Research on Climate Change and the Development Rate of Anthelmintic Resistance in Equine Cyathostomins

This research project found interesting results when looking at computer simulation models.

A warmer climate and longer parasite transmission season can also affect development of dewormer resistance. Courtesy Dr. Martin Nielsen

Dr. Martin Nielsen of the University of Kentucky’s Gluck Equine Research Center collaborates with leading scientists in New Zealand. In their most recent publication, they combined several computer simulation models to study how climate change could affect horse parasites and drug resistance. His New Zealand collaborators were Christian W. Sauermann, Dave M. Leathwick and MarkLieffering of AgResearch, Grasslands Research Centre.

These researchers combined their cyathostomin (small strongyle) simulation model with six different climate change prediction models and found very interesting results:

  • Shifting of seasonality will have a marked impact on parasite transmission patterns.
  • This can lead to larger parasite burdens in areas changing from temperate to warmer climates.
  • The warmer climate and longer parasite transmission season can also affect development of dewormer resistance.

“We evaluated three climatic regions within New Zealand, and found the most dramatic effects in the coldest one,” said Nielsen. “Horses grazing here will get more worms, and if you deworm them regularly, they will also see drug resistance more quickly. 

“This is because the climate is predicted to shift towards conditions more favorable for parasite eggs and larvae on pasture,” he continued. “And the grazing seasons will be longer. In the two other climates, which are both warmer, the changes were less dramatic.”


“Climate change is likely to influence livestock production by increasing the prevalence of diseases, including parasites. The traditional practice of controlling nematodes in livestock by the application of anthelmintics is, however, increasingly compromised by the development of resistance to these drugs in parasite populations. This study used a previously developed simulation model of the entire equine cyathostomin lifecycle to investigate the effect a changing climate would have on the development of anthelmintic resistance. Climate data from six General Circulation Models based on four different Representative Concentration Pathways was available for three New Zealand locations. These projections were used to estimate the time resistance will take to develop in the middle (2040–49) and by the end (2090–99) of the century in relation to current (2006–15) conditions under two treatment scenarios of either two or six yearly whole-herd anthelmintic treatments. To facilitate comparison, a scenario without any treatments was included as a baseline. In addition, the size of the infective and parasitic stage nematode population during the third simulation year were estimated. The development of resistance varied between locations, time periods and anthelmintic treatment strategies. In general, the simulations indicated a more rapid development of resistance under future climates coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages. This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages. 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.”

Their paper is available open access from the International Journal for Parasitology: Drugs and Drug Resistance.

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