5. SOCIAL AND ENVIRONMENTAL INFLUENCES ON EMERGING INFECTIOUS DISEASES: SOME EXAMPLES
The following section provides some examples of emerging infectious diseases, considered under major categories of environmental and social influences, pertaining to patterns of travel, trade and land use.
(a) Travel and trade
Wherever we travel, unseen microbes accompany us. The plague bacterium, Yersinia pestis, accompanied Roman legions returning from the Middle East. In London in the mid-nineteenth century, John Snow noted that cholera epidemics followed major routes of commerce between Asia and Europe, appearing first at seaports when entering a new region. However, the speed, volume and reach of today’s travel are unprecedented in human history, and offer multiple potential routes for microbial spread around the globe.
The African malaria vector mosquito Anopheles gambiae gained entry to Brazil in 1937. Apparently, the mosquito migrated from western Africa on the mail-boats that traversed the Atlantic in 3–4 days. In the ensuing years, this mosquito species spread along the Brazilian coastal region and inland, and caused up to 50 000 deaths. (Fortunately, an extraordinary eradication campaign, led by the American Fred Sopers, eliminated it in the early 1940s.)
In today’s globalizing economy, this story is often repeated. For example, a major mosquito vector for the dengue virus, Aedes albopictus (the ‘Asian tiger mosquito’), has been spread widely in recent years, particularly via the unwitting intercontinental exportation of mosquito eggs in used car tyres into Africa and the Americas (Reiter & Sprenger 1987).
Neisseria meningitidis, a global pathogen, causes seasonal epidemics of meningitis in parts of Africa: the so-called ‘meningitis belt’. The disease has spread more widely. Studies with molecular markers have shown, for example, how Muslim pilgrims that brought an epidemic strain of
N. meningitidis from southern Asia to Mecca in 1987 then passed it on to pilgrims from sub-Saharan Africa, who subsequently initiated, back home, strain-specific epidemic outbreaks in 1988 and 1989. Studies of molecular markers traced the spread of this epidemic clone to several other countries (Moore et al. 1989).
The globalization of the food market has accentuated the movement of pathogens from one region to another. The commercial movement of foods, particularly fruits and vegetables, also redistributes microbial resistance genes along with the microbes. For example, an outbreak of cholera in Maryland, USA, was traced to imported contaminated frozen coconut milk (Taylor et al. 1993). Alfalfa sprouts grown from contaminated seed sent to a Dutch shipper caused outbreaks of infections with Salmonella species in both the USA and Finland (Mahon et al. 1997).
A primary drive that underlies migration is the urge to enter the cash economy, allied with the international demand for both skilled and unskilled workers in a globalizing marketplace. Rapid urbanization tends to boost old infectious diseases such as childhood pneumonia, diarrhoea, tuberculosis and dengue. It also facilitates the spread of various ‘emerging’ diseases; for example, high-rise housing creates new risks, as seen for SARS in Hong Kong. Such housing also increases family breakdown, drug abuse, sexually transmitted infections and HIV (Cohen 2003).
The disease West Nile virus, newly emergent in North America, further illustrates the impact of long-distance trade and travel. The disease originated in Africa, and occurs sporadically in the Middle East and parts of Europe. It was unknown in North America until it arrived in New York in 1999, via an infected mosquito on an aeroplane. Birds were affected first, humans later. There were apparently favourable conditions for the virus to survive and spread within New York City. They were as follows.
(i) Early season rain and summer drought provided ideal conditions for Culex mosquitoes.
(ii) July 1999 was the hottest July on record for New York City.
(iii) Suburban/urban ecosystems supported high numbers of select avian host and mosquito vector species adapted to those conditions.
- (iv) High populations of susceptible bird species existed, especially crows.
- (v) Suburban/urban ecosystems were conducive to close interaction of mosquitoes, birds and humans.
West Nile virus then spread rapidly across the United States, and has now established itself as an endemic virus, harboured by animals including birds and horses and transmitted via mosquitoes. There was a sharp increase in the number of human cases, involving most US states, during 2002–2003. In July 2003, Mexico declared a state of emergency when West Nile virus arrived in that country. The concern that the disease could spread more rapidly in Central and South America than in North America reflects the awareness that Latin American countries could be ideal breeding grounds, because of their warmer climate, large bird populations and year-round mosquitoes. Ecologists anticipate adverse impacts on domesticated horses and on the diverse animal and bird life in the tropics.
(b) Land use and environmental change
As Rene Dubos noted long ago, humans have always changed their environments (Dubos 1980). We are, in the words of the ecologists, ‘patch disturbers’. The increasing scale of our intervention in the environment, both deliberately (e.g. land clearing, urbanization) and as collateral impact (e.g. global climate change, species extinctions), is inevitably accelerating the rate of emergence of new infectious diseases. The main human-induced environmental changes that affect infectious disease risk include: tropical deforestation; road building; irrigation; dam building; local/regional weather anomalies; intensified crop and animal production systems; urban sprawl; continued poor sanitation; and pollution of coastal zones.
A Working Group on Land Use Change and Infectious Disease Emergence, comprising several dozen scientists from around the world, met in 2002 and ranked the environmental factors, associated with land use, with most influence (‘public health impact’) on emerging diseases (Patz & Confalonieri 2004). The top 12 environmental changes, in descending order, were as follows:
- (i) agricultural development,
- (iv)population movement,
- (v) introduced species/pathogens,
- (vi)biodiversity loss,
(vii) habitat fragmentation,
(viii) water and air pollution (including heightened respiratory susceptibility),
- (ix)road building,
- (x) impacts of HIV/AIDS,
- (xi)climatic changes, and
(xii) hydrological changes, including dams.
Many natural systems—forests, drylands or cultivated systems—contain a distinct, exclusive set of infectious diseases. However, several major diseases, including malaria and dengue, occur across many ecosystems. Malaria is transmitted by 26 different species of anopheline mosquitoes, each of them dominant in particular habitats and locations. Because each species responds differently to a specified land use change, it is difficult to generalize the impact of ecosystem change effects across regions. Some other diseases such as yellow fever, however, can be transferred across ecosystems. The disease’s natural, sylvatic, zoonotic cycle is between mosquitoes and monkeys high in forest canopies, but yellow fever can move into savannah, agricultural, and even urban areas in the wake of human economic activities such as logging or forest clearing.