The Disease Impacts of Monsoons
The formal definition of monsoon is a "seasonal reversing wind accompanied by
corresponding changes in precipitation" (see Wikipedia for more), but most people, when
they think of "monsoon", think RAIN. Monsoon weather patterns are prevalent in several
regions of the globe, including parts of Africa, South and East Asia, Australia, and the
desert Southwestern United States. Many areas that experience monsoons depend on this
time of year for most of their water supply, but this distinct and dramatic wet season
can also be very destructive. Heavy rain can be preceded by strong winds and blowing
dust, and can spawn tropical storms, cause extreme flooding and overflow drainage and
sewage systems in a matter of hours.
Perhaps a more insidious danger associated with monsoons, however, is an increase
in disease incidence among people, animals, even plants. A number of infectious diseases
have strong seasonal components that can be correlated with rain patterns, including
airborne, vector-borne and dysentery diseases.
The seasonal progression between wet and dry monsoonal periods is commonly
associated with strong blowing winds and dust storms, which can stir up fungal spores
present in the soil in semi-arid areas and increase the incidence of respiratory
infections such as coccidioidmycosis, and may coincide with meningitis epidemics in
parts of Africa. Additionally, the blowing dust may further aggravate bronchitis and
other respiratory illnesses in populations in these areas.
Following storms, the accumulation of stagnant water in open containers and low-
lying areas provides convenient breeding sites for mosquitoes and other insect
populations, causing an increase in incidence of vector-borne diseases among human and
animal populations, including malaria, dengue, West Nile Virus, Rift Valley Fever, and
so on. Vector-borne diseases increase among crops as well, where insects such as
whiteflies transmit mosaics and other viruses, spreading gradually through fields and
orchards in the direction of prevailing winds.
Many dysentery diseases also show an uptick in incidence coinciding with monsoon
weather. Regional rainy seasons regularly bring about an increase in outbreaks of
cholera, typhoid, hepatitis-A and leptospirosis, particularly in areas without
sufficient sewage systems in place to prevent rapid multiplication of bacteria in
The manner in which monsoon effects are incorporated into mathematical disease
transmission models can vary widely. For person-to-person transmitted diseases, a
periodic seasonal forcing function is frequently applied to the transmission rate
parameter. This seasonal function is built to accommodate higher and lower rates of
disease transmission based on the time of year, and can also be employed to describe
non-weather-related events such as school sessions and holidays.
When modeling vector-borne diseases, the impact of heavy rains on human, animal
or plant infections is indirect, moderated through changes in the vector population. In
this situation, the seasonal forcing applies more to vector birth and death rates than
to transmission rates, though the latter will increase indirectly simply through the
presence of greater numbers of viable vectors in the area. For infections spreading
through crops, additional accommodation must be made for wind patterns to model
spatial transmission from one plant to another within a given field or orchard.
Water- or food-borne dysentery diseases are commonly modeled as having an
environmental component, within which the bacteria multiplies and through which human
infections occur via direct contact (wading, swimming) or consumption (both water and
food). In these settings, the sudden influx of large volumes of water can increase the
carrying capacity and potential for bacterial growth, increase the rate of contact
between humans and the contaminated source, or dilute the concentration of bacteria in
the environmental reservoir, depending on disease- and locale-specific conditions. In
some settings, all three effects can come into play at different points during and
after the monsoon rains.
For those regions of the world that experience monsoons, this strong seasonal
component cannot be neglected in mathematical models as it is a major driver of
transmission dynamics, and will play a major role in any successful effort to control or
prevent disease in these areas. If your organization is working to explore the impact of
strong seasonality such as monsoons on infectious disease transmission, contact
MathEcology to learn more about how mathematical modeling can help!
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