Pathologic and Immunohistochemical Findings in Naturally Occurring West Nile Virus Infection in Horses

Abstract

Abstract. The pathologic and peroxidase immunohistochemical features of West Nile flavivirus (WNV) infection were compared in four horses from the northeastern United States and six horses from central Italy. In all 10 animals, there were mild to severe polioencephalomyelitis with small T lymphocyte and lesser macrophage perivascular infiltrate, multifocal glial nodules, neutrophils, and occasional neuronophagia. Perivascular hemorrhages, also noted macroscopically in two animals, were observed in 50% of the horses. In the four American horses, lesions extended from the basal nuclei through the brain stem and to the sacral spinal cord and were more severe than the lesions observed in the six Italian horses, which had moderate to severe lesions mainly in the thoracolumbar spinal cord and mild rhombencephalic lesions. WNV antigen was scant and was identified within the cytoplasm of a few neurons, fibers, glial cells, and macrophages. WNV infection in horses is characterized by lesions with little associated antigen when compared with WNV infection in birds and some fatal human infections and with other important viral encephalitides of horses, such as alphavirus infections and rabies.

West Nile virus infection of horses

“…In vivo it can grow in a variety of cells from different tissues depending on the host species. These tissues include neurons, glial cells, and cells from spleen, liver, heart, lymph nodes and lung [24]. Virus replication takes place in the perinuclear region of the rough endoplasmic reticulum (ER) where the newly synthesised E, NS1 and prM are translocated to the ER lumen where prM and E heterodimerise [30,31,100].…”

“…Except for fever, clinical signs of WNV in horses are almost exclusively of neurological nature and reflect the pathology in the central nervous system (CNS). These occur predominantly in the spinal cord, rhombencephalon and mesencephalon being the cerebral cortex less often affected [24]. A transitory febrile period might occur after infection although this is not always observed in some epidemics, e.g.…”

“…Mortality rates among clinically affected horses have been estimated around 38%, 57.1% and 42% during outbreaks in the USA in 2000, France in 2000 and Italy in 1998 respectively [24,66,74]. In contrast to the human disease, severe neurological disease in horses does not appear to occur preferentially in old individuals.…”

“…The pathology of WNV in horses has been studied in natural cases during the epidemics of Italy in 1998 and the northeastern states of the USA in 1999 [23,24]. As briefly indicated earlier in the text, lesions of WNV infection in the horse are predominantly, if not exclusively, limited to the CNS, and lesions affecting extraneural tissues are rarely described, which contrasts with widespread lesions observed in many internal organs of WNV infected birds [84].…”

West Nile Virus Infection and its Neurological Manifestations

“…Notably, these findings were similar to necropsy findings of spinal cord in animals infected with the West Nile virus. 28,29 These data seem to suggest a predilection of this virus for the anterior horn cell and the production of a clinical state similar to that of acute poliomyelitis.…”