Gerald Hauer, DVM
Bison Production Specialist
Alberta Agriculture, Food and Rural Development
Bison Centre of Excellence, Leduc, Alberta.
Phone: (780) 986-4100

Reprinted from The Tracker, November 1998


Internal parasites can cause a lot of problems in bison herds. This article discusses parasites in general terms and gives some hints on ways that you can reduce their impact on your herd.


Parasites have been a problem for livestock producers for many years. Elk, deer, and bison ranchers are no exception and may even be faced with a larger problem than traditional livestock producers. Parasites tend to be associated with confined rearing and high stocking rates; elk, deer, and bison traditionally have not been exposed to many parasites and therefore have not developed much resistance. Taking these species from their wild range and putting them in fenced pastures has meant an increased exposure to parasites in animals with poor natural resistance. Since a discussion of all the different parasites would be beyond the scope of this article, I will discuss parasite infestations in general terms and concentrate on the significance to you as the producer.

Parasites come in many different descriptions, anywhere from intestinal roundworms that live entirely in the intestinal tract to external parasites like some ticks that require several different hosts to complete their life cycle. One thing that they do have in common is that they all derive their nourishment from the host, which in this case happens to be your animal. A few small parasites in/on an animal as large as a deer or bison are probably not significant, but problems arise as the numbers increase and they start to take their toll on the host.

Losses can occur in a number of ways when there is a parasite problem. Death loss can occur if parasite infestations are high enough, especially if there are some other stresses at the same time. I have seen herds that have lost elk calves at weaning because of high parasite loads coupled with the stress of weaning and bad weather. Fading elk syndrome has been associated with heavy loads of internal parasites. Death of animals may be the most obvious type of loss but a greater source of loss may be poor growth and performance caused by lesser infestations. In such cases young animals don’t die from their parasite load but they grow more slowly and have an unhealthy appearance (like a dull, shaggy hair coat). In older animals failure to breed or raise a healthy calf can be the result of parasite burdens. Some veterinarians feel that one of the leading causes of infertility in bison cows is parasitism.

Young animals are at more risk of developing problems associated with parasitism as they have an immature immune system that isn’t strong enough to fight off invaders. Older animals that have experienced several seasons of infestation usually have a strong enough immune system to keep their parasites numbers low. The problem is that the older animals can’t completely rid themselves of the parasites and therefore are a significant source of infection for the newborn animals. A typical scenario is when a herd of elk or bison cows with a low level of parasitism shed small numbers of eggs onto the pasture in the spring. Young calves eat the larvae that emerge from these eggs and infect themselves with low numbers of parasites. Because their immune system can’t suppress the parasites, large numbers of eggs are produced and are shed onto the pasture by mid to late summer. As calves are now picking up large numbers of larvae there is the potential for serious health problems. This scenario fits intestinal roundworms and lungworms most closely but the general pattern can be applied to many of the parasites that we deal with today.

With this knowledge of what these parasites can do to your herd we must now examine what we can do to reduce the losses. A good parasite control program consists of several things. Firstly, you need to know what types of parasites are on your farm and how heavily the animals are infested. For external parasites close examination of animals while they are in a squeeze is the most useful technique in identifying problems. For most internal parasites fecal examinations done either at a vet clinic or a lab can give a good indication of the parasite population. Talk to your veterinarian to devise a plan because there are several important factors to consider when collecting your samples such as; obtaining a proper fecal sample (very fresh feces from a calf is the best in most cases), the time of year, the type of test performed (centrifugation techniques generally yield more accurate results), and special tests for certain parasites (lungworm).Secondly, a sound deworming program is important. There are many products available for this purpose that work very well and kill a wide range of parasites.The product you choose should match the type of parasites that are present in your herd. Timing of the deworming is also important and I generally recommend 2-3 times per year; once in the fall to kill the parasites that were picked up over the summer, once in late spring to reduce the amount of pasture contamination, and one more time if the fecal tests suggest that it is necessary. Thirdly, strategic pasture rotation can be used to control some parasites such as ticks.Ticks in the immature stages are found in the pasture at certain times of the year. Through pasture rotation their numbers can be minimized. Lastly, by maintaining good health status in your herd through proper nutrition, vaccination, and minimizing stress you will develop a herd with a strong immune system so that parasites can’t get established.

As you can see, I have only scratched the surface of this huge topic but hopefully I have made you aware of some of the reasons why it is important to consider parasite control as an important part of a health management plan for your herd.

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