Halicephalobiasis is a rare and fatal parasitic disease in horses caused by the free-living nematode, Halicephalobus gingivalis (H. gingivalis)
. The disease is poorly understood and thought to also affect humans. Affected horses develop progressive neurological signs that appear similar to that seen in horses with West Nile virus (WNV) infection.
has been reported to infect horses and humans. It is found in decaying organic matter, horse manure, and soil. Little is known about how horses become infected with H. gingivalis
or its life cycle. The method of how it enters the horse is thought to be related to penetration of the mucous membranes, particularly oropharyngeal and skin wounds. Once inside the horse, H. gingivalis
migrates to different organs in the horse's body, where it reproduces. Once clinical signs of CNS involvement develop, the disease is rapidly fatal.
Historical Cases of Infected Horses
: A 12-year-old Arabian gelding was treated with ivermectin (1.2 mg/kg [0.55 mg/lb] of body weight, PO, every 2 weeks for 3 treatments). The granuloma was surgically debulked 2 days after the first dose of ivermectin. The granuloma resolved with no evidence of nematode infection after 18 months.
: Two Icelandic stallions died of the disease. One stallion sustained injuries to the mouth after an accident, developed severe neurological signs and had to be euthanatized. Histological examination revealed mild inflammation and malacia in the cerebellum associated with the presence of numerous H. gingivalis
nematodes. The second stallion started swerving to one side and lost balance was euthanatized due to lack of response to therapy and rapid deterioration. Histological examination revealed numerous H. gingivalis
nematodes in the cerebellum, brain stem, cervical spinal cord and in the meninges, with minimal reactive changes.
: A 13-week-old Thoroughbred colt from central Kentucky was euthanized after an acute onset of ataxia, blindness, head tremors, leaning to the right, recumbency, and seizures. Microscopically, there was a verminous meningoencephalitis characterized by an eosinophilic and granulomatous inflammatory reaction primarily affecting the cerebellum. Dispersed within regions of inflammation were numerous cross and longitudinal sections of intact and degenerative small nematodes. The nematodes had dorsoflexed ovaries and ventroflexed vulvas, which are distinguishing features of Halicephalobus gingivalis. Intact nematodes, compatible with H. gingivalis
, also were recovered and identified from portions of the brain that had been frozen for 5-week post-necropsy examination via tissue maceration and additional laboratory techniques.
: A 6-year-old Thoroughbred gelding in Korea was euthanized after a 2-month period of abnormal neurological signs, such as circling left in his pen and hitting his head and body against the wall. After the horse was euthanized on the farm, a half of the brain and whole blood were submitted for diagnostic tests. Histopathological examination of the brain revealed granulomatous and eosinophilic meningoencephalitis with numerous intralesional nematodes, predominantly affecting the cerebrum. Multifocal malacic foci were scattered in the brain parenchyma. The intralesional parasites were identified as H. gingivalis
by morphological features and PCR testing.
: Two Lipizzaner stallions living at two different breeding farms in Romania died from the disease. At necropsy, granulomatous and necrotizing lesions were observed in the kidneys, lymph nodes, brain, retroperitoneal adipose tissue, and lungs, indicating a systemic infection. Parasitological and histopathological analyses evidenced larval and adult forms of rhabditiform nematodes consistent with Halicephalobus species.
: A 13-year-old Koninklijk Warmbloed Paard Nederland stallion in Piedmont, Italy died. The post-mortem examination revealed neuropathological findings consisted of granulomatous meningoencephalitis and larvae and adult females of H. gingivalis
were isolated and identified from the digested brain.
Two unrelated horses living in Southern California were diagnosed with halicephalobiasis in July 2017.
- Case 1: An 18-year-old Gypsy Vanner mare who had been experiencing progressive neurological signs which included:
The horse was euthanized on day 3. The necropsy results revealed no gross lesions. The histopathologic examination of the horse's brain and spinal cord showed the presence of multifocal granulomatous meningoencephalitis and neuritis of cranial and spinal nerves, with abundant intralesional nematodes and eggs.
- Day 1: Ataxia and head pressing
- Day 2: Recumbency
- Day 3: Dog paddling
- Case 2: An Arabian mare presented with a history of chronic, severe hematuria (presence of blood in the urine) and deterioration. The horse was euthanized. The field necropsy performed by the veterinarian revealed large, firm, off-white nodular masses in both of the horse's kidneys, and scattered small firm to gritty nodules in the lungs. The histologic exam revealed the kidney masses were exuberant granulation tissue and inflammation with large numbers of intralesional nematodes, larvae, and egg. The lung nodules consisted of smaller granulomas with nematodes.