Are Bison Deworming Practices Based on Sound Scientific Principals?

Ken H. Dies and Jim M. Henderson

Alberta Agriculture, Food and  Rural Development
Animal Health Laboratories Branch
Box 197
Fairview AB TOH 1LO


The majority of information on bison parasites originates from the United States. Many bison producers use this information to make their parasite control decisions. Geographical location greatly affects the type and numbers of parasites which are present. Parasite control programs should be based on the parasite situation in your geographic location.

This article presents the results of some studies we have conducted in the Peace River region of northern Alberta. We will point out where our findings differ from more southerly climates. In addition to reporting our findings, this article will also provide general parasite control information that should be considered when making decisions on parasite treatment.

What information is available on intestinal worms or worm egg counts in Canadian Bison?

There is no information in Canadian scientific literature on worm egg counts in manure samples of bison. There is limited information on a few specific adult intestinal parasites in Canadian bison.

In addition to the lack of information on manure worm egg counts, problems occur in the interpretation of the significance of total egg counts. Only some types of parasites can be identified as to genus, based on the production of eggs that are sufficiently different to be recognized under the microscope. Unfortunately many of the common intestinal worms of bison produce eggs which are indistinguishable in appearance. These eggs will be referred to astrichostrongyle eggs in this article. Identification of the genus or species ifTrichostrongyle worms requires hatching of the eggs followed by examination of their characteristic larvae. Because different intestinal worms vary greatly in their ability to cause disease, it is important to know what genus or species ofTrichostrongyles are present in your herd or area.

What studies did we conduct?

Most of the information in this article is based on a study conducted on a 250 cow/calf bison herd, located in the Peace River Region of north western Alberta. This study was designed to determine changes in worm egg counts, over a 12-month period, and to identify the species or genus of worms that contributed to these egg counts. It also contains preliminary information from a study, in progress, designed to obtain parasite information from other bison herds in the Peace River Region. In this project, to date we have examined representative samples from close to 1000 animals in 12 herds: Herds which we refer to as untreated in this study, have not been dewormed for at least 12 months before our collection.

How were these studies conducted?

In the primary study, 35 individual manure samples were collected from the ground, immediately after a bowel movement, from both cows and calves m January 14/97. Two days later a pour-on dewormer was given to the cows and calves. Starting two weeks after deworming 10 random samples were collected from both cows and calves on a monthly basis. The last collection was made on January 6, 1998.

Yearlings in this herd were placed in a feedlot environment. They were not dewormed. Random fecal samples from % of these animals were collected on Jan 14/97 and Sept.15/97.

Total worm egg counts were determined on all samples collected over this time period. Following determination of the worm egg counts the monthly samples were mixed together and incubated to hatch the eggs so that the resulting larvae could be examined to determine the species or genus or worms contributing to the Trichostrongyle egg count. Separate pools of cow and calf samples were maintained to determine what differences there were, if any, in both groups.

The study currently in progress involves the collection of rectal manure samples from 30% of the animals in each herd. Collections are being done when these herds are run through for Brucellosis or TB. Total worm egg counts and larval identification are being performed on these bison.

What did we find?

In the primary study herd, the most common eggs encountered were in theTrichostrongyle group. There was an average of 9 Trichostrongyle eggs per gram (epg) in the cows and 78 epg in the calves in the first samples collected before treatment (Fig.I). Average Trichostrongyle counts, in the samples collected two weeks after treatment were less than 1 epg in cows and 40 epg in calves. Over the next six months egg counts, in the cows gradually increased to pretreatment levels, while levels in the calves fluctuated between 40- 72 epg. An overall decrease in epg was noted in both cows and calves between September and November then counts rose slightly in December and January (Fig.l).

Trichostrongyle Worm Egg Counts for Bison Calves and Adults in 1997

pre treatment values


Percentage Of Trichostrongyle Type Larvae Examined From Each Pooled Fecal Culture

pre treatment values

Examination of the larvae revealed the presence of 3 species ofTrichostrongyles, namely the small intestinal worm ( Cooperia), the brown stomach worm (Ostertagia) and the small stomach worm (Trichostrongylus). Cooperia was by far the most common Trichostrongyle encountered. This worm accounted for 96% of all larvae identified in calves and 92% of larvae in cows.Ostertagia was only identified in a few collections. The highest numbers reached 4% in adults and 2% in calves (Fig.2). Trichostrongylus larvae were only detected in very low numbers, with an average of3% to 6% infection rate over the year in both groups.

