Comparison of the transmission characteristics of low and high pathogenicity avian influenza A virus (H5N2)

Abstract

Low pathogenicity avian influenza A strains (LPAI) of the H5 and H7 type are noted for their ability to transform into highly pathogenic counterparts (HPAI). Here we compare the transmission characteristics in poultry of LPAI H5N2 (A/Chicken/Pennsylvania/83) and corresponding HPAI virus by means of transmission experiments. In the experiments, five inoculated animals are placed in a cage with five contact animals, and the infection chain is monitored by taking blood samples, and samples from the trachea and cloaca. The data are analysed by final size methods and a generalized linear model. The results show that HPAI virus is more infectious and induces a longer infectious period than LPAI. In fact, fully susceptible animals are invariably infected when confronted with HPAI virus and die within six days after infection. Animals previously infected with LPAI virus, on the other hand, survive an infection with HPAI virus or escape infection all together. This implies that a previous infection with LPAI virus effectively reduces susceptibility of the host to infection and decreases transmission of HPAI virus. We discuss the implications of these conclusions for the control and evolution of avian influenza viruses.

Effect of H7N1 vaccination on highly pathogenic avian influenza H7N7 virus transmission in turkeys

“…It is assumed that within each treatment all susceptible birds are equally susceptible, all infectious birds are equally infectious, and that each infectious bird is equally infectious over time. Furthermore, it is assumed that each infected bird constitutes an independent risk to infect a susceptible bird.ˇfollows from the intercept of the model, and subsequently the reproduction ratio R 0 can be estimated by taking the product of the estimatedˇand the estimated mean length of the infectious period, with the latter directly observed from the unvaccinated contact turkeys [17,18]. Virus shedding was compared between inoculated unvaccinated and vaccinated birds by means of a Fisher's exact test, and the NPAR1WAY procedure in SAS ® was used to compare HI antibody titres between groups.…”

Estimating the day of highly pathogenic avian influenza (H7N7) virus introduction into a poultry flock based on mortality data

“…For flock size, values were chosen based on the range of farms in the total field data set; 1 000 and 40 000 birds. The mean length of the latent period was set at 1.5 and 2.5 days; the latent period does not seem very variable according to the results from transmission experiments [21,22]. The basic reproduction ratio, R 0 , is an expression for the average number of new cases arising from one infectious chicken in a totally susceptible population [15].…”

“…In the transmission experiments with H7N7, a latent period of two days was used [22]. This latent period was based on previous transmission experiments, in which a model based on a latent period of one to two days fit the data better than a latent period of one day [21]. Also, we decided that we could not ignore a latent period which is a third of the infectious period.…”

“…To our knowledge, within-flock transmission has only been studied under experimental conditions, resulting in an indication of bird-to-bird transmission parameter values for the HPAI virus strain used in these experiments 1 [21,22]. However, there is a considerable gap between transmission experiments with a few animals and the field conditions of farms containing tens of thousands of animals.…”

Transmissibility of the highly pathogenic avian influenza virus, subtype H5N1 in domestic poultry: a spatio-temporal estimation at the global scale

“…However, R 0 values estimated under field conditions differ considerably from controlled experiments, e.g. the numbers of animals within herds or flocks, and the contact structure among animals, cannot be replicated in controlled experiments (van der Goot et al, 2003Goot et al, , 2005Tiensin et al, 2007). R 0 estimated using actual field data has therefore more practical applicability for disease control.…”