Minimize Your Risks When Introducing New Animals Into Your Herd
Introducing new animals is a way to expand and improve the quality of your herd. But it can also introduce diseases you weren’t counting on. Knowing the potential costs—and your risk tolerance—can help you determine which biosecurity efforts are right for your operation.
As dairy herd sizes have grown over the past 20 years, the practice of purchasing cows from other operations has become more and more common. In California alone, dairy producers import about 120,000 dairy cows each year from other states and countries. Such purchases provide an avenue to expand the herd, as well as a way to shift its genetic composition, thereby enhancing quality, variety or productivity.
Along with these advantages, though, comes the risk of exposure to infectious organisms that may be present in new animals.
It’s a risk that usually doesn’t become a reality. But when it does, it can spell big trouble—and big losses—for those involved, says Dale Moore, DVM, Ph.D., clinical professor and director of Veterinary Medicine Extension at Washington State University. One such “train wreck,” that she observed involved Staphylococcus aureus (S. aureus).
“A very nice herd purchased some animals and unbeknownst to them, 30 percent had S. aureus,” she explained. “Mastitis wasn’t noticeable at the time, but in the process of milking, it got transferred to other cows in the herd. It’s a tough organism to treat, so there was a lot of therapy. Some animals became chronically infected and had to be culled. Then, there were the losses from milk that had to be withheld.”
Moore said she had also personally witnessed a Salmonella Newport infection acquired from purchased cattle. Again, some cows had to be culled due to the multi-antibiotic-resistant problem.
S. aureus and Salmonella are just two examples of the disease threats that can inflict significant economic losses as a result of introducing new animals into a herd. Bovine viral diarrhea (BVD), Mycoplasma bovis, Streptococcus agalactiae and digital dermatitis (foot warts) can also cause clinical disease outbreaks. Other diseases, such as Bovine leukosis virus (BLV), a cancer-causing retrovirus, and Johne’s disease, do not cause outbreaks but can cause clinical disease and reduce the value of animals affected.
Interestingly, despite such risks, many dairy producers do not test incoming animals for disease, according to studies that Moore and her co-authors reviewed for a research article on the topic. In spite of increased attention to farm biosecurity—prompted by various initiatives, disease outbreaks and terrorism threats—less than 20 percent of Wisconsin dairies tested purchased animals. Similarly, only 20 percent of expanding herds in Idaho required health testing aside from mastitis detection.
And, in a study she and other researchers conducted with California dairy producers, while the majority knew the herd of origin, none routinely collected specific disease history or did any testing. The few exceptions included a producer who tested for Johne’s disease and several who checked for foot warts. Further, less than one third isolated or quarantined the animals they purchased.
So, why aren’t dairy producers taking more biosecurity precautions? Moore thinks it has to do with personal experience and attitudes toward risk.
“Most of the time you don’t see big wrecks,” she commented. “So, purchasing policies on the farm might remain the same for a long time. They might have a cattle buyer go out and find bulls or heifers and not think much about it because they haven’t had a problem. When you find someone getting interested in biosecurity, it’s usually because they have had a problem.”
At the same time, producers, in general, vary widely in their attitudes about screening new animals. The primary differentiating factor appears to be their tolerance for risk, says Moore.
“Some say, ‘I don’t want to ever see mycoplasma in my herd, and I will take milk samples from everything I purchase.’ Other folks say they’ve never seen it, so it must not be a problem,” she explained. “Everyone’s risk perception and tolerances are very different. That was eye-opening for me, as a probability-based person working in epidemiology.”
Nevertheless, when asked if they would have purchased the animal if they had known it was disease positive, most producers in Moore’s California study said ‘No.’
Moore said there are several steps producers can take to reduce the introduction of infected animals. The steps they choose to adopt will depend on their individual risk tolerances, as well as the investment required.
Know the source. The first level is simply to know the herd of origin. If producers buy from neighbors or others they know and trust, they can parlay their risk.
Review records. The next level is examining the records of potential herd replacement sources. Although many producers don’t keep comprehensive health records, they would probably have information on vaccines used, any testing done and lameness or mastitis events. Milk production/quality records could indicate high somatic cell counts, which could be an indicator of mastitis problems.
Pre-purchase examination/testing. Examinations and testing can detect many problems, but most producers don’t exercise this option, probably due to the cost involved, according to Moore. However, when you consider the cost of treating, lost production and potential culling as a result of diseases such as intramammary infections, Johne’s disease and BLV, the benefit of testing becomes clear.
In addition, many producers forget about bulls as a potential source of infection, she offered, because they purchase bulls on a routine basis. “They say they have a closed herd, then when I ask them about the bulls, they smack themselves on the forehead and say, ‘Oh yeah, I buy bulls.’”
Post-purchase quarantine. Isolating new animals makes sense, Moore said, but some diseases have a much longer incubation period than others, so each disease requires a different waiting period for animals to show clinical signs. Other diseases are subclinical, so they don’t show signs at all. This makes isolation a confusing recommendation, and perhaps testing for subclinical infections makes more sense. Isolation is very important in the midst of an outbreak, but usually animal purchases are suspended at such times.
Other considerations. In addition, Moore said human visitors can bring disease pathogens onto the farm. Farms can have 50-100 visitors in a given week. There are construction workers, milking machine repair people, milk truck drivers, veterinarians, foot trimmers, nutritionists and breeders. She recommends limiting the number of visitors and being aware of situations where they might have been exposed to diseases. Those who have been in foot-and-mouth-disease-infected areas, for instance, shouldn’t make contact with livestock for five - seven days. She said the disease could be carried not only on shoes and clothing, but also even in nasal passages.
Individual producers will decide to test incoming cattle for infectious diseases based on their risk tolerance and on their perception of the costs and benefits. She said she regards testing incoming animals as an investment, like an insurance policy—a policy that is especially important when the herd of origin is unknown or health history is lacking.
“People have crop insurance, fire insurance and liability insurance, but they often don’t think about the insurance of keeping diseases off the farm,” Moore stated. “Testing isn’t prohibitively expensive, and most state diagnostic laboratories have reduced fees for in-state livestock producers. We paid full price (in the California study) and only spent about $50 per head for physical exams on bulls and testing for Salmonella, Johne’s, Mycoplasma, S aureus, Strep ag and BLV.”
That same study revealed one third of the animals were positive for BLV and 15 percent of bulls had breeding abnormalities. Nationally, Johne’s disease is present in 22 percent of cattle, although the percentages vary by region.
If farmers choose not to test incoming cattle, she suggests they at least (1) know the herd of origin, (2) carefully examine cows and bulls and (3) isolate and visually screen for abnormalities or illness for up to three weeks. All such efforts will help to reduce the incidence of disease and save farmers significant amounts in treatment, lost production and culling.
Sources: Moore, DA, Adaska, JM, Higginbotham, GE, Castillo, AR, Collar, C, and Sischo, WM, Testing 2009. Testing new dairy cattle for disease can boost herd health, cut costs. California Agriculture 63(1):29-34 Ibid., Ibid., Ibid., Ibid.
It’s Good Biosecurity to:
• Know the health history of “selling” herds
• Know the health status of purchased animals
• Have your veterinarian talk to theirs
• Don’t purchase unvaccinated animals
• Don’t buy from herds with mixed-origin cattle
• Control animals (such as rodents) that can spread disease
• Load and unload new animals on the perimeter of the farm
• Have an isolated pickup area for rendering trucks
• Limit visitors with access to cattle, feed, storage and treatment areas
• Record visitors on the farm