Multiple anthelmintic resistance is a significant threat to the small ruminant industry worldwide.
This resistance is defined as the heritable ability of a parasite to tolerate a normally effective dose of anthelmintic, a deworming drug used to treat infections caused by parasitic worms.
According to Paris-based veterinary scientist and author David Bartram, anthelmintic resistance in ovine gastro-intestinal parasites affects the health and productivity of sheep globally.
Bartram, last year’s recipient of the British Veterinary Association’s Chiron Award for outstanding contributions to the profession, is the European marketing director for ruminant business at
global animal health company, Zoetis. He was in South Africa recently to address the Cape Wools 9th World Merino Conference in Stellenbosch.
Speaking on anthelmintic resistance management, he said: “Production losses occur even when the parasitism caused by anthelmintic resistance is subclinical.”
Bartram explained that anthelmintic resistance develops through the selection of genetic material that passes on resistance to parasite populations.
Parasites that survive the anthelmintic pass the genetic resistance on to subsequent generations, if selection is maintained.
The first reports of anthelmintic resistance date back to 1964, when evidence of resistance to the benzimidazole (1-BZ) anthelmintic group was recorded in UK sheep.
There are now six main groups of anthelmintics available, each using different active chemical compounds. Cases of resistance to certain dewormers from five of these groups have been documented. Reports of anthelmintic resistance to multiple drugs in individual parasite species are also increasingly common.
Although resistant worms do not cause more serious disease than susceptible worms, anthelmintic resistance makes it more difficult and more expensive to prevent production losses through parasitic infestation, he said.
Bartram suggested these guidelines for sustainable anthelmintic use:
- Farmers must plan an annual dosing strategy with the help of their medicine prescriber. Parasite control must be tailored to specific farms.
- Farmers should avoid implementing ‘blueprints’ for parasite control and use dewormers only when necessary.
- Dewormers should be administered at the recommended dose.
- Animals must be weighed to determine the dosage.
- Underdosing causes a build-up of parasite resistance.
- Dosing equipment must be calibrated and the correct dosing technique used. The drenching gun nozzle must be placed as far back in the mouth as possible so that the administered dose is swallowed.
- Products must be stored correctly and used before the expiry date. Dewormers must not be mixed with other drugs.
The role of Refugia
Bartram stressed the importance of refugia in the development of anthelmintic resistance. Refugia refers to an area in which a population of organisms, parasites in this instance, can survive despite unfavourable conditions.
For example, during the larval stage (a free-living stage), parasites survive outside host animals on pastures. Therefore, during this stage, they should not be exposed to the dewormers administered to livestock.
Parasites in ‘refugia’ are the proportion of a parasite population not exposed to dewormers during a treatment, and therefore not developing resistance to the drugs.
“To delay the development of anthelmintic resistance, it’s important to ensure that some susceptible worms survive in refugia, because treatments carried out when there are few worms in refugia are likely to select strongly for resistance,” said Bartram.
To ensure that some susceptible worms survive, animals should not be moved to clean pastures immediately after dosing – this will ensure that livestock continue to be exposed to non-resistant parasites in refugia, after dosing.
“After dosing, return animals to the same pasture for a few days, then move them to the cleanest, most high-quality pastures available,” he explained.
Farmers must also avoid bringing animals carrying resistant worms onto the farm. When new sheep are brought onto the farm from another environment, they should first undergo anthelmintic treatment in quarantine, and then be isolated from the rest of the flock for 30 days to prevent the introduction of drug-resistant worms.
In situations of low refugia, a portion of the ?ock should be left untreated. One of the best ways to decide which animals should be chosen for this, is targeted selective treatment, in which the farmer identifies individual animals that require treatment on the basis of their relative performance against the rest of the flock.
“Once identified, treatment is only given to those individuals not performing as expected, while animals that are meeting or exceeding their expected rate of live-weight gain, are left untreated,” Bartram explained.
Targeted treatment can also be carried out by using faecal egg count as a guide. Another option is the FAMACHA system. This treatment is based on the fact that small ruminants suffering from infestation or a disease caused by nematode worms show varying degrees of anaemia. This can be clinically evaluated by examining the mucous membranes of the eye. With the help of a colour chart, animals are scored in five colour categories, ranging from red (indicating the absence of anaemia) to very pale (indicating severe anaemia and probable haemonchosis). Only animals that need treatment are dosed.
The advantages of a targeted treatment system are savings in anthelmintic use, slowing down the development of drug resistance, and identifying animals that repeatedly require treatment and should be culled.
Bartram said that once resistance to a particular anthelmintic has emerged in a flock or herd, worms can no longer be effectively controlled using that anthelmintic class. However, resistant parasites can still be adequately controlled by using an alternative anthelmintic or a combination anthelmintic.
Combination anthelmintics or multiple active anthelmintic dewormers (that combine different chemical anthelmintic classes) can be used to control worms where there is single or multiple drug resistance. This also delays the development of resistance to different anthelmintic classes (see box on page 2).
Studies show that combinations provide more sustainable control of sheep nematodes than when used separately. A paper published by Bartram in 2012, stated that “combinations slow the development of a resistant parasite population by reducing the number of resistant genotypes which survive treatment, because multiple alleles conferring resistance to all the component anthelmintic classes must be present in the same parasite for survival”.
It further explained that parasites carrying multiple resistance alleles are rarer than those carrying single resistance alleles.
“Thus, treatment with a combination anthelmintic will be more effective and lead to greater dilution of resistant genotypes by the unselected parasites in refugia, reducing the proportion of resistant parasites available to mate with other resistant adults that have survived treatment.”
However, Bartram emphasised that the use of multiple-active anthelmintics in resistance management is not a magic bullet and should be used in combination with other sustainable anthelmintic practices.
For more information, email David Bartram at [email protected].