Worming Treatment for Sheep and Cattle: Wormer Groups and Resistance

Worming sheep and cattle: the short version

Worming every animal on a calendar is how wormer resistance was built. Current advice from SCOPS (Sustainable Control of Parasites in Sheep) and, for cattle, COWS (Control of Worms Sustainably) is to treat when there is evidence of need, use the right product for the parasite, dose accurately and check the treatment worked.

Agree a written parasite plan with your vet or SQP (Suitably Qualified Person) and review it each year. This guide covers the principles only: it gives no dose rates or product recommendations, so read the label and take advice before any worming treatment.

The main worms and flukes

SCOPS says around 20 species of roundworm live in British sheep, and how much harm they do depends on the species, the burden and the animal’s age, nutrition and condition. The ones that matter most are:

• Gut roundworms (parasitic gastroenteritis, PGE): scour, poor growth and lost condition in lambs. In cattle, COWS names Ostertagia ostertagi and Cooperia oncophora as the economically important species, with first-season grazers most at risk because they have not built immunity.
• Nematodirus battus: a spring threat to young lambs, covered below.
• Haemonchus contortus (barber’s pole worm): a blood-sucking stomach worm that kills sheep through anaemia rather than scour.
• Liver fluke (Fasciola hepatica): a flatworm spread through mud snails, affecting both sheep and cattle.
• Lungworm (Dictyocaulus viviparus): causes husk or hoose in cattle. COWS warns that it is not picked up by faecal egg counts, so watch for coughing and call your vet straight away if you hear it.

The five wormer groups and their colours

Roundworm wormers (anthelmintics) fall into five groups. The SCOPS anthelmintic groups guide (January 2024) lists them as:

• Group 1-BZ, white: benzimidazoles
• Group 2-LV, yellow: levamisole
• Group 3-ML, clear: macrocyclic lactones
• Group 4-AD, orange: monepantel
• Group 5-SI, purple: derquantel, a spiroindole

Products in the same group work in the same way, so check the group number on the label rather than the brand. COWS notes that levamisole does not kill larval Ostertagia in cattle. Flukicides are separate from these five groups, and different flukicides kill different ages of fluke.

Wormer resistance

Resistance means a wormer no longer kills enough of the worms it should, and SCOPS warns that it sneaks up on you. Every treatment selects for resistant worms, so the aim is to treat less often, treat well, and leave some worms unexposed so susceptible worms stay in the population.

SCOPS advice is to use narrow-spectrum products where they do the job, use combinations only when they are needed, and choose a narrow-spectrum flukicide rather than a combined fluke and worm product when fluke is the target. The two newer groups, 4-AD orange and 5-SI purple, are mainly recommended for two jobs: quarantine treatment of bought-in sheep, and a single late-season break dose for lambs. They are not for routine use: overuse abroad has already produced resistance to them.

Faecal egg counts and drench checks

A faecal egg count (FEC) measures worm eggs in dung as eggs per gram (epg). It shows whether to treat, whether a treatment worked and how contaminated pasture is likely to be. For a reliable result:

• Collect at least 10 individual samples at random. Do not pick the poor doers.
• Use fresh dung, less than an hour old, picked up after holding the group in a field corner for a few minutes.
• Keep samples cool but not frozen, and get them to the lab within 48 hours.

For lambs where Haemonchus is not involved, SCOPS guides that under 250 epg is low, 250 to 750 is medium and over 750 is high, but interpret results with your vet or adviser. For cattle, COWS says counts below 200 epg may not need treatment, but should be rechecked in two to three weeks because counts can climb quickly in July and August.

A drench check, or worming treatment check, repeats the FEC after treatment to see whether it worked. COWS suggests a post-treatment count at 14 days in cattle, and SCOPS suggests checking a white drench for Nematodirus seven to ten days after treatment. Ask your vet about the right timing for other groups.

Nematodirus in lambs and the SCOPS forecast

Nematodirus battus larvae develop inside eggs dropped by the previous year’s lambs, then hatch after a cold spell followed by temperatures of 10°C or more. That can cause a sudden mass hatch in spring. SCOPS says healthy lambs are most at risk at 6 to 12 weeks old, though stressed younger lambs, such as triplets, can be hit earlier. Disease strikes fast, killing some lambs and stunting others.

