Occurrence of second and third instars of Gasterophilus intestinalis and Gasterophilus nasalis in stomachs of horses in Kentucky.

The research article investigates the occurrence of various growth stages of the Gasterophilus intestinalis and Gasterophilus nasalis fly larvae in horse stomachs over a 22-year period in Kentucky.

Study Overview

• The researchers examined and compared the occurrence of the second (2nd) and third (3rd) instars stages of Gasterophilus intestinalis (G intestinalis) and Gasterophilus nasalis (G nasalis), in horse stomachs over a 22-year period from 1951 to 1973.
• The instar stage refers to the development stage between moults for insects that undergo a complete metamorphosis.
• A total of 476 horses were sampled over this long-term period.

Key Findings

• Overall, 98.7% of the horses were infected with G intestinalis, averaging 168 larvae per horse, while 80.7% of the horses were infected by G nasalis averaging 52 larvae per horse.
• There was a monthly variation in the average total numbers for G intestinalis and G nasalis. For G intestinalis, the lowest average was 50 in September, reaching a peak of 229 in March. For G nasalis, the lowest average was 14 in September, increasing to a peak of 82 in February.
• The horses were infected by 2nd or 3rd larval stages of both fly species throughout the year.

Understanding Infestations

• By differentiating the stages of larvae in horse stomachs, the study provided insights into the dynamics of these infestations.
• Data showed that for G intestinalis, 2nd instars from last year’s fly generation continue to arrive in the horse’s stomach until April. The new year’s crop of 2nd instars start appearing from July onwards, and 3rd instars from the previous year’s generation are not expelled in significant numbers until August.
• For G nasalis, it was found that arrival in the horse’s stomach of the new year’s generation of 2nd instar larvae starts in July and ends in January. The 3rd instar larvae from last year’s generation are expelled over a period from March to August.

Rate of Infection

• The study found that horses can acquire infections from G intestinalis nearly all year round, except in April. This is mainly because the larvae in the eggs can survive for a long time after development and fly activity has ceased in the fall.
• In contrast, infections with G nasalis are interrupted between December and May. This is due to the eggs hatching naturally after a short period of development and subsequent cessation of fly activity in the fall.