Gastrointestinal Nematodes in Ontario Cow-Calf Herds

Gastrointestinal nematodes and anthelmintic use:

Gastrointestinal nematodes (GIN), also known as roundworms, are fairly common in beef cattle. They have a direct life-cycle, including one host, and primarily spread through oral transmission on pasture. We generally find a diverse collection of GIN genus and species in one host animal. Some of the most common GIN found in cattle are Ostertagia spp., Cooperia spp., and Nematodirus spp. The make-up and number of GIN varies between animals, herds and geographical regions.

Depending on the species and the resilience of the host, GIN infections can supress immune response and change gastrointestinal motility, digestion and absorption(1). Today because of the widespread knowledge and use of anthelmintics (aka dewormers), clinical signs of GIN parasitism like weight loss and diarrhea are uncommon in cattle. Our bigger concern is subclinical infections (i.e., those that you can't necessarily see) and the effect that these have on animal performance.

There are well documented advantages to including a deworming program in your beef operation. Two of the commonly used anthelmintics in the cattle industry are ivermectin (e.g., Ivomec®, Noromectin®) and fenbendazole (e.g., Safeguard®). Studies have shown weaning weight advantages of 7-22 kg (15.4-48.4 lbs) (P < 0.05) (2,3), increased average daily gains (3,4), as well as improved milk production and reproduction (5-7). In addition to these production increases, an economic benefit has also been seen with the use of anthelmintics in cow-calf operations (8,9). It is hard to accurately measure economic benefits because of all the variables associated with cattle, weather, geography, management, and infection levels as well as the forever changing markets.

Are your dewormers doing their job?

Anthelmintic resistance is currently a significant concern to small-ruminant producers, particularly sheep farms, with several cases of resistance found in Ontario sheep flocks (10). However, there is lack of agreement that anthelmintic resistance is occurring in cattle. Reports of anthelmintic resistance in cattle vary with drug class, GIN species, and geographical region. At this time, there has been no literature published that looks at anthelmintic resistance to fenbendazole or ivermectin in cattle in Ontario, Canada.

Fecal egg count reduction tests (FECRT) can be used to get an idea of how dewormers are working, by measuring the portion of GIN eggs or adults that are able to survive treatment. To perform a FECRT, fecal egg counts (FEC) are conducted before treatment with an anthelmintic and 14-days afterwards.

Microscope slide image showing GIN eggs in fecal sample

Figure 1: GIN eggs extracted from a beef cow fecal sample for a fecal egg count

Producers should consult their herd veterinarian about submitting fecal samples for fecal egg counts. Samples should be collected from a minimum of ten different animals and can be mixed together. The samples should be collected from fresh fecal pats and placed in a clean bag. They need to be kept cool and submitted to the laboratory within 48 hours of collection. A pooled sample the size of a golf ball is all that is required to conduct the count.

Research update:

In 2014 and 2015 we conducted a randomized clinical trial at the University of Guelph Elora Beef Research Centre looking at GIN infections. The study objectives were to: 1) examine the epidemiology of GIN in an Ontario cow-calf herd, to determine the most appropriate time to apply anthelmintics, and 2) determine any difference in efficacy of two common anthelmintics used in cattle production; ivermectin and fenbendazole.

In the spring of both years, a total of 128 cows and calves were randomly assigned to one of three groups - one was given a pour-on ivermectin product, one orally administered a fenbendazole product, and the third group did not receive an anthelmintic. Cows and calves were treated in the spring and randomly assigned a rotationally grazed pasture for the summer. One treatment per field was used to decrease the risk of cross-contamination.

Beef cows in pasture

Figure 2: Trial animals on rotationally grazed pasture at the Elora Beef Research Centre

Weights, fecal samples, and cow body condition scores (cows) were collected before turnout on pasture and at 28-day intervals throughout the grazing season. Fecal egg counts were conducted using the Modified Wisconsin Sugar Floatation Technique.

Over the two years no clinical cases of GIN parasitism were seen - all counts were at subclinical levels. On average, calf FEC was higher than cow FEC. About 25% of the fecal samples collected from the calves throughout the summer had zero or undetectable levels of GIN eggs. This highlights the pattern of over-dispersion typical of GIN in a grazing herd. It is typically felt that about 80% of the parasites are carried by only 20% of the animals. There was a peak in FEC seen in cows and calves after about two months on grass (P > 0.05). This finding would suggest a possible benefit to treating later in the pasture season around late June and early July rather than at turnout. Though this could be beneficial in supressing infection, treating later in the pasture season does present a challenge to cattle producers in terms of labour and infrastructure complications.

Cattle treated with an anthelmintic in the spring before turnout had significantly lower FEC than those that were not treated. In the calves there was no difference in FEC between fenbendazole or ivermectin treatment groups (P = 0.42). Cows treated with the fenbendazole product had significantly higher FEC than the ivermectin group (P = 0.028), however the FEC for the cows were all very low. In this study treating with an anthelmintic had no effect on cow or calf performance. Treatment did not have a significant effect on the unadjusted weaning weight of the calves in this trial (P = 0.35). In addition, treatment had no effect on pregnancy rates of the cows (P = 0.18).

In addition, we performed a FECRT to look for evidence of anthelmintic resistance in the second year of the trial. We used four different versions of the FECRT different methods for performing a FECRT. All the methods showed evidence of resistance or suspected resistance, to both drug classes, since the lower confidence intervals were all <90%, following guidelines by Coles, et al (11). Since there is no standard FECRT for calculating anthelmintic efficacy, caution needs to be taken when interpreting anthelmintic efficacy statements. No animals in this trial had clinical signs of GIN parasitism (e.g. diarrhea, weight loss, bottle jaw). We only saw subclinical infections and in addition there was no significant advantage to treating animals in terms of performance. Given these findings, it is questionable if there is biological or economic advantage to treating cows-calf pairs under similar conditions with similar parasite infection levels.

Future research:

Fecal egg counts are a useful tool for producers and veterinarians however little work has been done in cattle to differentiate what constitutes a low, medium, or high level of GIN infection in different areas in Ontario. More work is needed to distinguish this threshold level of when treatment is necessary. In addition, more work looking at timing of treatment and anthelmintic efficacy could be beneficial to producers.

Both fenbendazole and ivermectin resistance in cattle nematodes has potential to be a future challenge in the Canadian cattle industry. There is a responsibility of those involved in the industry to use current anthelmintics appropriately. This further supports the importance of developing, implementing, and monitoring GIN control programs in you beef operation.

Acknowledgements: Thank you to committee members Drs. Paula Menzies, and Ken Bateman with the Department of Population Medicine, OVC, University of Guelph, the Elora Beef Research Centre and the Agricultural Adaptation Council for funding through Beef Farmers of Ontario.


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