Anthelmintic resistance in small strongyles (cyathostomins) of equids: The uncertain future of ivermectin

By Emily Huff

Edited by Kyle Lesack

Historically, large strongyles, such as Strongylus vulgaris were the most prevalent parasitic nematodes of horses, but widespread use of anthelmintic drugs has greatly diminished this threat (Love et al., 1999). In their place, cyathostomins (small strongyles) in the subfamily Strongyloidea are now an increasing cause for concern due to their pathogenicity in young and mature horses (Love et al., 1999) and ability to develop resistance to certain anthelmintic medications, such as fenbendazole and pyrantel pamoate (Tarigo-Martinie et al., 2001). Infections are most commonly found in younger horses less than five years of age (Giles et al., 1985). Cyathostomins have a direct lifecycle, where eggs are shed within the feces and develop into L3 infective larvae, which are then ingested by the host (Fig. 1) (Corning, 2009). The larvae migrate into the large intestine and penetrate the intestinal lining where they encyst and eventually emerge into the intestinal lumen as a male or female adult (Corning, 2009). An important aspect to note is that once encysted within the intestinal mucosa, the larvae can arrest their development for up to two years and are resistant to anthelmintic treatments while in this stage (Corning, 2009). In addition, most species of cyathostomins have higher levels of pathogenicity as larvae, however they cannot be detected through fecal egg counts during this period (Nielsen, 2012).

Figure 1. Life cycle of cyathostomins. Image from Corning (2009).

Clinical presentation of cyathostomin infection typically includes rapid weight loss (Fig. 2), recurrent diarrhea episodes, reduced appetite, subcutaneous ventral abdominal oedema, hypoproteinemia (Lyons et al., 2000; Murphy et al., 1997), and occasionally colic (Uhlinger, 1990). In severe cases, larval cyathostomiasis (sudden mass emergence of larvae from the wall of the intestine) can result in fatal colitis (Peachey et al., 2017). Diagnosis is typically achieved by examination of a fecal smear or by observing larva directly on the sleeve following a rectal examination (Murphy et al., 1997). Hematology and blood serum chemistry can also be used to support diagnosis, as neutrophilia and hypoalbuminemia are frequently present (Murphy et al., 1997) in cyathostomin infections. Since the inhibited larvae within the mucosal lining are not susceptible to anthelmintics, ivermectin or fenbendazole are commonly used to treat the developed larvae in the lumen (Murphy et al., 1997).

Figure 2. A colt with larval cyathostomiasis in poor body condition. Image from Lyons et al. (2000).

The three types of anthelmintics used most frequently for the treatment of helminths in horses are benzimidazoles, tetrahydropyrimidines, and avermectin/milbemycins (macrocyclic lactones) (Tarigo-Martinie et al., 2001). Cyathostomin resistance to benzimidazoles is the most common and widespread, however there is also an increasing incidence of resistance to pyrantel pamoate (Tarigo-Martinie et al., 2001). Previously, ivermectin was not shown to illicit resistance in cyathostomins, although this was predicted to change as its use became more frequent as a treatment and is already ineffective to other gastrointestinal nematodes of ruminants (Kaplan et al., 2004). Moxidectin (a type of macrocyclic lactone) was also shown to have some efficacy in the removal of encysted cyathostomin larvae (Xiao et al., 1994). More recently, reports of macrocyclic lactone resistance in cyathostomin infections have emerged (Kooyman et al., 2016), making this an increasing cause for concern in the equine veterinary community. Quantifying anthelmintic resistance can be done by measuring the egg reappearance period (ERP), which is the amount of time following treatment that eggs are once again present in a fecal sample (Kooyman et al., 2016). Reduced susceptibility to an anthelmintic treatment manifests as a shortened ERP (Fig. 3), which was found to be the case when horses infected with cyathostomins were treated with ivermectin (Molena et al., 2018) and moxidectin (Kooyman et al., 2016). Reduced ERP’s are thought to be caused by the survival of L4 larvae within the lumen of the intestine that reach sexual maturity and reproduce, however more research is needed to confirm this theory (Molena et al., 2018).

