Avian influenza is the most important historical viral infection in poultry industry causing huge economic losses. Ducks act as natural host while the virus can be transmitted from these natural hosts to highly prone species such as chickens and turkeys.
A specialised feature of influenza viruses is that they can emerge as novel subtype due to special characteristic of "mutation", thus are capable to produce disease in large population throughout the world. Avian influenza viruses (AIV) are placed in the family of orthomyxoviridae which consists three influenza genera named as A, B and C. Influenza viruses which are responsible for infections in birds, belongs to genus influenza virus A. AIV are well known to produce two different types of diseases in domestic poultry birds on the bases of their virulence and pathogenicity. Due to this unique characteristic, the viruses are classified into two types known as a highly pathogenic avian influenza virus (HPAIV) and a low pathogenic avian influenza virus (LPAIV). HPAIV are responsible for rapid and fatal systemic infection inducing mortality upto 100% especially in broilers, layers and breeders while LPAIV produce asymptomatic infection.
All HPAI viruses belong to H5 and H7 while all other subtypes including H9 are non pathogenic in nature. H9 viruses were first time reported in Pakistan in 1999, but are now endemic in poultry industry in Pakistan causing 5-30% mortality in field conditions. Interestingly under experimental conditions these viruses do not produce any disease. Possible factor associated with this elevated mortality is the involvement of secondary pathogens. The main pathogens involved in coinfection are Mycoplasma gallisepticum and pathogenic Escherichia coli while immunosuppression is another important factor. Another momentous feature of H9N2 viruses is that they are capable to cross transmission barriers existing between different species and can produce infection in human beings and in other mammalian species.
The genetic sequence of these viruses has 98% homology with the genetic sequence of H9N2 isolates recovered from children in Hong Kong. These findings indicate that these subtypes pose a similar threat as H5 viruses to human population. Keeping in view the importance of the disease the author designed a project at University of Veterinary and Animal sciences Lahore to reproduce disease as observed in field conditions and to study pathogenicity of H9 virus coinfection with Mycoplasma and E. coli infection and under immunosuppressive condition in broiler chickens and to asses its zoonotic potential in human beings.
In first part of study, H9 virus, Mycoplasma gallisepticum and E. coli were isolated and experimental inocula of each were prepared. In second part, pathogenesis of H9 virus was carried out under controlled environmental conditions. The virus prevailing in Pakistan was found non pathogenic with Intravenous Pathogenicity Index (IVPI) 0.12/3, produced significant reduction in body weight and had tissue tropism for kidney and respiratory system. Immunohistochemical detection of viral antigen and rapid molecular detection of virus from tissues by using Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) revealed that RT-PCR is more efficient method than immunohistochemistry.
The pathogenicity of H9 virus was also studied along with coinfection of secondary pathogens such as Mycoplasma gallisepticum and E. coli and in chemically induced immunosuppressive condition. It was found that E. coli and Mycoplasma gallisepticum induced exacerbation in pathogenicity and caused mortality 40% and 25% respectively, while under immunosuppressive condition only 5% mortality was observed.
In third part, it was intended to determine zoonotic potential of H9 virus infection among human beings working in close association to poultry industry by a serological survey. From this project, it seems reasonable to conclude that virus H9 prevailing in poultry industry in Pakistan belongs to non pathogenic Asian sublineage which did not produced mortality but produced significant reduction in weight gain of infected birds. Its pathogenicity is aggravated due to presence of secondary pathogens such as coinfection with E. coli and Mycoplasma gallisepticum. On the bases of serological study it is found that virus has potential to produce infections in human beings and this indicates that virus could be a candidate for future pandemic.