A model for the dynamics of the parasitic stages of equine cyathostomins

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand

“…The large increase in EL3 in Southland reflects the increase in larval ingestion due to the much better development of eggs on pasture. In the model the EL3 population is strongly influenced by the number and seasonal pattern of larvae on pasture ( Leathwick et al, 2019 ) while the adult worm burden remains relatively constant. EL3 populations are known to be highly seasonally variable between animals and can reach enormous numbers ( Nielsen et al, 2010 ).…”

“…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 burden increases with the number of infective stage larvae ingested ( Leathwick et al, 2019 ), 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. This can be seen in the positive relationship between the time resistance took to develop and the number of parasitic EL3 (as a measure for the parasitic phase) or free-living L3 population (as a measure for the free-living phase and reproductive success) ( Fig.…”

“…The development of the model and incorporation of genetic mechanisms for anthelmintic resistance have been described in detail elsewhere ( Leathwick et al, 2019 ; Leathwick et al, 2015 ; Sauermann et al, 2019 ). Briefly, the model replicates the dynamics of the cyathostomin free-living and parasitic stages, the administration of anthelmintics and the subsequent build-up of resistant genotypes.…”

“…All simulations assumed a group of 16 horses (1–19.75 years old in increments of 1.25 years). Age is used as a proxy for horse immune status, which influences the values of several parameters ( Leathwick et al, 2019 ), including the establishment of new infection and egg count levels. Thus, the 16 horses used in the simulations represent a range in immune status, between that of a naïve foal or yearling to a mature, immune competent horse.…”

“…The recent development of a whole lifecycle simulation model for the cyathostomin species complex infecting horses Leathwick et al, 2019 ; Leathwick et al, 2015a , Leathwick et al, 2015b ; Sauermann et al, 2019 ) enabled the investigation of the development of anthelmintic resistance under different treatment regimens and for different climatic locations ( Leathwick et al, 2019 ; Nielsen et al, 2019 ). In these modelling studies the size of the parasite population increased, and anthelmintic resistance developed more rapidly in warmer climates when compared to colder climates.…”