What is the current situation with sheep and goat nematodes?

Sheep and goat farmers, whether that be in a com­mercial or a smallholder setting, are inadvertently endoparasite (“worm”) farmers. Endoparasites are hugely costly and it was estimated back in 2005 that the cost to the UK sheep industry was £84 million (Nieuwhof and Bishop, 2005). Initial reports in the 1950s showed concerns of Haemonchus contortus resistance (Drudge et al., 1957). Kaplan (2004) showed that resistance was first documented within three to nine years of group 1 to 3 anthelmintics becoming available. The WAARD project showed that only 2 percent of Welsh sheep flocks had no levels of anthel­mintic resistance (WAARD project consortium, 2015). Triple resistance in the spring/summer of 2015 was present on 60 percent of farms. Charlier et al. (2020) has published data to suggest the cost of anthelmintic-resistant nematodes in sheep alone, excluding susceptible nematodes, is costing the UK sheep industry €3.6 million.

So, how do you know if nematodes are production-limiting on-farm? A valuable National Sheep Association (NSA) webinar held in July 2020 stressed the importance of “invest and test – don’t guess. Work with your vet, and one that is a sheep enthusiast”. Routine regular faecal egg counts (FEC) are the essential first step in responsible use of anthelmintics and their use is increasing (Figure 1).

FEC – when and how best to do it?

Regular FEC should be done every two to three weeks in lambs from six weeks of age to monitor parasite burden. Samples need to be fresh, “still warm”, and refrigerated until they reach the lab. Pooled worm egg counts need to be done on 10 to 15 individual samples taken from the aver­age animals in the group. If worming is deemed necessary, post-drench checks are essential and should be done at the appropriate times as stated in Table 1.

Nematodirus can occur acutely and may cause a problem before eggs are seen on FEC. The Nematodirus Forecast is a great tool for monitoring the risk of mass hatchings in your practice area (SCOPS, 2020a).

It is important that lambs and ewes have separate pooled FEC. Ewes are generally quite resist­ant to endoparasites unless there is a high worm challenge at pasture (eg Haemonchus contortus), immunosup­pression around lambing resulting in the periparturient rise or concurrent disease in the ewes (eg iceberg dis­ease such as maedi-visna).

Lungworm

Lungworm is generally thought of as a parasite that does not cause clinical disease in sheep. However, the author has seen several clinical cases this year. It is important to note that not all anthelmintics are effective against the two common lungworms: D. filaria and M. capillaris. Perhaps our responsible use of anthelmintics and regular FEC will pose a new challenge with regards to less common endoparasites in sheep and goats in the future.

Goats

Goats are much more susceptible to endoparasites than sheep and never form an immunity to the parasites unlike their ovine counterparts (Edwards et al., 2007). So, with that knowledge, all the principles to ensure responsible anthel­mintic usage are even more important in goats as there are many factors that lead to resistance (Harwood, 2018).

There are very limited licensed products for goats in regard to anthelmintics. Licensed group 3 treatments exist for goats; however, to protect this anthelmintic class it is essential for veterinary surgeons to be able to use the vet­erinary prescribing cascade to vary the anthelmintic classes used (Table 2). Of course, appropriate withdrawals must be set for the prescription of off-licence medication.

It is important to note that group 4 and group 5 anthel­mintics should be used with caution in goats to ensure we protect the newer classes from developing resistant nema­todes. There is a place for quarantine treatments, knock-out drenches and use following confirmation of multi-resistance nematodes in sheep; however, please discuss with the rele­vant company before using in goats.

How to keep your flocks healthy and endoparasites resistance free

1. Quarantine treatments

The SCOPS website now advises three levels of quarantine treatment: gold, silver or bronze (SCOPS, 2020b). Gold stand­ard treatment is with a group 4 and a group 5 wormer with or without a moxidectin injection (dependent on scab risk and inability to organophosphate (OP) dip). The use of the sheep scab blood test ELISA two weeks following arrival, providing able to remain in isolation, will be a useful tool in helping to avoid unessential anthelmintic treatments. Following gold standard treatment has had some difficulties due to the cur­rent availability within the UK of group 5 wormers.

2. Avoid underdosing

Dosing guns are one source of underdosing due to inac­curacy (Geddes et al., 2017) and therefore need regular calibration. The other common cause of underdosing is due to inaccurate weights.

Goats pose a risk with underdosing for two main reasons: incorrect dosages of treatments (Table 2) and underesti­mating weight (Harwood, 2018).

3. Avoid overuse

Avoiding overuse is essential and echoes the NSA's state­ment of “invest and test – don’t guess”. Regular FEC moni­toring is key to establish if/when flocks require anthelmintic treatment. Proactive flocks are pasture mapping to establish the “risk” for each field they graze. Another method to avoid overuse is to regularly weigh lambs, dosing those that are not hitting expected growth targets. It is thought that 20 percent of your sheep will have 80 percent of your worm population (NSA, 2020).

An open letter by Stocker et al. (2020) urges the careful use of group 4 and 5 anthelmintics to protect their lon­gevity and efficacy for the sheep industry. Two main times that these groups should be used are (1) as “quarantine” drenches and (2) as mid-late season “knock-out” drenches. The aim of the “knock-out” drench is to kill any resistant worms that have developed over the season through the use of groups 1, 2 and 3.

4. Reduce exposure

Forming a closed flock is the most effective way to reduce exposure. For flocks with show animals, a simple invest­ment in plastic matting/boards in the show pen will help prevent sheep and goats from grazing when housed at shows. If also strict on preventing grazing in the show ring, it will minimise their risk of exposure to endoparasites. Remember, vets should be recommending that all show animals require quarantine treatments on their return. Do we need to consider the idea that virtual shows are the way forward for the future, eliminating risk of disease and para­site spread and enabling high health status closed flocks to participate as well?