Changing weather causes change in worm patterns

Few would deny the UK has experienced some extraordinary weather in recent years – but many farmers may be unaware of the impact this has had on internal parasites in their stock.

Intestinal worms affect all species of livestock, and, because part of their lifecycle is spent outside the host animal, their breeding habits are inextricably linked to weather conditions. And with the recent changes in Britain’s climate, including wetter and cooler summers and generally milder springs, some worm species have thrived and spread remarkably quickly.

There is no active surveillance of worms in livestock, but working on anecdotal evidence from farmers and vets, there are some clear changes in seasonality and geographic spread of some species, says David Bartley, a research scientist at the Moredun Institute.


The roundworm nematodirus, which causes watery diarrhoea and sometimes death in lambs, used to be most active early in the spring as it requires a cold spell followed by warm conditions to hatch. However, cool and damp weather in May and June can extend larval survival on pasture well into the summer, leading to an increase in cases in older lambs.


Another roundworm, haemonchus, was formerly associated with tropical climates, but is becoming increasingly prevalent in the UK, says Moredun’s Philip Skuce.

“It is being found anywhere from south-east England to north-east Scotland, and if weather patterns continue to change it may lead to more clinical cases. It is a pathogenic, blood-sucking parasite, which is prone to anthelmintic resistance, so it is a real threat. These parasites are incredibly adaptable; if the weather gets hotter and drier as predicted, and doesn’t suit our endemic species, something else like haemonchus may come in.”

Wormer recommendations

  • Use anthelmintics only when necessary
  • Administer anthelmintics effectively
  • Select the appropriate anthelmintic for the task
  • Test for anthelmintic resistance on your farm
  • Reduce dependence on anthelmintics
  • Adopt strategies to preserve susceptible worms on the farm
  • Use effective quarantine strategies to prevent importing resistant worms
  • Develop a control strategy with your vet

Liver and rumen fluke

Liver fluke is another opportunist that has made the most of our warm and wet weather, says Dr Skuce. “It used to be most common in wet parts of south-west England and Wales, but there is plenty of evidence that it is marching north and east – we are even finding it on sandy soil in north-east Scotland, where it was formerly considered to be too dry and cold for fluke.”

Affecting both sheep and cattle, liver fluke cases are spreading rapidly, with 25% of beef livers condemned in Scotland due to fluke damage, he adds.

An emerging parasite, the rumen (or stomach) fluke, is also starting to be noticed in sheep and cattle nationwide. “Like liver fluke, it has a snail intermediate host, but this is thought to be a water snail which thrives and spreads in flooding events. And we are starting to see real problems in Ireland, where 40% of sheep and cattle are affected – but it is also present in Wales and Scotland,” says Dr Skuce.

Although it is not yet clear whether the stomach fluke causes clinical disease, it could trigger hidden production losses, he adds. “It doesn’t respond to the commonly used liver fluke drugs, so you need to be sure which type of fluke you are dealing with.”

Faecal egg counts

In all cases, it is essential that producers monitor their farm’s worm burden by taking frequent faecal egg counts to identify which species are present. “You need to know what you are treating and whether the anthelmintic has been effective, so take egg counts before and after treatment to be sure it has worked,” says Dr Bartley.

Drug resistance is already a widespread problem in sheep, and is becoming a greater concern in cattle, particularly with regard to roundworm and fluke. “If you are bringing animals on to your farm you could import new or resistant parasites, so it’s really important that you treat and then quarantine new stock before turning them on to pasture,” he adds.

Organisations such as the National Animal Disease Information Service offer parasite forecasts according to regional weather patterns, he adds. “Keep an eye on the forecasts and your local weather, and be aware of the implications. Don’t assume that you know what to expect at a certain time of year, because climate change is very much driving changing seasonality and geographical spread of parasites.”

For more information see EBLEX’s publications on worm control see

 Worm control in ruminants

The advice for worming cattle and sheep has changed in recent years, to avoid breeding anthelmintic resistant strains. Resistance is becoming a major problem in sheep, with most farms showing resistance to benzimadazoles, or white drench, and an increasing number finding resistance to levamizoles and macrocyclic lactones (yellow and clear drenches).

“In cattle the main issues have been with pour-on products (macrocyclic lactones) and triclabendazole (flukicide),” says Mary Vickers, senior beef and sheep scientist at EBLEX.

A common problem is underdosing animals, which not only reduces efficacy but also promotes the development of resistant strains. “A low level of worm burden is fine, as that is what allows animals to develop natural immunity.” Producers should use a narrow spectrum product to target liver fluke, and always carry out faecal egg counting to assess worm burdens instead of just using routine treatments.

“The old dose and move strategy is also no longer recommended because it has the potential to select for resistance. By delaying the movement to new pasture livestock can become lightly reinfected with susceptible worms, which avoids any resistant worms gaining an advantage.”

Worm control in pigs 

Pigs are less affected by worms than sheep and cattle, with the main threat being roundworm.

Ascaris suum normally peaks in the autumn, causing scarring in the liver and sometimes pneumonia, says Derek Armstrong, head of research and development at BPEX. However, just 4-5% of livers show signs of infection upon slaughter, so it is not a major problem. “It is more difficult to control in outdoor pigs, but is not really changing with the climate,” he says. 

Through the BPEX pig health scheme, producers are informed of any worm damage identified at slaughter, and should use that to guide targeted worm treatment. Lungworm can occasionally be a problem in outdoor pigs, with trichuris and strongyloides seen even less frequently. “From a food safety point of view we need to keep an eye on trichinella, but it is not currently a problem in the UK.”

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