Climatological influence in the epidemiology of leptospirosis in humans in SVG

INTERPRETATION OF THE INVESTIGATIVE RESULTS – Part 4

by Dr Eric Alanzo Audain Tue, Sept 18, 2012

LEPTOSPIROSIS is endemic to SVG, as a result of the population of infected RATS, which act as the main vector of the bacteria.

The evidence is overwhelming with respect to the level of correlation between the number of confirmed clinical cases of leptospirosis in humans and precipitation (rainfall) in St Vincent and the Grenadines during the period 2001-2010.{{more}}

There is a definite pattern regarding the occurrence of this disease in our country, where the number of confirmed clinical cases of the disease shows a significant increase as the accumulation of rainfall increases.

The manifestation of leptospirosis in humans in SVG does not depend on the amount of rainfall recorded in a day or in a month, but its accumulation over a period of time. Our investigative study shows that when the country experiences an accumulation of rainfall that exceeds 550 mm, the number of confirmed clinical cases of leptospirosis in humans increases. This pattern was seen during the period 2001-2010. We have seen cases of the disease when there is an accumulation of rainfall below 550 mm; however, the number of confirmed clinical cases is minimal.

There are several explanations regarding the correlation between confirmed clinical cases of leptospirosis in humans and accumulation of rainfall. Firstly, as the rainfall increases over time, the rats tend to move from their natural habitat, which suddenly becomes wet and, as a result, they are forced to seek dry ground, which, on most occasions, is found in farm houses, old abandonned buildings and any other area where they may be protected from the rain, thus bringing them closer to us. When this transpires, almost all accessible food material will be consumed and urinated upon. They also urinate in water that is stored in uncovered containers, on the floor or on any other surface area. Subsequently, when a person comes into direct contact with that contaminated food, the bacteria then penetrate the intact or broken skin and enter the blood stream.

Secondly, as the rainfall accumulates, the humidity of the soil increases; this creates conditions that are favourable for the survival of the bacteria while in the environment. There is also an increase in the amount of stagnant water that may be found under trees or any other area where the water is shaded from the direct effect of the sun. The radiation from the sun has a negative effect on the survival of the bacteria while in the environment. This water may become contaminated with urine of infected RATS or other infected animals. A person may become infected with the bacteria if any form of contact, whether directly or indirectly, is made with this contaminated water.

Thirdly, food availability to these rodents increases as the rainfall continues, since moving water brings along with it a large amount of food particles from various sources, including farms and homes. Finally, as the rainfall accumulates, much of the stagnant water that has been contaminated with the urine of infected animals is now moving and may end up in rivers, streams and on many pathways, hence increasing the dissemination of the bacteria.

Extensive emphasis has been placed on RATS, because they are the maintenance host of the bacteria. Most other animals acquired the bacteria (leptospirosis) as a result of their exposure to the urine of infected RATS. Dogs, cats, pigs, horses and cattle play a very significant role in the spread of leptospirosis in SVG and their role must not be underestimated.

Series to be continued next Tuesday