Controlling avian influenza with public health measures

Abstract

Avian influenza is a respiratory disease caused by avian influenza virus infection. Avian influenza viruses circulate among wild birds and often reach domestic poultry leading to important economic losses in the poultry sector. Because avian influenza viruses are not yet capable of sustainable human-to-human transmission, reported human infections are often the results of human exposure to infected poultry or poultry-related premises. However, in the event of a co-infection in pigs or humans with both avian and human influenza viruses, re-assortments between the viruses could make the new avian influenza virus capable of efficient human-to-human transmission, becoming a pandemic threat.

This thesis focuses on the epidemiology of avian influenza viruses and public health interventions against its spread. A review of human infections with avian influenza viruses was performed to study differences in age, sex and exposure of human cases with different subtypes. Seasonality of human infections with avian influenza viruses was also investigated in different locations reporting human cases.

The effect of public health interventions against avian influenza outbreaks was studied based on publications of estimates on real outbreaks. Raw data were also used to provide further estimates, and a meta-regression analysis of interventions was performed. Live poultry market closures were found to be highly effective in reducing human infections, whereas the effect of biosecurity improvements and education were mixed and not significantly associated with reduction of virus isolation or transmission.

To investigate the effect of alternative interventions against avian influenza along a poultry chain and considering poultry transportation as a major source of spread of the disease, an agent-based model was developed. The model encapsulates multiple dynamics such as the poultry transportation and trading, the spread of the disease within and between poultry bands, the risk of introduction of the virus from wild birds, and implementation of control measures depending on the status of an outbreak. Different scenarios were explored to analyse avian influenza transmission and showed different effectiveness depending on the parties participating in the poultry chain targeted for the analysis. Control measures also had reduced effectiveness against outbreak of low pathogenicity avian influenza, with silent transmission making the surveillance of the disease more difficult.

This thesis identifies significant differences in the exposure, age, and sex of the distribution of human avian influenza viruses per subtype. It also shows important differences in the estimates of effectiveness of an intervention depending on the methodology used for the implementation of the intervention, methodology used to assess the effectiveness, or target of the analysis. This may have implications for the assessment of the effect of interventions and the control of avian influenza in domestic flocks and at the human-animal interface.