NEW PRODUCTS on the market that treat both internal and external parasites, including one that can be used on lactating cows with no milk withdrawal, are causing dairy producers and veterinarians to take a second look at the effect of parasites. But a lack of research in this area, especially with external parasites, makes it difficult to know exactly what to do.
Add to that the many different variables, and you’ve really got confusion. Variables such as: cows versus heifers; internal versus external parasites; drugs that treat one or both types of parasites; drugs that can be used with lactating animals versus those that can’t; grazing versus confinement systems; climatic differences across the U.S. or from year to year.
In this article we’ll try to sort through the confusion, answering those questions that can be answered.
Lice and mange. Five species of lice affect cattle in the U.S. One species is a biting or chewing louse; the other four are sucking lice.
There are two species of mange mites that most often affect cattle. Chorioptes bovis is a mange mite that infests cattle and is commonly known as “tailhead mange.” It is the most common type of mange found in the U.S. It is a problem primarily in winter in all types of cattle but especially in dairy cattle which are housed in closed quarters.
Sarcoptes scabiei var. bovis is a burrowing mite that infests cattle and is frequently referred to as “neck and tail mange,” although it may be found on any part of the body.
Lice bring about itching and skin irritations, causing animals to scratch, rub, and bite infested areas. Mange also causes skin irritation and itching, with more severe cases resulting in thickened skin, hair loss, and lesions.
Losses can occur from these parasites due to irritation, blood loss, depressed appetite, and decreased rate of gain. Mange can affect the mammary gland and interfere with milking. However, there is little research to indicate the extent of these economic losses.
Intestinal roundworms (nematodes), with the brown stomach worm (Ostertagia ostertagi) being the most economically detrimental. Lungworms are a widespread problem in Europe and are being increasingly recognized as causing problems in the U.S.
For intestinal parasites, appetite suppression leads to poor weight gain in dairy heifers or beef cattle and reduced milk production in dairy cows. A severe outbreak of lungworms has a marked impact on productivity and can even kill the animal.
Dr. Bill Ryan, senior director of technical services for Merial Limited (a Merck and Rhone-Poulenc Company), has been working with parasite control in livestock for many years. He explains the effect of parasites on dairy cows and heifers this way: “The most susceptible animal to parasitism is the young animal,” he says. “Grazing heifers are most at risk. If there’s no effective control mechanism, generally they will show clinical signs of parasitism. They’ll look in terrible condition. They’ll have diarrhea. They won’t be thriving. Usually people do something to control that level of parasitism before it happens.
“If that control is not fully effective, you still have the risk of subclinical or covert parasitism,” Ryan continues. “With this sort of parasitism, the animal may look like it’s doing well. But it’s still suffering from some productivity losses that you won’t pick up unless you’re actually measuring the body weight progression.”
Dr. Victor Cortese, a veterinarian with Pfizer Animal Health, points to another potential loss from internal parasites: less effective vaccinations.
“To prevent disease, vaccinations require a cow’s immune system to mount a sufficient response,” he explains. “Unfortunately, cows that are infected by parasites cannot mount an effective response because their immune systems are compromised by the negative nutritional impact caused by gastrointestinal parasites.”
Yes, but it’s never a complete resistance.
What generally has been believed is that calves are most important to treat. Maybe you have to treat your yearlings. But, after that, you just don't see effects in older animals,” Ryan continues.
“That’s generally true,” he admits. “But you don’t see it because you’re not looking for it. This is what researchers in the 1970’s began to do--look for an effect on milk production. As these trials have progressed along with:
In the past, study results were mixed; some showed a statistically significant improvement in milk production when cows were treated for internal parasites, while others showed a statistically insignificant improvement or no improvement.
More recently, Dr. Ryan did a literature review of 66 studies going back to the mid 1970’s. We’ve taken all the published references from an enormous number of scientific publications and pulled together all the data,” he explains. “For the first time, we had a statistician look at the overall data.
“When you pool the 66 studies, you find that 52 showed a positive response. That’s 79 percent; that figure in itself is significant.”
“By putting these 66 studies together, the overall impact on milk production is over a pound of milk per day throughout the lactation,” Ryan points out.
Dr. Louis Gasbarre is a USDA immunologist who studies internal parasites. “There’s a whole gamut of feelings on this question, from people who think you’re absolutely wasting your money to those who feel it’s important,” he says.
“Based on our research here with an intensive rotational grazing system, my feeling is that, if grazing is a major component of the dairy system, you can’t get by without deworming.
“But, in a confinement system, the results are ambivalent (indecisive),” he adds. “You can find studies that say there is an effect and others that say that there’s absolutely none.
“If you remove any kind of an insult from the gut, the animal is probably going to be in slightly better shape,” Gasbarre continues. “The question is, will you get adequate return for the expense of treating?”
“Yes,” according to Dr. Ryan. “And I’d like to make two points here. One, we have cows that are genetically selected to produce more milk, and we also have improvements in dairy management that put more demands on cows.
“What we need to do is to keep assessing the impact of parasitism to see if it is the same or greater on the modern dairy cow,” he continues. “Research shows that parasitism has greater impact on high-producing cows.
“The second point is, of the 66 studies in the review, only 10 percent involved use the modern anthelmintics now available,” Ryan points out.
Studies have shown that beef and dairy calves and heifers that are treated for internal parasites have a better rate of gain, better feed conversion, and reach breeding weight more quickly than untreated controls. The effects of this improved growth can follow the animal through its whole lifetime of milk production.
In the second article, we’ll look at how to decide whether or not animals need treating; when to treat; and some of the different products available for treatment.
Reprinted with permission from HOARD’S
DAIRYMAN.
Written by Lori Ward Bocher