Nematodirus in Sheep

Nematodirus in Sheep
23 April 2020
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Nematodirus (nematodirosis) is an important disease affecting young lambs at grass. It is a particularly nasty disease, causing high numbers of deaths and stunting growth in many others. 

Sheep and lambs grazing in a field

It is caused by the Nematodirus battus worm, which has a different lifecycle to other sheep worms and if the conditions are right, it can strike very quickly with little or no warning. Unlike with other worms, the development of the larvae takes place within the egg, which can survive on the pasture for up to 2 years. In order to hatch, the eggs have to undergo a period of cold weather followed by warmer tempatures of 10°C or more. A mass hatch can occur if these conditions occur over a short period of time and cause high levels of disease if these conditions coincide with lambing causing devastating effects. Due to the worms dependency on the weather infection rates vary from region to region. Here in Somerset the risk period is highest in early April/May and with recent frosty mornings and hot days the risk is high - so now is the time to be vigilant!

Lambs grazing infected pasture will be at risk once they reach 6 weeks, as they will be eating enough grass to put them at risk. Younger lambs may also be at risk if the ewe is not producing sufficient milk. Other risk factors to consider are lambs in mixed age groups as they can be predisposed to other disease such as coccidiosis. Lambs under other stresses such as triplets or lambs fostered on to young, or old ewes should also be considered at risk. 

Once the lambs reach around 3 months of age they will not be at as much of a risk as they will have ingested enough larvae to develop their own immunity.

Check the NADIS parasite forecast to assess the risk on your farm

Clinical Signs

Nematodirosis will only affect lambs, ewes will not show signs of the disease. You will see a sudden onset of profuse watery scour, with faecal staining on the tail and around their back end. The affected lambs will be dull and depressed, stop sucking and rapidly develop a gaunt appearance with obvious dehydration and condition loss. If left untreated during the early stages of the disease lambs can die of dehydration, and the lambs that survive will suffer considerable weight loss. In situations where there is a very high larval challenge up to 5% of lambs deaths can occur within a few days. Lambs that are treated effectively can take an extra 2 - 3 months to reach their target weight, contributing to financial losses. 


Sheep grazing

Faecal worm egg counts are not an effective method of diagnosis because the disease is caused by the larvae and adults before they start laying eggs. An effective diagnosis is based on clinical signs, such as severe scour in lambs grazing infected pasture. If there is no history of nematodirus on the pasture then a post mortem can be used to diagnose, as large numbers of developing larval stages and adults can be found within the small intestines. 

Nematodirosis is not the only cause of scour in young lambs however, there is also coccidiosis. It is therefore recommended to discuss any cases with a vet to ensure you select the most appropriate treatment. 

Management and Prevention

The best method of prevention is to avoid grazing young lambs on pastures that were grazed by lambs the previous season. Avoidance is the best method on control but if you are unable to avoid high risk pastures and decide to treat for nematodirus SCOPS recommend that you treat with a white (1-BZ) drench. There wormers are suitable for lambs and are still highly effective against this parasite on most farms. There is some reported resistance however, with the first confirmed case of nematodirus resistant to the 1-BZ wormer group being reported in 2011. Farmers are therefore advised to follow the SCOPS guidelines on correct drenching technique and dose correctly to the correct weight of the lambs, as well as carrying our a post treatment  faecal egg count 7 - 10 days post treatment.

If the lambs ages vary throughout the group they may need to be treated more than once. For advice on this please contact us. 

Read the SCOPS guidelines on correct drenching

If you have any questions or concerns please contact the practice and we can discuss a tailored parasite management plan.