Other parasite eggs observed included the whip worm (Trichuris), capillary worm(Capillaria), thread necked worm (Nematodirus), threadworm (Strongyloides)and tapeworm (Moniezia). These were only present in a few animals and in very low numbers.

On June 18/98, 18 months after their last deworming, manure samples were examined to determine present parasite loads. Results from this examination revealed 18 Trichostrongyle/epg in the cows and 15 Trichostrongyle/epg in the calves.

On Jan. 14/97, the yearling bison in the feedlot had an average of 39Trichostrongyle/epg. On Sept. 15/97, 1 day before shipping, they had an average count of less than 2 Trichostrongyle/epg. This large decline in fecal egg count was attributed to poor conditions for parasite development in an area free of vegetation.

In our study of Brucellosis and TB tested herds, treated herds had a high of 78Trichostrongyle/epg and a low of less than 1 epg. Samples from untreated herds had a high of 56 epg and a low of 2 epg.

Why is it important to know what parasites are present?

It is important to know the types of re worms present because they vary in their ability to cause disease. A summary of the disease causing capability le of the species encountered in our study is as follows.

Of the Trichostrongyle species encountered, the brown stomach worm 9 (Ostertagia) is the only one capable of

causing disease when present in moderate numbers.

The small stomach worm (Trichostrongylus) is usually found in mixed infections with Ostertagia, and is rarely a leading cause of parasitism. Diarrhea and weight loss can occur when this worm is present in large numbers. In well nourished, unstressed animals this parasite is unlikely to cause harm. The small intestinal worm, Cooperia, by itself is considered to be relatively harmless. In cattle it has been associated with problems in calves and only rarely causes disease in animals more than 4 months old. Calves must be exposed to massive numbers of larvae on pasture before signs of disease occur. At one year of age, cattle will develop an immunity to this parasite and their numbers will drop. Our findings suggest this may also be true in bison.

Disease caused by whipworms (Trichuris) is rare in ruminants and tends to be an individual animal problem. Large numbers in young animals can cause problems.

The Capillary worm (Capillaria), a very small worm which lives in the small intestine of ruminants, usually in low numbers, is not considered to be an important cause of disease.

Thread necked worms (Nematodirus) are not usually associated with clinical disease in adult cattle. On rare occasions, calves less than six months old with extremely heavy infections may experience dehydration and weight loss.

The threadworm of sheep and occasionally cattle (strongyloides), were observed in only three animals in .the study herd. In bovines, this parasite is not considered to be a problem in temperate regions. This parasite has not been previously recorded in bison of North America.

Tapeworms (Moniezia) are commonly found in cattle and bison. They have a worldwide distribution but have never been associated with any evidence of disease.

What effects does climate have on parasites?

In most of the continental United States, climatic conditions are favorable for the development of parasites and their transmission throughout most of the year. In the northern areas of the prairie provinces conditions are less favorable for completion of the life cycle of many parasites, other than for a few months in the summer. This is reflected by lower worm burdens and lower manure egg counts. Temperatures above 10° Celsius are required for the development of most Trichostrongyles. In the Peace River region of northern Alberta it is not uncommon to see frost ten months of the year. Frequent frosts greatly limit the number of days suitable for development and transmission. Most larval stages ofTrichostrongyles, including Ostertagia do not withstand prolonged freezing. In northern Alberta where the winters are long and severe, it is doubtful many larvae survive over winter on pasture. This means that mature bison, although usually only carrying a few parasites, are the major source of pasture contamination for spring calves.

In most of the southern United States, Ostertagia is one of the predominant parasites found in cattle and bison. The small stomach worm (Trichstrongylus),and intestinal worm (Cooperia), are also found in these animals, but usually in lower numbers. Based on studies done in cattle, there is a gradual change in the relative numbers of different

Trichostrongyles as you move north. Reports have shown that cattle on pasture south of Calgary harbor slightly more Ostertagia than any other Trichostrongyle.Approximately, 200 km north of Calgary, near Red Deer, Cooperia is the most common strongyle present with Ostertagia second. In our studies, 700 km north of Red Deer, Cooperia was the predominant worm encountered and Ostertagiawas observed only in trace amounts. This confirms the trend of increased numbers of Cooperia and reduced numbers of Ostertagia as you move north. This geographic difference is particularly important when one considers the relative disease causing capabilities of these two parasites.

