**Part 1**

**General Problems of Epidemiology** 

**1** 

**Zoonotic and Animal Vector** 

When discussing human diseases in general, the majority (61%) of them are zoonotic, or are able to be transferred between animals and humans (Taylor et al., 2001), either through the bite of an arthropod, exposure to the pathogen through direction contact with animal products (urine, feces, milk, afterbirth) or when humans are a part of the pathogen's life cycle directly. In regard to diseases that are considered emerging, 75% of them are zoonotic (Taylor et al., 2001), which places more weight on the study of these pathogens and their evolution in order to better understand the risk of infection to a new host (Alexander and Day, 2010). This chapter is designed to give the reader an overview of the wide array of pathogenic etiologies, whether viral, bacterial or parasitic, that have the potential to develop into encephalitis, though in some cases, this is a rare side effect, or limited to specific groups (i.e., immunecompromised individuals). Of these diseases, all have a life cycle intimately connected with an animal vector or host in some way. Some disorders are very well known and studied as a means of vector transmitted encephalitis (e.g., Eastern Equine Encephalitis), while others are just emerging as serious health risks associated with encephalitic symptoms (e.g., Chandipura Virus Encephalitis, Nipah and

Viral infections are important in the induction of encephalitis. Arboviruses are transmitted by arthropods (e.g., mosquitoes, ticks, sand flies) and are maintained through biological transmission between a vertebrate host (Kuno & Chang, 2005). Biological transmission of such a pathogen involves several factors (Reviewed by Scott, 1988). The virus must be able to reproduce in both the arthropod and vertebrate host, then be able to produce a high enough viral titer in the blood of the vertebrate to be passed back to the vector Reviewed by Scott, 1988). It is well known that RNA viruses tend to have a high mutation rate because of unfaithful replication in host cells, among other things (Domingo, 1997). However, to maintain a relationship with both vertebrate and arthropod vectors, observed mutation rates are reduced, as is with New World Alpha viruses (Weaver et al., 1991). When selective pressures are applied to the virus between two alternating hosts, the virus population responds with adaptations fit for both environments in order to optimize suitability in a

dual host system (Weaver et al., 1999; Cooper & Scott, 2001).

**1. Introduction** 

Henipa virus).

**2. Viral related diseases** 

**Mediated Encephalitides** 

Colleen Mikelson and Sean Richards *University of Tennessee Chattanooga* 

*United States of America* 
