Verminous mastitis caused by Cephalobus cubaensis infection in a mare

A 7-y-old American Quarter Horse mare was presented to a veterinary hospital for ongoing reproductive monitoring and management of endometritis. During physical evaluation, several nodular lesions of unknown origin and duration were discovered in the mammae. Histologically, biopsies from the lesions were confirmed as severe granulomatous mastitis with intralesional rhabditid nematodes. A putative diagnosis of Halicephalobus gingivalis or Pelodera strongyloides infection was assigned, and ivermectin (1.2 mg/kg PO) treatment was given every 2 wk. When lesions persisted after the initial dose of ivermectin, PCR testing of 18S and 28S gene targets confirmed Cephalobus cubaensis infection. Given the lack of response after 4 doses of ivermectin and the zoonotic concern, euthanasia was elected. Analysis of sediments of the skin tissues recovered eggs, larvae, and female nematodes with a blunt tail terminating in a mucro and the rhabditid esophagus of Cephalobus spp. Spread to the left inguinal lymph node was confirmed. Cephalobus sp. is a saprophytic nematode that has very rarely been reported as a cause of verminous mastitis in horses and in a single fatal case of meningoencephalitis in a human. Differentiation between C. cubaensis, H. gingivalis, and P. strongyloides by histopathology alone is unreliable because of morphologic similarities; molecular identification should be pursued.