Computer modeling paints a worrying picture of the likely effects of climate change on cyathostomins, the most common parasite in horses.
A just-published New Zealand study used a previously developed simulation model of the entire cyathostomin lifecycle to investigate how a changing climate would affect development of resistance to dewormers.
The study team input climate-change projections out to the end of the century for three areas in New Zealand, which reflect the diversity of the country’s climatic conditions.
Three researchers with the New Zealand agricultural science agency AgResearch, parasitologists Christian Sauermann and Dave Leathwick, and climate specialist Mark Lieffering, together with parasitologist Martin Nielsen, with the M.H. Gluck Equine Research Center at the University of Kentucky, modeled outcomes from 2006 to 2100, factoring in different deworming treatment strategies to see how these would affect the results.
Sauermann, Leathwick and Nielsen were the scientists who developed the model used in the study, which replicates the dynamics of the cyathostomin free-living and parasitic stages, the administration of dewormers and any subsequent build-up of resistance.
All simulations in the study, led by Sauermann, were based on a group of 16 horses aged 1 to 20, with the age being a factor in the modeling in terms of immune status. The model assumed a single grazing area of 1 hectare per horse, cumulatively grazed throughout the year with a predetermined pasture cover and fixed daily dry matter intake of 8kg by each animal.
Two treatment strategies were modeled. The first involved the use of a dewormer every six months; the second involved their use every second month.
Data from six different climate-change models were input for the Waikato region, which has a warm, humid summer and a mild winter; Hawkes Bay, which has a warm, dry summer and a mild winter; and Southland, which has a mild summer and a cool winter. The projections pointed to a warming climate in all areas.
The study team evaluated two outcomes: The size of the parasite burden (that is, would the horses get more worms?), and whether development of drug resistance would be accelerated by climate change.
The answer to both was yes, but it depended on the climatic zone, with the most dramatic parasite-related changes seen in the Southland modeling.
The study team found that, in general, the simulations pointed to more rapid development of resistance under future climates, coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages for cyathostomins, which are also known as small redworms or small strongyles.
“This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages,” the study team wrote in the International Journal for Parasitology: Drugs and Drug Resistance.
“A longer period suitable for larval development resulted in an increase in the average size of the parasite population, with a larger contribution from eggs passed by resistant worms surviving the anthelmintic treatments.
“It is projected that climate change will decrease the ability to control livestock parasites by means of anthelmintic treatments and non-drug related strategies will become increasingly important for sustainable parasite control.”
Non-drug related strategies will become increasingly important for parasite control, they suggested.
The authors noted that the changes in the Southland region were notably larger than those predicted in the more northerly (generally warmer) regions. The large increase in third-stage larvae in Southland reflected the increase in larval ingestion due to the much better development of eggs on pasture.
“The predicted milder Southland winters with less cold day events will create environmental conditions more suitable for the free-living parasite stages for a longer part of the year and give any worms that survive treatment a greater opportunity to successfully produce viable infective stage offspring.
“It follows then that the increased rate of development in anthelmintic resistance is linked to the changes in parasite dynamics.”
There was an association between treatment frequency and development of resistance, they said, with resistance developing faster under six annual treatments than under two.
“With more anthelmintic treatments the proportion of resistant adult parasites in the host is increased as is the time over which no susceptible eggs are passed onto pasture.
“Furthermore, with an extended transmission period, these resistant adult worms also have a higher likelihood to reproduce successfully to infective stage offspring, thereby escalating development of anthelmintic resistance.”
Because adult worm burdens increase with the number of infective stage larvae ingested, higher survival and therefore numbers of free-living stages on pasture also increases the number of adult worms available to be selected for resistance by anthelmintic treatments.
“The likely dilemma ahead will be that if parasite populations, in many areas, are going to increase in size under climate change, horse owners and veterinarians are likely to respond by increasing the number of administered anthelmintic treatments, thereby making an already increasing problem of anthelmintic resistance, even worse.”
It is widely accepted, they said, that greater use of deworming drugs never has, nor ever will, constitute a sustainable solution, but only results in a faster selection for resistance.
“The solution will be to find ways in which parasite populations can be managed without more frequent administration of anthelmintic products.”
Mitigation strategies that need to be explored and/or implemented include more evidence-based management, ensuring the current resistance status is identified and effective treatments used, and avoiding unnecessary use of drugs.
“In conclusion,” they wrote, “the study indicated that climate change will influence both the level of parasitism and development of anthelmintic resistance, however, the scale will vary with the magnitude of climate change and between climatically different locations.
“The results suggest that the changing environmental conditions in temperate climates will further lessen the ability to sustainably control livestock parasites merely by means of anthelmintic treatment. Non-drug related parasite control strategies will be increasingly necessary with predicted climate changes.”
Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand
Christian W. Sauermann, Dave M. Leathwick, Mark Lieffering and Martin K. Nielsen.
International Journal for Parasitology: Drugs and Drug Resistance, Vol 14, December 2020, Pages 73-79 https://doi.org/10.1016/j.ijpddr.2020.09.001