The importance of paying attention to anthelmintic resistance on your farm

Following the success of the National Sheep Welfare Scheme (NSWS), the Sheep Improvement Scheme (SIS) was introduced under Ireland’s CAP Strategic Plan, offering financial support and incentives to farmers to go beyond mandatory health and welfare standards to improve the overall welfare of their flocks.

It also is enhancing the working relationship between farm owners and their vets/local laboratories. One of the implemented measures in the scheme includes parasite control involving faecal egg count (FEC) testing of a group of lambs. Different types of targeted actions are required depending on whether it is a lowland or hill flock. (See here for more on the SIS)

FEC testing is a very useful, inexpensive and effective tool to assess parasite burdens as well as the level of pasture contamination with parasites.

Therefore, it is important to sample animals at appropriate times to gain the maximum information to benefit from the design of a specifically tailored parasite control plan for the farm.

Both the NSWS and the SIS were very successful in improving animal welfare standards, in particular highlighting the importance of parasite control.

These schemes have demonstrated the usefulness of FEC testing as a hugely useful tool in assessing parasite exposure in a single or group of animals.

Many flock holders now have a growing appetite and understanding of the importance of an individualised and specifically tailored parasite control plan, from both an economic standpoint and welfare point of view.

Continuous exposure

Grazing sheep are continuously exposed to roundworm challenge at pasture, which may result in clinical disease (especially in lambs in their first grazing season) resulting in a range of clinical signs such as diarrhoea, ill-thrift and sudden weight loss.

Although parasite control has always been part of a farmer’s general flock management programme, parasite control in sheep has become a growing concern within both the agricultural and veterinary industries. This is due to the increasing resistance of nematodes to various classes of wormers and the limitations of current deworming practices. No new classes of wormers have been discovered, and the persistent issue of wormer resistance remains a serious challenge in farm animal and equine husbandry.

A survey of wormer effectiveness as part of the sheep technology adoption programme (STAP), administered by the Department of Agriculture between 2013 and 2015 which involved more than 4,200 sheep farms found widespread resistance to all of the chemical drug groups. The results showed that only 31.5%, 51.9%, 62.5% and 84% of treatments were considered effective for white – benzimidazole; yellow – levamisole; and white drenches –avermectin and moxidectin, respectively.

New approach

The historical approach of repeated blanket treatment to control parasites is no longer sustainable and appears to be growing in understanding among flock owners.

Changing weather patterns, combined with different housing and grazing systems used on Irish farms now necessitate a more tailored parasite control programme. The farming population is acknowledging the value of regular monitoring of parasite exposure in conjunction with a more strategic treatment approach.

The SIS has shown that one simple but useful tool can be implemented by farmers to highlight whether their current parasitic treatment practices are effective, assess the level of pasture contamination and if there is an emerging anthelmintic resistance issue within their flock.

Within the diagnostic laboratories in the University College Dublin (UCD) School of Veterinary Medicine, the most frequent clinical diagnostic request in farm animal parasitology is FEC analysis. While there are limitations to this testing, it can be extremely useful as an indicator of the presence of egg laying adult worms in the gut.

It cannot, however, determine the quantity of infective larvae that are ingested and in the prepatent period (time between infection and egg shedding).

Fresh dung from either a single animal or targeted pool (groups of 10) is analysed using a counting method (McMaster, mini Flotac, FECPAK, or an AI commercial analyser) to determine, both qualitatively and quantitatively the number and species of parasite eggs in that selected cohort of animals.

The most common parasites present in Irish flocks are identified, eg typical strongyle eggs (morphologically similar roundworm eggs – ie, Teladorsagia circumcincta, trichostrongylus spp, haemonchus contortus and cooperia spp); Nematodirus spp; Moniezia spp (tapeworm); and coccidia (coccidiosis causing protozoan species), excluding lungworm.

Typical parasitology reports would indicate an ‘eggs per gram’ (epg) breakdown, useful for estimating parasite exposure to an animal or pooled (composite) group.

There is a growing perception that Haemonchus contortus – commonly referred to as the ‘barber’s pole worm’ – is becoming more widespread in Ireland, though data on the actual prevalence is not known at this stage.

Unfortunately, routine FEC methods cannot easily differentiate H contortus eggs from other strongyle eggs and specific techniques are required.

H contortus eggs can be identified with a specific staining technique (peanut agglutinin, or PNA, staining) – a service which is provided by the UCD parasitology laboratory.

One single FEC is not particularly useful for assessing the efficacy of a particular anthelmintic.

A more accurate and reliable approach is to conduct a faecal egg count reduction test (FECRT). This test compares the faecal egg counts before and after a specific treatment time point.

The percentage reduction is then calculated to assess treatment efficacy. Although factors such as underdosing and the timing of sample collection can influence results, the FECRT is a valuable tool for developing a farm-specific parasite control plan, helping to determine which anthelmintics are effective or ineffective for a particular operation.

UCD veterinary diagnostics is one of the Department-approved laboratories which must be used for FECs collected for scheme compliance.

The full list can be found here. Kits and further information on services provided by UCD parasitology laboratory are available here.

https://www.gov.ie/en/service/98a53-sheep-improvement-scheme/#approved-laboratory-list

Amanda Lawlor is a senior technical officer, parasitology, in the School of Veterinary Medicine, UCD and Theo de Waal is Professor and EVBS® European Veterinary Specialist in Parasitology, UCD.