Incorporating strategies

To help slow the problem of parasite resistance to deworming products, the American Consortium for Small Ruminant Parasite Control (ACSRPC) developed the Sustainable Integrated Parasite Management (SIPM) toolbox. This management combination helps reduce drug use and slows the rate of drug resistance in small ruminants thorough a chemical and non-chemical approach.

“Targeted selective treatment is deworming only animals that require treatment,” said Dr. Dahlia O’Brien, Virginia State University. “It uses two tools: FAMACHA and the five-point check which includes FAMACHA. This helps decrease the number of animals dewormed.”

The technique slows resistance by increasing refugia – the number of worms left in refuge from the drug. If all the grazing animals have both susceptible and resistant worms in their digestive system, and all animals are dewormed based on weight with the proper dosage and drugs are highly effective, all the susceptible worms would die. What’s left are resistant worms, so if ineffective drugs are used, the resistance problem becomes worse.

“If we deworm based on clinical symptoms and FAMACHA scores, we’re leaving some of those worms in refuge from the drug,” said O’Brien. “Instead of pastures being populated with only resistant worms, pastures are populated with both susceptible and resistant worms. The susceptible worms dilute the population of resistant worms and ultimately decrease dewormer use.”

The FAMACHA eye score was developed in response to severe anthelmintic resistance. FAMACHA assesses the level of Haemonchus contortus (barber pole worm) infection in sheep, goats and camelids. The score assesses the level of anemia, which is the main symptom of barber pole worm infection.

The five-point check addresses the limitations of FAMACHA and considers clinical signs such as bottle jaw, body condition score, dag (manure in hind end wool) score and nasal discharge (nose bots) in addition to FAMACHA.

Fecal egg counts (FECs) should not be used to determine individual animals’ need for deworming, but are useful for monitoring the rate of pasture contamination. O’Brien suggested taking fecal samples from pastures for FECs, then using the information for level of pasture contamination and resistance testing.

Some aspects of deworming require judgement by the flock or herd owner. If an animal with a FEC of 2,000 eggs/gram of feces (egf) is in a group of open females that are not susceptible, deworming may not be necessary. If an animal’s FEC is above 4,000 and it lives in a pasture or pen with more susceptible animals (those in late pregnancy or lactating or young animals), it’s probably worth treating.

“Not every animal with a high FEC will have clinical signs,” said O’Brien. “An animal can be highly resilient and have a high FEC, but the parasite burden is not affecting it. However, by dropping parasite eggs on pasture, larvae are now present to potentially infect other susceptible animals.”

When a flock or herd owner identifies animals that need to be dewormed, it’s critical to administer the correct weight-based dose of an effective dewormer to ensure treatment is successful. If a scale is not available, use a weight tape on the heaviest animal and use that weight for all animals. Use caution in weight estimation for levamisole treatment because it’s easy to overdose with that drug.

O’Brien emphasized the importance of correct drenching for maximum efficacy. Use a dosing syringe with a long metal nozzle rather than a needleless plastic syringe. After filling the syringe with the correct dose, place the nozzle over the animal’s tongue toward the back of its mouth.

“We don’t want the drug to bypass the rumen,” said O’Brien. “We want the drug to go where the parasites are.”

Because most dewormers used alone are no longer effective, the recommended practice is to use drug combinations. “As the effectiveness of a dewormer decreases, it provides less and less benefit,” said O’Brien. “We now know that rotating dewormers does not prevent resistance from worsening and is no longer recommended.”

Rather than rotating products, select two or three dewormers from different classes and use them at the same time. Draw up the correct dose for the animal in separate syringes and administer one after the other.

“If we use a drug that is 80% effective, there are still 20% resistant parasites, so you are still repopulating pastures with the resistant worms,” O’Brien explained. “If we use two drugs that are 80% effective, we’ve brought up the effectiveness to 96%, and 99% with three drugs.”

The increased efficacy is due to the additive effect: one drug kills 80% of the worms, the next drug kills 80% of the remaining worms, and the next drug kills 80% more. By using only one drug that is 80% effective, a fairly large population of resistant worms will be left. With multiple dewormers, be sure to follow the longest withdrawal period of drugs in the combination.

Find Part 1, “Using Drugs Effectively,” here. Part 3 will explain incorporating non-chemical deworming options.

by Sally Colby