Getting to the wriggly bottom of “Refugia”. What is it and how does it benefit our horses?

A recent headline in a “Horse and Hound” publication caught the attention of the global equine community. “Large numbers of horses will die if we do not change the way we worm”. This dire warning comes from BEVA president and internal medicine specialist David Rendle. He admonishes that if we do not embrace sound practices around the use of equine anthelmintics then our current treatment options may soon lose efficacy. As controversial as this may seem, when we look at outdated deworming practices that are still being used in our own country, it may be time to take note. The New Zealand equine community is in a prime position to heed these warnings and learn from examples in Europe and the States where there is now documented resistance to all classes of anthelmintics. The situation has become so problematic that several European countries have placed restrictions on anthelmintic over the counter sales and effectively restricted anthelmintics to a prescription basis dependent on recent faecal egg count (FEC) results.

We also have the benefit of experience from our peers within the New Zealand sheep, beef and dairy industries. The fallout from anthelmintic resistance (AR) in production animals is of huge economic significance and research has been expedited due to these production losses through key initiatives such as Wormwise™ and pioneering modelling data from AgResearch.

In the equine industry, our focus is on athletic performance and animal welfare, rather than production, and it is more difficult to measure changes in these areas specifically attributable to AR. However, if we lose access to efficacious anthelmintic options and therefore the ability to treat clinically affected animals, then both welfare and performance will be significantly compromised.

To mitigate these risks there are two concepts that are crucial to become comfortable with: Resistance and Refugia. Most of us are familiar with AR which is the genetic ability of worms to survive an anthelmintic treatment they were previously susceptible to. Selection pressure for resistance arises from high frequency exposure to often unnecessary anthelmintic treatments.

Refugia is a concept whereby a portion of a worm population is left unexposed to chemical treatment and therefore protected from selection pressures for resistance development. The aim is to create a large “refuge” of susceptible parasites within the environment and subsequently the horse. The presence of susceptible parasites within the horse allows adult worms to dilute the genetic expression of any resistant strains during reproduction.

The whole process begins with two adult worms reproducing in the lumen of the large bowel. The subsequent eggs that are released into the environment carry the genetic blueprint for the future adult’s response to any given anthelmintic. The higher the reservoir of adult worms in the gut with susceptible genes, the higher the chance eggs will not carry resistant genes. This concept has been illustrated in work performed in lambs through AgResearch (Waghhorn et al., 2008).

How can we safely apply these principles to minimise selection for AR whilst maximising horse health? Treating less is ideal but how do we practically achieve this whilst managing parasite pasture contamination?

To help answer these questions we will be focusing on the cyathostomin species in adult horses which lends itself easily to explain the concept of refugia.

Sources of refugia

Environmental refugia

Seasonally the proportion of eggs and larvae on pasture versus larvae and adults in the horse can vary. At times up to 90% of the cyathostomin population will be on pasture (Nielsen and Reinemeyer, 2018). This is a great example of environmental refugia as these stages are free from exposure to any anthelmintic the horse may receive. In New Zealand our warm and wet environment lends itself to favourable conditions for larval development on pasture for most of the year.

Horse Refugia

There are two key examples of cyathostomin refugia that occur within the horse: larval inhibition and selective therapy.

Cyathostomin larval refugia

Horses ingest infective L3 larvae from pasture which soon migrate through the digestive system to the colon and caecum. The larvae may complete their lifecycle and emerge through the bowel wall becoming reproductively active adults within the lumen. Alternatively, if significant numbers of adults are already present within the lumen or host/climatic conditions are unfavourable, larvae may stay in a state of arrested development within the bowel wall. The larvae form cysts within the mucosa and can in some cases remain in this state for up to 2 years (Nielsen and Reinemeyer, 2018). These encysted stages are largely refractory to penetration from most anthelmintic treatments and therefore in themselves a source of refugia.

Larvicidal pharmaceuticals that have some efficacy against mucosal stages include moxidectin and historically a 5-day fenbendazole (Panacur™) course. However, there is now widespread resistance to fenbendazole and moxidectin is the preferred larvicidal anthelmintic (Steinbach et al., 2006).

Prudent use of larvicidal versus adulticidal anthelmintics is crucial to protect this reservoir whilst at the same time minimising the risk of host disease. This applies especially to larval cyathostominosis which is associated with a 50% mortality rate in acute clinical cases (Love et al., 1999). An annual larvicidal treatment is good practice to manage this risk, whilst still maintaining a controlled population of larvae in a state of refugia.

Selective therapy

The most important contribution to refugia we can make as clinicians is implementing selective anthelmintic therapy versus fixed interval blanket treatments. This looks at the individual host’s ability to suppress egg production via FEC. It then identifies their egg shedder status and allows us to advise on the most appropriate frequency of treatments based on this. The strongyle egg shedder status of adult horses appears to be a stable trait and can be readily classified following a well-timed FEC (Nielsen and Reinemeyer, 2018). When testing we must ensure horses are not within the egg reappearance period for their last anthelmintic to avoid false negatives or low readings. Veterinarians should also be aware of overinterpreting the FEC. It is not a useful predictor of disease and the FEC result does not directly correlate to the number of adult worms present within the horse.

