Helminth infections: economic impact and worming strategies

Helminth infections represent a ubiquitous and significant challenge in grazing livestock systems, particularly in temperate regions such as the United Kingdom where climatic conditions favour parasite survival on pasture.

Parasitic infections reduce growth rates, milk yield, reproductive performance and overall productivity, resulting in major economic losses for the livestock sector. Gastrointestinal nematodes, lungworms and trematodes affect sheep and cattle of all production types, causing both clinical disease and significant subclinical production losses, and they also impact on welfare and profitability.

Anthelmintic drugs have historically formed the cornerstone of parasite control; however, widespread and increasing anthelmintic resistance now threatens their long-term efficacy, while awareness is increasing of the secondary negative effects their use has on pasture insect populations. Historical routine blanket worming and prophylactic use of anthelmintics, which has driven the emergence of anthelmintic resistance in many parasite species and resistance to multiple drug classes, is now widespread in sheep nematodes in the UK and increasingly reported in cattle parasites.

Lately we have been looking towards integrated, diagnostics-driven strategies that aim to preserve drug efficacy while maintaining production. In addition, grazing management and the impact of dung beetles and ecosystem services has come to the fore more in the past few years, allowing us to look beyond the drugs to control strategies.

This article reviews the major parasites affecting sheep and cattle in the UK, their economic significance and the latest evidence-based worming strategies relevant to veterinary practice, including targeted selective treatment, refugia preservation, faecal egg count monitoring and integrated grazing management. The article aims to provide practising vets with an overview of contemporary parasite control strategies and the evolving role of anthelmintics, while reducing infection pressure through management interventions. It will start by looking at some of the common parasites of sheep and cattle in the UK.

Gastrointestinal nematodes

Gastrointestinal nematodes are the most economically significant parasites of grazing ruminants in the UK. They infect animals through ingestion of infective third-stage larvae (L3) present on pasture, where they develop from eggs shed in faeces as part of their seasonal pasture based life cycle.

Key species in sheep

• Teladorsagia circumcincta (brown stomach worm).
• Trichostrongylus species (black scour worms).
• Haemonchus contortus (barber’s pole worm).
• Nematodirus battus.

Collectively, these parasites (with the exception of Haemonchus contortus) cause parasitic gastroenteritis (PGE), characterised by diarrhoea, reduced appetite, weight loss and poor growth in lambs. Subclinical infections are particularly important economically. Even relatively low worm burdens can significantly reduce growth rates in lambs without overt clinical disease.

Seasonal epidemiology and immunity is critical; for example:

• Larvae generally require warm and wet conditions to develop and disperse in pasture.
• Nematodirus is thought to typically cause outbreaks in spring when temperatures exceed about 10°C, but in the past few years has been seen in autumn as well.
• Teladorsagia and Trichostrongylus dominate during the summer and autumn-winter grazing periods respectively.
• Haemonchus contortus is increasing in incidence from spring to late autumn in some areas.
• Young lambs are most susceptible to all worms due to their limited acquired immunity.
• Adult sheep will gain some immunity to most gut worms but remain susceptible to Haemonchus contortus following infection.
• The periparturient dip in immunity leads to increased pasture contamination on farms with breeding ewes.
• Towards the end of the grazing season the larvae of Teladorasagia and Trichostrongylus are able to hypobiose (encyst in the wall of the abomasum or intestine) until conditions are favourable the following year, which can lead to mass emergence in animals with high numbers of arrested larval stages and can cause severe clinical disease similar to type-II ostertagiosis in cattle.

Epidemiology

Key epidemiological features can include:

• Overwintering larvae on pasture.
• Periparturient rise in egg output from ewes.
• Seasonal larval peaks in spring and summer.
• Climate-driven outbreaks (for example, N battus seen in autumn now rather than just spring as traditionally thought).
• Older animals generally develop some immunity.

The interaction between host immunity, grazing management and climatic conditions determines infection pressure.

