Gasterophilus nasalis infection: prevalence and pathological changes in equids in south-west England.
A licensed horse abattoir in south west England was visited on 3 occasions in December 1995 and January 1996 (period 1) and on 2 occasions in February 1997 (groups A and B, period 2). In the first period, the stomach and proximal 15 cm of the duodenum were collected from 62 healthy equids (adult horses, untreated brood mares, foals and Dartmoor ponies). In the second period, the stomach and upper duodenum from 27 healthy Dartmoor ponies were examined. The prevalence of Gasterophilus spp. is tabulated. The dominant spp. was G. intestinalis, present in all animals that the owners stated had not been treated with a boticide in the previous 5 months. Dartmoor ponies were take straight off the moor and had received no treatment. G. nasalis was found only in these ponies and a horse of the owner of the ponies. In period 2, 3.6 and 6.6% of the G. intestinalis in groups A and B respectively and 0 and 11.5% of the G. nasalis were second instar larvae, the remainder being third instar. There was a single specimen of G. pecorum in 1 horse in period 1. G. nasalis were found in the duodenum of the Dartmoor ponies in period 2, adjacent to the pylorus, in clusters up to 6 cm in diameter. At the site of attachment of the larvae, elevated craterform lesions were visible on the surface. In a cross-section of the intestinal wall, the submucosa was variably thickened, firm and white. On histological examination, polymorphonuclear neutrophils and eosinophils were observed in the lamina propria and submucosa, along with slight to moderate fibrosis. Ulceration was present in 14 of 18 cases and was frequently associated with the attached larvae. Lymphoid cell aggregates were observed within the submucosa and lamina propria, but their number and size were variable. Similar aggregations were present in 3 animals in which G. nasalis were absent and they were used as controls. A focal, submucosal accumulation for eosinophils was seen in 2 infected animals, representing early abscess formation.