LOW rainfall in late spring and early summer coupled with the increase in temperatures throughout May and June is providing ideal conditions for infective larvae on grass.
According to the latest NADIS Parasite Forecast, the recent warm/hot temperatures could speed up the development of larval stages on pasture too, as the drier than average conditions in May are thought to have bolstered their long-term survival. 

Parasitic Gastroenteritis (PGE)
Parasitic Gastroenteritis (PGE) is caused by roundworm species within the digestive tract. Symptoms include loss of appetite, diarrhoea, dehydration, weight loss and death if heavily infected.
“Lower levels of infection may have no obvious signs but can have a profound impact on performance, particularly weight gain in lambs,” says Sioned Timothy, ruminant technical manager at Boehringer Ingelheim. 
Infection occurs when infective larvae from contaminated pasture are ingested. These larvae develop from eggs passed in the faeces of infected animals. Early in the grazing season, pregnant or recently lambed ewes are the predominant source of pasture contamination due to increased egg output during pregnancy-associated immunosuppression (referred to as the periparturient rise). 
These larval burdens continue to accumulate on pastures where ewes and lambs continually graze. Weaned lambs are particularly at risk during the summer months due to increased pasture contamination, their exposure increasing as their grazing activity increases and since they have yet to develop a protective immunity.
Where possible weaned lambs should be moved to ‘safe grazing’ (hay or silage aftermath).  
Treatment with an anthelmintic is indicated in cases of clinical disease, where average egg counts exceed 500-700 eggs per gram and/or lower than expected weight gain. Consider SCOPS guidelines2 to reduce selection for anthelmintic resistance. 
If treating and moving animals to safe pasture, moving should be delayed for a short period (4-5 days) post-treatment to allow lambs to become lightly re-infected with larvae from the contaminated pasture. 

Haemonchus contortus, or the barber’s pole worm, is another roundworm found in the digestive tract. Infection in the UK is sporadic, but onset of disease can be sudden and severe.  Disease is caused predominantly by adult worms which feed on blood. 
Treatment of Haemonchosis can be carried out using most anthelmintic products, although some evidence of resistance to white drenches (1-BZ) has been reported in the UK. Some flukicidal products, such as nitroxynil and closantel are also effective against Haemonchus.
Monezia tapeworms
Adult tapeworm segments may be seen in the faeces of lambs during summer months. These are acquired through consuming infected intermediate hosts, orbatid mites, which live on the pasture.