**4. Conclusion: EHV-9 infection as a model of cross-species viral transmissions**

Based on previous experimental studies in different animals inoculated via the nasal route, the olfactory pathway (i.e. through the olfactory nerves) is the major route of transmission of EHV-9 into the CNS. However, recent study that compared different routes of inoculation clearly indicates that virus can enter the CNS after administration of EHV-9 via the oral, peritoneal, and ocular routes, and that there are differences in the distribution of antigenpositive cells and in the location and severity of the cerebral lesions. Thus, EHV-9 may gain access to the CNS through a non-olfactory route as these animals inoculated via these nonnasal routes did not exhibit EHV-9 induced rhinitis, and the olfactory bulbs showed milder lesions and fewer viral antigen-positive cells than observed in the animals infected via the nasal route.

One of the striking finding was that animals infected via the ocular route had mild and localized lesions in the rhinencephalon, which indicated that the virus had traveled to the CNS through the optic nerve. Similar to what is reported about fatal infections by Cercopithecine herpes virus 1 (B virus) in humans via ocular exposure from biological fluid from macaque monkeys (CDC, 1999).

The differences in the incubation period and paths of travel to the CNS among the various routes in resulting in a variety of clinical signs and histopathological features, suggests a dependency on the replication of the virus at the site of entry and its opportunity to access regional nerves to travel to the brain. A similar hypothesis was proposed in the case of pigs infected orally by EHV-9 (Narita et al., 2000), where the virus was thought to travel centripetally in the nerve fibers from the oral mucosa to the trigeminal ganglion, eventually entering the olfactory lobes (Chowdhury et al., 1997; Kritas, et al., 1994; Narita et al., 1976). Previously, EHV-9 antigen was found in the neurons and neural fibers but not in the glial cells in the brain, indicating that neurons are the susceptible cells to EHV-9 in the CNS of hamsters (Fukushi et al., 2000). However, other neurotropic herpesviruses such as herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV) are known to infect glial cells as well as neurons (Johnson, 1998). Astrocytes infection of EHV-9 was described in the cases of a giraffe and a polar bear (Donovan et al., 2009; Hoenerhoff et al., 2006). Immunohistochemistry demonstrated the presence of EHV-9 antigen in the neurons and neuronal fibers including the axons and dendrites in the brain of the goat and the naturally infected bear which indicate a trans-synaptic spread of EHV-9 from neuron to neuron via the neuronal fibers (Taniguchi et al., 2000b; Donovan et al., 2009). Similar transmission has been shown in pseudorabies viral infection (Card et al, 1990). In suckling hamster study, there was necrosis of some of trigeminal ganglion cells as well as detection of the viral antigen in the same ganglion and in the connection between trigeminal sensory nerve root and the brain stem at the level of the pons and medulla oblongata. Based on previous findings, the neurotropism might be the most characteristic property of EHV-9, differentiating it from other neurotropic herpesviruses. Similar transneural passage was suggested after intranasal infection with IBR virus (Narita et al, 1979).

EHV-9 infection can be regarded as one of cross-species viral transmission. In nature, natural barriers exist to prevent the cross-species transmission as well as natural clearance such as predation by carnivors can hide the lethal cross-species transmission. An artificial situation of zoos or farms can cause cross-species transmission among carrier animals and susceptible animals. However, some species do not show clinical symptoms even though they can be infected by viruses. In EHV-9 infection, all of the animals infected by EHV-9 caused various degree of meningo-encephalitis. It is not clear what kind of factors are involved in the clinical course of EHV-9 infection.
