Working Group II: Impacts, Adaptation and Vulnerability

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9.7. Infectious Diseases

The ecology and transmission dynamics of infectious diseases are complex and, in at least some respects, unique for each disease within each locality. Some infectious diseases spread directly from person to person; others depend on transmission via an intermediate "vector" organism (e.g., mosquito, flea, tick), and some also may infect other species (especially mammals and birds).

The "zoonotic" infectious diseases cycle naturally in animal populations. Transmission to humans occurs when humans encroach on the cycle or when there is environmental disruption, including ecological and meteorological factors. Various rodent-borne diseases, for example, are dependent on environmental conditions and food availability that determine rodent population size and behavior. An explosion in the mouse population following extreme rainfall from the 1991-1992 El Niño event is believed to have contributed to the first recorded outbreak of hantavirus pulmonary syndrome in the United States (Engelthaler et al., 1999; Glass et al., 2000).

Many important infectious diseases, especially in tropical countries, are transmitted by vector organisms that do not regulate their internal temperatures and therefore are sensitive to external temperature and humidity (see Table 9-1). Climate change may alter the distribution of vector species—increasing or decreasing the ranges, depending on whether conditions are favorable or unfavorable for their breeding places (e.g., vegetation, host, or water availability). Temperature also can influence the reproduction and maturation rate of the infective agent within the vector organism, as well as the survival rate of the vector organism, thereby further influencing disease transmission.

Changes in climate that will affect potential transmission of infectious diseases include temperature, humidity, altered rainfall, and sea-level rise. It is an essential but complex task to determine how these factors will affect the risk of vector- and rodent-borne diseases. Factors that are responsible for determining the incidence and geographical distribution of vector-borne diseases are complex and involve many demographic and societal—as well as climatic—factors (Gubler, 1998b). An increase in vector abundance or distribution does not automatically cause an increase in disease incidence, and an increase in incidence does not result in an equal increase in mortality (Chan et al., 1999). Transmission requires that the reservoir host, a competent arthropod vector, and the pathogen be present in an area at the same time and in adequate numbers to maintain transmission. Transmission of human diseases is dependent on many complex and interacting factors, including human population density, housing type and location, availability of screens and air conditioning on habitations, human behavior, availability of reliable piped water, sewage and waste management systems, land use and irrigation systems, availability and efficiency of vector control programs, and general environmental hygiene. If all of these factors are favorable for transmission, several meteorological factors may influence the intensity of transmission (e.g., temperature, relative humidity, and precipitation patterns). All of the foregoing factors influence the transmission dynamics of a disease and play a role in determining whether endemic or epidemic transmission occurs.

The resurgence of infectious diseases in the past few decades, including vector-borne diseases, has resulted primarily from demographic and societal factors—for example, population growth, urbanization, changes in land use and agricultural practices, deforestation, international travel, commerce, human and animal movement, microbial adaptation and change, and breakdown in public health infrastructure (Lederberg et al., 1992; Gubler, 1989, 1998a). To date, there is little evidence that climate change has played a significant role in the recent resurgence of infectious diseases.

The following subsections describe diseases that have been identified as most sensitive to changes in climate. The majority of these assessments rely on expert judgment. Where models have been developed to assess the impact of climate change, these also are discussed.

Table 9-2: Effect of climate factors on vector- and rodent-borne disease transmission.
Climate Factor Vector Pathogen Vertebrate Host
and Rodents
  • Decreased survival, e.g., Culex. tarsalis
    (Reeves et al., 1994)
  • Change in susceptibility to some pathogens
    (Grimstad and Haramis, 1984; Reisen, 1995);
    seasonal effects (Hardy et al., 1990)
  • Increased population growth (Reisen, 1995)
  • Increased feeding rate to combat dehydration,
    therefore increased vector-human contact
  • Expanded distribution seasonally and spatially
  • Increased rate of extrinsic
    incubation in vector
    (Kramer et al., 1983; Watts
    et al., 1987)
  • Extended transmission
    season (Reisen et al., 1993,
  • Expanded distribution (Hess
    et al., 1963)
  • Warmer winters favor
    rodent survival
Decreases in
  • Increase in container-breeding mosquitoes
    because of increased water storage
  • Increased abundance for vectors that breed in
    dried-up river beds (Wijesunder, 1988)
  • Prolonged droughts could reduce or eliminate
    snail populations
  • No effect
  • Decreased food
    availability can reduce
  • Rodents may be more
    likely to move into
    housing areas, increasing
    human contact
Increases in
  • Increased rain increases quality and quantity
    of larval habitat and vector population size
  • Excess rain can eliminate habitat by flooding
  • Increased humidity increases vector survival
  • Persistent flooding may increase potential
    snail habitats downstream
  • Little evidence of direct
  • Some data on humidity
    effect on malarial parasite
    development in Anopheline
    mosquito host
  • Increased food
    availability and
    population size (Mills
    et al., 1999)
Increase in
  • Heavy rainfall events can synchronize vector
    host-seeking and virus transmission (Day and
    Curtis, 1989)
  • Heavy rainfall can wash away breeding sites
  • No effect
  • Risk of contamination
    of flood waters/runoff
    with pathogens from
    rodents or their excrement
    (e.g., Leptospira from
    rat urine)
Sea-level rise
  • Coastal flooding affects vector abundance for
    mosquitoes that breed in brackish water (e.g.,
    An. subpictus and An. sundaicus malaria
    vectors in Asia)
  • No effect
  • No effect

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