Be a Virus, See the World

Abstract

As the recent SARS epidemic has shown, viruses are able to migrate with remarkably high speed, endangering countries around the globe within hours. Since viruses are obligate parasites, their migration speed is dependent on the mobility of the respective host. Several examples of pathogenic viruses with different patterns of migration will be discussed. Lassa virus is endemic in West Africa and causes a highly pathogenic hemorrhagic fever among humans. Lassa virus is transmitted by urine and feces of rodents that are persistently infected. Lassa virus, dependent on its rodent host, seems not to spread significantly outside its endemic regions. The human infection rather represents a dead end for the virus. West Nile virus entered the American public awareness in 1999 with the first human cases in the United States appearing in New York City. The virus is transmitted by mosquitoes that parasitize migrating birds, and also, e.g., crows. Transmitted to humans, West Nile virus can cause encephalitis that is especially dangerous for the elderly. West Nile virus spread remarkably fast over the North American continent. Nowadays, even several Canadian provinces are facing a severe problem with infected birds and human cases. The SARS coronavirus caused not only epidemics with local transmissions in mainland China, Hong Kong, Taiwan, and Toronto, but was also spread to almost every country worldwide by infected patients. So far, this was the most impressive example for the speed a virus can achieve by the mobility of its host. The natural host of the SARS coronavirus is still unknown and, thus future outbreaks cannot be excluded. The risk imposed by an emerging virus to the human population is a product of migration velocity, transmission route, and speed of detection. Most dangerous for the human population are highly pathogenic viruses that are transmitted from human to human via the air (SARS coronavirus). However, equally dangerous are viruses that are highly pathogenic, transmitted by blood to blood contact but have a long incubation period and, thus, detection and surveillance are complicated (human immunodeficiency virus). It is still not completely clear where SARS-CoV came from. However, investigations of a Chinese group showed that the novel coronavirus could be detected in a cat species in Guangdong province where the outbreak originated. The sequence of the cat SARS-CoV is not 100% identical to most of the human isolates. In the latter, a 29-nucleotide deletion is found when compared to the cat isolate. Thus, the current hypothesis is that SARS is the product of an interspecies viral transmission with dramatic consequences for the new involuntary human host. Today, it is not understood whether the nucleotide deletion in the genome of the human isolates is related to the high pathogenicity of SARS-CoV (Guan et al. 2003). It is sometimes questioned whether the measures taken by the WHO and the countries with local transmission chains were justified, considering the relatively low number of global cases and deaths. However, at the time of the outbreak, the public health services were confronted with a highly pathogenic virus that was transmitted via the air and had high impact on the public health sector by infecting mainly health care workers. Thus, the fact that SARS so far resulted in a relatively small number of diseases and deaths is mainly the result of the global efforts to contain the outbreak -- not the benign character of the SARS-CoV.