Roundworms

Roundworms (nematodes) have a significant effect on the economic performance and health of sheep flock. It is crucial that control strategies are implemented as part of the flock health plan. Historically, the industry has relied on anthelmintics but in order to limit the development of anthelmintic resistance, it is important to adopt monitoring approaches.

Understanding roundworms in sheep

Learn more about the causes and signs of roundworm in sheep and how to treat and control the parasites.

Roundworm life cycle

There are a number of species of roundworms which affect lambs and hoggets and most roundworms follow a similar life cycle to the below.

Important worms

There are about 20 different species of nematode parasites of sheep commonly found in the UK¹. Some of the most important worms are detailed below.

Teladorsagia:

Teladorsagia circumcincta, small stomach worms, formerly known as Ostertagia that cause reduced appetite, diarrhoea, dehydration, weight loss and death. Seen from mid-summer onwards.

Haemonchus:

Haemonchus contortus, (the 'Barber's Pole worm') is a widespread worm that feeds on blood and can cause anaemia and death in sheep. Most prevalent during the summer in warm, humid climates.

Nematodirus:

Nemastoirus battus, a small intestinal worm that causes serious health problems in young lambs during late spring/early summer. The condition can strike very quickly under certain weather conditions with little warning.

Trichostongylus:

Trichostongylus is a black scour worm that was traditionally prevalent in the autumn but has now been seen in the summer among young lambs.

Signs of worms

It is important to quickly spot the signs of disease within flocks so that treatment can start quickly. Disease caused by roundworms may be acute in onset, with outbreaks of clinical disease affecting 10% or more of a flock, and some even suffering mortality².

Clinical signs include:

Reduced appetite
Diarrhoea
Anaemia

Sub-clinical disease causes:

Reduced growth rates
Reduced milk yield
Reduced wool production
Reduced body condition

Treatment

There are a myriad of products available to help control parasites. Aim to choose the right product for your livestock and farm by:

Making the right choice to target the parasite(s) in question: 'Getting it right first time'
Effective administration - Use correctly calibrated equipment, the correct dose for weight and the right application method
Group large flocks of sheep that are similar in weight and weigh and dose based on the heaviest weight per group
Consult with your vet/animal health advisor
Read the product insert and packaging prior to use

Group*	Group Type	Group Colour
Group 1	Benzimidazoles (BZ)	White
Group 2	Levamisole (LV)	Yellow
Group 3	Macrocyclic Lactones (ML)	Clear
Group 4	Amino Acetonitrile Derivatives (AD)	Orange
Group 5	Multi-actives (SI)	Purple

* Products that fall into these groups will cover the major species of gut roundworms and lungworms. At appropriate doses, white drenches can also treat for adult tapeworms and fluke.

Your choice of anthelmintic (wormer) will be influenced by what you are looking to treat. Anthelmintics can be broad spectrum covering many parasites such as gut roundworms, lice, mites, fluke etc. or narrow spectrum covering for example fluke only or tapeworm only. It is important to understand that anthelmintic resistance is widely reported in sheep. Sustainable Control of Parasites in Sheep (SCOPS) have developed strategies which aim to reduce the rate of resistance development.

Control of roundworms

There are a number of measures that can be implemented to limit and avoid a potential outbreak of Nematodirus.

Pasture management

During the key season, pastures should be assessed to determine whether the risk is high, medium or low.

Early identification of roundworms

Early Identification of roundworms by season

Faecal Egg Count (FEC)

A Faecal Egg Count (FEC) counts the number of worm eggs in faeces. The results of this FEC will indicate the number of adult worms in the gut of the sheep. Consult your vet or animal health advisor on how to carry out an FEC test.

It is extremely important to carry out an FEC before sheep are wormed to ensure that sheep do not receive any unnecessary treatment.

Interested in learning more?

Download our best practice guide to learn more about how to control worms in sheep.

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References:

_{¹ SCOPS Internal parasites [ONLINE]: https://www.scops.org.uk/internal-parasites/worms/species-and-lifecycles/ Accessed 19/06/24}

_{² SCOPS Sustainable Worm Control Strategies for Sheep, 3rd Edition 2009}