A model for the dynamics of the free-living stages of equine cyathostomins

Anthelmintic resistance in strongyle nematode parasites of horses is an expanding global problem and steps need to be taken to slow its development before control becomes more problematic. A move away from traditional deworming programmes, involving frequent whole-herd treatments with broad spectrum anthelmintics, to a more strategic or targeted use of chemicals is required. However, anthelmintic resistance management strategies which also maintain effective control are invariably more complicated and often require a greater understanding of both nematode epidemiology and grazing management, than does the simple routine use of chemicals. Here, as a first step in applying a modelling approach to resistance management in horses, a model is proposed to describe the dynamics on pasture of the free-living stages of equine cyathostomins. Firstly, the development and survival of the pre-infective stages is considered as a single process driven by temperature, and secondly, two populations of infective stage larvae (L3) are considered; those within the faecal pat and those on the herbage. Both are modelled using the box-car train approach which allows for variable development rates within a cohort of individuals and full overlap of generations. Uniquely, L3 survival is modelled as an ageing process where larvae progress through physiological age classes at a rate determined by temperature and rainfall. Model output reflects the dynamics of free-living stages under a range of environments. Under extreme cold, there is no development to L3 but eggs can survive for long periods to develop if conditions become favourable, while L3 survival is reduced under repeated freeze–thaw cycles. Under tropical conditions, development is rapid and a large number of L3 can be produced but survival of L3 is short. In temperate climates development tends to be slower, with large numbers of L3 produced over the warmer months but fewer over winter, and L3 survival tends to be higher all year round. Although attempts to validate model output against field studies were compromised by the lack of published detail or an inability to access specific weather records, outputs averaged over multiple sets of weather data was often appropriate for that location. Variation in model output when using weather data sets which started on different days within the same week suggests that day-to-day differences in weather may affect the number of L3 developing on pasture and that optimisation of anthelmintic use to minimise pasture infectivity may require a more detailed understanding of weather effects than previously thought.