Anthelmintic Resistance – A Serious Set Back To Small Ruminant Production
Anthelmintics, or deworming medications, play a crucial role in maintaining the health and productivity of small ruminants, including sheep and goats. However, the emergence of anthelmintic resistance has become a significant concern in the livestock industry worldwide. This phenomenon poses a serious setback to small ruminant production, as it challenges the effectiveness of traditional deworming practices and necessitates a reevaluation of parasite control strategies. In this article, we explore the causes, consequences, and potential solutions to anthelmintic resistance in small ruminants.
“Anthelmintics are used traditionally as an integral part of helminthic control strategies for grazing livestock to prevent production losses from parasitic infections. The continuous and indiscriminate use of the same anthelmintics over years together as the sole means of control are now failing due to the emergence of resistance strains of helminths. Resistance to the commonly used anthelmintics in gastrointestinal nematodes of sheep has become an increasingly wide spread problem throughout the world. Diagnosis of Anthelmintic resistance can be diagnosed by the history and a failure to respond to treatment, with confirmatory testing such as an on-farm trial using a faecal worm egg count reduction test (WECRT). This paper summarizes the ways and means to avert and delay this anthelmintic resistance.”
In India, sheep and goat production plays a vital role in augmenting socio – economic status of the rural masses, particularly the small land holders and landless farmers, who rely on these animals for their animal protein source and income for their livelihood (Lateef, 2003). Sheep and goats are susceptible to many dreadful diseases causing heavy economic losses in terms of reduction in weight gain, delay in maturity, reduction in lambing percentage, poor hide quality and mortality in extreme cases. Among the diseases of small ruminants, gastrointestinal helminthic problems always come to the fore because majority of species reach their host directly through vegetation, without involvement of intermediate host. This factor, together with ideal climatic conditions for survivability of pre parasitic stage, favour high prevalence of gastrointestinal nematodes in tropical countries and hence, they are recognized as a major constraint to livestock production throughout the tropics and elsewhere (Githiori et al., 2004). Direct and indirect losses due to nematode infections are estimated to be high and control of these parasites is therefore considered important. Winrock International (1992) indicated that over $4 billion loss in animal productivity as a result of animal diseases and half of this loss was due to internal parasites such as helminths. Effective parasite control has become heavily dependent on anthelmintics. The compulsory and often excessive use of these anthelmintics in combination with poor managemental practices has resulted in resistance to most of the available anthelmintics. The development of anthelmintic resistance by roundworms to these anthelmintics, poses a potential crisis for sheep producers and measures to avert and delay this are highly essential.
What is resistance?
Resistance (AR) is the heritable change in the ability of some nematode parasites to survive treatment with a therapeutic dose of anthelmintic drugs. The genes responsible for resistance are present in many of the important pathogenic round worms of animals. The first report of anthelmintic resistance has been described in 1957 against phenothiazine. Since then, it has become a world wide problem especially in sheep and has been documented to almost all classes of anthelmintics. The parasite once developed resistance to one anthelmintic, will also become resistant to other drugs which have similar mode of action (Side resistance). Some parasites develop resistance against different classes of anthelmintics at the same time (cross resistance).
Understanding Anthelmintic Resistance
- Definition
Anthelmintic resistance occurs when parasites develop the ability to survive exposure to doses of anthelmintic drugs that would normally be effective in eliminating them. This resistance can be specific to particular drug classes or may manifest as multidrug resistance.
- Causes
The primary drivers of anthelmintic resistance include:
- Overuse and Misuse: Frequent and indiscriminate use of anthelmintics without proper management practices can contribute to the selection of resistant parasites.
- Underdosing: Administering insufficient doses of anthelmintics may fail to eliminate all parasites, allowing resistant individuals to survive and reproduce.
- Genetic Predisposition: Some parasites naturally possess genetic traits that confer resistance, and these traits can be passed down to future generations.
Consequences of Anthelmintic Resistance
- Reduced Efficacy of Treatment
As resistance develops, the efficacy of anthelmintic treatments diminishes. Infestations persist, leading to suboptimal growth rates, decreased reproductive performance, and overall poor health in small ruminant populations.
- Economic Impact
Anthelmintic resistance has economic implications for small ruminant producers. Increased treatment costs, decreased productivity, and potential losses due to parasitic infections can significantly impact the profitability of farming operations.
- Animal Welfare Concerns
Resistant parasites can cause chronic infections, leading to prolonged suffering in affected animals. This compromises the welfare of small ruminants, as they experience the negative effects of parasitism without effective treatment options.
Management and Prevention Strategies
- Integrated Parasite Management (IPM)
Implementing an IPM approach involves combining various strategies to control parasites, reducing reliance solely on anthelmintics. This includes pasture management, genetic selection for resistance, and targeted treatment based on diagnostic assessments.
- Fecal Egg Count Monitoring
Regular monitoring of fecal egg counts helps assess the level of parasite burden in small ruminant populations. This information guides targeted deworming efforts, reducing the risk of unnecessary anthelmintic use.