The following table illustrates the average distribution of shedder types within the adult horse population.

Table 1. Population spread of egg shedding status and treatment interval recommendations in adult horses

Shedding Status


Estimate % of population

Treatments per year













Here we can see that 50% of horses generally fall into the low egg shedder category and current recommendations suggest these individuals only require 1 to 2 treatments per year! Interestingly the 20% of high egg shedding individuals make up an 80% contribution to pasture contamination. Strategic treatment of this higher shedding group mean that we can encourage refugia in the lower egg shedding groups with extended treatment intervals, whilst minimising pasture contamination.

Gone are the days of 6 to 8 weekly interval treatments, “drenching by the season” or using the dreaded “worming wheel”. The ability to identify individual shedder status is an advantage we have in our industry compared to production animals as we have a far greater ability to assess individual animals versus large flocks or herds. Obtaining samples from broodmares during breeding season is obviously another very simple method.

We now have computer generated modelling available to predict possible AR in cyathostomin species placed under different anthelmintic treatment regimes. Some simulations suggest that there is no significant reduction in AR development until treatment frequencies are getting closer to two treatments per year! This work highlights how important selective therapy and extending treatment intervals is to curb the acceleration of AR (Leathwick et al., 2019).

Many owners may be concerned about placing their horses at risk of worm related disease if using selective therapy. A recent study conducted in New Zealand followed a group of foals and broodmares on two North Island stud farms under variable treatment regimens (Nielsen et al., 2020). Foals were split into two groups and either treated at 8 weekly intervals or once at 2 months and once at 5 months. Mares were split into 3 groups. Group1 was treated twice in 1 year, group2 when FEC thresholds exceeded 300epg and group 3 at 8 weekly intervals. There was no significant difference exhibited in foal growth rates, mare weight, clinical disease or general health parameters between any of the treatment groups. This study indicates that less treatment does not necessarily lead to increased clinical disease. Limitations included the fact that it was conducted over just one year and did not consider the possibility of increased pasture contamination going forward.

Maintaining a large reservoir of parasites in refugia through understanding the cyathostomin lifecycle and adopting selective therapies is crucial in the battle against AR. Through every step of setting up a farm management programme client communication is of utmost importance. If clients understand that there are no increased risks with selective therapy, compliance is generally high, especially as we are advising less anthelmintic as a result which equates to real savings for them.

Each farm should have a tailored programme outlined annually, there is no “one size fits all” solution. There are many other tools including quarantine drenching, cross grazing, the use of efficacious products and timing of treatments that help to support refugia and reduce chemical input. The most important principle to apply across all farms is to identify adult horses as individuals and treat strategically. If owners are on board with these strategies their farm contamination should be greatly reduced, resulting in a controlled population of worms that are still susceptible to treatment. Remember a zero-egg count is not the aim of the game!

Food for thought: A common myth amongst horse owners is that “horses should be treated and moved to clean pasture. Every time we use any anthelmintic there will be a small proportion of worms that survive treatment even if this product is highly efficacious. By moving horses to prepared “worm free” pasture post treatment we will only be encouraging a resistant sample of infective larvae to populate the new pasture and ultimately the horse.
Instead by allowing horses to remain on pasture with safe levels of exposure to susceptible worms post treatment, the effect of any remaining resistant larvae will be greatly diluted.

In summary, in order to combat anthelmintic resistance, the aim is not to eliminate all worms with a zero-egg count. The aim is to strike a balance between preventing illness related to infection and maintaining a population of worms that are susceptible to anthelmintics. To achieve these aims we need to:

  • Refrain from over treating, particularly mucosal (larval) stages of the cyathostomin lifecycle.
  • Target treatments by identifying horses on the basis of their shedder status and developing treatment regimens specific to individual horses.
  • Advise against moving horses to clean pasture after treatment.


Leathwick D.M., Sauermann C, Nielsen M.K. Managing anthelmintic resistance in cyathostomin parasites: Investigating the benefits of Refugia-based strategies. Int.J.Parasitol.Drug Resist.10:118-124, 2019

Love S, Murphy D, Mellor D.  Pathogenicity of cyathostome infection. Vet. Parasitol. 85:113-122, 1999

Nielsen M.K., Reinemeyer C.R. Handbook of Equine Parasite Control. John Wiley & Sons, New Jersey, 2018

Nielsen, M.K., Gee, E.K., Hansen, A., Waghorn T., Bell J., Leathwick D.M.. Monitoring equine ascarid and cyathostomin parasites:Evaluating health parameters under different treatment regimens. Equine vet. J. 53 (5) 902-910, 2021

Steinbach T, Bauer C, Sasse H, Baumgartner W, Rey-Moreno C, Hermosilla C, Damriyasa I, Zahner H. Small strongyle infection: Consequences of larvicidal treatment of horses with fenbendazole and moxidectin. Vet. Parasitol. 139:115-131, 2006

Waghorn T.S., Leathwick D.M., Miller C.M., Atkinson D.S. Brave or Gullible: Testing the concept that leaving susceptible parasites in refugia will slow the development of anthelmintic resistance. NZVJ. 56 (4) 158-163, 2008