Figure 3. Mean fecal eggs per gram (EPG) for each horse (H) studied. No treatment was administered on Day 0 (D0). Ivermectin was administered on D14, and EPG was measured at D21, D28, D35, D42, and D49 following treatment. Image from Molena et al. (2018).

As resistance to ivermectin and moxidectin emerges in cyathostomins, it is crucial that we continue researching ways of slowing the progression of resistance in individual pastures and developing new anthelmintics or combinations of treatments to combat resistance. Strategies for controlling the populations of cyathostomins in pastures frequently involve the maintenance of pasture hygiene to control the abundance of free-living larval stages (Nielsen, 2012). This could include the biweekly removal of feces, however the practicality of this is questionable as it would greatly increase workload and would possibly require expensive machinery (Nielsen, 2012). Frequency of anthelmintic dosage is also an important factor in creating sustainable cyathostomin control. For instance, parasite transmission and abundance appear to be correlated with climate and seasonality (Nielsen et al., 2007). Depending on the location, optimal timing for parasite treatment may vary, and treatment should only be administered during this time to reduce the chances of resistance being selected for (Nielsen, 2012). 

Another possible method of reducing cyathostomin transmission is biocontrol through nematophagous fungi (Baudena et al., 2000). Duddingtonia flagrans is a species of fungi that has been shown to control populations of cyathostomins in temperate climates by reducing the number of free-living infective larvae in pastures (Baudena et al., 2000). This fungi species is ideal as it can withstand the harsh conditions of passage through the mammalian gastrointestinal tract and it is relatively easy to harvest chlamydospores from a culture (Baudena et al., 2000). Over the course of a year, the percent survival and percent reduction of infective L3 nematode larvae in the presence of D. flagrans was measured, showing that the percent reduction in the fungus treated grass plots were significantly greater than in the control plots (Fig. 4). By utilizing this method of biocontrol, reliance on anthelmintic drugs may be reduced, decreasing the rate of resistance in these cyathostomin populations (Baudena et al., 2000).

Figure 4. Percent survival and percent reduction of infective L3 larvae in grass plots treated with D. flagrans over a year (11 time periods). Figure modified from Baudena et al. (2000).

In order to produce effective anthelmintic treatments among developing resistance, the genetic mechanism for resistance in cyathostomins must be identified and understood. Other studies on parasitic nematodes have shown that ATP-binding cassette (ABC) transporters and P-glycoproteins (P-gps) are involved in resistance mechanisms through gene knockout experiments (Peachey et al., 2017). From this previous knowledge, a study was conducted that aimed to determine if P-gps played a part in reduced sensitivity to ivermectin in cyathostomins (Peachey et al., 2017). It was found that in cyathostomins displaying ivermectin resistance, pgp-9 mRNA transcription was upregulated, and the use of P-gp inhibitors improved the overall efficacy of ivermectin (Peachey et al., 2017). Based on these results, it is suggested that ivermectin use in horses be paired with a P-gp inhibitor to combat resistance (Peachey et al., 2017). Different parasite populations have been shown to respond differently to P-gp inhibitors, and different combinations of these drugs may have to be employed to increase the sensitivity of larvae (Peachey et al., 2017).

As anthelmintic resistance continues to become more prevalent in global cyathostomin populations, our research and responses need to adapt accordingly and rapidly. This current situation provides a great example of how the arms race affects our advancements in veterinary medicine and why it is so important that research into resistance mechanisms and drug development continues to be funded. Educating farmers and horse owners will also become more important as resistance progresses, as they are the people directly experiencing the negative impacts of cyathostomin infection and administering deworming medication. Providing them with alternative methods of preventing cyathostomin infection as opposed to treating it is also crucial for combating resistance, such as the use of nematophagous fungi as biocontrol agents and maintenance of pasture hygiene. As these nematodes can have devastating effects on equids, small strongyle emerging anthelmintic resistance is a growing concern among veterinarians and alternative methods for control must be developed.

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