Controlling gastrointestinal nematodes (GIN), or what many farmers call gastrointestinal parasites or worms, can be a major challenge in sheep, goat and cattle production. Fortunately, Organic Science Cluster 4 research might provide both organic and non-organic farmers with tools to easily identify animals with greater parasite immunity, and thereby develop herds or flocks with greater immunity.
Controlling gastrointestinal nematodes in Canada
The gastrointestinal nematodes (GIN) that commonly cause serious disease in Canada are Teladorsagia circumcincta (Brown stomach worm), Trichostrongylus colubriformis (Bankrupt worm), and Haemonchus contortus (Barber’s pole worm). These parasites live in the digestive tract and can literally suck the blood of an animal, as well as damaging tissues in the abomasum (true stomach) or intestines. The host animal might lose weight, have slower growth rates, and might even die from the infection.
For many farmers, their goal is to eliminate all exposure to parasites. A more realistic target is to minimize exposure to parasites while improving the flock or herd’s immunity to infections.
Approaches to control parasitic infections in livestock include the following:
- Reducing how many parasites the animals consume. This can be done by pasture rotation and ensuring the most vulnerable animals are not put out on pasture when parasite levels are high;
- Inhibiting or killing the parasites within the body of the animals. Farmers can use synthetic dewormers (anthelmintics), natural substances (e.g. copper boluses, wormwood, pine needles, etc.) or both;
- Improving immunity of the herd/flock. This enables animals to reject parasite infections despite the same exposure, preventing a negative impact on their health. Farmers can achieve this by improving overall health of the animals, as well as through selective breeding for parasite immunity. We will focus on the latter in this article.
Both organic and non-organic farmers are becoming increasingly interested in preventing parasitic infections through pasture management and improving flock immunity. In organic production, immunity is particularly important since the use of synthetic parasiticides is restricted to certain conditions (see Box below). Meanwhile, non-organic farmers are finding it increasingly difficult to rely on parasiticides because parasites are becoming resistant to the drugs.
Immunity to gastrointestinal nematodes
Acquired immunity happens in individual animals as they build a response to the parasites. When young healthy animals are frequently exposed to small numbers of infective larvae over a long period, their immune systems develop mechanisms to help protect the animals from the parasites. In most cases, the fecal egg count will be lower in the animals with the greatest immunity. However, for Teladorsagia (brown stomach worms), immune animals have smaller worms, which leads to lower reproductive rates of the parasites.
Innate immunity reflects the immediate protective barriers to parasites, such as a more resistant gut lining. Both acquired and innate immunity are under genetic control. Certain breeds and certain bloodlines within a breed develop antibodies to internal parasites faster or to a greater degree than other animals. This allows immune animals to remain healthy while grazing on pastures that have relatively high parasite loads.
In addition, the animals with greater immunity to gastrointestinal nematodes have lower worm burdens and shed fewer eggs. So not only are the highly immune animals healthier, they also don’t contaminate the pasture with parasites to the same degree as less immune individuals.
These are all reasons why farmers would prefer to choose immune animals as their replacement breeding stock. But how can they tell which animals are most immune? Fortunately, there might soon be a simple way to assess parasite immunity.
CARLA testing
The CARLA Saliva Test®, which was developed in New Zealand, can be used in ruminants to assess protective immunity to gastrointestinal nematodes (GIN). The test assesses an animal’s immunity as indicated by the level of antibodies (IgA) against CarLA, a carbohydrate found in the infective third-stage larvae of all GIN species.
New Zealand researchers found that sheep with high levels of the antibody typically had 20-30% lower GIN fecal egg counts than individuals with low levels of the antibody. Also, the animals with high antibody levels grew faster after weaning and had “no tendency for increased breech-soiling.” [1] Moreover, it appears that such protective immunity is moderately heritable.
The immunity indicated by the CARLA Test® partially reflects acquired immunity. This means that animals need to be exposed to the parasites before they can develop the immunity, which develops after repeated exposure. Consequently, young animals have lower levels of antibodies compared to yearlings and older individuals.
Immunity is complicated. Antibody levels can fluctuate over the year depending on exposure to GIN. This isn’t a problem in New Zealand where sheep are on pasture year-round. However, levels of parasite exposure and antibody levels drop in flocks that are housed indoors in the winter. Results from a French study of GIN resistance in cattle suggests that the CARLA Test® is most effective at the end of the first grazing season compared to earlier in a calf’s life or after animals were housed over winter.[2]
Dr. Emma Borkowski and other members of an Organic Science Cluster 4 research team at the University of Guelph studied the effectiveness of the CARLA Test® on sheep housed indoors in the winter. Their research builds upon the work of Dr. Andrew Peregrine, who researched parasite loads in Canadian sheep flocks in the first Organic Science Cluster and remains involved in the research.
