• Beef Cattle Research Council on Facebook
  • Beef Cattle Research Council on Twitter
  • Beef Cattle Research Council on Youtube

Looking for foragebeef.ca? Click here for more info

Research   »   Internal Parasites

Internal Parasites

There are many different parasites that cause production impacts and disease in Canadian beef cattle. Parasite control is an important part of maintaining health, welfare and production and different parasites require specific control measures. Internal parasites, such as roundworms and coccidia, live inside the gastro-intestinal tract whereas external parasites, such as lice and flies live on, or around, the animal. It is important that all parasites are not considered as a single group when planning control measures. Instead, each should be considered separately within an overall integrated parasite control program. 

On this page:

Common parasites in Canada 

Before developing a parasite control program it is important to understand which parasites are impacting your herd, and what that impact is. 

Common parasites in Canada


Impact on Animal

Roundworms: Brown stomach worm (Ostertagia ostertagi), instestinal worms (Cooperia oncophora and punctata, Nematodirus helvetianus)


Roundworms are the most common class of parasites in beef cattle and impacts can be insidious. They cause depressed weight gains, poor feed efficiency, diarrhea in calves and reduced milk production and reproductive inefficiency in cows.

Lungworms (Dictyocaulus viviparous). Sporadic disease outbreaks


Lungworms in upper airways of respiratory system cause nasal discharge, coughing and difficult breathing. It is a sporadic disease in Canada but can be severe affecting both calves and adult cattle.

Fluke (flatworm) (Fascioloides magna)


Fascioloides magna has a regional distribution (eg. Foothills of Rockies, Great Lakes). It is carried by elk/deer and transmitted in wet boggy areas via semi-aquatic snails. It causes liver condemnation and impact on production poorly defined.

Tapeworms (Monezia benedeni)


Tapeworms are several metres long and are common. Segments can often be seen in manure but are not considered harmful.

Coccidia  (Eimeria bovis, Eimeria zuernii and othe Eimeria spp)


Coccidiosis is caused by single celled parasites that invade and destroy cells lining the intestine. It is very common in Canada, causing acute dysentery and diarrhea, neurological signs, chronic diarrhea and reduced growth. Disease most commonly occurs in 1-6 month old calves.

Lice (Damalinia bovis, Linognathus vituli, Haematopinus eurysternus, Solenopotescapillatus)


There are two types of lice, biting and sucking lice. Numbers increase with cooler temperatures reaching maximum levels in late winter. Lice cause coat discolouration and hair loss, and sometimes anemia and production loss.

Stable Flies (Stomoxys calcitrans)


Typically affect confined livestock, but can also on be bothersome on pasture. Bites are painful and often bleed when fresh. Feeding activity results in stress to the animals, pain and reduced production due to reduction in feeding time.

Horn flies (Haematobia irritans)


Horn flies congregate around and on cattle at pasture all summer. They bite and suck blood, affecting livestock behaviour, causing reduced performance and reduced milk production.

Cattle grubs (Hypoderma bovis, H. lineatum)



Internal & External

Ivermectin has reduced the prevalence of cattle grubs in Canada to very low levels but they are still present in localized areas. Adult female flies lay eggs in the hair of the animal which hatch into grubs and migrate deep into the tissues where a painful warble develops that causes pain to animals and holes in the tissue.

Internal roundworms

Unlike external parasites such as lice and flies, which are often clearly visible, internal roundworm parasites are less obvious. Consequently, Canadian beef producers are often more concerned with controlling visible external parasites. A common misconception is that it is too cold in Canada for roundworm parasites to be a problem, which is not the case. There are several species of roundworm parasites that are well adapted to cold weather, including Ostertagia ostertagi, Cooperia oncophora, and Nematodirus battus, which are common and widespread in Canadian beef cattle and result in production impacts. The life cycle of the most damaging of these, Ostertagia ostertagi, Although cattle younger than two years of age typically have the highest worm burdens, and are most impacted by internal roundworms, even adult cattle carry worms and contribute to pasture contamination. Life Cycle of the Cattle Gastrointestinal Parasie Ostertagia ostertagi

