Animal models of highly pathogenic RNA viral infections: Encephalitis viruses

Abstract

The highly pathogenic RNA viruses that cause encephalitis include a significant number of emerging or re-emerging viruses that are also considered potential bioweapons. Many of these viruses, including members of the family Flaviviridae, the genus Alphavirus in the family Togaviridae, and the genus Henipavirus in the family Paramyxoviridae, circulate widely in their endemic areas, where they are transmitted by mosquitoes or ticks. They use a variety of vertebrate hosts, ranging from birds to bats, in their natural life cycle. As was discovered in the United States, the introduction of a mosquito-borne encephalitis virus such as West Nile virus can cause significant health and societal concerns. There are no effective therapeutics for treating diseases caused by any of these viruses and there is limited, if any, vaccine availability for most. In this review we provide a brief summary of the current status of animal models used to study highly pathogenic encephalitic RNA viruses for the development of antiviral therapeutics and vaccines.

Introduction

The objective of this review is to provide a brief guide to animal models used for the study of highly pathogenic RNA viruses that cause encephalitis (HPRVE). An accompanying review article in this issue discusses RNA viruses associated with viral hemorrhagic fever (Gowen and Holbrook, 2008). The HPRVE are found primarily in three families: Flaviviridae, Togaviridae and Paramyxoviridae and single genera within each family: Flavivirus, Alphavirus and Henipavirus, respectively. In humans, these diseases usually take the form of meningioencephalitis, with limited involvement of the limbs. However, some diseases, such as Japanese encephalitis and Russian spring–summer encephalitis viruses, can cause a polio-like illness with significant peripheral neurologic involvement.

All of the viruses discussed in this review are zoonotic agents, for which human infection is incidental and is a dead-end in the virus life cycle. All of them, with the possible exception of West Nile virus, are considered potential biothreat agents. The majority are transmitted by arthropods with the viruses transmitted to humans in mosquito or tick saliva. The amount of virus required to infect humans is unknown. The route of progression through the host is not clearly established, but it appears that the virus progresses first from the site of the bite to draining lymph nodes, where it replicates and is amplified. The virus then enters the circulation and crosses the blood–brain barrier (BBB) and enters the central nervous system (CNS) through unknown mechanisms. Several hypotheses have been offered for mechanisms of CNS penetration. These include virus penetration as a result of inflammation and damage to vascular integrity (Lossinsky and Shivers, 2004), entry through the olfactory bulb (Cook and Griffin, 2003), toll-like receptor mediated entry (Wang et al., 2004) and transcytosis across vascular endothelial cells (Lossinsky and Shivers, 2004).

The henipaviruses, which are also discussed in this review, are not transmitted by arthropods, with fruit bats (Pteropus spp.) serving as the primary reservoir. With Nipah virus, pigs play an important role in viral transmission to humans and it seems likely that transmission to pigs and humans may occur via feces or urine from bats, with transmission from pigs to humans possibly following a similar route (Eaton et al., 2005), but this mechanism is not clearly established.

Viruses that cause encephalitis in humans also frequently cause severe neurologic disease in mice, making the mouse model a fundamental tool for studying viral pathology and for the development of vaccines and antivirals. The US Food and Drug Administration (FDA) has stipulated that vaccines and therapeutics considered for use in humans against some highly pathogenic viruses may bypass human efficacy testing if they show efficacy in animal models that are adequately representative of the human disease. This so-called “Animal Rule” is discussed more extensively elsewhere in this issue (Roberts et al., 2008). The “Animal Rule” provides an important avenue for the development of vaccines and therapeutics that would have been impossible if clinical or field efficacy studies were required, and makes the development and recognition of animal models that mimic human disease vital for effective drug development.

A number of animal models have been tested for the HPRVE ranging from guinea pigs to cats and also non-human primate models such as rhesus and cynomolgus macaques. For the viruses discussed here, challenge by peripheral routes is generally sufficient to cause neurologic disease in the established models. One of the limitations of using animals to model human disease is that in order to effectively test antiviral or vaccine efficacy, the animal model needs to have an easily measurable end-point for efficacy evaluation. This end-point is typically debilitating disease or death and many of the viruses discussed here are uniformly lethal in various animal models if a sufficient virus dose is given. Clearly these viruses are not 100% lethal in humans as many of these viruses cause a large number of subclinical infections for each case of disease.

Unlike hemorrhagic fever viruses, the development of encephalitis and neurologic disease in animal models is generally obvious, with paralysis and seizures being measurable indications of illness. The question that arises is whether the mechanisms (e.g. host immune response) of disease development in the animal model are the same as, or similar to, what occurs in humans. Given that some therapeutics and all vaccines rely on the host immune response, gaining a clear understanding of how viral infection affects the host is a major component of effective drug development that is often overlooked or ignored. A second component of effective drug development for encephalitis viruses is the ability to develop drugs that cross the blood–brain barrier and are able either to limit viral replication in the CNS or to reduce the inflammation stimulated by infection.

There are currently no therapeutic interventions and limited, if any, vaccine availability for the diseases discussed in this article. The development and use of animal models for the study of pathogenesis is vital for the development of effective therapeutics and vaccines. Furthermore, the inability to directly determine antiviral or vaccine efficacy in humans makes the use of animal models a vital part of drug development.

The goal of this review is to provide a brief summary of the available animal models for the HPRVE with selected references describing the use of these systems. More comprehensive reviews containing extensive lists of primary references, such as articles covering arboviral encephalitides by Nalca et al. (2003) or flavivirus mouse and hamster models by Charlier et al. (2006), should be consulted for more complete information.