Occurrence of second and third instars of Gastrophilus intestinalis and Gastrophilus nasalis in horses in the mid-atlantic united states

Seventy-four ponies and horses were necropsied over a 4-year period (April 1973–April 1977). Prevalence data on second and third instar larvae of Gastrophilus intestinalis and G. nasalis were collected. Second instar G. intestinalis were found from late July through March of the following year. Third instar G. intestinalis were found throughout the year with minimum numbers in August, September and October. Second instar G. nasalis were recovered from two animals in February and November respectively. Third instar G. nasalis occurred too infrequently to provide sufficient data for analysis except to note they appeared to be present all year. Four animals were not infected with either species.