Haemonchus contortus

A few points on H contortus include the following:

• Haemonchosis is the disease specifically caused by this parasite.
• H contortus is an abomasal blood-sucking roundworm.
• Adult Haemonchus worms, and worms approaching adulthood (L5), can potentially remove large quantities of blood (0.05ml/day) from their host.
• A sheep carrying 5,000 H contortus may lose 250ml of blood per day leading to acute disease – anaemia and death.
• Females can produce 5,000 to 15,000 eggs per day, compared to 400 eggs for the Teladorsagia roundworm.
• It has a short life cycle (20 days), which – combined with high egg output – can lead to a very rapid build-up of  parasites on pasture given suitable weather conditions.
• It is found in diverse climatic conditions from the Arctic to the tropics, although conditions have a significant effect on the extent, timing and frequency of disease outbreaks.
• Because female H contortus worms breed so quickly and in such large numbers, there is potential for large genetic variation in the population, giving it the ability to adapt and enhancing its survival strategies (in particular, its ability to overwinter).
• Overwintering inside the host used to be considered the main overwintering strategy, but possible overwintering on pasture in warmer climates has implications for management and control.
• While it is predominantly a parasite of sheep and goats, it can cause severe disease in alpacas and also be ingested by cattle and deer. These potential reservoirs need to be considered when investigating outbreaks and looking for sources of the parasite.
• Acute infections, resulting from the ingestion of many infective larvae over a short period of time, can result in:
  • Weak animals that are likely to collapse if gathered.
  • Pale mucous membranes.
  • Quick, shallow breathing and increased heart rate.
  • Sudden death.
  • Possible slight constipation (and certainly not diarrhoea as seen with the other gastrointestinal nematodes).
• The onset of clinical signs may be so sudden that affected animals are still in good body condition.
• Sub-acute infections are characterised by bottle jaw (sub-mandibular oedema), which means it can resemble liver fluke.
• Chronic infections are characterised by a more general failure to thrive, showing signs of weight loss, poor body condition, bottle jaw, lethargy and weakness. The chronic nature of the blood loss leads to an exhaustion of iron reserves and the development of anaemia.

Key species in cattle

In cattle, the most economically significant gastrointestinal nematodes are:

• Ostertagia ostertagi.
• Cooperia oncophora.
• Trichostrongylus axei.

Again, seasonal epidemiology and immunity is critical:

• Calves in their first grazing season are the most vulnerable to gastrointestinal nematodes because they lack protective immunity.
• Ostertagia larvae invade the gastric glands of the abomasum, disrupting digestion and leading to reduced weight gain, diarrhoea and poor feed conversion.
• Arrested larvae acquired late in the grazing season can cause type-II ostertagiasis during winter or early housing.
• Subclinical infections frequently reduce growth rates in youngstock.
• Adult dairy or beef cattle will rarely succumb to infection and have a limited input to the life cycles of gut worms.

Tapeworms

Moniezia expansa quite commonly infects lambs as its host, but it is usually of limited clinical significance. Although segments may be visible in faeces, treatment is rarely necessary unless very heavy infections occur, which may cause, loss of condition, scouring or intestinal blockage in extreme cases. Its intermediate host is the pasture mite.

Remember that both sheep and cattle can be the intermediate host for other tapeworms. If they become infected they usually develop muscular cysts that may not cause clinical symptoms and cannot be treated. However, these cysts may be a cause of carcase condemnation at the abattoir, resulting in economic implications for the producer.

Lungworms

Lungworms affect both sheep and cattle and can be responsible for respiratory disease in grazing animals.

Cattle lungworm

Dictyocaulus viviparus is the most common parasite. It causes parasitic bronchitis, also known as husk, with clinical signs including coughing, tachypnoea, respiratory distress, reduced weight gain or milk production, and occasionally sudden death in heavy infections.

Outbreaks are most common in young grazing cattle, but they can occur in adult cattle when immunity wanes or when climatic conditions favour larval survival.

Adult lungworm may cause clinical signs before D viviparus sheds larvae in the faeces, so diagnosis by the Baermann technique – looking for live larvae in the faeces – is only possible from 25 days post-infection.

It is also a consideration in adult animals that have repeat infections that the cow’s immune system may stop the life cycle before the larvae are produced.

A definitive diagnosis may therefore require postmortem examination, although generally clinical signs and history are good indicators of lungworm disease.

A vaccination is available.

Sheep lungworm

The most common lungworm to infect sheep is Dictyocaulus filaria. The clinical picture is usually less severe than in cattle, but will still reduce growth rates and production and it needs to be identified if animals are coughing or have laboured breathing while at grazing.

Liver fluke

The trematode Fasciola hepatica is widespread in the UK due to the favourable climate for the intermediate host snail (Galba truncatula), which is an amphibious, air-breathing freshwater snail. It primarily inhabits shallow, slow-moving aquatic ecosystems and moist mud.

Fluke infection in ruminants leads to:

• Acute hepatic damage.
• Chronic weight loss.
• Reduced milk production.
• Poor fertility.
• Anaemia.
• Bottle jaw – but beware of similarities in presentation to subacute or chronic haemonchosis.
• Production losses.
• Liver condemnation at slaughter.

