Disease related issues

In recent years emerging infectious diseases, such as the avian influenza and severe acute respiratory syndrome (SARS), have rapidly caused many infected cases in humans, negatively affecting human life and the economy. There are some studies focusing on reviewing infectious diseases and analyzing their impacts. Capua and Alexander (2002) reviewed the evolution of avian influenza and discussed its relation to human health. They showed that new strains mostly certainly emerged after reassortment of genes of viruses of avian and human origin in a permissive host, and an H5N1 avian influenza virus was recognized as the cause of death of 6 of 18 infected patients, a very high mortality rate, during 1997 in Hong Kong. Centers for Disease Control and Prevention (2003) showed that

of the 131,132 persons who were quarantined during the SARS epidemic in Taiwan, the highest percentage of persons who had suspect or probable SARS diagnosed were persons who sat near a SARS patient on the same airplane flight. Olsen et al. (2003) indicated a high rate of transmission of SARS on aircraft, and believed that the most plausible explanation for the development of SARS in the passengers and crew members of a given flight was that they were infected while on board the aircraft, although other explanations were possible.

Brahmbhatt (2005) pointed that the avian influenza H5N1 will change into a form that is highly infectious for humans and spreads easily from person to person, if given enough opportunities. Given that H5N1 would be a global disease and would last longer than SARS, he predicted that H5N1 would result in a loss of around $800 billion around the world.

Since the bird flu virus could also be carried to another region over the world by wild birds or by illegal trade, he said this may lead to genetic reassortment and the start of a human pandemic in some other region, not restricted to East Asia area. Ligon (2005) reviewed information about the virus itself and its spread among poultry, migrating birds, mammals and humans, and indicated that the next pandemic would result in the deaths of at least 2 to 7 million people. He defined an influenza pandemic as a global outbreak of disease that occurs when a new strain of influenza A virus emerges in the human population, causes serious illness and then spreads easily from person to person worldwide. To prevent and control the spread of H5N1, he showed several activities implemented by Hong Kong, such as preparing a pandemic preparedness plan, conducting human and poultry surveillance programs, stockpiling antiviral drugs for treatment and prophylaxis, and raising public awareness of issues related to an influenza pandemic.

There are also some studies focusing on the design and evaluation of disease control measures. For example, King et al. (2004) established animal and human virological

surveillance systems to monitor evolutional changes of influenza viruses in the same and different hosts/geographical areas over years. They showed that the integrated surveillance was very useful in disease control and understanding conditions for the emergence of novel influenza viruses with pandemic potential. Ferguson et al. (2005) modeled the spread of a pandemic in Thailand by incorporating random contacts associated with day-to-day movements to work within the country, and then evaluated the potential effectiveness of containment strategies. They showed that containment and elimination of an emergent pandemic strain of influenza at the point of origin was feasible using a combination of antiviral prophylaxis and social distance measures. Rapid identification of the original case cluster, effective delivery of treatment to a high proportion of the targeted population, and sufficient stockpiles of drug were the key criteria must be met for a high probability of success in disease control.

In addition, numerous studies have proposed mathematical epidemic models to evaluate and describe the dynamic evolution and the severity of epidemics in the population.

For example, Massad et al. (2005) proposed a dynamical model to assess the expected burden of the epidemic in the absence of control measures and the impact of adopting well-known precaution methods. The proposed model was applied to the communities of Hong Kong and Toronto, and projected that the final number of cases would be 320,000 in Hong Kong and 36,900 in Toronto in the absence of control. With control measures, the expected final number of cases was reduced to 1778 in Hong Kong and 226 in Toronto.

Colizza et al. (2006, 2007) developed a metapopulation stochastic epidemic model on a global scale, and used a stochastic transport operator to describe the dynamics of individuals based on travels between cities. They concluded that a cooperative strategy where countries with large antiviral stockpiles shared a part of their resources with other countries resulted in a global deceleration of the pandemics. For highly contagious viruses, even the

unrealistic use of antiviral drug supplies corresponding to the treatment of approximately 20% of the population, there was still 30%-50% of the population infected. They also demonstrated that in the case of limited antiviral drug supplies, the more cooperative the strategy, the more effective were the containment results in all regions of the world. Table 2.5 summarizes the main issues and features as well as important results in the existing literature on disease related issues.

Table 2.5 Main issues, features and results on disease related literature Authors Main issues and features Important results Olsen et al. (2003) Investigate the transmission

of SARS on aircraft

There is a high rate of transmission of SARS on aircraft

King et al. (2004) Establish animal and human virological surveillance systems

The integrated surveillance is very useful in disease control and understanding conditions for the emergence of novel influenza viruses Brahmbhatt (2005) Discuss the impact of the

avian influenza H5N1

H5N1 would result in a economic loss of around $800 billion around the world

Containment and elimination of an emergent pandemic strain of influenza at the point of origin is feasible using a combination of antiviral prophylaxis and social distance measures

Ligon (2005) Review information about the virus and its spread

The next pandemic would result in the deaths of at least 2 to 7 million people Massad et al. (2005) Propose a dynamical model

to assess the expected burden of the epidemic

The pandemic is effectively mitigated by the practice of precaution methods in Hong Kong and Toronto

Colizza et al. (2006, 2007)

Develop a stochastic epidemic model on a global scale and assess control strategies

A cooperative strategy where countries with large antiviral stockpiles share a part of their resources with other countries results in a global deceleration of the pandemics

Source: this dissertation

Summary:

Past studies have demonstrated the impacts of diseases on the human population. At the same time, there are many works developing transmission models to analyze the spread of diseases. However, the influences of the air travel activities of passengers and the small-world properties of air transportation network on the spatiotemporal evolution of diseases have rarely been incorporated into the construction of transmission models, and have rarely been explored in the literature. In fact, the transmission of diseases may occur during air travel activities of passengers, such as consolidation, conveyance and distribution on scheduled flights and at airport terminals. Besides, the small-world properties of air transportation network may facilitate the spread of diseases. So, the influences of these activities and distinctive properties deserve further investigation.