New approach to horse worms needed

June 23, 2008
 

An Australian report on horse farm pests suggests that a new approach to controlling horse worms is needed.

Researchers at the Centre for Environmental Stress and Research Adaptation at the University of Melbourne have been working on a system of integrated pest management (IPM) for use on horse farms. A report was made for Australia's Rural Industries Research and Development Corporation.

The past 50 years have seen an increasing reliance on the use of chemicals to control intestinal parasites in horses and other species. But worms are becoming resistant to most, if not all, of the drugs used against them. Chemicals alone can no longer be relied upon to control horse parasites. A more sustainable worm control system is needed, the report found.

By adopting a controlled or integrated approach, the aim is to reduce the reliance on chemical treatment. Instead of regular treatments at set times based on the properties of the wormers, chemicals could be used only when necessary.

Most chemical control measures target the parasitic stages of the worms - by treating the horse. However, the free-living larval stages on the pasture account for the vast majority of the total population. They are susceptible to predators, both natural and introduced.

Control measures to disrupt the free-living stage of the life cycle include removing the dung from the pasture, rotating pastures, and harrowing in dry weather. These play a part in an integrated pest management system. However there are other aspects to consider - such as the insects that live on the pasture and take part in breaking up the dung pats.

The study led by Cindy Edward involved 10 horse farms with differing worm control regimes. It looked at how to take advantage of the natural enemies of the free-living stages of parasites.

Among the insects found on the pasture were various species of dung beetle. These were considered beneficial as they break up and bury the dung, exposing the larvae to desiccation.

The researchers found that the species of dung beetle present were active during the summer but did little in early spring or autumn and winter. So they tried introducing two new species, one of which was active in the summer and the other in the winter.

They also looked at the role of birds in control of the free-living larvae. When horses are fed whole grain, some may pass through the gut undigested. Birds scavenging for grain in the dung, scatter it and allow it to dry out. This may help reduce the infective larvae on the pasture. Indeed, in one experiment, the researchers retrieved fewer larvae from the pasture surrounding the dung when it contained grain (and so was scattered by birds) than when it contained no grain.

Bird activity may help reduce pasture contamination in dry conditions, but under cool moist conditions it may actually favour the spread of larvae onto pasture.

Although bird activity may help reduce pasture contamination in dry conditions, the researchers warn that under cool moist conditions it may actually favour the spread of larvae onto the pasture. They also found that bird activity interfered with dung beetles establishing successfully. So if you are trying to establish a population of dung beetles it is best to discourage scavenging by birds. Rolling, crushing or soaking grain before feeding it to horses will make it more easily digested, resulting in fewer grains in the dung.

The study also demonstrated the value of faecal worm egg counts in reducing the frequency of treatment and giving more effective control.

On one farm, which had been drenching every six weeks, the introduction of monthly worm egg counts allowed the number of treatments to be greatly reduced.

"Horses may develop a low level of tolerance to parasites without health concerns," said Cindy Edward. "A continual negative FEC through drenching is not necessary for a healthy horse."

Another farm had been drenching for worms in spring and winter. In the second year of the study, the timing of dosing was based on FEC. This resulted in the winter treatment being brought forward by three months. Although the same amount of medication was used it resulted in lower pasture contamination overall.

The need for a different approach to controlling horse worms was reinforced when the regular FECs identified worms that were resistant to ivermectin.

Dung beetles are adversely affected by ivermectin. So another advantage of reducing the frequency of worming would be to encourage dung beetles to become established. Indeed, the researchers found fewer dung beetles in pastures where horses were treated every 6 weeks compared with those treated 3 monthly or once a year.

"The integrated pest management strategy developed in this study will enable a more sustainable, environmentally friendly horse farm," Edward concluded. "The key to controlling internal horse parasites is to maintain a low burden of parasites in the horse, and most importantly on pasture."