South-East Asia Regional Conference on Epidemiology

46 | <strong>South</strong>-<strong>East</strong> <strong>Asia</strong> <strong>Regional</strong> <strong>Conference</strong> on Epidemiology
Chikungunya, an African virus, has been found in <strong>Asia</strong> for many years. It was introduced into <strong>Asia</strong>
some time in the 1950s or ‘60s. Some of the first epidemics of a haemorrhagic disease reported in
Kolkata in the 1960s turned out to be associated with chikungunya. The virus then spread to a number
of south-east <strong>Asia</strong>n countries. Epidemics occurred in the 1970s in Myanmar, Thailand and Indonesia.
The virus was distributed widely in the 1980s but it was not reported for many years. Periodically, small
focal outbreaks occurred, mainly in Indonesia, but it was not a major epidemic disease. That changed
in 2004 with the emergence of a new virus subtype in Kenya, which subsequently spread over the next
couple of years to islands off the <strong>East</strong> African coast, causing major epidemics. It was introduced into
India some time in 2005 or 2006 and has caused major epidemics throughout south-east <strong>Asia</strong> in the last
four to five years. It was introduced into Italy in 2007 and caused a small outbreak of about 250 cases.
An important question is how this virus was maintained in <strong>Asia</strong> for over three decades? Interestingly,
chikungunya virus was recently documented in a non-human primate in Malaysia, suggesting that
there is a non-human primate reservoir similar to dengue, i.e. a zoonotic cycle.
There are many factors contributing to the dramatic re-emergence of vector-borne diseases over
the past 30 years. However, the major drivers of this global emergence are 3-fold: 1) demographic
changes; 2) modern transportation; and 3) lack of effective public health infrastructure. Population
growth and its movement have been the major drivers of environmental change, and one of the most
important environmental changes that has contributed to the increase in epidemic infectious diseases,
especially vector-borne diseases, is uncontrolled urbanization. Agricultural and land-use changes
such as deforestation have also contributed. Population growth has been the major driver of changes
in animal husbandry, which has contributed to a number of major epidemics. The jet airplane has
increasingly become the principal mode of transportation over the last 40 years. As a result, there
has been a dramatic increase in the movement of people, animals and commodities during this time,
providing the mechanism to move vectors and pathogens around the world. When pathogens move
into areas where there is little or no vector control, there is an increased probability of secondary
transmission, and thus occurrence of epidemics.
The number of people living in urban areas of the world has also shown a dramatic increase. In 2007,
the world crossed the 50% line with over half of the world’s population now living in urban areas. Most
of this growth has been due to migration from rural to urban areas. People who move into cities bring
their rural lifestyles and pathogens with them. The United Nations has defined urban agglomerations as
cities with five million people or more. In 1950, there were only eight such cities in the world; by 2000,
there were 40 such cities. Most of the new cities were in the <strong>Asia</strong>n region. As projected over the next
five to ten years, another 20 cities will be in this category, and most of these cities are projected to be in
the <strong>Asia</strong>n region. The United Nations thus projects that most of the urban growth in the world is going to
occur in <strong>Asia</strong>n cities. This creates poor living conditions for the people, especially crowding and lack of
basic services, i.e. housing, piped water, sewage, waste management, etc. Poor urban slums create ideal
conditions for increased transmission of all kinds of infectious diseases pathogens, especially the urban
mosquito-borne diseases. Combine this to the fact that all of these urban centres have modern airports
through which tens of millions of people travel every year, creating an ideal mechanism for increased
movement of pathogens. Every year, over two billion people get on the air and fly somewhere. Many
of these people carry pathogens with them. Dengue is a good example to illustrate this phenomenon. In
1970, only south-east <strong>Asia</strong> had hyperendemicity of dengue with all four virus serotypes co-circulating
in most countries. It was in this setting that dengue haemorrhagic fever emerged. The African and
American tropical countries were either non-endemic with no viruses or hypoendemic with a single
virus serotype circulating. In 2010, however, the whole of the tropical world is hyperendemic (Fig. 4A
& 4B). And this increased transmission has led to the emergence of epidemic dengue haemorrhagic
fever in many countries of <strong>Asia</strong> and the Americas. And it is not just dengue; a lot of other viral, bacterial
and parasitic pathogens have taken advantage of easy transportation around the world.