I’ve been hearing many complaints from cattle producers and veterinarians about major issues with cattle and potential external parasites this winter.
This year there seems to be an exceptionally high number of complaints about this issue, and in many cases the problem is associated with lice infestations.
There are two specific types of lice that infest cattle in Western Canada and these are very descriptively named sucking lice and chewing (or biting) lice.
Sucking lice feed on the small capillaries in the skin. They can lay two to six eggs per day and these eggs hatch into nymphs in about five to 11 days. The nymphs go through three stages before they become adults and each stage takes two to four days.
All of this development is highly temperature dependent and so as a result we see very few lice during the hot weather of the summer and a gradual buildup in the winter until lice reach their maximal numbers in late winter. Many adults are shed in the spring with winter hair loss.
Chewing lice feed on dead skin cells and oil secretions and lay less than one egg per day. These eggs take seven to 10 days to hatch and again go through three nymphal stages before they become adults.
Unlike sucking lice, chewing lice can survive off the animal for up to two weeks, which might allow infestations to persist and build up in a population despite being treated.
Lice are transmitted from animal to animal by direct contact, and calves can rapidly acquire infestations from the dam. Hopefully, once we get some longer days and warmer weather, the louse populations will start to decrease significantly.
All lice cause irritation, which results in rubbing, licking, itching and hair loss.
There is some debate about the level of production losses that occur but evidence indicates louse infestations cause lower weight gain. There is no doubt that itchy cattle will be very hard on fences, buildings and other fixtures in their vicinity. Heavy infestations of sucking lice may cause anemia in young calves.
Diagnosing lice infestations takes careful visual inspection and often requires a bright flashlight or headlamp.
The hair needs to be parted and the best areas to inspect include the head, sides of neck, dewlap, flanks and tail switch.
Biting lice are very mobile and move quickly away when the hair is parted. They are light brown and have a rounded head. They are often seen on the dorsal surface and the flanks of the animal.
Sucking lice are usually fixed in place and are somewhat easier to find. They are grey or blue-grey and have pointed heads. They are more likely to be found in the head and shoulder area of the animal.
The Chorioptic bovis mite can also cause issues this time of year. It feeds on the skin surface and isn’t visible to the naked eye. It can only be diagnosed by taking a skin scraping and examining it under the microscope.
These mites cause a condition known as chorioptic mange, which is basically an allergic response to the mites’ saliva.
The cattle will develop small nodules or scabby areas that exude a small amount of serum and some matting of the hair may occur. These scabs can coalesce into larger scabs and all of the lesions tend to be around the tail head, udder area and down the back legs.
The chorioptes mite can survive for several weeks in the environment, which means that re-infestations can occur after treatment.
There are a number of reasons why animals that were treated with pour-on endectocides in the fall or early winter may now have external parasite infestations.
Biting lice and the chorioptes bovis mite can both survive in the bedding or other areas of the environment for a period of time, which can prevent the complete elimination of the organism from a herd of cattle and allow the parasites to re-establish.
If producers don’t treat all of their cattle within a certain window of time or if they introduce untreated animals into a population, infestations can also be perpetuated in a herd. If fall treatment was done earlier, such as in August or September, you can almost always expect to have to re-treat later in the winter.
There is also the possibility that some external parasites have developed levels of resistance to pour-on products. There isn’t a lot of scientific evidence available for this theory because it is difficult to measure levels of resistance in external parasites.
If you have itchy cattle, make sure you get a correct diagnosis before treatment. Your veterinarian can help you choose from a variety of products that will provide control for external parasites. Note that the pyrethroid products work against all types of lice whereas the ivermectins mainly kill sucking lice.
Some products may not have a sustained action over a period of weeks and more than one treatment may be needed, usually spaced two to three weeks apart. Follow the label instructions and apply the products at the correct dosage in the recommended fashion.
Try to treat the entire herd within as short a time period as possible so that you have the best chance of eliminating the infestation.
If our weather stays cold through March, we may have lice infestations persisting later than usual. Once the weather warms, we will likely see a rapid drop in the numbers of lice on cattle.