The prevalence of Gasterophilus intestinalis in horses in northern England and Wales.

The stomachs of 448 horses from northern England and Wales were examined for Gasterophilus larvae, and 237 (52.7%) were found to be infected with G. intestinalis. Larvae were present in stomachs examined during each month of the year except August. Second instar larvae occurred from September through February and third instars were present from November through July. Adult fly activity began in August as indicated by the presence of eggs on horses. The life-cycle of G. intestinalis in northern England and Wales is outlined from the data presented. The mean instar burdens were 15.7 second and 38.0 third instars, and more than 75% of the infections consisted of up to 50 larvae. Prevalence of infection and mean larval burdens declined with increasing age of host. Only one of 258 duodena examined was infected with G. nasalis and this horse originated from the south coast of England, outside of the catchment area of the other horses examined.