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
|
| |
Increased
temperature |
- 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,
1995)
- Expanded distribution (Hess
et al., 1963)
|
- Warmer winters favor
rodent survival
|
| |
Decreases in
precipitation |
- 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
|
|
- Decreased food
availability can reduce
populations
- Rodents may be more
likely to move into
housing areas, increasing
human contact
|
| |
Increases in
precipitation |
- 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
effects
- 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
precipitation
extremes |
- Heavy rainfall events can synchronize vector
host-seeking and virus transmission (Day and
Curtis, 1989)
- Heavy rainfall can wash away breeding sites
|
|
- 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)
|
|
|
| |
