ABSTRACT
The term ‘‘influenza’’ describes an acute viral disease of the respiratory tract is caused by viruses that belong to the orthomyxovirus family, which includes the genera of influenza virus A, B, and C as defined by the antigenicity of the nucleocapsid and matrix proteins.1-6
Generally, influenza A viruses are associated with more severe human illness, epidemics, and pandemics.7,8 Influenza A virus is a negative sense, single-stranded RNA virus, with an 8-segment genome that encodes for 10 proteins. Influenza A virus is further subtyped by 2-surface proteins haemagglutinin (H), which attaches the viral particle to the host cell for cell entry, and neuraminidase (N), which facilitates the spread of progeny virus.1,9-12 It is the latter that is a target for the class of antiviral therap y referred to as neuraminidase inhibitors, which will be discussed shortly.9-11
There are 16 H and 9 N subtypes that make up all the subtypes of influenza A by various combinations.1,4,11,12 The term ‘‘antigenic drift’’ refers to the various mutations and changes in surface antigenicity of these surface proteins as a response to host immunity. This is why every year, usually in February, the World Health Organization (WHO) decides the strain of viruses to be incorporated into the annual influenza vaccine.13 More worrisome is the potential for ‘‘antigenic shift’’; an event that can lead to the creation of a novel virus against which humans have little or no immunity. This can occur because influenza has a segmented genome shuffling of gene segments can occur if 2 different subtypes of influenza A virus infect the same cell. If a human flu virus, such as H3N2 and an avian H5N1 virus coinfect a human or pig, a new virus ‘‘H5N2’’ could emerge a hybrid that could bring the high virulence and case fatality rate of H5N1 with the efficiency of human-to-human transmission found in the ‘‘parent’’ human virus.1,2,8 Studies suggest this is what happened in the 1957 and 1968 influenza pandemics. A pandemic is considered an epidemic that crosses continents. Of note, while Mother Nature has demonstrated its capacity to do this, such viral reassortment also could be accomplished in a laboratory for bioterrorism purposes.14-16 Therefore, bioterrorism preparedness, increased surveillance, and efforts to enhance physician training in unusual or emerging diseases is of significant importance.