The lifecycle of other internal roundworm parasites such as Cooperia oncophora, Cooperia puntata, Nematodirus helvetiatianus, are very similar. The key to parasite transmission and control for these roundworm species is pasture contamination. Adult roundworms living in the gastro-intestinal tract produce eggs which pass out in the feces and then develop to infective L3 larvae in the fecal pat. This can occur in as little as 7 days under optimal conditions but can take several weeks in cooler weather. The L3s then migrate into the soil and onto the grass where they are ingested by cattle during grazing. Transmission only occurs on pasture because L3s do not survive in indoor or feedlot pen environments. Although the development from egg to L3 only occurs in the spring/summer/early fall, infective L3 larvae can survive over the winter in the soil and be a source of infection to cattle grazing the following spring. Parasites also survive over the winter inside the host as adult worms and inhibited larvae.  The pasture contamination builds up over the grazing season to maximum levels in the late summer/early fall. Parasite egg counts in cattle in Canada tend to be in the range of 1- 50 eggs per gram of feces (epg) but in situations of heavy pasture contamination counts up to 200-300 epg in a few individual animals can occur.  If you consider a typical beef cow might produce 30kg of feces each day, a single cow with an egg count of 10 epg would shed ~300,000 eggs onto the pasture each day!

The relative importance and success of roundworm “overwintering” strategies will vary depending on the temperature, moisture and snow cover in a particular year. During the spring, summer and fall, the numbers of infective larvae build up on pastures at levels that again, depend in the temperature and moisture during the grazing season. Typically, warmer, wetter summers and regions building up more pasture contamination leading to higher worm burdens in the cattle.

Effectiveness of parasite control

There is a limited ability to accurately assess worm burdens in live animals and there are practical challenges of implementing the best control practices. In addition, the longstanding use of anthelmintic (dewormer) products, particularly ivermectin, has led to parasites becoming increasingly resistant to these products further complicating control and threatening sustainability. We know that pour-on dewormer treatments used in Canadian beef cattle are often only partially effective at clearing worm burdens, as demonstrated in the image below, This may be due to the products not being properly applied as well as the presence of drug resistant parasites. 

roundworm egg counts cattle canada

The chart shows the percentage reduction in roundworm egg counts following an ivermectin or doramectin pour-on treatment in ~50 beef herds across Canada. Fecal samples were taken from 20 calves in the herd before pour-on treatment and 2 weeks after pour-on treatment and fecal egg counts were conducted. The different coloured bars indicate the numbers of herds in each of the following categories; less than 50%, 50-85%, 85-95% and greater than 95% reduction in fecal egg counts following treatment. It can be seen that the percentage reduction in fecal egg counts sub-optimal in many herds. 

Drug resistant parasites are an inevitable consequence of dewormer use and the more we use these products the more resistance develops over time.. Consequently, it is important that dewormers are used in a way that maximize health and production but prevent overuse in order to maintain their efficacy in the longer term.

Overall, we know that internal roundworm parasites are not well controlled in Canadian beef cattle at present. Work funded by Alberta Livestock and Meat Agency in collaboration with Merck Animal Health demonstrates average fecal egg counts for common parasite species in groups of grazing calves from approximately 50 beef herds across Canada.  The majority off parasite burdens seen in this study were enough that they were likely to be associated with production losses.

Economic impact

A largest academic assessment of production impacts of internal roundworm parasites occurred in North America in 2007[1]. The study evaluated the economic impact of parasite control, growth promoter implants, sub-therapeutic antibiotics, ionophores and b-agonists. The study found that, of these practices, deworming had the biggest positive impact in cow-calf (23% for weaning rates), stockers ($20.77 per head in breakeven prices) and the second highest benefit after growth promoter implants at the feedlot (5.6% improvement in average daily gain and 3.9% reduction in the feed-to-gain ratio). 