Fluke risk depends strongly on environmental conditions conducive to snail habitats, and it has increased in the past few decades due to wetter conditions and changes in grazing patterns. Strategic flukicide use and pasture management remain key control measures, but newer diagnostic tests, such as fluke coproantigen testing, are now commonly used, allowing earlier intervention when infection is present.

Economic impact of helminth parasites

Parasitic infections represent a major economic burden for the UK livestock sector due to both clinical disease, subclinical productivity, treatment costs and animal losses. A large European modelling study published in 2020 estimated that parasitic worms cost the UK livestock industry approximately £270 million annually, including losses in beef, dairy and sheep production¹.

The breakdown of estimated annual losses includes:

• Beef cattle: approximately £149 million.
• Dairy cattle: approximately £78 million.
• Sheep: approximately £43 million.
• Economic losses arise through:
• Reduced weight gain.
• Lower carcase weights
• Reduced milk yield.
• Increased mortality.
• Increased veterinary costs.
• Reduction in reproductive performance.

Additionally, drug-resistant parasites contribute millions of pounds in further losses through reduced treatment efficacy and increased disease incidence, costing an estimated £7.5 million per year¹.

Subclinical parasitism may have the greatest economic impact; even moderate worm burdens can reduce youngstock growth rates significantly and impair feed conversion efficiency.

The scale of these losses underscores the importance of sustainable parasite control strategies to maintain both farm profitability and welfare.

Anthelmintic drug classes

Several classes of anthelmintic drugs are licensed for use in ruminants in the UK.

Group 1 – benzimidazoles (1-BZ): white

Benzimidazoles (BZ) act by binding to parasite ß-tubulin and disrupting microtubule formation.

Examples include:

• Albendazole.
• Fenbendazole.
• Oxfendazole.

They are effective against many nematodes and some cestodes, but there is widespread resistance in Teladorsagia and Trichostrongylus in sheep.

Group 2 – Levamisole (2-LV): yellow

Levamisole acts as a nicotinic acetylcholine receptor agonist causing spastic paralysis of nematodes.

It has a relatively narrow safety margin, but currently remains effective against many BZ-resistant parasite populations. It works rapidly and has short persistence.

Group 3 – macrocyclic lactones: (3-ML) clear

These compounds act on glutamate-gated chloride channels causing parasite paralysis.

Examples include:

• Ivermectin.
• Moxidectin.
• Doramectin.
• Eprinomectin.

They have broad activity against nematodes and ectoparasites, but resistance issues and also environmental impacts are increasing due to persistence in dung affecting insects.

Newer classes

Newer compounds have been developed to combat resistance and two additional groups are licensed for sheep in the UK:

• Group 4 – amino-acetonitrile derivatives (4-AD): orange
  • Monepantel.
• Group 5 – spiroindoles (5-SI): purple
  • Derquantel (often combined with abamectin).

These drugs provide additional treatment options for resistant nematode populations and are important tools in managing resistance. They are useful for strategic treatments or quarantine dosing.

Knowledgeable evidence-based veterinary guidance is critical in selecting appropriate drug classes to both maximise efficacy and delay the development of resistance.

Anthelmintic resistance

Anthelmintic resistance occurs when parasites survive doses that would normally be effective and pass on resistance traits to their offspring.

Resistance is now widespread in sheep parasites in the UK and it is well documented that most lowland flocks are likely to have some species resistance to BZ, with increasing resistance to levamisole and macrocyclic lactones being found. Multiple-class resistance has also been documented.

The key drivers of resistance include:

• Frequent blanket treatments.
• Underdosing.
• Treat-and-move grazing practices.
• Lack of refugia.

With these issues in mind, for a number of years veterinary advice on using anthelmintics has included:

• Using faecal egg counts to check if animals require treatment.
• Checking the weight of animals to be treated and the calibration of the drench guns to ensure that animals aren’t being underdosed.
• Advising farmers to return animals to dirty pasture for a few days prior to moving to new pasture to ensure that potentially resistant worms are mixed up with a large population of larvae on the pasture.
• Leaving 10 per cent of the groups untreated to again ensure dilution of potentially resistant worms.

Resistance threatens the long-term viability of chemical parasite control as treatments fail to reduce worm burdens, production losses increase and available drug options become limited. As a consequence, as vets we need to increasingly emphasise sustainable parasite control strategies.

Contemporary worming strategies

Targeted selective treatment

Targeted selective treatment (TST) involves treating only animals demonstrating evidence of infection or poor performance rather than the entire flock or herd.

