Transmission Dynamics of Low Pathogenicity Avian Influenza Infections in Turkey Flocks

Abstract

Low pathogenicity avian influenza (LPAI) viruses of H5 and H7 subtypes have the potential to mutate into highly pathogenic strains (HPAI), which can threaten human health and cause huge economic losses. The current knowledge on the mechanisms of mutation from LPAI to HPAI is insufficient for predicting which H5 or H7 strains will mutate into an HPAI strain, and since the molecular changes necessary for the change in virulence seemingly occur at random, the probability of mutation depends on the number of virus replicates, which is associated with the number of birds that acquire infection. We estimated the transmission dynamics of LPAI viruses in turkeys using serosurveillance data from past epidemics in Italy. We fitted the proportions of birds infected in 36 flocks into a hierarchical model to estimate the basic reproduction number (R 0) and possible variations in R 0 among flocks caused by differences among farms. We also estimated the distributions of the latent and infectious periods, using experimental infection data with outbreak strains. These were then combined with the R0 to simulate LPAI outbreaks and characterise the resulting dynamics. The estimated mean within-flock R0 in the population of infected flocks was 5.5, indicating that an infectious bird would infect an average of more than five susceptible birds. The results also indicate that the presence of seropositive birds does not necessarily mean that the virus has already been cleared and the flock is no longer infective, so that seropositive flocks may still constitute a risk of infection for other flocks. In light of these results, the enforcement of appropriate restrictions, the culling of seropositive flocks, or pre-emptive slaughtering may be useful. The model and parameter estimates presented in this paper provide the first complete picture of LPAI dynamics in turkey flocks and could be used for designing a suitable surveillance program.

Evaluating Surveillance Strategies for the Early Detection of Low Pathogenicity Avian Influenza Infections

“…*values derived from Comin et al [10].§values based on van der Goot et al [13]. Footnote: values of parameters a, b, s L , r L, s I , r I , s and r are those previously reported in Table 1.…”

“…To this end, we carried out simulations with a model parameterized with data from the Italian LPAI epidemics [10]. Although future outbreaks will probably be with different strains and in different areas, this is the model that combines most available field data and therefore represents the best available knowledge.…”

Risk based surveillance for early detection of low pathogenic avian influenza outbreaks in layer chickens

“…A total of 27 outbreaks were selected for analysis, based on sufficient information about antibody and/or virus (PCR) prevalence, egg production data and tracing. This information was used to reconstruct the within-flock infection dynamic (Comin et al, 2011a;Gonzales et al, 2012b) and the between-flock transmission events in case of secondary spread. Twenty out of these 27 outbreaks were primary introductions; the remaining seven were secondary cases.…”

Field Monitoring of Avian Influenza Viruses: Whole-Genome Sequencing and Tracking of Neuraminidase Evolution Using 454 Pyrosequencing

“…This case suggests further that the turkey is a very peculiar host among poultry species and shows very high receptivity and sensitivity to influenza A viruses (21). Recent epidemiological modeling data suggest a very high withinflock transmissibility of LPAIVs in turkeys (1). This case also emphasizes the risks of breeding together different avian species, i.e., terrestrial birds (chickens or turkeys) and waterfowl.…”