There have been no large studies of the economic impact of internal roundworm parasites in Canadian beef cattle in recent years, however studies from the northern United States showed that production gains occurred when parasite burdens were eliminated using a long-acting anthelmintic known as eprinomectin, a slow-release injectable formulation. Untreated cattle had fecal egg counts of 2-84 eggs/gram. Once treated, cattle had effectively zero egg counts. This resulted in production gains averaging between 0.16-.54 1lb/day over 120 days of grazing. The average fecal egg counts of the untreated control groups in these studies varied between 2 - 84 egg per gram. Therefore, parasite burdens represented by those egg counts have production impacts, and egg counts within this range are commonly seen in Canadian beef cattle with the major species present being round worm species like Osteragia ostertagi, Cooperia oncophora and Nematodirus helvetianus. This suggests that good roundworm control should produce significant production gains in Canadian beef cattle.

Controlling internal roundworm parasites

A good internal roundworm parasite control program should maximize production gains, minimize disease risk but avoid indiscriminate and unnecessary dewormer use. The aim is to use the correct product at the correct time on the animals that need it most. The issues and practicalities of roundworm parasite control differs significantly between cow-calf, stocker and feedlot cattle as well as with the grazing and management strategies of each individual herd. 

Recommended practices are outlined below however producers should consult with their veterinarians to assess their full operation, environment and animal health inputs in order to develop a parasite control program appropriate for their specific herd.

Grazing management

Avoid overstocking and overgrazing. Heavily stocked pastures leads to increased pasture contamination with infective parasite larvae. Overgrazing increases the number of parasite larvae ingested since cattle graze closer to fecal pats and closer to ground where the numbers of parasite larvae are highest.

If possible, avoid grazing the same pastures in the fall of one year and the spring of the next. Infective parasite larvae from eggs deposited in manure in fall may survive the winter on the pasture and be a source of pasture contamination for cows and calves grazing in the spring.

If possible, harrow pastures only when it’s hot and dry. Harrowing under other conditions will increase the potential exposure of cattle as infective larvae are scattered from fecal pats across the area.

Consider parasite control when planning rotational grazing strategies. For example, if a twice-over or rotational grazing system is implemented, be aware that pastures previously grazed by yearling or stocker cattle may be heavily contaminated with infective parasite larvae and so be a risk to younger cattle.

Monitor parasite burdens

Conduct fecal egg counts on your herd to assess internal roundworm parasite burdens and determine which parasites you need to target. Collect fresh fecal samples from pastures to perform fecal egg counts in consultation with your veterinarian, who can advise on sampling strategy, perform the fecal egg counts, and help interpret the results. Typically, fecal samples from 20 cows in the spring and from 20 calves in the fall will provide useful information on parasite burdens in the herd and the effectiveness of current control programs. Fecal egg counts are only an indirect, and often insensitive, measure of worm burdens therefore fecal egg count results need to be interpreted in the context of your grazing management, husbandry and parasite control regimes.

Use dewormers effectively and responsibly

Choose the correct dewormer. Common internal and external parasite controls are highlighted in Table 2 below. Each dewormer has its own strengths and weaknesses and varies in effectiveness against specific parasite species.  For example, macrocyclic lactone (i.e. ivermectin) dewormers are becoming less effective against Cooperia parasites due to resistance whereas fenbendazole or albendazole are less effective against inhibited larvae of Ostertagia. External parasite control also needs to be considered. For example, ivermectin targets many external parasites whereas fenbendazole does not. Dewormers come in several formulations that differ in convenience and effectiveness including injectables, oral pastes or drenches, in-feed pellets or minerals or topical pour-on products. It is important that the correct formulation is chosen for the specific application and this will differ between herds and at different times of year.

Administer the dewormer at the correct time. Dewormers should be used strategically to minimize pasture contamination and so prevent the build up of roundworms in the cattle during the grazing season. Treatments are often given when it is convenient – when cattle are being processed - rather than at the best time for control. Consequently, in many herds, roundworm control depends on pour-on treatments applied in the fall, partly because of the need to also control external parasites. However, spring treatments can sometimes provide added benefits leading to lower roundworm burdens in the fall. Producers should consult their veterinarians to plan a strategic worm control program that balances best practice with the practical realities of herd management.

