Parasitology

Strategic worm control

Dr Frank Jackson

Helminth infections are an important cause of poor welfare and lost production and nematode infections alone have been estimated to cost the UK sheep industry around £65 million each year. To date, control of helminths has almost exclusively been achieved through the use of anthelmintics. However despite the availability of numerous anthelmintics the number of cases of parasitic gastroenteritis and fluke reported by the veterinary investigation services have continued to increase. The explanation for this increase may lie in climate changes, prevalence of drug resistant parasites and farm management systems. In Scotland, benzimidazole resistance has more than trebled in the last ten years and a recent Moredun survey identified resistant Teladorsagia populations on over 80% of our lowland farms. Recently we have seen the first reports of multiple anthelmintic resistance in sheep and the emergence of triclabendazole-resistant fluke populations on sheep farms in the south west of Scotland. These findings suggest that sole reliance upon chemoprophylaxis for controlling helminthoses may not be a sustainable approach and have led to research into alternative prophylactic approaches. Researchers at Moredun are currently investigating methods of enhancing the efficacy of available drugs against a background of multiple resistance and, through pharmacokinetic studies, means of enhancing bioavailability of the drugs within the host.

Researchers at Moredun are investigating a number of alternative prophylactic approaches to the control of nematodes that utilise either host immunity, plants with anthelmintic properties to regulate parasite populations within the host or fungal predation to regulate nematode populations on the pasture.

Optimising Nutrition and Immune Responses

Recent research at Moredun involving collaborators in Scotland and New Zealand has highlighted the importance of protein availability in the maintenance of immunity against gastrointestinal nematode infections in the periparturient ewe. These studies have shown that ewes supplemented with non rumen degradable protein can maintain their immunity to worms during the periparturient period.

Figure 1
Percentage protection as measured by reduction in egg count and worm burden in vaccinated and control animals used in nine vaccine trials.

Vaccine Development

Hidden antigen studies at Moredun have identified and patented a number of promising candidate antigens from Haemonchus contortus, a haematophagous parasite that has a huge impact on sheep production throughout the world. Several studies using one microvillar extract H-gal-GP have provided more than 90% reduction in egg count and 70% reduction in worm burden (see Figure 1). The challenge now facing researchers at Moredun is to produce recombinant forms of these antigens that are immunogenic and confer similar levels of protection as the native protein.

Genetic Selection

Another form of immunoprophylaxis relies upon enhancing the development, expression and maintenance of immunity at the herd level through genetic selection. Research at Moredun in collaboration with researchers at the Macaulay and Roslin Institutes has demonstrated the benefits of genetic selection in Scottish Cashmere goats, where egg output has been reduced by over 40% following several years of selection. Protocols developed in this research and research on Texel lambs have been adopted by commercial companies who now offer this service to the sheep industry.

Figure 2
Effect of incorporating condensed tannin-rich extract into diet on week 5 in lambs continuously infected with T.colubriformis

Control through Pasture Management

Parasite populations can also be regulated using bioactive forages ie plants containing secondary compounds that either act directly on the parasites or indirectly through effects on nutrition. Studies at Moredun using a commercial product rich in condensed tannins were the first to demonstrate direct anthelmintic effects, reducing both egg count (see Figure 2) and worm burden. Grazing studies using bioactive forages such as chicory, sulla and sanfoin are currently examining the impact that these can have on gastrointestinal nematode populations.

Figure 3
Worm burdens accumulated in 4 weeks grazing by tracer lambs grazing paddocks where animals had either been fed or not fed D.flagrans chlamydospores.

Parasite populations on pasture can also be regulated using either grazing management or predatory nematophagous fungi such as Duddingtonia flagrans. Trials at Moredun feeding D. flagrans chlamydospores to grazing ewes and lambs have shown some benefits over the season as a whole (see Figure 3) but no benefit when used solely to control spring ewe-derived contamination. Failure at this time may be due to the effects of temperature (spores take longer to germinate at low temperatures), the persistence of faecal pats (trapping will not occur outside the faecal pat and soft ewe faeces tends to break down very quickly) and changes in the faecal environment (ewes almost always scour for several weeks after turnout).

Although none of the alternative approaches offers a means of replacing anthelmintics, they do offer a way of reducing our reliance upon chemoprophylaxis and, if used in an integrated approach, should minimise parasite adaptation and hence offer benefits with regard to sustainability.

This research is funded by SEERAD, EU & BBSRC.