H5N1 is highly contagious between birds of different species. H5N1 easily mutates creating variants which are capable of infecting birds across different species. There are some of these variants which infect human beings. The avian virus is transmitted between birds by binding to galactose receptors located in their respiratory tract. The receptors are found in human alveoli and cannot therefore be transmitted through coughing and sneezing. The virus is therefore contagious in humans through physical contact with an infected person.
H5N1 virulence is determined by genetic variation of the various pathogenic profiles. H5N1 mutates through antigenic drift resulting in the development of a pathogenic strain, genotype z which is efficiently transmitted among birds of different species in various regions of the world. Mutations of genotype Z are considered to be the most virulent increasing pathogenicity of the H5N1 virus. Bird populations are favourable hosts for the H5N1 virus being in a position to carry different strains for a longer period of time. Infected H5N1 birds “transmit the disease through their saliva, nasal secretions, faeces and blood”.
Transmission results in acute neurological dysfunction, severe flu and eventually death. Birds can infect other animals through direct contact with the above mentioned body secretions and fluids. Migratory birds are attributed to the global spread of the avian flu. H5N1 is more virulent and pathogenic due to its potential to be transmitted between both birds and animals. Birds which transmit the pathogenic avian flu virus include chicken, crows, pigeons and the waterfowl. Poultry also acts as a carrier which can transmit the virus to mammals. Influenza A viruses being transmitted through poultry hosts can be divided into two main groups according to their ability to cause disease in birds.
The most virulent strain causes the “fowl plague” currently referred to as the highly pathogenic avian influenza (HPAI) which can lead to death of all infected birds. HPAI variant is further classified into subtypes H5 and H7. Low pathogenicity avian influenza (LPAI) causes milder respiratory disease which if combined with other infections may cause death. Haemagglutinin glycoprotein for either HPAI or LPAI requires further posttranslational cleavage in order to release infectious virus particles (Loscalzo et al, 2008). HPAI viruses are cleavable because they posses multiple amino acids at their cleavage sites where host proteases can bind. These viruses are therefore able to replicate at a higher rate in the host bird causing significant damage on vital organs and tissues.
On the other hand, LPAI contains fewer amino acids at the cleavage sites. LPAI viruses are therefore limited as far as replication is concerned within the host bird. Replications sites are only available where proteases are found, that is, in the respiratory and intestinal tracts. Human influenza hemagglutinins replicate within the lower respiratory tract where they bind to alpha sialic acid receptors. The avian H5N1 transmits influenza to humans causing “viral pneumonia, fever, cough, sore throat, muscle aches, conjunctivitis and fatal pneumonia”. The severity of human influenza depends on the state of patient’s immune system and the rate of exposure.