Administer the dewormer properly.  Weigh cattle to ensure you administer the correct dose for the particular class of cattle you are treating. Underdosing is ineffective and leads to resistant parasites. Use the proper route of delivery for the specific product (oral, injectable, fed, topical). Follow veterinary advice and label instructions for administration, storage and withdrawal times prior to slaughter. Adhere to recommended practices in Canada’s Verified Beef Production+ on-farm food safety program. Properly dispose of expired product, empty containers and used needles.

Check the effectiveness of dewormer treatments. Do not assume a dewormer treatment has been effective. It is increasingly important to check the effectiveness of treatments to prevent parasite resistance. This can be done by taking fecal samples and performing fecal egg counts approximately 2 weeks after deworming. Collecting and analysing 20 samples will provide a good estimate of the treatment efficacy for the group. 

Treat new cattle entering your herd health

New cattle introduced onto your farm are a source of parasites that will contaminate your pastures. Treating such cattle with a combination of dewormers from the two major drug classes ( eg. Ivermectin plus fenbendazole) will minimize the risk of bringing resistant parasites into your farm. Consult your veterinary surgeon to develop this approach as part of your herd health and biosecurity programs.

Progressive ideas

Leave a small proportion of the herd untreated. In most well managed herds, the 10-20% of cattle that are in the best condition actually don’t benefit from dewormer treatments. This is because they only carry low worm burdens and so, if left untreated, there will be no significant reduction in herd production gains. As an added benefit, there will be less selection for drug resistant parasites as less dewormer is used and the eggs shed in the feces of the untreated animals will “dilute” the population of resistant parasites on the pasture.

Use a combination of two dewormers. Dosing cattle with two dewormers of different classes at the same time both maximizes the effectiveness of treatment and slows the development of resistance. It is important not to mix products before dosing but to administer each sequentially, using the correct administration route and following the label instructions. Consult your veterinary surgeon to discuss using dewormer combinations as part of your herd health and parasite control programs.

Common parasite control products available in Canada

Common Parasite Control Products

Parasites Controlled

Mode of Administration

Examples of Brand Name of Products Registered for Use in Canada


Internal Roundworms

Feed, Mineral, pellets

Oral Drench


Safeguard, Pancur


Internal Roundworms

Oral drench



Internal Roundworms & External

Topical pour-on


Bimectin, Ivomec, Noromectin


Internal  Roundworms & External

Topical pour-on




Internal Roundworms & External

Topical pour-on





Topical pour-on




Topical pour-on





Eliminator, Protector, Optimizer


Internal Coccidia


Rumensin, Coban, Monensin


Internal Coccidia


Bovatec, Avatec,


Internal Coccidia




Internal Coccidia

Oral drench


The first five drugs on the list are dewormers with activity against internal roundworms.
1 Fenbendazole and albendazole belong to same drug class (Benzimidazoles).
2  Ivermectin, Doramectin, Moxidecitn  belong to same drug class (Macrocyclic lactones).

Learn More
Don't Ignore drug resistant parasites 
Canadian Cattlemen The Beef Magazine 
Good riddance to roundworms on pasture 
Canadian Cattlemen The Beef Magazine 
Rethinking strategic deworming in beef cattle
Canadian Cattlemen The Beef Magazine 

Lawrence, J. D., and M. A. Ibarburu. 2007. “Economic Analysis of Pharmaceutical Technologies in Modern Beef Production.” Proceedings of the NCCC-134 Conference on Applied Commodity Price Analysis, Forecasting, and Market Risk Management. Chicago, IL. [https://www.agrireseau.net/bovinsboucherie/documents/confp05-07.pdf]

B.N. Kunkle et al. / Veterinary Parasitology 192 (2013) 332–337


Feedback and questions on the content of this page are welcome. Please e-mail us at info [at] beefresearch [dot] ca.


Thanks to Dr. John Gilleard from the University of Calgary Faculty of Veterinary Medicine for contributing his time and expertise in writing this page.

This topic was last revised on May 3, 2018 at 8:05 AM.

© 2019 BCRC. All Rights Reserved  |  Council Login

Privacy Policy | Terms of Service | Sitemap | info [at] beefresearch [dot] ca | Site By Media Dog