Dr. Borkowski’s team found that even though antibody levels drop during the winter, sheep maintained detectable antibody levels even when kept off pasture (i.e., not exposed to infective GIN larvae) for more than five months. The researchers concluded that assessing salivary CarLA levels at the end of the first grazing season appears to be a promising tool to identify sheep with greater GIN immunity.[3]
This is significant news for sheep farmers. In the fall, when farmers are deciding which ewe lambs to keep as replacement ewes, they can sample saliva using cotton swabs on the inside cheeks of the animals. They would send the chilled samples to a lab for testing. The CARLA Test® results could be an invaluable tool to help in selecting breeding stock.
The CARLA Test® is now commercially available in New Zealand for sheep and deer, but it is prohibitively expensive for Canadian farmers given they would need to send samples on dry ice all the way to New Zealand. The test might be available in Canada soon, at least for sheep, which will dramatically reduce shipping costs.
Dr. Borkowski’s research team is now exploring the potential for using the CARLA Test® in goats. Goats tend to have weaker immune systems than sheep. Consequently, it might take longer for goats to build up sufficient immunity to the nematodes for the test results to be meaningful.
Preliminary results suggest that the CARLA Test® is:
- an effective way to assess immunity to gastrointestinal nematodes in goats.
- most effective in goats that are at least a year old (in contrast, the test is effective in sheep in the fall of their first year).
The research on goats is in its early stages. Data from the next few years will enable farmers to learn how to best use the CARLA Test® on goats. By that time, it’s possible that the test will be available for Canadian farmers (at least for sheep), and data from this research will pave the way for expanding availability to Canadian goats.
Canadian Organic Standards on using parasiticides for ruminants
6.6.11 Organic livestock operations shall have a comprehensive plan to minimize parasite problems. The plan shall include preventative measures, such as genetic selection, pasture management, fecal monitoring and assessments of tissue at slaughter, and emergency measures in the event of a parasite outbreak. Hygienic cleaning and disinfection methods for barns, such as power washing, steam washing, floor burning and lime washing, shall be included in the plan as well as down time (i.e., when the barn is vacant). By way of an exception, if preventative measures fail due to, for example, climatic conditions or other uncontrollable factors, the operator may use parasiticides that are not listed in Table 5.3 of CAN/CGSB-32.311, provided that:
- a) observation of the animal, fecal test results, or assessment of tissue as appropriate for the species indicate that livestock is infected with parasites;
- b) the operator provides a written action plan, with a timeline, describing how they will amend their parasite control plan to avoid similar emergencies;
- c) the operator has written instructions from a veterinarian indicating the product and method to be used, including provisions to avoid developing parasite resistance, such as rotation of parasiticides;
- d) withdrawal times are twice the label requirement or 14 days, whichever is longer;
If these conditions are met, the following restrictions apply:
- e) the exception cannot be granted for a group of animals or an entire production unit for more than two years in a row for the same problem;
- f) a dam from any species may receive only one treatment of parasiticides during gestation;
- g) meat animals from any species less than 12 months old shall receive at most one parasiticide treatment. Meat animals 12 months of age or older that receive more than two parasiticide treatments in their lifespan shall lose their organic status;
- h) dairy animals that receive more than two treatments in a 12-month period, whether of parasiticides, antibiotics or one of each, shall lose their organic status and go through a 12-month transition period;
- i) dairy cull animals that receive more than two treatments with parasiticides over their lifespan shall never be considered organic for meat;….
6.6.12 Poultry or breeding livestock treated with a parasiticide or veterinary drug not listed in Table 5.3 of CAN/CGSB-32.311 shall be considered non-organic meat animals. Exceptions pertaining to parasiticide use may apply (see 6.6.11).
References
[1] https://pubmed.ncbi.nlm.nih.gov/22153121/
[2] https://www.sciencedirect.com/science/article/abs/pii/S0304401717302625
[3] https://www.sciencedirect.com/science/article/abs/pii/S0304401720301631
Published January 21, 2025
Organic Science Cluster 4 is an industry-led research and development endeavour co-managed by the Organic Federation of Canada and the Organic Agriculture Centre of Canada at Dalhousie University and supported by the AgriScience Program under Agriculture and Agri-Food Canada’s Sustainable Canadian Agricultural Partnership together with over 80 funding partners.
