Livestock News

Don’t let external parasites peak this winter


Populations of external parasites such as mange and lice typically peak in the winter months, so it’s important to have strategic deworming protocols in place prior to when they can do the most harm.

“The presence of external parasites can affect a cow’s milk production immediately, because cows will spend their time scratching instead of eating or resting,” said Mark van der List, DVM, Boehringer Ingelheim. “We often see a jump in milk production within days of treating a herd, even in herds that did not show signs of an obvious mange or lice problem.”

External parasites threaten cow comfort

Typically found in colonies on the tail, shoulders and back, biting lice feed on cattle by scraping the skin or hair. Sucking lice, such as the short-nosed, long-nosed and small blue lice, extract the blood of their host. Both biting and sucking lice are known to reduce weight gain and decrease general thriftiness.

Clinical signs of lice include intense itching and hair loss. The greater the percentage of animals with clinically apparent lesions within a herd, the greater the herd’s milk production losses. And while they present a year-round threat, lice burdens are greatest during the winter months.

Mange mites attack and damage the skin and hair. Chorioptic mange (C. bovis) can cause hair loss, scabbiness and dermatitis around the feet, legs and tailhead. Mange-infested skin typically swells and can become inflamed.

Sarcoptic mange (S. scabiei), a quarantinable mite, causes even more severe skin lesions than C. bovis. Infection by just a small number of these mites can be detrimental to an animal’s comfort, health and production.

Clinical signs of mange are the development of crusty, asbestos-like lesions near the tailhead, caudal udder and inner thighs. These lesions cause intense itching and restlessness, leading to decreased cow comfort and milk loss.

Horn flies are a costly external parasite for producers. The sheer number of flies typically found in a herd during the biting fly season renders them a constant threat to cow comfort. Cattle are frequently playing defense against the flies, which can lead to reduced milk production.

Designing a cost-effective external parasite program

Dr. van der List recommends producers include the following steps when developing a whole-herd external-parasite control program that includes a pour-on dewormer:

  • Complete a whole-herd pour annually. Because lice and mange spread through direct contact, being proactive and treating the entire herd — not just those showing obvious clinical signs — is the best way to prevent an outbreak. External parasites typically peak in January and February, so late fall is the ideal time to treat the herd.
  • Dose properly. Incorrect dosing is the No. 1 reason pour-on programs fail. Proper dosing depends on the weight of the animal, so be prepared to reset the applicator gun as needed to ensure each animal receives the appropriate dose of pour-on.
  • Apply properly. A proper application includes pouring along the back of the animal, from the poll to the tailhead.
  • Pour new additions. As new animals join the adult herd, they should be poured upon arrival and kept isolated from the herd for 14 days. This would include replacement heifers, as these animals may have been exposed to mange and lice, putting the entire herd at risk of having external parasites reintroduced. Selection of the appropriate pour-on product will also ensure that any internal parasites the heifer may have acquired while on grass will be eliminated.
  • Re-pour as needed. Animals that are the most severely infected with lice and mange at the time of the whole-herd pour should be re-poured three weeks later for optimal control. To control biting flies, additional treatments during the biting fly season can prove to be cost-effective. For horn fly treatments, you often do not have to apply product to the entire milking herd. Instead, you can focus on the transition animals, as they are most vulnerable to discomfort.
  • Choose a proven product. For best economic return it is important to choose products with long periods of activity after application. While generic dewormers are often less costly, Dr. van der List recommends looking closely at the label and the data used to back up the product’s performance claims. For a lactating herd, it’s important to select a product with a zero milk withhold time in addition to long duration of effectiveness. Choosing a weatherproof pour-on product also ensures that the treatment will continue to work when cattle are under misters or exposed to rain or snow.

“Any interference with feed intake or resting time due to parasites will be reflected in lost milk production,” concluded Dr. van der List. If you see a significant milk production response after treatment, parasites have most likely been impacting your herd for some time. Be sure to consult your local veterinarian to determine ideal treatment timing for your operation.

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