The word salmonella strikes fear into any animal caregiver. It is a disease that spreads quickly, has limited treatment options, and can make animals, particularly calves, very sick.
A new strain of salmonella has surfaced among dairy calves, and its impact in some herds has been devastating. Its name is Salmonella Heidelberg.
According to Donald Sockett, D.V.M., with the Wisconsin Veterinary Diagnostic Laboratory, calves infected with Salmonella Heidelberg can appear perfectly healthy at the morning feeding but be deceased by midafternoon.
“This is very hard on the animal caregivers because these calves are dying very quickly,” said Sockett.
He explained that affected calves are typically 1 to 2 weeks of age. Diarrhea is not consistently found with these animals. Instead, what kills them is generalized bacteremia or septicemia, where bacteria leave the gut and spread through the bloodstream quickly. Farms affected by Salmonella Heidelberg have experienced 20 to 65 percent death loss, with an average around 35 percent.
Identified by an outbreak
Salmonella Heidelberg first appeared on the radar during the summer of 2016 when a number of animal and human cases were identified. Sockett said that this strain affects calves but usually not cows, and most of the animal cases were dairy beef calves.
Unfortunately, there are no antibiotics approved for use in dairy calves in the United States that are effective against this organism, as it is multi-drug resistant. “No antibiotic can be used to treat it. All you can do is supportive care,” he said. “Even if you do everything right, the best of the best still lose 20 to 25 percent of their calves.”
Sockett said that vaccination is an important tool, but the efficacy is only 25 to 60 percent in adult animals and less than 50 percent in calves. Vaccines may provide protection against getting sick, but they do not protect animals from getting infected, he explained. They also do not defend against shedding of the organism.
People were impacted, too
Salmonella Heidelberg is not only a concern to animals, as it has affected people as well. Rachel Klos, D.V.M., an epidemiologist with the Wisconsin Department of Health Services, explained that salmonella is a zoonotic disease. The transmission of bacteria, parasites, or viruses spread it between animals and people. Transmission can go either direction, but the cases more commonly identified are from animals to humans.
Klos shared that in Wisconsin, there are approximately 850 confirmed cases of salmonella in humans each year and 42,000 nationwide. For every confirmed case, an estimated 29 additional illnesses are not reported. Children less than 5 years of age have a high infection rate, and approximately 25 percent of all infections lead to hospitalization. An estimated 11 percent of salmonella infections are zoonotic.
An outbreak of the multidrug-resistant Salmonella Heidelberg was identified in August 2016 when human patients reported contact with sick or dying dairy bull calves. Between January 2015 and November 2017, 56 people in 15 states were confirmed sick, and 17 people were hospitalized.
Due to its multidrug-resistance, human treatment options are also very limited. Fortunately, no deaths were reported.
Contact with dairy calves or cattle was reported by 63 percent of the sick people. In Wisconsin, that number was 75 percent.
Klos recognized that avoiding contact with animals is impossible for people who live and work on farms; however, there are practices that can help protect against Salmonella Heidelberg and other zoonotic diseases. See the sidebar for some recommendations.
Searching for the source
Most of the sick calves that infected people had contact with were traced back to farms in Wisconsin. What we need to figure out is if these farms had any practices in common.
Jason Lombard, D.V.M., with the USDA National Animal Health Monitoring System (NAHMS), is involved with a multi-department study of Salmonella Heidelberg looking to answer these three questions:
1. What are the sources?
2. What practices spread it?
3. What practices can control it?
The investigators designed a case-control study of operations that either had Salmonella Heidelberg isolated in dead calves or did not. They also confirmed that Salmonella Heidelberg pathogens were present on case farms and not on control operations. Farms filled out an extensive questionnaire, and samples were collected.
Of the 21 farms that have completed the investigation so far, herd size did not seem to be a factor in whether or not infection was present. More of the controls were dairy farms, while more of the cases were dairy steer or dairy calf raisers.
The study is ongoing and the results are still preliminary, but one commonality between infected farms was the introduction of new cattle into the herd. Every one of the case operations had added cattle, compared to 73 percent of the control herds. “Any time you bring cattle onto the operation they come with the risk of disease introduction,” Lombard said. “The more you bring on, the greater the risk.”
Half of the case farms purchased animals from a cattle dealer or through a cattle market, making cattle purchases through such facilities a possible source of Salmonella Heidelberg. Lombard was quick to point out that it’s not something that cattle marketers are doing intentionally. “Calves that are sold are not only exposed to cattle from other operations but also other pathogens,” he said.
“When we mix cattle, we are causing stress. Stressors build up until we have shedding of these pathogens, and calves not shedding are even more susceptible,” he further explained.
Distance also appeared to be a risk factor. Three-fourths of the farms had cattle that had been transported. On control farms, a majority (75 percent) traveled less than 50 miles to get to the farm. For the case farms, the same percentage of calves traveled 50 miles or more before arriving at the farm.
Of the cultured samples taken on farm, the most positive samples (46 percent) came from calf housing. Samples taken from the ground and from individual calves each represented about 20 percent of the positive samples. The milk preparation area was responsible for 15 percent of positive results.
In herds where salmonella is present, cleaning with a low pressure foaming and disinfection are a necessity. Contact time and concentration of products used must be considered. Sockett recommended verification of cleanliness using an ATP meter.
There are some other strains of salmonella that can be dangerous and deadly to calves, including Salmonella Dublin and Salmonella Newport. Sockett emphasized that not all salmonellas are the same, and that it is very important to identify the serotype of salmonella you are dealing with.
Salmonella Heidelberg, though, is proving to be in a league of its own in terms of how fast it can take down calves. More work to better understand the strain and how to protect against it are certainly needed.
“We’ve never seen a salmonella kill calves this fast before,” Sockett said.
To learn more, a free webinar, “There’s a new calf killer in town,” is available online.
Practices to prevent human infections
- Always wash hands thoroughly with soap and water after touching livestock, handling equipment used on animals, or coming into contact with anything present in animal areas.
- Use separate shoes, work gloves, and clothes when working with livestock.
- Supervise small children during any animal encounter, and discourage behaviors that can elevate their risk of disease.
- Do not eat or drink in areas where livestock are present.
- Work with your veterinarian to keep your livestock healthy.
- Do not drink unpasteurized (raw) milk.