Does My Dewormer Still Work?

Many sheep and goat producers struggle with internal parasites in their flock or herd each year. One of the questions they often ask themselves is Does this dewormer still work? Producers can perform a Fecal Egg Count Reduction Test to check product effectiveness.

The Fecal Egg Count Reduction Test, FECRT, requires two fecal egg counts. The first fecal sample should be taken before an animal receives treatment, and then the second fecal sample should be taken 10 to 14 days later. Producers should always first identify animals exhibiting symptoms of parasite infections before treating them. This can be accomplished by using a five-point check system.

This system starts with evaluating anemia based on a FAMACHA score. Keep in mind that this score only assesses anemia, most likely caused by the Haemonchus contortus worm, also called the barber pole worm. Not all internal parasites cause anemia; hence, it is essential to consider other points.

Parasite infections can also cause swelling under the jaw, often called bottle jaw. This occurs when fluids accumulate under the jaw due to fluid imbalances related to anemia. While the most likely cause would be barber pole worm, this condition can also result from malnutrition.

Another area producers should assess is body condition. Animals with internal parasite infections often lose weight due to reduced appetite or issues with decreased nutrient absorption. Parasites can damage the stomach and intestinal lining, causing a reduced ability to absorb nutrients. This damage can be permanent, so it is essential to prevent it. All types of parasites can cause a loss in body condition.

Another point for producers to check is called a Dag Score. Producers can Dag Score animals by evaluating the area around and under the tail for diarrhea. Only certain types of parasites cause diarrhea. The brown stomach worm, the bankrupt worm, and coccidia cause diarrhea.

The last check area for sheep producers is evaluating the nose for any nasal discharge. Nasal bots cause this issue. However, goats are not affected by nasal bots. Therefore, goat producers should consider the condition of the haircoat. Animals with a rough, dull coat may have issues with internal parasites or could be experiencing problems with external parasites.

Once you identify animals that require treatment, collect samples from 15-20 animals in a similar group. For example, samples can be collected from weaned lambs or kids. Or, you could sample yearlings or mature females as a group. Be sure to record the animal identification for each sample. Treat these animals with a deworming product and then collect a second sample from each of these animals 10 to 14 days later.

Your veterinarian should be able to perform the fecal egg count for you. Or, if you have had sufficient training, you can perform the fecal egg counting procedure yourself. Ideally, the number of eggs per gram (EPG) in the second fecal sample compared to the first will decrease by at least 90% for the flock or herd. If the reduction is less than 90%, the flock or herd has developed internal parasites with resistance to the product you treated with.

To calculate the fecal egg count reduction (FECR) percentage, subtract the number of eggs per gram after treatment from the number of eggs per gram before treatment, divide that number by the number of eggs pretreatment, and multiply by 100.

(Pre-treatment EPG – post-treatment EPG) ÷ pre-treatment EPG * 100 = FECR %

The calculations should be averaged across the animals sampled to arrive at a FECR% for the flock or herd. For example, if the average number of EPG before treatment was 500 and the average number of EPG after treatment was 50, the FECR would be 90%. This indicates that the deworming product is still working well.

However, if, for instance, the pre-treatment average EPG was 500 and the post-treatment EPG was 200, this would only be a 60% FECR and would indicate that parasites within that flock or herd are resistant to the deworming product.

If a producer encounters a product that no longer works sufficiently, the same process could be repeated with a product from a different anthelmintic class. Currently, producers in the United States can choose from three different classes of products: benzimidazoles (such as Safe-Guard or Valbazen), macrocyclic lactones (such as Ivomec or Cydectin), and imidazothiazoles (such as Prohibit).

All producers should work with their local veterinarian to determine the best protocol for managing internal parasites in their flock or herd. Producers should also follow good prevention practices such as grazing forages no lower than four inches in height, rotating pastures, providing good nutrition, and handling animals quietly in a low-stress manner. Treating only animals that exhibit parasite infection symptoms will also help preserve dewormer effectiveness.

This article was first printed in Lancaster Farming.