Editor's note: This article first ran in the November 2011 edition of Dairy Herd Management.
"I'll never forget when a herd I was working with lost 17 cows to acute salmonellosis and three of the farm’s employees were hospitalized with Salmonella,” recalls Gary Neubauer, senior veterinarian with Pfizer Animal Health. At the time, Neubauer was practicing veterinary medicine in Minnesota.
Like Neubauer, if you’ve dealt with a clinical outbreak of Salmonella, you’ll never forget how devastating the disease can be. But did you know that your herd could be carrying Salmonella even if the cows don’t show clinical signs?
“Your cows may look very healthy, but they can still be carriers,” says Bhushan Jayarao, extension veterinarian with Penn State University. The majority of Salmonella infections in herds are subclinical, meaning that cattle shed bacteria but show no visible signs of the disease.
The 2007 National Animal Health Monitoring (NAHMS) Study for Dairy shows that 47.2 percent of herds surveyed tested positive for Salmonella.
While this is only a sampling of the herds in the U.S., these data indicate that Salmonella is an issue the industry should be concerned about.
Salmonella doesn’t discriminate
No herd is immune to Salmonella.
If you think you don’t have to worry because you’ve never had a clinical case of Salmonellosis on your operation, you’re wrong. “It’s still possible that you have Salmonella,” says Simon Peek, veterinarian with the University of Wisconsin-Madison. “If you start looking for it, you’ll very likely find it.”
Salmonella does tend to be more prevalent in herds with 500 cows or more —61 percent —according to the NAHMS 2007 dairy study. But smaller herds are still at risk; 41.5 percent of herds with less than 500 cows tested positive for Salmonella.
Research also shows that organic operations are just as susceptible to Salmonella. “Research in the Midwest shows that organic dairies do not differ significantly from larger free-stall farms in the prevalence of environmental and individual cow and calf Salmonella infection,” says Peek.
There are more than 2,200 different serotypes of Salmonella, ranging from the “garden variety” types to the moreserious Salmonella Dublin and Salmonella Newport. “Of increasing concern is the frequency with which Salmonella Dublin is identified, especially in heifer-rearing operations, because of its propensity to create a chronic carrier state,” explains Peek. However, any Salmonella serotype isolated should be cause for concern.
“Even your ‘garden variety’ types can cause disease in humans and animals,” explains David Wolfgang, extension veterinarian with Penn State University.
It spreads easily
Ease of infection is what makes this disease difficult to control.
The source of infection is usually feces from infected animals. But it can be difficult to tell which cows are shedding bacteria because asymptomatic and subclinically infected animals can shed just as many organisms in their manure as the cows that are sick with salmonellosis, says Peek. Clinically infected animals can shed more than a 100 trillion organisms per day.
Contaminated feed or water, rodents, birds, flies, cats, dogs, raccoons, and human traffic (visitors, vehicles) are also possible sources of infection, although less significant, notes Peek.
It’s important to note that Salmonella can survive in the environment for four to five years in water, soil, dust, and moist areas. Salmonella Dublin can survive in dry feces for more than a year.
Human health alert
Increased presence of Salmonella on-farm is also a concern for human health.
Transfer from animals to humans is possible. “We’ve seen quite a few transfers of Salmonella from cats and dogs to humans. But, in the case of large animals, there is very little documentation of Salmonella infecting employees,” says Jayarao.
Yet, employee exposure to Salmonella on-farm is a concern.
Salmonella is also a concern from a public-health standpoint. Salmonella is one of the two most common bacterial causes of food-borne illness in humans in the U.S. Each year, Salmonella is estimated to cause 1.4 million cases of disease, with 500 deaths.
“The U.S. Food and Drug Administration is taking an increased interest in Salmonella to make sure it stays on the farm and does not enter the food system,” says Wolfgang.
No direct dollar figure is associated with Salmonella. But there is an indirect cost from both clinical and sub-clinical cases that impact your bottom-line.
Salmonella can cause reduced milk yield, weight loss, poor reproduction, abortion and an increase in metabolic disease. “Currently, there are no accurate estimates of what Salmonella costs the dairy industry every year,” says Peek. “I wish there was, because it would help reinforce the importance of this disease.”
“It is true that Salmonella is very common on farm, but that doesn’t mean we have to surrender to it,” says Jayarao. “There are many practices that can help to limit animals’ exposure to Salmonella and, thus, keep them healthy and productive.”