Internal parasites suppress appetite, reduce weight gain

The adage, “the more we look, the more we find,” is a good one when it comes to internal parasites.

The problem is that we have no idea internal worms are a problem in the herd unless horses have a tremendous burden of internal worms or we see tapeworm sections shed in the manure. Parasites decrease production subtly.

Cattle shed parasite eggs in lower numbers, so doing an accurate fecal count on several animals in the herd is the only way to determine the actual parasite burden.

Animal health technicians at veterinary clinics are highly trained in this procedure.

Producers can take several golf ball sized individual samples to their veterinarian for fecal analysis. Most clinics do the analysis in house, but they can also be sent to several good labs across Canada.

Parasites can be the root cause of many problems in cattle, but at first they might not be obvious. Cattle may not appear sick or in poor shape or with rough hair coats.

Small weight loss or gain is hard to determine just by looking at the animal. For example, can you see the extra 20 to 30 pounds that growth implants provide? I can’t.

Deworming can prevent a 20 lb. loss, which makes the extra revenue from that extra 20 lb. a wise investment.

Parasites also affect the immune system, which increases the susceptibility to respiratory disease, and hinders feed conversion as they rob cattle of needed nutrients.

Older trials in the United States found that reproductive performance in breeding heifers suffered.

You would think infected cattle would want to eat more because parasites rob them of nutrients.


However, heavily parasitized cattle don’t feel well and suffer from suppressed appetites, which worsens the problem of weight gain.

All species of animals are affected by parasites. Fortunately, I seldom see clinically affected cattle or camelids with internal parasites. The problem is usually subclinical, with production losses being the main consequence.

However, horses, goats and especially sheep and bison can be severely affected, sometimes fatally. Producers of any of these species should check them periodically.

Your veterinarian and technician will know what is significant to your operation with each species.

Ivermectin revolutionized the way producers treat parasites when it came on the market.

Western Canada had a special problem with lice. While previous medications controlled lice and warbles, they did nothing for internal parasites. The avermectin products that followed controlled all three.

These medications were expensive at first, but producers could see the difference in their cattle and em-braced them.

Ease of administration was paramount when they became a pour-on. Cattle could be treated quickly and conveniently, and twice yearly applications were often adopted when the price decreased.

The issue has changed. We became complacent and stopped checking manure samples for worms.

Over the past 25 years, the pour-on products have become less effective against internal parasites. They are still effective against lice but are becoming less so against internal worms, which are developing resistance. In some cases, other animals were licking off the product.


These medications don’t work nearly as well today as they did a few years ago. This was first noticed in the United States close to 10 years ago.

It was believed at the time that our cold Canadian winters killed lots of the parasites on pasture. That is not the case and producers in Eastern Canada, where problems have surfaced earlier, have developed different strategies for de-worming.

This often includes using pour-ons and another internal dewormer called fenbendazole, or Safeguard.

The product can be administered orally with a hook applicator, mixed in the feed over several days or scripted into the minerals over several days. Safeguard works by direct contact with parasites and kills them quickly. This combination will reduce external and internal parasites to zero.

Merck Animal Health is doing a study across Canada that samples cattle at processing with the endectocides and then again two weeks later to determine the effectiveness of the various treatment methods.

Effectiveness of the pour-on products varies from almost totally ineffective to close to 90 percent. Many products are 30 to 70 percent effective, which means many of the worms are left, reducing production and recontaminating pastures next spring.

The take home message is to collect manure samples and have them checked, even if you have applied an endectocide. Use the Safeguard for internal worms if worm burdens are significant.

Your veterinarian can devise a strategy next spring to decrease worm burdens over the summer and start to clean up the parasites on the pastures.

This may involve letting cattle pasture for a time and then deworming them through a prescribed mineral.


Roy Lewis has a veterinary practice in Westlock, Alta. and works part time as a technical services veterinarian with Merck Animal Health.