Treatment options for parasite control in cattle

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

Article Summary:

• Management practices – pasture management, stocking density having animals in good condition – can help reduce parasite populations in both the environment and the animal. However, treatment interventions may still be necessary.
• The goal of treatment is not to eliminate parasites altogether but rather to keep levels low enough to minimize their impacts. Elimination is rarely achievable or comes at too high of a cost.
• To get the most out of any treatment, be sure to use the right drug at the correct dose in the appropriate group of animals.
• Timing of treatment is critical – too early will result in little to no benefit and too late means the parasites have already had some impact on the animal.

Management practices can help reduce parasite loads in both the environment and the animal. However, it may be difficult to achieve adequate control of parasites through management alone. Animals will often require a form of treatment to help reduce the impacts on health and performance.

It is best to develop a treatment program in consultation with your veterinarian, who is familiar with the specific conditions on your farm. There are several things to consider:

1. Do all animals require treatment, or are there specific groups or individuals that can be targeted?

For lice, treatment of all the animals within a group provides the best control. Untreated individuals can reinfest the animals in a group once drug levels have declined in the treated group.

With gastrointestinal worms, targeting treatment of the younger animals may be more efficient. Yearlings (i.e. grass yearlings, heifer replacements, yearling bulls) are impacted the most as they have developed minimal immunity in comparison to adult animals. Calves born earlier in the year (January or February) may also be at risk. Mature animals have developed immunity over several grazing seasons and are generally less impacted by worms.

2. When is the best time for treatment?

This is probably one of the most important considerations. If treatment occurs too early, it could be wasted, but leaving it too late may result in the animal already being compromised. Any treatment decisions need to take into account the seasonality of the parasite you are dealing with and how long the treatment is effective for.

Flies are most active in mid to late summer. A treatment at spring turnout, while convenient, will have little to no impact on fly numbers in mid-summer, as drug levels will have declined by this time.

Lice do not become active until temperatures get colder. Treatment in early fall when weather is still mild may be ineffective, as the drug may be gone by the time the lice become active. Waiting until temperatures get cooler will make treatment more effective.

Gastrointestinal worms need to spend part of their lifecycle outside the host on pasture. Because of this, their biggest impacts will be seen later in the grazing season and into the fall when their numbers are greatest. Using a conventional product at turnout will have minimal impact, as drug levels will have disappeared by the time the worm populations build on pasture. While treatment in the fall is effective, animals may have already experienced some impacts during the grazing season. A treatment that is effective later in the grazing season would be more beneficial but is not always practical or easily done.

3. What drug or product should be used?

Some products are effective against both internal and external parasites (i.e. macrocyclic lactones like ivermectin or eprinomectin) while others may be more specific (i.e. benzimidazoles are only effective for internal parasites while pyrethrins are only effective for external parasites). Make sure to use the right product for the parasite(s) you are targeting.

The route of administration – applied topically, injected or administered orally - is another consideration.

How long the product is effective for should also be considered. Short-acting products (a day or two) may require more than one treatment. Intermediate (weeks) and long-acting (months) products may provide better control over time, but require longer withdrawal periods.

Common errors to avoid when treating animals for parasites

Use the correct dose, ideally based on the actual weight of the animal. Be as accurate as possible when estimating weights, and if using the same dose for a group of animals, base it on the heaviest animals, not the average weight of the group. This will avoid or reduce the number of animals that are under-dosed.

It is important to apply the treatment according to the label directions with pour-on products. Avoid over-spraying, where the pour-on product bounces off the back of the animal and on to the chute or ground. If the directions indicate to treat from the withers to the tailhead, do not apply in a smaller area or spot as this can affect the effectiveness of the product.

Be sure to administer injectable products in the appropriate location according to label directions (i.e. subcutaneously versus intramuscular). It is important to treat at the right time. If treatment occurs at the wrong time, it may be ineffective.

If there are concerns that a treatment has not been effective, talk to your veterinarian. They can help investigate what may have gone wrong.

Hair loss and itchiness in cattle in winter is often assumed to be due to lice. However, this can be due to other conditions. It is important to check animals to confirm lice are present.

If there are concerns regarding efficacy of deworming, your veterinarian may suggest doing a Fecal Egg Count Reduction Test (FECRT). This compares fecal egg counts (FECs) before and after treatment. It is important to remember some of the limitations of the FEC.

As always, talk with your veterinarian, who can advise on which treatments may be most suited to your farm and situation.