- Rotational Grazing
Rotational grazing systems can help break the parasite life cycle by allowing pastures to rest between grazing cycles. This reduces the exposure of small ruminants to infective parasite larvae.
- Selective Breeding for Resistance
Genetic selection for resistance to internal parasites is a long-term strategy to enhance the natural ability of small ruminants to resist infections. Breeding programs can focus on identifying and promoting animals with inherent resistance traits.
- Diversification of Anthelmintics
Where necessary, rotating between different classes of anthelmintics can slow the development of resistance. However, this should be part of a comprehensive parasite control strategy.
How resistance occurs?
Ø Indiscriminate use of different classes of anthelmintics at short intervals. Ø Prolonged use of a single class of anthelmintic drug. Ø Improper calculation of dose of anthelmintics leading to under or excess dosing. Ø Improper dosing due to spillage or faulty equipments. Ø Drenching of anthelmintics without knowing the type of worms present in animals
How can diagnose the resistance?
Anthelmintic resistance can be suspected when there is poor response to anthelmintic treatment. A number of in vitro and in vivo tests can be used for detecting the resistance in gastrointestinal nematodes. Egg hatch assay, larval migration inhibition assay and larval development test are the most commonly used in vitro tests. These assays involve examining the development of eggs and larvae, in various concentrations of the anthelmintic. The larval development test can be used to detect resistance to both benzimidazole and levamisole. The in vivo technique, faecal egg count reduction test (FECRT) is the most practical method of determining resistance by nematodes in sheep and other animals. It is a simple and field friendly test that does not require highly trained personnel, extensive resources, sophisticated equipment or facilities. It can be applied to detect the development of resistance to all class of anthelmintics and in all species of animals. The susceptibility or resistance of worms to anthelmintics is determined by comparing the faecal egg output (epg) in animals prior to and 15 days after anthelmintic medication. When there is reduction of egg per gram (epg) by more than 95 per cent after 15 days of treatment, the drug is susceptible. A reduction of epg less than 90 per cent is indicative of resistance. If the anthelmintic kills 90 percent or more of the worm eggs, it is considered to be effective. If it kills 60 to 90 percent of worm eggs, it is considered to have a moderate level of resistance. Anthelmintics reducing less than 60 percent of worm eggs are considered to as severe resistance (Coles et al. 1992).
Current and future strategies for combating anthelmintic resistance
The challenge to veterinarians and producers is to utilize known and emerging technologies to control anthelmintic resistance of gastrointestinal nematodes of livestock especially small ruminants such as sheep and goats. The below mentioned are some of the techniques commonly employed to control or delay the development of resistance. Ø Use the most appropriate anthelmintic of reputed companies. Ø Check the drenching gun is delivering the correct volume or dose of drug. Ø Animals should be weighed or measured with a tape to determine the proper dosage since under dosing is a leading cause of development of anthelmintic resistance. When deworming a group of animals, the dose should be set for the heaviest animals in the group, not the average. Ø Anthelmintic treatments should be targeted to the most susceptible animals in the herd such as lambs/kids, lactating ewes/does, and high producers. Leaving some animals untreated will help to slow anthelmintic resistance by maintaining anthelmintic susceptible population of worms (Refugia – not exposed to anthelmintics to reduce the intensity of selection for drench resistance in environments where there is a high risk (Van Whk, 2001). Ø Frequent deworming is costly. It accelerates the development of anthelmintic resistant worms and leads to a false sense of security, which may result in unnecessary production losses and animal deaths. Ø Double the cattle/sheep dose when deworming goats for all dewormers, except levamisole which should be dosed at 1.5 times the cattle/sheep dose in goats since goats metabolize anthelmintics differently (it clears their system faster) than sheep and cattle and require higher doses. Ø Maintain the animal on dry fodder or fasting prior to dosing. Ø Anthelmintics should be administered orally, over the tongue of the animal. Research has shown that benzimidazoles are more effective when the animals are fasted 12 to 24 hours before treatment or when two treatments are given 12 hours apart (repeating the drench 12 hrs after the first dose). Ø Anthelmintics should not be used indiscriminately. Ø Avoid using the same class of anthelmintics years together; rotate the anthelmintics annually with different mode of action Ø Test for anthelmintic resistance regularly and also after deworming by monitoring the egg count Ø Avoid the introduction of resistance onto a farm by treating purchased stock on arrival followed by a quarantine period. Ø Provide safe pasture for grazing animals. Ø Minimize the stocking rate of animals in farm/pasture land.
Conclusion
Anthelmintic resistance poses a significant threat to small ruminant production, impacting the health, productivity, and economic viability of farming operations. To address this challenge, a holistic and integrated approach is essential. Small ruminant producers must adopt sustainable parasite control practices, including targeted anthelmintic use, pasture management, genetic selection, and ongoing monitoring. By embracing these strategies, farmers can contribute to the preservation of anthelmintic efficacy and the long-term health and sustainability of small ruminant production systems.
Compiled & Shared by- This paper is a compilation of groupwork provided by the
Team, LITD (Livestock Institute of Training & Development)
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Reference-On Request.
