The unusual weather this year has meant we are seeing significant problems with gastrointestinal worms in sheep coming through the lab much later in the season than usual. Many farmers found they did not need to worm sheep as much as usual over the summer as the hot dry conditions restricted successful larval development on the pasture so sheep did not have much worm challenge.

However, the warm weather extending well into November has allowed a late population boom for many worm species and this has resulted in many flocks seeing problems with parasites much later than we would normally expect. As the sheep have had little exposure over the summer they have not built immunity to the worms and this has caused severe clinical signs on some farms with weight loss, severe scouring and deaths.

Many of the worm species can undergo hypobiosis within the sheep when the weather does get colder. This is when the intermediate life stages of the worm arrest their development inside the sheep’s tissues over the winter and then re-emerge and continue developing into adults in the spring once environmental conditions
become more favourable to larval survival on the pasture. This may have repercussions next spring as animals which have gone into the winter with high worm burdens may have amass emergence when the weather warms up causing clinical signs and high levels of pasture contamination.

It’s important to note that many wormers are not effective at treating the hypobiotic stages of the larvae so worming during the winter may not help reduce the problem.

Climate change is altering the epidemiology of some worm species too. Nematodirus battus is traditionally thought of as a “lamb to lamb” infection with disease occurring in young lambs in the springtime. However in recent seasons we have seen an increase in well grown lambs having problems with high Nematodirus burdens in the Autumn with resultant clinical signs of scouring and weight loss. Historically N. battus eggs required a significant period of chilling followed by an average day/night temperature exceeding 10oC for 10 days in order to trigger the mass hatching, however, there are strains of N. battus that no longer require this factor and therefore multiple generations are now possible within the same grazing season.

Haemonchus contortus (aka the Barber’s pole worm) is another parasite which is becoming more significant. Traditionally thought of as a tropical parasite this worm is now well established in many areas of the UK. The adult worms suck blood in the abomasum of the host and can cause severe anaemia and even death. A
burden of 5000 worms can cause a sheep to lose 250ml of blood perday resulting in rapid development of anaemia. H. contortus is a prolific egg layer and infections can build quickly but in contrast to most other worm species they rarely cause scouring. Affected animals suffer ill-thrift and weakness and may have pale mucous membranes and “bottle jaw” with soft swelling under the chin. This species is able to undergo hypobiosis meaning larvae can survive the winter in the
colder parts of the UK although in warmer areas development right through to the adult stage is possible all year round.

As ever prevention is better than cure and you can keep an eye on your flock’s worm challenge with regular faecal egg counting. We are happy to do these at any time from individual or pooled samples, either from healthy animals or any you have a concern about, please have a chat with your vet for more information. Remember the Dump Days are there for a reason.