What are some common factors which affect parasitism in bison?

Grazing density and pasture rotation are very important keys in relation to parasitism. The higher the grazing density, the more likely you will have a high parasite population on the pasture. Animals grazed at low density do not consume herbage close to the ground nor near manure pads, thus ingesting fewer larvae than bison placed on high density grazing. Stock should be rotated to new pastures at regular intervals to prevent excessive pasture contamination.

Average temperature and humidity influences the processes of hatching, movement and survival of Trichostrongyle larvae on pasture. If climatic conditions are favorable for constant worm development, more parasite problems are likely to occur. In contrast, temperate regions with distinct summer and winter seasons are limited to just a few parasite generations.

Most, if not all, ruminants including bison, become more resistant to parasitic infections as they reach maturity. Clinical problems created by intestinal worms are usually associated with young animals which have little or no immunity to no specific parasites. As these individuals grow older their immunity develops and they are basically resistant by a year of age.

General nutrition can also affect an animal’s susceptibility to parasites. Livestock on a low level of nutrition are more susceptible than animals which are well nourished.

Should you deworm your bison?

Ideally, this decision would be based on the types and total number of parasites found in your bison. Next best is a consideration of the types of bison or cattle parasites found in your region. If worm egg counts are very low, it may not be economical to deworm. On the other hand, if egg counts are high with a high percentage of Ostertagia deworming would be advisable. A count of 200-300 epg of mixed of Trichostrongyles has been used for many years to indicate significant sub clinical infection in cattle. These levels are frequently used as guidelines in bison. Some producers believe that bison are more susceptible to parasitism than cattle. There is no scientific evidence to prove or disprove this belief. Numbers of eggs per gram of feces is only one of many factors used to determining whether parasite treatment is of benefit. The general health status of the herd, feed quality, age, grazing practices, past history and egg laying characteristics of parasites involved are other factors which will assist you and your veterinarian to decision to treat or not to treat.

When is the best time to deworm?

The majority of bison producers in northern Alberta routinely deworm their animals in the fall or winter, during weaning or blood testing. Although this may be a convenient time, it is not the best time for the most effective parasite control. Deworming temporarily reduces the existing population of adult and in some cases immature parasites. Fall treatment may do very little to prevent pasture contamination. Many animals released on to spring pasture may harbor parasite loads close to pre-treatment levels. To minimize pasture contamination the ideal time for deworming would be just before the animals are turned out on spring pasture. The requirement of another trip through, the chute is a problem with this recommendation but based on our information, spring treatment should have more benefit. Long winter haircoats may reduce the effectiveness of pour on products. In our study, there was almost a 100% reduction ofTrichostrongyles and other parasites in the cows but only a 50% reduction in the calves. The calves had a much heavier hair coat which prevented the pour on from contacting the skin, thus, accounting for the poorer treatment results.

What is the owner of the primary study herd going to do now and why?

All manure samples had very low egg counts throughout the 12 months of the study and in samples collected 18 months after the last treatment. The herd remained in excellent body condition with no evidence of any symptoms of parasitism. Calving percentages for 1997 and 1998 were 92% and 94%, respectively. As a result of low egg counts in this herd consisting almost entirely of Cooperia, the producer decided not to deworm in the winter of 1998. He intends to monitor the parasite level in his herd through his practicing veterinarian and will not deworm unless egg counts rise substantially.

This owner routinely grazes at low density and rotates the herd to new pastures every five days. This not only prevents overgrazing of pasture but also greatly reduces the number of parasite larvae on the pasture.


Many bison producers have never tested their herd to determine numbers and types of worms present, but deworm once a year, based on information from a different geographic area. All bison herds examined in our studies to date, had low worm egg counts consisting primarily of Cooperia. Is treatment necessary when egg counts are extremely low caused by worms which may cause little or no harm? Small numbers of intestinal worms normally harbored by domestic ruminants, including bison, are an important factor in the development of immunity.

Deworming will not harm your bison but is expensive. If other management factors are not addressed, deworming may do very little for prolonged parasite control.

In our survey of random herds, no significant differences were seen in parasite egg loads between treated and non-treated herds. Bison herds should be monitored regularly to determine worm egg count and type of parasites present. If levels are high, treatment is recommended in addition to the implementation of other management practices to reduce parasite numbers. If worm egg counts are low and good management practices are in place, deworming is probably not required.

There is a need for further scientific study to determine the significance of various intestinal worms in Canadian bison.

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