A case of a rare and dangerous parasitic infection in horses has been described by Danish researchers, who said the evidence suggested the nematode spread to multiple organs via the bloodstream.
Halicephalobus gingivalis can infect humans and horses. More than five cases in humans – all fatal – and 65 cases in horses have been reported since the infection was first described in 1954.
Most H. Gingivalis infections in horses have been fatal and were usually not diagnosed before necropsy. Knowledge about its disease-causing mechanisms in horses were therefore limited, the researchers said. This had resulted in an on-going discussion about the port of entry and subsequent dissemination of H. gingivalis within the host.
University of Copenhagen researcher Christina Henneke and her colleagues reported on their investigations into a case in an 11-year-old Icelandic stallion.
The scientists, reporting their findings in the journal, Acta Veterinaria Scandinavica, diagnosed the infection at the university before the horse’s death.
Surgical intervention followed, but the horse later developed neurological signs and blindness, and was euthanized because of the grave prognosis.
Describing their post mortem findings, the scientists said the distribution pattern of H. gingivalis nematodes in the brain, a high prevalence of inflammation close to blood vessels, and the presence of the nematodes in multiple organs with a disseminated pattern of distribution strongly suggested that it spread internally via the animal’s blood vessels.
In both kidneys, a high density of parasites with the same characteristics as the nematodes in the meninges, brain and sinuses were found. The inflammatory reaction of the kidneys was similar to that in the sinuses, they said.
The researchers said the infection was comparable to other cases of equine H. gingivalis-infection, with the nematode identified in different organs.
That said, reported clinical manifestations in horses with H. gingivalis infections were highly variable, they said, complicating any possible conclusions regarding the port of entry and subsequent spread of the parasite within the host.
However, during recent years the infection has been diagnosed in countries worldwide, and the growing number of reported cases had created a clearer picture of the route of infection and the distribution pattern of the nematodes in horses.
Several studies suggested that penetration of mucosal membranes or the skin was the most likely port of entry.
The nematodes were often found in various parts of the brain and meninges, and the high occurrence of neurological manifestations in horses with H. gingivalis infection was probably caused by a high affinity for the meninges and brain tissue, the researchers suggested.
“The pattern of distribution of meningeal and brain lesions together with a high prevalence of inflammation and presence of nematodes in relation to blood vessels strongly suggest, that H. gingivalis was spread haematogenously [via the blood] to the brain and meninges in this case,” they reported.
This, they said, was supported by others, who observed the nematode close to blood vessels, and occasionally in the heart, and within the wall of blood vessels.
Although H. gingivalis has been isolated from urine and semen, it remains to be found in blood samples.
The possibility of spread via the blood was also supported by the presence of lesions in the kidneys, which have also frequently been reported by others.
Their microscopic tissue examinations showed that all lesions contained a high number of reproducing mature nematodes, which might give rise to the formation of new infection sites.
In single cases, the nematode has been found in the heart, blood vessels, testicles, femur, lymph nodes, parathyroid glands and preputium.
Christina Henneke, Anna Jespersen, Stine Jacobsen, Martin Nielsen, Fintan McEvoy, and Henrik Jensen.
The distribution pattern of Halicephalobus gingivalis in a horse is suggestive of a
haematogenous spread of the nematodeActa Veterinaria Scandinavica 2014, 56:56 doi:10.1186/s13028-014-0056-0
A provisional PDF of the study can be read here.