What is the most important aspect of parasite testing and treatment for autumn/winter?

Through the grazing season we can rely on a regular schedule of worm egg counts to target strongyles and ascarids (every 8-12 weeks) and EquiSal tests for tapeworm (every 6 to 12 months) to monitor and manage horse parasite burdens. These are the most common parasites of horses, and the testing intervals are based on the lifecycle of these worms and each horse’s relative risk of parasite infection. Keep an eye out for signs or symptoms of more incidental horse parasites too during this time, such as pinworm, bots, lungworm and liver fluke and test and treat accordingly. 

However, as we go into late autumn/winter there is one dangerous parasitic stage of the small redworm that we need to be particularly aware of. Some small redworm species have an unusual developmental stage, where the larvae burrow into the horse’s gut wall and ‘encyst’. This is a state of hibernation inside our horses. Research shows that tens of thousands of these encysted larvae can line the intestine, staying for a period of up to three years. While there they can impair absorption of nutrients and cause weight loss and in large numbers are a ticking time bomb for serious disease. 

Certain changes thought to be temperature and number of daylight hours, particularly linked to the shift in season from winter to spring can trigger their ‘mass emergence’ from the gut wall. This has the potential to prompt larval cyathostominosis, an acute colitis and severe colic combined with blood streaked diarrhoea which is very difficult to treat and often fatal.

Because these larval worms aren’t egg laying, they can’t be detected on a worm egg count. But there is now a small redworm blood test that you can request through your vet. This test measures antibodies that the horse produces against any worms present and, as such, detects encysted larval redworm stages as well as adults dwelling in the gut of the horse. This test is very sensitive and so not all horses are suitable candidates.

We need to carefully weigh up our approach to managing encysted small redworm to balance protecting against potential disease while also minimising the use of wormers. Firstly, we need to assess the risk profile of each horse for parasite infection [link to blog 1]

• Horses classed as low risk – healthy adult horses with worm egg count results of less than 200 e.p.g. over the previous year, living in a closed herd with good paddock management, meet the criteria to have a blood test first to determine whether treatment is needed. We would encourage this to help preserve our key medicines that are developing resistance.
• For all other horses we would recommend a proactive dose at this time of year. Two chemicals are licenced in the UK to treat the larval stages of the small redworm - moxidectin is the preferred choice for most horses because of lower rates of resistance in this parasite. A 5-day course of fenbendazole is the alternative which may be considered especially for youngsters or those who are very lean. We can also use this opportunity to reduction test (a worm egg count carried out before treatment and a follow up 10-14 days after the last day of treatment) to ensure our worming has been effective, particularly if using the latter chemical.

Timing your treatment

Treatment for encysted redworm was traditionally done in the autumn, but climate change is adjusting our advice around this. Daytime temperatures now persist into plus figures through November and December in many parts of the UK. This keeps parasites more active on the pasture too. We only see a marked lowering of infection potential when daytime temperatures fall below 6 °C. Waiting until a sustained temperature drop occurs in late autumn/winter therefore reduces the potential for reinfection and further small redworm encysting after you have given the larvicidal dose.

Advice you may have heard around waiting for the first frost applies if you are also treating bots. This cold snap is what kills off the bot flies and so again prevents reinfection. If we have a mild winter and temperatures never drop below the magic numbers, then ensure to treat regardless if this is required. 

Every horse that does not require a wormer dose is helping to protect key medicines and delaying the development of wormer resistance. Resume worm egg counts again in the Spring for all horses. This approach is appropriate for single horses and whole yard worming programmes. 

More information available from Westgate Labs - www.westgatelabs.co.uk