Internal roundworms

Unlike external parasites such as lice and flies, which are often clearly visible, internal roundworm parasites are less obvious, however they are extremely common. Cattle that are affected by roundworms can have depressed weight gains, poor feed efficiency, diarrhea in calves, and reduced milk production and reproductive inefficiency in cows. Parasite species also vary by region.

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 most damaging of these is 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. Other species, more common in warmer regions such as the southern US, such as Cooperia punctata and Haemonchus placei are present in Canada and can occasionally cause problems. 

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, and 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 animal host as adult worms and inhibited larvae.  The pasture contamination builds up over the grazing season to maximum levels in the late summer and early fall. Parasite egg counts in cattle in Canada tend to be in the range of 1- 50 eggs per gram (epg) of feces 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 infective L3 larvae can survive the winter on pasture to varying degrees, even in Canada, and adult worms and inhibited larvae can survive during the winter inside the host. 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 also depend on the temperature and moisture during the grazing season. Warm and wet summers, and regions building up more pasture contamination, can lead to higher worm burdens in beef cattle.

Economic impact of internal roundworms

The largest academic assessment of production impacts of internal roundworm parasites occurred in North America in 2007.  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). 

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 (Kunkle 2013). In the US study, untreated cattle had fecal egg counts ranging from 2-84 eggs/gram. Once treated, cattle had effectively zero egg counts. This resulted in production gains averaging between 0.16-.54lb/day over 120 days of grazing, providing calves an additional 14-65 lb overall gain. Similar egg counts within this range are commonly seen in Canadian beef cattle with the major species present being roundworm species like Osteragia ostertagi, Cooperia oncophora and Nematodirus helvetianus. This suggests that good roundworm control should produce significant production gains in Canadian beef cattle as well. This is supported by a recent study showing similar gains for stocker cattle in Western Canada (Rademacher et al., 2018).  

Effectiveness of parasite control

There is a limited ability to accurately assess worm burdens in live animals and there are challenges to implementing recommended control measures at practical times. In addition, the longstanding use of anthelmintic (dewormer) products, particularly ivermectin, has led to parasites becoming increasingly resistant to these products which further complicates control and threatens 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. Recent collaborative work between the Western College of Veterinary Medicine and the University of Calgary Faculty of Veterinary Medicine has confirmed the presence of ivermectin resistant Cooperia oncophora , Cooperia  punctata and Haemonchus placei in Western Canadian Beef Cattle (Eranga de Seram, Fabienne Uehlinger and John Gilleard , unpublished data). 

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. From this study, we know that internal roundworm parasites are not well controlled in Canadian beef cattle and the majority of parasite burdens detected were enough to likely cause production losses.

Drug-resistant parasites are an inevitable consequence of using dewormer. 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 benefits but prevent overuse in order to maintain their efficacy in the longer term.

Prevention and control measures

A good internal roundworm parasite control program should maximize production gains, minimize disease risk but avoid haphazard 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 develop a parasite control program appropriate for their specific herd and environment.

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.

When 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.

When 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 loads and determine which parasites you need to target. Consult your veterinarian, who can advise on sampling strategy. Collect fresh manure samples and submit to your veterinarian who can perform fecal egg counts and interpret your 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 measure of worm burdens therefore results need to be interpreted in the context of your grazing management, production practices, 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 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 fresh fecal samples and performing fecal egg counts approximately two weeks after deworming. Collecting and analysing 20 samples from cows and 20 samples from calves will provide a good estimate of the treatment efficacy for the group. 

Biosecurity

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 veterinarian to discuss your parasite biosecurity program.

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

*Every effort has been made to ensure the accuracy of the information above. However, it remains the responsibility of the readers to familiarize themselves with the product information contained on the Canada product label or package insert. Ensure label directions and veterinarian instructions are followed when using any veterinary product.

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).