How do you know if your cattle have parasites?

Tim Nickel - Bovine Technical Services Veterinarian, Boehringer Ingelheim Animal Health Canada Inc.

Article Summary:

• It would be fair to assume a Canadian cattle herd has worms rather than assume they are worm-free. Young animals tend to have higher burdens, as older animals have acquired some natural immunity to the worms over several grazing seasons.
• The Fecal Egg Count (FEC) is the only practical test available in Canada to diagnose internal parasites.
• FECs are better used to estimate the parasite load of a group of animals rather than assessing an individual animal’s worm burden. It is recommended that multiple animals in a group are sampled.

Parasites are organisms that live in or on an animal, at the expense of the host. External parasites are easier to diagnose than internal ones. Flies are readily visible, and others like lice and mites can result in signs that can be seen on the exterior of the animal. While they are visible to the naked eye, lice are much smaller than flies and it requires restraint of the animal and close examination under good lighting to see these tiny parasites. Mites, which are the parasites that cause mange, are microscopic. Skin scrapings of affected animals and exam under a microscope is necessary to make a diagnosis.

While these parasites can cause scratching and hair loss, they are not the only cause of these signs. Irritants in the animal’s environment, the cold and dry condition of the skin that occurs during winter, shedding of the winter haircoat, and allergies are some of the other possible causes. It is important to take the time to confirm the presence of parasites rather than making an assumption, especially with regards to lice. If they are causing a problem, one should be able to find them at relatively high numbers on an animal.

Internal parasites are more challenging to diagnose since they cannot be seen. Gastrointestinal worms or nematodes (GIN) are very common in Canadian cattle herds. In fact, a recent study in Western Canada¹ showed that 100% of the farms that were enrolled were positive for worms.

Based on this data it would be fair to assume a Canadian cattle herd has worms rather than assume they are worm-free. Young animals tend to have higher burdens, as older animals have acquired some natural immunity to the worms over several grazing seasons.

The Fecal Egg Count (FEC) is the only practical test available to diagnose worms in cattle. Female worms inside the digestive tract of the animal lay eggs, which are passed in the feces. It is these eggs that are counted in a fecal sample using a microscope. It sounds simple, but the reality is more complex.

There are several factors that may result in a zero count on an FEC, even when worms are present.

• If worms are immature, they will not pass any eggs.
• Eggs are not uniformly distributed in feces, and sometimes the sample may come from an area of low egg density or none at all.
• At low parasite loads, typically seen in Canadian cattle, the concentration of eggs in a sample may be too low to detect.

As a result, it is possible to have zero eggs on a FEC, and yet an animal could still have worms.

Because of these limitations, FECs are better used to estimate the parasite load of a group of animals rather than assessing an individual animal’s worm burden. It is recommended that multiple animals in a group are sampled.

Another limitation of FECs is the inability to identify different species of worms, with a few exceptions, as cattle usually are infested by a mixed population of worms. For many of the common species that affect cattle, the eggs look similar, and it is impossible to differentiate on a routine FEC.

Some species have bigger impacts than others, and specialized tests are needed to tell them apart. Unfortunately, these tests are not readily available. This limits the useful information that a FEC can provide.

Despite these issues, FECs are the only practical test available in Canada to diagnose internal parasites. Understanding the limitations to this test is important when determining treatment options. For example, a frequently asked question is at what threshold should one consider treating animals.

Given the limitations discussed, the FEC is useful to confirm if worms are present, but not reliable enough to determine if a threshold has been reached where animal health and performance may be impacted.

It is best to discuss with your veterinarian regarding your specific situation and risk factors in order to develop a parasite control program that is tailored to your herd.