Haycocknema-like nematodes in muscle fibres of a horse

A 14-year-old horse (imported to Switzerland from Ireland 8 years earlier) showed signs of chewing muscle atrophy. A severe chronic myositis, caused by numerous immature and mature female nematodes, was diagnosed in muscle samples obtained by biopsy and subsequently at necropsy. Most of the nematodes had invaded muscle fibres of the masseter, root of the tongue and anterior breast, only a few were found in the intermuscular interstitium. Isolated nematodes and parasite sections were clearly different from muscle larvae of Trichinella spp. but showed morphological similarities to Haycocknema perplexum, a nematode species (order Enoplida, family Robertdollfusidae) recently found in the musculature of a human patient in Australia. However, our material did not allow the precise identification of the nematode genus nor the unequivocal differentiation from Halicephalobus gingivalis. This species infects horses and humans and can cause severe granuloma formation in muscles and many other organ systems, but has never been observed to invade individual muscle fibres. Our findings show that nematodes of another genus than Trichinella may invade muscle fibres of the horse and cause myositis. These nematodes are provisionally regarded as Haycocknema-like.