Parasite control and anthelmintic resistance
Livestock parasites, the barber's pole worm, anthelmintic classes, how resistance develops, refugia, FAMACHA, integrated control, and an egg count sum.
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Worms, flukes, ticks, mites and flies are among the most costly constraints on livestock production worldwide, especially for grazing sheep and goats in warm, humid regions. For decades they were controlled mainly with cheap, effective drugs given to every animal several times a year. That approach bred anthelmintic resistance: in many sheep and goat flocks around the world, some or all of the main worming drugs no longer work. Modern parasite control combines drugs with grazing management, breeding and monitoring, to keep animals productive while preserving the drugs that still work.
Parasites, anthelmintics and resistance management are core topics in veterinary parasitology and livestock management.
Main parasites of livestock
Internal parasites (endoparasites)
- Gastrointestinal nematodes (roundworms): Haemonchus contortus (barber's pole worm), Teladorsagia (Ostertagia), Trichostrongylus, Cooperia and Nematodirus in ruminants; Ascaris in pigs; Parascaris in horses.
- Lungworms (Dictyocaulus).
- Liver flukes (Fasciola hepatica and F. gigantica), with snails as intermediate hosts.
- Tapeworms and protozoa such as coccidia.
External parasites (ectoparasites)
- Ticks, which also transmit East Coast fever (theileriosis), babesiosis, anaplasmosis and heartwater.
- Mites (mange, scab), lice, flies (blowfly strike, nuisance and biting flies, tsetse).
The barber's pole worm
Haemonchus contortus sucks blood in the abomasum of sheep and goats. Heavy infections cause anaemia (pale gums and eyelids), bottle jaw (fluid under the jaw), weakness and death, often without diarrhoea. Each female can lay thousands of eggs a day, so pasture contamination builds up fast in warm, wet weather.
Diagnosis
Parasite problems are confirmed with faecal egg counts (such as the McMaster technique), larval culture to identify which worm species are present, post-mortem worm counts, blood tests for anaemia, and inspection of animals for ticks, mites and lice. Monitoring a sample of animals in each group every few weeks in the risky season shows when treatment is needed.
Anthelmintic classes
| Group | Examples | Notes |
|---|---|---|
| Benzimidazoles ("white" drenches) | albendazole, fenbendazole, oxfendazole | broad spectrum; some act on flukes; resistance very common |
| Imidazothiazoles and tetrahydropyrimidines ("clear" or "yellow") | levamisole, morantel, pyrantel | nicotinic agonists |
| Macrocyclic lactones | ivermectin, doramectin, moxidectin | also kill many external parasites |
| Amino-acetonitrile derivatives | monepantel | introduced around 2009 |
| Spiroindoles | derquantel (with abamectin) | newer combination |
| Flukicides | triclabendazole, closantel, oxyclozanide | act on liver flukes |
How resistance develops
Every treatment kills susceptible worms but leaves any resistant survivors to breed. Resistance builds faster when:
- animals are treated often;
- all animals are treated at once, especially before moving them to clean pasture, so the next generation on that pasture comes almost entirely from resistant survivors;
- under-dosing occurs through guessing weight or faulty dosing guns;
- the same drug class is used repeatedly.
Refugia: the key idea
Refugia are the worms not exposed to treatment: those in untreated animals and on the pasture. Their susceptible genes dilute the resistant survivors. Keeping refugia, for example by leaving some healthy animals untreated, slows resistance dramatically.
Integrated parasite control
- Targeted selective treatment: treat only animals that need it, using the FAMACHA eye-colour chart for Haemonchus anaemia, body condition, dag scores or production records.
- Faecal egg counts to decide when to treat and to test whether drugs work.
- Grazing management: rotation, rest periods, mixed or alternate grazing with cattle or horses, avoiding grazing very short grass, and keeping young animals on safer pasture.
