The main purpose of the paper was to review information about the most important mechanisms of influenza virus evolution and their consequences for immunoprophylaxis and the elaboration of diagnostic tests. The influenza virus is the pathogen that indicates tropism on the epithelial cells of respiratory tract, responsible for frequent seasonal epidemics, caused by the rapid evolution of the viral genome. There are two main mechanisms of evolution: antigenic shift and genetic or antigenic drift. Hemagglutinnin, the protein of the virus envelope, is the main place of these variations. The study concerning receptor binding site structure and the specificity of human and animal influenza viruses have brought information about the mechanisms of interspecies spread of infections. It was confirmed that human influenza A viruses do not spread in birds while the species barrier between human and pigs is relatively low. Therefore pigs might functions as “mixing vessels” for the creation of new pandemic reassortants. The variability of the influenza virus is very complex process. Antigenic drift and shift still cause the origin of immunologically distinct strains of influenza viruses. The rapid antigenic drift of new forming viruses explain why there is a need for regular monitoring of that process. Antigenic and genetic characteristics of currently circulating strains of influenza virus could be beneficial in evaluating new diagnostic methods as well as for vaccine composition, which have to be updated annually.
Influenza, which affects people and animals, is caused by type A influenza viruses, a member of the Orthomyxoviridae family. The viruses are characterized by a considerable antigen variability. Two kinds of the variability are known: antigenic shift and antigenic drift. Type A viruses appear in populations of birds worldwide. They may be pathogenic for some birds and non-pathogenic for others. Infected wild water fowl, migratory and marine birds which show no clinical signs are the largest natural reservoir for all types of influenza viruses and they are the main source of infection for domestic birds and other animals. The influenza is zoonosis. H5N1 influenza virus strain was initially transmitted directly to humans in Asia in 1997. The virus has subsequently appeared at the end of 2003 in Hong Kong and it has been diffused to several countries of Asia. According to WHO, the total number of affected persons in these countries on December 07, 2005 was 137 of which 70 of them died. In 2005 avian influenza was noticed in Russia, Kazakhstan, Mongolia, Romania and Turkey. In these countries no infections in humans were observed. According to WHO, FAO and OIE the avian influenza appearing in many Asian countries and expanding to the West is a serious threat for human health.
Equine influenza is highly contagious and spreads rapidly among susceptible horses. The disease occurs globally and is caused by two main strains: H7N7 and H3N8. The H7N7 strain has not been isolated since the 1980s, and H3N8 circulates in equine population throughout most of the world. The H3N8 virus has diverged into two antigenically and genetically different evolutionary lineages since the 1986s: the American and European ones. Equine influenza exists in an endemic form in many countries. Transmission of the influenza virus from one host species to another is a crucial feature of its ecology and epidemiology. Two basic mechanisms of interspecies transmission are possible. One is the direct transfer of an essentially unaltered virus from one species to another. The second mechanism is a consequence of the segmented nature of the influenza genome and genetic reassortment.
The worldwide dissemination of HIV, the cause of AIDS, over the past 25 years has been one of the most catastrophic examples of the emergence, transmission, and propagation of a zoonotic infection. Ongoing exposure of humans to simian lentiviruses and the potential for additional lentiviral epidemics should not be dismissed.
Human and animals influenza is caused by the virus possessing 8 RNA segments in the genome. The segmented nature is critical structural feature that allows influenza viruses to undergo genetic reassortment. Pigs are important reservoir of influenza viruses because they express two types of receptors in the trachea which can bind swine, human and avian influenza viruses during infection. Therefore reassortation of viruses’ segments from different origin and subtypes can occur and facilitate in this way the generation of the strains with pandemic potential. Pigs can infect human with influenza virus directly which was several times confirmed. However the monitoring of pig care takers health indicate that it happened occasionally. In April 2009 the first cases of influenza, caused by the new strain of influenza type A H1N1, possessing genes of swine origin of unknown combination, occurred in North America. Within few days influenza spread and over 1000 people in 21 countries developed the illness, including 27 deaths. New A H1N1 strain possesses pandemic potency and is transmitted between humans only. Up to now none of the cases reported worldwide have had any direct contact with pigs. More over infected person introduced the mutant into the pig herd which may increase the risk of further reassortation. Data concerning the epidemic of A H1N1 strain are presented in this paper.