Pathology in practice.

Halicephalobus gingivalis is a small, free-living saprophagous nematode belonging to the order Rhabditada (Family Panagrolaimidae) that is found worldwide. This nematode is sporadically associated with opportunistic infections of horses and humans, frequently with a fatal outcome; the infective organisms are consistent in their CNS localization. To date, approximately 50 cases of equine infection and 4 cases of human infection have been reported in the medical literature. The route of entry of H gingivalis in vertebrate hosts is still unclear. In humans, contamination of skin wounds by manure containing free-living parasites seems the most likely means of infection; in horses, the most common site of primary infection is probably the oral cavity, wherein the parasite passes through small lacerations of the oral mucosa. In some equine cases, the only reported lesion was granulomatous posthitis, suggesting a possible transcutaneous route of entry in the genital region. In the horse of the present report, neither skin wounds nor cutaneous granulomas were present; however, the severe involvement of inguinal superficial lymph nodes suggested the possibility of a transcutaneous route via the external genitalia or preputial skin. After entry to the host's body, hematogenous spread to highly vascularized tissues occurs, followed by secondary localization. Most of the cases reported in the veterinary medical literature describe lesions in the CNS (both brain and spinal cord) and kidneys and less commonly in the eyes, testes, mammary glands, bones, gingivae, liver, and stomach.10 In the horse of the present report, the lesions were confined to intracranial portions of the CNS, lymph nodes, kidneys, and, interestingly, the heart. Cardiac involvement in cases of halicephalobiasis is considered to be uncommon, and we are aware of only 1 report that describes a gray raised lesion (2.5 × 1.5 × 1 cm) beneath the endocardium of the right ventricle in the lateral wall, 5 cm from the atrioventricular valves, in a horse. In a case of human infestation, a single nematode was found in the myocardium with no inflammatory response. In the horse of the present report, a large (3 × 4-cm) granulomatous lesion was present in the myocardium together with microscopic scattered multifocal microgranulomas. Diagnosis of H gingivalis infection is often made after necropsy because of the difficulty in intravitam collection of samples from internal lesions. However, intravitam diagnosis can be achieved by microscopic examination of tissue biopsy specimens obtained from external (mostly gingival) lesions or, rarely, by percutaneous biopsy of the kidneys. To date, H gingivalis has not been observed in samples of body fluids (CSF, blood, saliva, and joint fluid) except for urine and semen. Verminous encephalomyelitis is a rare cause of neurologic disease in horses. Parasites that can invade the CNS during aberrant migration include strongyloid nematodes (Strongylus equinus, Strongylus vulgaris, and Angiostrongylus cantonensis), Draschia megastoma, Setaria spp, and Hypoderma spp; the most common is H gingivalis. In horses with verminous encephalomyelitis, antemortem diagnosis is often impossible, even if suspicion is raised by the sudden onset and rapid progression of clinical signs and the results of CSF analysis. In horses with halicephalobiasis, the CSF changes are nonspecific and are represented by high total protein concentration and mild to marked pleocytosis, with high numbers of mononuclear cells and lower numbers of nondegenerate neutrophils and lymphocytes; eosinophils are reported to be rare to prevalent, representing up to 42% of total nucleated cells. In the horse of the present report, most cells in the CSF sample were mononuclear cells, and eosinophils were rare. The presence and detection of the parasite in a CSF sample are considered exceptional, and can follow massive CNS tissue necrosis in the subependymal areas or CSF contamination by circulating larvae. In this horse, the larvae probably represented extension from cerebral lesions. In the CSF sample collected from the horse, parasite specimens and parasite DNA were recovered by cytologic and biomolecular investigations, respectively. Recently, a DNA-based approach has been reported to be reliable for parasite identification at the species level and for phylogenetic analysis; nevertheless, the diagnosis of H gingivalis infection is mostly based on physical characteristics of the parasites, which have specific morphometric and morphological features. The observation of parasite larvae in the CSF sample obtained from the horse of the present report has suggested that intravitam diagnosis is possible, which would then lead to appropriate treatment with specific anthelmintic drugs. However, the susceptibility of H gingivalis to the most common drugs remains to be assessed. The case described in the present report has highlighted the importance of including verminous meningoencephalitis caused by H gingivalis among the differential diagnoses for neurologic diseases in horses, and the potential value of CSF analysis for determination of a definitive diagnosis.