Silent Threat

Published on Mon, 11/16/2009 - 4:00pm

Whether a dairy farmer has 20 cows or 20,000, the need for a comprehensive biosecurity plan has never been more important. Even though bioterrorism threats have increased in recent years, international biosecurity is just one element of a sound plan to prevent the introduction or spread of infectious disease in a dairy operation.

"There are three levels of biosecurity that dairy farmers need to be aware of," Dr. Don Sanders, dairy expert and author of dairy related articles and books says. "Farmers with small herds may sometimes think there's no need for them to have a biosecurity plan because they aren't bringing cows into their dairy or selling them to other dairies. But there are biosecurity issues in the smallest herds, because dairy cows are immune to some diseases that their calves aren't. If they have a PI cow or Johne's in their herd, they will need to address that or they'll end up exposing their healthy animals to disease."
Foot and mouth disease is the most significant threat that comes from outside the United States. Sanders believes that, at some point, an outbreak of the disease is likely, in spite of the precautions taken to prevent it.
"I'm not a naysayer or fear monger," he says. "I just think that at some point it will be an issue. We may not recognize the holes in our biosecurity plans until we have to address it. But dairymen should consider how they could protect their herd if it did happen."
The next most significant threat to dairy herds is movement of dairy cows from one farm to another. The two major diseases threatening cattle through movement of cows are bovine viral diarrhea (BVD) and Johne's disease.
"While not every herd is screened for BVD, at some point most dairymen have implemented a program to protect their cows from it," Sanders says. "Persistently Infected animals, or PI cows, can act like Typhoid Mary’s in a dairy herd. Unless a dairyman's vet tests for BVD, he may not realize that it's making its way through his barn."
When unborn calves are exposed to BVD early in gestation, before their immune systems are developed, BVD can take up residence in the fetal tissues and survive in the calf's system after it's born. Though the calf doesn't display symptoms of the disease, they carry the virus and spread it to all the cows around them.
"A PI calf may look healthy in every respect," Sanders says. "But they are shedding that virus in their nasal secretions, feces, and ocular secretions. Unless the farmer identifies that PI animal, they will continue to have problems with BVD."
Vaccinations have reduced the number of PI animals that can go unnoticed in a herd, but haven't eradicated the problem. "Ear notch tests and blood tests are two of the most effective ways to test for a PI animal," Sanders says. "But neither one is foolproof. Dairymen are sometimes hesitant to test for PI animals in their herd, especially if they sell cows, because it means they have to make an ethical decision about destroying that positive PI animal and sustaining an economic loss rather than sending it to market. If it goes to market, it could end up in someone else's herd."
Blood test samples can often serve as a way for dairymen to test their cattle for brucellosis, BVD and Johne's.
"In my estimation, the ear notch test is a tad bit more reliable than the blood test," Sanders says. "Some producers automatically ear notch calves as they're born, with the strategy that as long as the calves test negative, they can rest comfortably."
Sanders worked with a 1000 head dairy herd that began experiencing high fevers and diarrhea in the fresh cows after being in operation for about six months.
"They experienced six or eight abortions and had calves born with diarrhea and I told the dairyman there was only one way to get to the bottom of the problem. He had to ear notch all the cows," Sanders says. "A couple of the cows were ultimately identified as PI cows. It cost between $6000 and $7000 to do the testing. But at that point it was a profit issue. He was seeing more and more BVD and he had to identify the source."
Cattle who graze in pastures are susceptible to BVD through contact with other animals, such as deer or feral pigs. "Some dairies put up a security fence around their entire dairy to control what comes in and out of it," Sanders says. Johne's disease can escape detection too if dairymen use an ELISA test to identify it. Test results won't be positive until the disease is relatively advanced.
"It's often difficult to track where the disease originated," Sanders says. "One practice dairy farmers can adopt to prevent spread of Johne's is to immediately remove newborn calves from their mothers. If the mother has Johne's, the calf is likely to pick it up from her feces. When colostrum is collected, dairymen should make sure they clean the teats thoroughly before they milk the cow. The colostrum can even be pasteurized before it's given to the calf."
The vast majority of dairymen, Sanders notes, don't test cows for either BVD or Johne's when they purchase them because the tests aren't completely reliable.
"Even if a dairyman tests for the diseases and removes the infected cows, there's no good research data that proves we can eradicate Johne's disease through that practice," Sanders says. "Bedding can be cultured in calving stalls too, to make sure there aren't organisms in the stalls related to these diseases."
Sanders recommends that dairymen arrange a veterinarian-to-veterinarian consultation before they buy dairy cows. A discussion between the two professionals can provide the buyer with verification of the instance of disease within a particular herd.
"If the veterinarians can talk about a particular herd's health, the purchaser will be able to make an informed decision about whether or not to buy the cows or perhaps make special provisions for these cows' entry into the herd."
Small dairy farmers also need a biosecurity plan that protects cows and calves from a variety of bacteria that can lead to infectious illness. Housing calves separately from cows reduces the chances that calves will come in contact with bacteria they're immune systems are not ready to combat. Udder infections can be controlled by milking and penning calls separately.
"A cow with mycoplasma can infect a newborn calf and the calf may develop pneumonia," Sanders says. "They can also develop otitis media (ear infection), which can lead to a droopy ear and head tilt. Small dairies may tend to think that biosecurity is someone else's problem, but they can cause the spread of disease themselves, so they need to have a plan in place."