Parallels among positive-strand RNA viruses, reverse-transcribing viruses and double-stranded RNA viruses

Viruses are divided into seven classes on the basis of differing strategies for storing and replicating their genomes through RNA and/or DNA intermediates. Despite major differences among these classes, recent results reveal that the non-virion, intracellular RNA-replication complexes of some positive-strand RNA viruses share parallels with the structure, assembly and function of the replicative cores of extracellular virions of reverse-transcribing viruses and double-stranded RNA viruses. Therefore, at least four of seven principal virus classes share several underlying features in genome replication and might have emerged from common ancestors. This has implications for virus function, evolution and control.     

Foodborne viruses

Foodborne and waterborne viral infections are increasingly recognized as causes of illness in humans. This increase is partly explained by changes in food processing and consumption patterns that lead to the worldwide availability of high-risk food. As a result, vast outbreaks may occur due to contamination of food by a single foodhandler or at a single source. Although there are numerous fecal-orally transmitted viruses, most reports of foodborne transmission describe infections with Norwalk-like caliciviruses (NLV) and hepatitis A virus (HAV), suggesting that these viruses are associated with the greatest risk of foodborne transmission. NLV and HAV can be transmitted from person to person, or indirectly via food, water, or fomites contaminated with virus-containing feces or vomit. People can be infected without showing symptoms. The high frequency of secondary cases of NLV illness and - to a lesser extent - of hepatitis A following a foodborne outbreak results in amplification of the problem. The burden of illness is highest in the elderly, and therefore is likely to increase due to the aging population. For HAV, the burden of illness may increase following hygienic control measures, due to a decreasing population of naturally immune individuals and a concurrent increase in the population at risk. Recent advances in the research of NLV and HAV have led to the development of molecular methods which can be used for molecular tracing of virus strains. These methods can be and have been used for the detection of common source outbreaks. While traditionally certain foods have been implicated in virus outbreaks, it is clear that almost any food item can be involved, provided it has been handled by an infected person. There are no established methods for detection of viruses in foods other than shellfish. Little information is available on disinfection and preventive measures specifically for these viruses. Studies addressing this issue are hampered by the lack of culture systems. As currently available routine monitoring systems exclusively focus on bacterial pathogens, efforts should be made to combine epidemiological and virological information for a combined laboratory-based rapid detection system for foodborne viruses. With better surveillance, including typing information, outbreaks of foodborne infections could be reported faster to prevent further spread.     

Psychotropic viruses

Viruses are increasingly being recognized as important factors in the pathogenesis of acute and chronic mental illness. Here we review clinical and epidemiologic data concerning viral infection and mental illness, as well as animal models that provide insight into the myriad of mechanisms by which infection can cause brain dysfunction.     

Rabies-related viruses

Five viruses related to rabies occur in Africa. Two of these, Obodhiang from Sudan and kotonkan from Nigeria, were found in insects and are only distantly related to rabies virus. The other three are antigenically more closely related to rabies. Mokola virus was isolated from shrews in Nigeria, Lagos bat virus from fruit bats in Nigeria, and Duvenhage virus from brain of a man bitten by a bat in South Africa. The public health significance of the rabies-related viruses was emphasized in Zimbabwe where in 1981 a rabies-related virus became epizootic in the dog and cat population. It is postulated that the ancestral origin of rabies virus was Africa where the greatest antigenic diversity occurs and that the ancestor may have been an insect virus. Questions are raised why rabies has not evolved more rapidly in the New World, given the frequency and ease with which antigenic changes can be induced in the laboratory, and how the virus became so extensively established in New World bats.     

Electron tomography of viruses

Understanding the molecular architectures of enveloped and complex viruses is a challenging frontier in structural biology. In these viruses, the structural and compositional variation from one viral particle to another generally precludes the use of either crystallization or image averaging procedures that have been successfully implemented in the past for highly symmetric viruses. While advances in cryo electron tomography of unstained specimens provide new opportunities for identification and molecular averaging of individual subcomponents such as the surface glycoprotein spikes on purified viruses, electron tomography of stained and plunge-frozen cells is being used to visualize the cellular context of viral entry and replication. Here, we review recent developments in both areas as they relate to our understanding of the biology of heterogeneous and pleiomorphic viruses.