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Worms pose a significant threat to sheep, making effective control measures essential for flock health and productivity. Dr Takula Tshuma, a specialist veterinarian in bovine herd health and a senior lecturer at the University of Pretoria’s Faculty of Veterinary Science, emphasises the importance of worm control, especially during the warm summer months.
The warm and wet conditions favour the rapid development and survival of larvae on pastures, creating a high-risk environment for infections. Uncontrolled infections may lead to sheep with high worm counts struggling to utilise and fully benefit from the high-quality summer grazing, thus reducing feed efficiency. This results in diminished growth rates, poorer body condition, loss of fertility and lower milk production, which means farm income will also take a knock. Worms can also compromise immune function, making sheep more susceptible to secondary infections.
While producers need to be extra vigilant in summer, worm control should be maintained year-round. Neglecting to follow an adequate worm control programme may lead to:
- Decreased weight gain: High worm burdens slow down growth and extend the time to market.
- Poor reproductive performance: Worms can lower fertility and increase lambing intervals.
- Increased disease susceptibility: A weakened immune system makes sheep more susceptible to infections.
- Economic impact: Slower growth and more frequent veterinary interventions raise costs.
Types of worms
Dr Tshuma says sheep in South Africa are highly susceptible to several types of gastrointestinal worms due to warm, moist conditions promoting parasite survival during summer. Producers are likely to encounter gastrointestinal roundworms (nematodes).
These include:
- Haemonchus contortus (barber’s pole worm): This is one of the most dangerous worms for sheep in South Africa. This blood-sucking parasite attaches to the lining of the abomasum, causing severe anaemia, weakness, weight loss, and often sudden death. It can lead to bottle jaw, reduced productivity, and significant flock mortality. Warm, humid conditions promote rapid development, increasing the likelihood of infection.
- Trichostrongylus spp (black scour worm): These worms primarily affect the small intestine. Infections result in dark, watery diarrhoea, weight loss, and poor growth rates, particularly in young lambs, leading to rapid dehydration and loss in condition. Although they can occur year-round, infection peaks in late summer and early autumn after rain.
- Teladorsagia circumcincta (brown stomach worm): Common in temperate and subtropical regions of South Africa, this worm targets the stomach. It causes gastroenteritis, poor digestion, reduced feed efficiency, weight loss, and diarrhoea, particularly in young or immunocompromised sheep. Summer rains increase larval loads on pasture, making sheep more susceptible.
- Nematodirus spp: Typically associated with cooler weather, these worms can affect sheep in early summer. They primarily target young lambs, causing severe dehydration, diarrhoea, and potential death if untreated. They hamper nutrient absorption and lead to rapid deterioration. While larvae numbers peak in spring, effects can persist into summer if lambs are infected during late spring.
Feed intake
Dr Tshuma emphasises that diet plays a vital role in the occurrence of worms in sheep. Placing pellets in raised troughs reduces heavy worm burdens in sheep, while lambs grazing near the kraal are at higher risk. Sheep graze close to the ground, where larvae of gastrointestinal nematodes are concentrated in the few centimetres of pasture near the soil. This grazing behaviour increases the likelihood of sheep ingesting larvae. Therefore, preventing overgrazing and overstocking is vital, and rotational grazing is essential for minimising infection risk.
Diet has a significant influence on sheep’s immune responses to worm infections. Proper nutrition, particularly sufficient protein and energy, enhances animals’ ability to resist worm infections. Protein is essential for producing immune cells and antibodies to fight parasitic infections. Undernourished sheep lack protein and struggle to mount effective immune responses against worms. Minerals such as copper, zinc, and selenium are essential for maintaining vital immune function. Deficiencies in these micronutrients can worsen parasitic burdens.
High-energy diets provide the necessary resources for sheep to maintain normal physiological functions while combating parasitic stress; low-energy diets increase vulnerability to worms. Nutritional stress during critical periods, such as pregnancy or lactation, increases vulnerability. The risk for lactating ewes, for instance, is higher due to the additional energy demands of milk production.
Control measures
Dr Tshuma highlights strategic deworming as a key control measure. Animals should be treated at critical times, such as before lambing and during peak worm seasons (late summer to autumn) to reduce worm burdens in ewes and prevent infections in young animals. Rotational grazing minimises the exposure of sheep to infective larvae. Allowing rest periods helps larvae die off before reintroducing animals.
Selective treatment (the Famacha method) focusses on treating animals exhibiting signs of anaemia, particularly relating to H. contortus. This approach minimises the risk of developing drug resistance. Faecal egg count monitoring assesses worm burdens and guides treatment decisions.
He recommends integrating the following strategies for effective worm control:
- Regular monitoring: Use faecal egg counts to track worm burdens and only treat when necessary.
- Grazing management: Implement rotational or deferred grazing to limit livestock exposure to infective larvae. Avoid overgrazing to reduce contamination.
- Selective breeding: Incorporate breeds or individuals with natural parasite resistance into breeding programmes to lessen reliance on chemical treatments.
- Nutritional management: Provide high-quality feed and supplements, especially during critical periods such as lambing or weaning.
- Environmental management: Maintain pasture hygiene by harrowing and resting pastures to reduce the build–up of infective larvae.
- Strategic deworming: Administer dewormers at optimal times, using appropriate anthelmintics and dosages. However, anthelmintics should never be used in isolation; they must complement other control measures.
Anthelmintic resistance
Dr Tshuma says anthelmintic resistance is a significant concern in sheep production. Overreliance on specific dewormers and improper dosing contribute to this issue. To manage resistance, he recommends rotating anthelmintic classes and using combination treatments when possible. The refugia strategy, which allows non-resistant worms to survive and breed, helps dilute resistant genes. Regular faecal egg count reduction tests are crucial for monitoring drug efficacy and detecting early resistance signs. – Christal-Lize Muller, Stockfarm
For more information, contact Dr Takula Tshuma at takula.tshuma@up.ac.za.
