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 contaminated water.

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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