Health Management: Internal Parasites

This is a fact sheet from the Health Management section of the Alberta Feedlot Management Guide, Second Edition published September 2000. The 1200 page guide is available for purchase on CD-ROM.

Take Home Message

The control of internal parasites in a cost-effective manner is an important issue in feedlot production. Scientific evidence indicates that it can be cost-effective to control internal parasites in Canadian feedlots. However, there are several factors involved in determining the most appropriate method of internal parasite control. The following article discusses the pertinent information with respect to internal parasite control in feedlot cattle.

Introduction

The economic importance of internal parasites, such as gastrointestinal worms and lungworms, in feedlot cattle has been a controversial topic. It is known that internal parasitism can decrease an animal’s productivity. However, the decision whether/which anthelminthics (dewormers) to use in feedlot cattle requires that the economic value of production improvements is greater than the cost of treatment.

Virtually all feedlot cattle have some level of internal parasitism (1, 2, 3). The primary parasites involved are gastrointestinal worms, such as Ostertagia ostertagi (brown stomach worm), Cooperia spp. (small intestinal worm or cattle bankrupt worm), Trichostrongylus (hair worms), and Nematodirus helvetianus (thread-necked or thin-necked worm). In certain areas, lungworms (Dictyocaulus viviparus) may also be present. Levels of parasitism are greatest in younger animals and in animals coming off pasture (1). Younger cattle are most susceptible to parasites, because they lack the previous exposure and immune system development to keep parasite burdens under control as compared to older cattle. Cattle coming off pasture tend to have higher parasite burdens because worms are transmitted and spread on pasture. The adult worms live and reproduce in the animal’s gastrointestinal tract. They lay eggs that are passed out in the manure. These eggs hatch on the pasture and develop into infective stage larvae. These larvae migrate up blades of grass and are consumed by the cattle as they graze. The larvae continue their maturation in the gut, thus completing the life-cycle.

Historically, gastrointestinal worms have not been considered a significant production problem in Canada, because harsh winter conditions keep parasite populations from reaching the higher levels seen in warmer climates. Additionally, cattle with clinical signs of parasitism (diarrhea, weight loss, rough hair coat, etc.) are rarely observed in Canada; therefore, it was assumed that internal parasites were not causing much damage. It is now known that subclinical (no obvious signs of infection) parasitism can cause great losses in terms of decreased productivity. In feedlot animals, subclinical parasitism can cause inferior rates of gain and feed conversion. It is assumed that losses occur as a result of a number of factors, including the diversion of nutrients to parasite growth and reproduction, interference with nutrient absorption by reducing available surface area and direct damage to the gut lining, and interference with digestion by damaging enzymeproducing cells necessary for the breakdown of feed. The true extent of subclinical parasitism is unknown and it is difficult to estimate because the effects are not visible. However, the economic results of a Canadian study conducted in 1987 (4) demonstrate the potential for economic losses. With this study as a guideline and knowing that approximately 2.6 million cattle were fed in Canada in 1995, subclinical parasitism could be costing the feedlot industry upwards of 17 million dollars annually (4).

A review of the pertinent literature reveals diverse opinions on the cost-benefit of deworming feedlot cattle (1-8). Early, small scale studies conducted in research facilities at various times of the year and utilizing cattle of mixed ages were inconclusive regarding the benefits of deworming feedlot animals. Research data obtained from large scale commercial field trials in western Canada demonstrate that deworming fall placed calves on arrival at the feedlot results in significant improvements in average daily gain (ADG) and feed conversion (4, 5). In the studies referenced, ADG was improved by 6.5% over cattle treated for external parasites only. The feed to gain ratio (F:G) was improved by 3.0%. These improvements in rate of gain and feed conversion resulted in a net economic benefit of approximately $7.00 per animal in one study (4). It should be noted that these data show a cost-benefit to using the specific deworming programs tested in each study (ivermectin and oxfendazole). It may not be appropriate to extrapolate these results to other deworming strategies.

In yearling cattle, parasite levels are generally not as high as in calves because older cattle have started to develop immunity to parasites. As a result, conventional wisdom held that deworming yearling cattle was not a cost-effective management decision. However, fall placed yearling feedlot cattle (usually coming off pasture) have higher parasite levels than yearling cattle placed at other times of the year. Moreover, recent research data obtained from a large scale commercial field trial in western Canada demonstrated that deworming fall placed yearling cattle on arrival at the feedlot resulted in significant improvements in rate of gain and feed conversion. In this study, cattle treated with ivermectin had a 3.5% improvement in ADG over controls treated with an external parasite control product only. In the same study, treated cattle also had an improvement in F:G of 2.7%. Thus, the use of anthelminthics in fall-placed yearling feedlot cattle can be a cost-effective management decision.

In animals arriving at the feedlot at times of the year other than fall, a strategic deworming approach may be appropriate. This strategy involves assessment of each group of cattle arriving at the feedlot. Factors to consider during the assessment should include: physical appearance, geographical origin, age, body weight, length of feeding period, and deworming history. In general, younger, lighter cattle from areas with high rainfall and densely stocked pastures are more likely to be heavily parasitized and more likely to benefit from deworming.

Summary

Research data from western Canada indicates that deworming fall placed cattle (calves and yearlings) is a cost-effective management strategy. In animals arriving at the feedlot at times of the year other than fall, a strategic deworming program should be used. The most appropriate product to use (Table 1) depends on a number of factors and should be discussed with a feedlot veterinarian.

Table 1. Available Anthelmintics.

References

1. Williams, J.C. 1983. Ecology and control of gastrointestinal nematodes of beef cattle. Vet Clin N Amer: Lg An Prac 5:183-205.
2. Schultz, R.H. 1994. Thoughts on current anthelmintic needs and usage. Bovine Practitioner 28: 150-153.
3. Gibbs, H,C. 1992. The effects of subclinical disease of bovine gastrointestinal nematodiasis. Compendium 14: 669-677.
4. Bauck, S.W., Jim, G.K., Guichon, P.T., Newcomb, K.M., Cox, J.L., Barrick, R.A. 1989. Comparative cost-effectiveness of ivermectin versus topical organophosphate in feedlot calves. Can Vet J 30: 161-164.
5. Jim, G.K., Booker, C.W., Guichon, P.T. 1992. Comparison of a combination of oxfendazole and fenthion versus ivermectin in feedlot calves. Can Vet J 33: 599-604.
6. Reinmeyer, C.R. 1994. Parasitisms of dairy and beef cattle in the United States. JAVMA 205: 670-680.
7. Myers, G.H., Grant, R.J. Effects of fenbendazole and ivermectin on performance of feedlot cattle. Agri-Practice 9: 39-42.
8. Stockdale, P.H.G., Harries, W.N. 1979. Treatment of feedlot cattle in Alberta for gastrointestinal nematodes. Can Vet J 20:223-226.
9. Bennett, K. 1995. Compendium of Veterinary Products. Fourth Edition. Hensall, Ontario: North American Compendiums Ltd.