Transmission of highly pathogenic avian influenza H7 virus

Abstract

Knowledge of the transmission of highly pathogenic avian influenza (HPAI) virus still has gaps, complicating epidemic control. A model was developed to back-calculate the day HPAI virus was introduced into a flock, based on within-flock mortality data of the Dutch HPAI H7N7 epidemic (2003). The method was based on a stochastic epidemic model in which birds move from being susceptible, latently infected and infectious, to death. Our results indicated that two weeks can elapse before a noticeable increase in mortality is observed after a single introduction into a flock. Furthermore, field data were used to estimate the transmission rate parameter beta and the influence of risk factors on within-flock transmission. Daily mortality data again were back-calculated to fit a susceptible – infectious – dead format. The method took into account the uncertainty of the length of the latent period, the survival of an infection by some birds and the influence of farm characteristics. The beta was estimated at 4.50 per chicken per day (95% CI: 2.68 – 7.57) with a bird-infectious period of four days. In contrast to general belief, none of the studied risk factors (housing system, flock size, species, age of the birds and date of depopulation) had significant influence on the estimated beta. The same method was used on field data of the HPAI H7N1 epidemic in Italy in 1999-2000. The bird-infectious period was assumed to be two days, and all birds were assumed to die after this infectious period. The estimated beta for HPAI H7N1 virus transmission in turkeys was 1.43 (95% CI: 1.17 – 1.74). Farm risk factors such as flock size and age of the turkeys again did not influence beta. The effect of vaccination on HPAI H7N7 transmission in turkeys was estimated by means of a transmission experiment. Cloacal and tracheal swabs (for real-time RT-PCR) and serum samples (for hemagglutination inhibition test) were taken to monitor the infection both in inoculated and in susceptible contact turkeys, which were all either unvaccinated, vaccinated once or vaccinated twice with H7N1. Unvaccinated contact birds had a mean infectious period of 6.2 days, and an estimated beta of 1.26 per infectious bird per day. However, no virus shedding was found in inoculated vaccinated turkeys and thus we concluded that vaccination with H7N1 protected against challenge with HPAI H7N7 virus. During the Dutch epidemic a large number of human infections were reported. We estimated infection probabilities for persons involved in disease control on infected farms. Case definition was based on self-reported conjunctivitis and positive hemagglutination inhibition test. A high infection probability during a farm-visit was associated with clinical inspection of poultry in the area surrounding infected flocks (7.6%, 95% CI: 1.4 – 18.9%) and active culling during depopulation (6.2%, 95% CI: 3.7 – 9.6%). Low probabilities were estimated for managing biosecurity (0.0%, 95% CI: 0.0 – 1.0%) and cleaning assistance during depopulation (0.0%, 95% CI: 0.0 – 9.2%). No significant association was observed between the probability of infection and some exposure variables (number of birds present, housing type, poultry type, depopulation method, period during epidemic).