with scours, possibly accompanied by pneumonia and/or septicemia, in the first 5-7 days of life.
  • Unexplained, periodic “abortion storms.”
Keeping Salmonella at bay
While completely eliminating the presence of Salmonella on a dairy probably is not a realistic goal, Smith says the key to controlling the disease is to keep the numbers down and limit animals’ ingestion of the organism, while maintaining as little feed and production stress on cows as possible. The experts advise these steps:
  • Maintain separate facilities for fresh cows, sick cows and maternity areas. Do not put sick cows in with fresh cows in the hospital pen.
  • Keep lots, free stalls and loafing areas as clean and dry as possible.
  • Protect calves by calving in a clean environment; removing calves from dams and the maternity area as quickly as possible; pasteurizing waste milk fed to calves; and housing and managing calves separately from the adult herd.
  • Store feed in clean, cool, dry, shaded areas and limit access to feed by rodents and birds.
  • Manage nutrition and regularly clean feed bunks for all ages of cattle so that they are not forced to eat stale feed. Salmonella numbers can double every 20 minutes in hot, damp feed.
  • Don’t buy it. Select herd replacements carefully by purchasing animals only from known sources with a complete health history.In addition, quarantine replacements for at least four weeks.
  • Use medicated feed additives. 
Research has shown that herds feeding medicated milk replacer, and monensin in heifer rations, have a lower incidence of Salmonella infections. Until recently, there were only killed Salmonella vaccines and a modified-live Salmonella Dublin vaccine commercially available for cattle in the U.S. Smith believes these vaccines can be effective under certain circumstances, but all possess the risk of adverse reactions (endotoxemia). He says the new Salmonella vaccine from AgriLabs using SRP technology should present lower risks for adverse reactions.
 
All three veterinarians are impressed with the new vaccine, and are looking forward to experiences with it in the field. The unique SRP technology interferes with the Salmonella bacteria’s ability to absorb iron, thus essentially starving it to death. Although the primary target organism is Salmonella Newport, the structures upon which the vaccine acts are common among all Salmonella bacteria.
 
“Previous attempts to vaccinate against Salmonella have been poor,” says Peek. “While the SRP vaccine will not eliminate the organism, it is showing excellent efficacy in lessening the incidence and severity of clinical and subclinical Salmonella infections, and also appears to provide cross serotype protection. The new SRP vaccine shows real promise for limiting when used in conjunction with other good management practices.”