- Breeding for resistance: some breeds (such as Red Maasai sheep and St Croix hair sheep) and some individuals within breeds resist worms better.
- Good nutrition, especially protein, which improves resilience and immunity.
- Combination drenches of different classes, used carefully.
- Correct dosing: weigh animals, dose for the heaviest in the group, and calibrate equipment.
- Quarantine drenching of purchased animals to avoid importing resistant worms.
- For ticks: integrated strategies with acaricides, resistant cattle breeds, pasture management, and vaccines where available.
Testing whether a drench works
A faecal egg count reduction test (FECRT) compares egg counts before and after treatment, usually 10 to 14 days later.
A group of goats has a mean count of 1,200 eggs per gram (epg) before treatment with a benzimidazole, and 240 epg after.
- Reduction = (1,200 − 240) ÷ 1,200 × 100 = 80 per cent.
A reduction below about 95 per cent (with the lower confidence limit below about 90 per cent) indicates resistance under widely used guidelines. This drug is no longer effective on this farm, and the farmer should switch to a different, tested class.
Dosing. Goats metabolise many anthelmintics faster than sheep, so they often need higher doses than sheep labels state; follow veterinary advice for each species. Dosing a group by the average weight of 35 kg when the heaviest goat weighs 50 kg under-doses the heavier animals by up to 30 per cent, selecting for resistance; dose for the heaviest animals, or weigh and group them.
Key points
- Major internal parasites: gastrointestinal nematodes (notably Haemonchus), lungworms, liver flukes, tapeworms and coccidia; external: ticks, mites, lice and flies.
- Anthelmintic classes: benzimidazoles, imidazothiazoles, macrocyclic lactones, monepantel, derquantel; flukicides.
- Resistance grows with frequent, whole-flock treatment, under-dosing and treating before moving to clean pasture.
- Refugia of untreated worms slow resistance; targeted selective treatment using FAMACHA and egg counts preserves them.
- Integrate grazing management, breeding, nutrition, correct dosing and quarantine.
- FECRT: reduction below about 95 per cent suggests resistance.
What the exams ask
- School-leaving and entrance papers: common parasites of livestock and their control.
- Degree and research-fellowship entrances: life cycles, pathogenesis of haemonchosis and fasciolosis, anthelmintic classes, mechanisms and detection of resistance, and refugia.
- Veterinary and professional examinations: designing parasite control programmes, FECRT interpretation, tick control and tick-borne diseases.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Should I deworm all my animals every month?
No. Frequent whole-flock treatment rapidly breeds resistance. Treat animals that need it, based on signs, FAMACHA scores and egg counts, and combine with grazing management.
What is FAMACHA?
A colour chart, developed in South Africa, for scoring the colour of the inner eyelid of sheep and goats. Paler colours indicate anaemia from Haemonchus, so only those animals need treatment.
Are natural dewormers effective?
Some plants rich in condensed tannins, such as sericea lespedeza, can reduce worm burdens. They help as part of integrated control but rarely replace effective drugs entirely.
Sources
- Kaplan, R. M. and Vidyashankar, A. N. "An inconvenient truth: global worming and anthelmintic resistance." Veterinary Parasitology 186, 70–78 (2012).
- Van Wyk, J. A. and Bath, G. F. "The FAMACHA system for managing haemonchosis in sheep and goats by clinically identifying individual animals for treatment." Veterinary Research 33, 509–529 (2002).
- Coles, G. C. et al. "World Association for the Advancement of Veterinary Parasitology (WAAVP) methods for the detection of anthelmintic resistance in nematodes of veterinary importance." Veterinary Parasitology 136, 167–185 (2006).
- Taylor, M. A., Coop, R. L. and Wall, R. L. Veterinary Parasitology, 4th edition. Wiley-Blackwell, 2016.
- Waller, P. J. "Sustainable nematode parasite control strategies for ruminant livestock by grazing management and biological control." Animal Feed Science and Technology 126, 277–289 (2006).
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