Nematodirus in Lambs
Introduction
Nematodirus battus is one of the most significant parasitic worms of UK sheep. It typically affects young lambs aged between six and 12 weeks old in late spring and early summer, but may occasionally cause problems in the autumn as well. A strong immunity usually develops over the first grazing season making this a disease almost exclusively of lambs.
.
Figure 1: Only lambs are affected by Nematodirus, ewes do not show disease
Figure 2: Nematodirus eggs are readily identified by faecal worm egg counts (FWEC)
Cause
Unlike the other gut roundworms, Nematodirus battus infection passes directly from one season’s lamb crop to the next. This occurs through the survival of eggs passed in the faeces of infected lambs the previous year on pastures over the winter. Infective 3rd stage larvae develop within the eggs once on pasture, but usually these will not hatch until temperatures rise again in the spring. Following a period of further development in the new year as temperatures increase eggs will hatch en masse, leading to high levels of pasture infectivity. If this occurs at a time when lambs are beginning to graze extensively, typically from 6-12 weeks of age this can result in widespread and severe disease, which typically occurs within 10-12 days of infection. Once hatched, the infective stage larvae are relatively short-lived and usually die off to low levels on pastures within 2-3 months of hatching. Patency is usually around 2 weeks after initial infection. Occasionally, in cooler, wetter summers these eggs may develop and hatch in the autumn potentially leading to further clinical disease in lambs towards the end of the grazing season, although in most cases the cycle will complete the following year.
Figure 3: Sudden onset diarrhoea in a lamb caused by Nematodirus
Welfare and Economics
Where outbreaks of clinical disease occur, lamb illness and deaths can be significant, resulting in serious reduction in growth and reduced numbers of lambs weaned. Even mild infections will impact on production - lambs which recover from nematodirosis have a slower growth rate and take much longer to achieve their market weight. Diarrhoea caused by nematodirosis can also lead to heavily soiled fleece and an increased risk of flystrike.
Figure 4: Severely affected lambs frequently die from dehydration
Clinical signs and diagnosis
As with most other PGE-causing roundworms affecting growing lambs, the main clinical signs seen with Nematodirus infection are diarrhoea, loss of appetite and weight loss. As a result of dehydration caused by sudden onset diarrhoea, affected animals may also show a profound thirst and are often seen congregating around water troughs. In severe cases lambs may deteriorate rapidly and die as a result of this dehydration. Due to its unique life cycle, nematodirosis generally occurs earlier in the grazing season than other causes of PGE.
Disease is caused by intestinal damage due to the development of larval stages in the gut wall. Consequently, clinical disease may present before eggs are shed in the faeces, meaning diagnosis of acute disease is usually based on clinical signs, age of affected animals, grazing history, time of year and recent weather conditions. It is important to note, however, that due to similarities in presenting signs and the age of animals affected coccidiosis should also be considered. Later into the season patent infections and are useful in demonstrating hatch has occurred and to monitor adult burdens and levels of pasture contamination.
Figure 5: Adult worms can be found in large numbers in the intestines of affected lambs at post mortem. Image credit: Ben Strugnell, Farm Post Mortems Ltd.
Prevention and control
The life cycle of Nematodirus battus is reliant on direct lamb-to-lamb transmission over sequential years. Selecting grazing for young lambs early in the season to avoid the same pastures grazed by the previous years’ lamb crop reduces the risk of disease being perpetuated from year to year. A risk forecast based on daily local temperatures from the beginning of January is available through both NADIS and SCOPs. This provides an indication of when peak hatch is likely to occur in a given area and provides an early warning for farmers with lambs grazing high risk pastures that preventative dosing is indicated.
Figure 6: Grazing lambs on different pasture each year reduces the risk of infection
Figure 7: The NADIS Nematodirus forecast alerts vets and farmers when the period of peak risk occurs in their local area
Treatment
Whilst most common roundworm products are effective against Nematodirus battus, selection of the correct product for treating nematodirosis is still important. There is widespread and growing resistance to group 1-BZ, 2-LV and 3-ML wormers in other PGE-causing roundworm species (e.g. Telodorsagia and Trichostrongylus). Resistance in N. battus, however, remains uncommon. Therefore, group 1-BZ (white) drenches are still considered an effective treatment option for control of nematodirosis in young lambs in the late spring and should be used in preference to group 2-LV and 3-ML products to help preserve their efficacy against other roundworm species. As with all anthelmintic treatments, these should be administered following the manufacturer’s recommendations at the correct dose by weight. Timing of dosing for nematodirosis is generally early in the season to prevent clinical disease, and can be informed by local temperatures and risk forecast predictions. In the event of clinical outbreaks treatment is indicated in the affected group. FECs 10 days post treatment are also useful in checking whether treatments have been effective. Where safe grazing alternatives are not available, repeated dosing of animals grazing heavily contaminated pastures may be necessary.
Figure 8: Treating lambs early in the season based on clinical signs and forecasts can reduce disease levels