Co-evolution of viruses and host defence mechanisms
In our daily research, we use viruses to study the host's defence system and employ immunological tools to examine viruses. Despite of this mutual usefulness, the interaction between viruses and their hosts is most often pictured as a battle, a war-like situation. Therefore, only a total victory, the complete destruction of the enemy, i.e. total clearance of the virus from the organism, the population, the world, is considered as a basis for lasting peace. In veterinary virology, this view is reflected in political strategies to protect farm animals from highly contagious disease agents, such as those recorded in List A of the Office International des Épizooties (O.I.E.). Once the label "DANGEROUS!" has been attached to a particular virus, it can be removed only with great difficulty, mostly only after a change in the political climate. This fact leads to strange situations, as e.g. in the pestivirus field, where classical swine fever virus (CSFV) is in List A of the O.I.E. As a consequence, CSFV-infected herds must be destroyed and control measures must be taken to prove that the virus has been eliminated from the farm, the region, the country in question. In contrast, the closely related bovine virus diarrhoea virus (BVDV) and border disease virus - also pestiviruses - are considered as innocuous and consequently neglected by the veterinary authorities. This example emphasizes the most important argument, the seeming truism that neither the virus nor its host are static entities. Who can tell whether the vacuum created by the extinction of CSFV will not be filled soon by another virus?

The first encounter between a new or exotic virus and a particular host may result in a fatal outcome for both. This situation is illustrated by reports of encounters between seemingly new viruses and unprepared hosts, recently [5,18] and in the past (e.g. [23, 24]). The host's inadequate defence and/or the virus' poor adaptation may result in death of the infected organism, and the virus would soon become extinct if it destroyed the susceptible population too rapidly. Hence, to survive in nature, any virus needs the host's functional defence as much as the host itself - both the host and the virus are on a path of co-evolution (Fig. 2) (e.g. [12], and references therein). Again: both are essential, the defence mechanisms for the survival of host and virus, and the virus for the constant education and evolution of the defence mechanisms.

A dramatic example of this co-evolution, the HIV/AIDS epidemic, is occurring before our eyes. In Botswana, a shocking 35.8% of adults are infected with HIV (http://www.unaids.org/fact_sheets/files/Africa_Eng.html), which means that Africa will probably be repopulated by progeny of HIV-resistant survivors of the infection. Genetic determinants for long-term survival of AIDS have indeed been described [4,31], as have viral determinants, which influence the same [1].

In my opinion, the eradication of a given virus will create an empty ecological niche, soon to be taken by another agent, probably by one less adapted to the host than the original virus. Thus, if extermination of viruses is not the ultimate goal of veterinary virology, what is its goal? Obviously, the direction of co-evolution is influenced by external factors; understanding these factors is essential for steering the co-evolution of host and virus a course that is beneficial for humans and animals.

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