The SCOPS Nematodirus forecast tracks temperatures from weather stations across the UK. Peak hatch at one station has ranged from late March to late May, so check the forecast every spring, and adjust by about seven days per 100 metres of altitude. Where possible, graze young lambs on fields that did not carry lambs the previous spring. SCOPS says a white (1-BZ) drench remains highly effective against Nematodirus on most farms.

Haemonchus: barber’s pole worm

Haemonchus contortus feeds on blood in the abomasum. SCOPS says each adult takes about 0.05 ml of blood a day, so 5,000 worms remove around 250 ml daily, and as few as 500 adult worms can cause disease. Females lay 5,000 to 15,000 eggs a day and the lifecycle takes about 20 days, so numbers build fast.

Signs are pale gums and eyelids, fast shallow breathing, weakness, bottle jaw, weight loss and sudden death. Scour is not a typical sign, which is why it is easily missed. Once mostly seen in south east England, it is being reported more widely, with more outbreaks later in the autumn. Ask your vet about testing for it.

Liver fluke

Liver fluke needs the mud snail to complete its lifecycle, so risk rises after wet, mild weather and on wet grazing. SCOPS says sheep face the greatest risk of acute fluke in autumn and early winter, which makes them a useful early warning for cattle on the same farm.

Dung tests can miss fluke too young to lay eggs, so SCOPS advises retesting every three to four weeks through risk periods and following up deaths and abattoir liver reports. The right flukicide depends on the age of fluke present: SCOPS noted in 2021 that four actives were available for sheep and five for cattle, and that some kill only adults. COWS also flags resistance to triclabendazole, so agree fluke treatment with your vet.

Quarantine treatments for bought-in stock

Bought-in stock can bring resistant worms with them. For sheep, SCOPS sets out a gold standard: treat with both group 4-AD (orange) and group 5-SI (purple), given one after the other with separate guns and never mixed. A silver standard uses one of those two with a group 3-ML (clear) product.

• Yard new arrivals for 24 to 48 hours after treatment so fewer eggs reach the pasture.
• Turn them onto pasture already grazed by your own sheep, so any survivors are diluted.
• Keep them apart for three to four weeks and check the treatment worked.

Bought-in cattle need a quarantine plan too, including fluke status. Agree it with your vet before the animals arrive.

Dosing correctly

Underdosing drives resistance, and SCOPS says underestimating weight is a common cause. Its anthelmintics guide says to weigh the biggest animal in the group, check the weigh crate is accurate, and check that the drench gun is delivering the right amount before you start. Where weights vary widely, split the group and dose each group for its heaviest animal.

• For oral drenches, restrain the sheep properly and put the nozzle into the gap between the front and back teeth, then over the back of the tongue.
• For cattle, COWS advises weighing or using a weigh band to work out the right dose for each animal.
• Read the label every time and check the meat and milk withdrawal periods.
• Record what was given, to which animals, the batch number and the date.

Frequently Asked Questions

How often should sheep be wormed?

There is no fixed interval. Treat when faecal egg counts, growth rates, the Nematodirus forecast or your vet show a need, rather than on a calendar.

What are the five wormer groups?

Group 1-BZ (white, benzimidazoles), 2-LV (yellow, levamisole), 3-ML (clear, macrocyclic lactones), 4-AD (orange, monepantel) and 5-SI (purple, derquantel). Products in the same group work the same way.

What is drenching sheep?

Drenching means giving a liquid wormer by mouth with a drench gun, with the nozzle over the back of the tongue so the full dose is swallowed.

How do I know if a wormer has worked?

Do a drench check: a faecal egg count before treatment and another after it. Timing depends on the wormer group, so agree it with your vet. A poor drop in egg count points to resistance.

When should cattle be wormed?

First-season grazing calves are most at risk from gut worms. Use faecal egg counts, growth and signs such as coughing (lungworm) to time treatment, and agree a plan with your vet.

BritFarmers Weekly

One email a week: what’s moved on schemes, prices, disease control and policy, with links to primary sources. Free. No sponsors. Unsubscribe anytime. Subscribe.

Source: SCOPS: Worms