Selection criteria may include:

• Body condition scoring.
• Growth rate monitoring.
• Faecal egg counts.
• Clinical signs such as scouring.

TST reduces selection pressure for resistant parasites by leaving a proportion of the parasite population unexposed to anthelmintics.

Refugia management

Refugia refers to the proportion of the parasite population not exposed to anthelmintic treatment, that is those parasites currently on the grazing or of the wrong stage of development, or species to be targeted by the anthelmintics used.

Maintaining refugia is essential for slowing resistance development because susceptible parasites dilute resistant genes.

Strategies include:

• Avoiding whole-flock treatments where unnecessary.
• Leaving a subset of animals untreated.
• Avoiding immediate movement to clean pasture after treatment.

Faecal egg count monitoring

It should be gold standard to use regular faecal egg count (FEC) testing to guide parasite control decisions.

Applications include:

• Assessing parasite burden.
• Determining treatment thresholds.
• Evaluating anthelmintic efficacy (faecal egg count reduction testing [FECRT] carried out post-anthelmintic treatment)

Routine monitoring allows more targeted and evidence-based treatment protocols.

Grazing management

Pasture management plays a crucial role in reducing parasite exposure.

Effective strategies include:

• Rotational grazing.
• Mixed or alternate grazing with cattle and sheep.
• Maintaining lower stocking densities.
• Use of “safe” pastures for youngstock – pasture that was not grazed by that age of stock in the previous year.

Such measures reduce the number of infective larvae available for ingestion.

Quarantine treatments

Introducing new animals poses a major risk of introducing resistant parasites.

Recommended quarantine protocols include:

• Treatment with new or multiple anthelmintic classes.
• Holding animals off pasture for 24 to 48 hours.
• Monitoring via faecal egg counts.

These practices reduce the likelihood of resistant parasites entering the farm population.

National sustainable parasite control initiatives

The UK livestock industry has developed coordinated programmes to address anthelmintic resistance.

SCOPS

The Sustainable Control of Parasites in Sheep (SCOPS) initiative provides evidence-based guidelines for parasite control and resistance management in sheep production systems.

Key recommendations include:

• Use wormers only when necessary.
• Maintain parasite refugia.
• Use accurate dosing.
• Monitor treatment efficacy.
• Quarantine new animals.

COWS

The Control of Worms Sustainably (COWS) programme provides equivalent guidance for cattle producers, focusing on integrated parasite management strategies.

Emerging approaches to parasite control

Genetic selection for parasite resistance

Breeding for parasite resistance is gaining increasing attention as a sustainable control strategy and has the potential to improve productivity and reduce reliance on anthelmintics.

Nutritional management

Adequate nutrition enhances host immunity against parasites. Protein supplementation has been shown to improve resilience and reduce parasite establishment in young animals.

Vaccination

While a vaccine for the cattle lungworm (D viviparous) has been available for a number of years and is used successfully on many farms, research into vaccines against gastrointestinal nematodes is ongoing.

A vaccine against H contortus is also now commercially available in the UK via import.

Role of vets in sustainable parasite control

Vets play a critical role in implementing sustainable parasite control strategies on farm.

Key inputs include:

• Designing farm-specific parasite control plans.
• Interpreting diagnostic data.
• Educating farmers on resistance management.
• Monitoring treatment efficacy.

Regular veterinary involvement helps ensure parasite control programmes remain effective and evidence-based.

Conclusion

Helminth parasites continue to represent a major challenge to sheep and cattle production in the UK. Gastrointestinal nematodes, liver fluke and lungworm contribute to substantial production and economic losses, and animal welfare concerns.

While anthelmintic drugs remain essential tools for parasite control, widespread resistance threatens their long-term effectiveness. As a result, modern parasite management strategies should increasingly include sustainable approaches that integrate targeted treatment, diagnostic monitoring, grazing management and genetic selection.

Veterinary surgeons are central to implementing these strategies and guiding clients toward evidence-based parasite control programmes. By combining responsible anthelmintic use with broader management interventions, the livestock industry could preserve the efficacy of existing treatments and ensure sustainable parasite control for future generations.

This article appeared in Vet Times Livestock (21 July 2026 issue, VT56.29), Volume 12, Issue 2, Pages 18-23.

Clare Eames has worked in the south-west since qualifying from the University of Bristol in 2001. She works part time at Synergy Farm Health and farms with her husband the rest of the time. Clare is particularly interested in regenerative farming and improving whole farm health.