Mosquito populations respond favorably to above-average rainfalls and temperatures. | Photo: iStock

The annual transition from spring to summer typically heralds warmer weather and reminds us of the risk of human and horse exposure to various ­vector-borne diseases. Some of these diseases are mosquito-transmitted and some tick-transmitted. In any year temperatures and seasonal rainfall can greatly influence the bionomics of these arthropods (how they adapt to their environment).

Researchers have recognized for a significant number of years that a country’s climatic conditions can fluctuate greatly due to phenomena such as El Niño. For instance, a number of years ago scientists conducted a retrospective study on the frequency of major African horse sickness epizootics (relating to or denoting a disease that is temporarily prevalent and widespread in an animal population) in the Republic of South Africa. (This disease is spread by Culicoides midges.) They found that all but one of these historically recorded events since the early 19th century occurred in El Niño years. The U.S. climate has been similarly affected in such years.

Mosquito and tick populations respond very favorably to above-average rainfalls and increased temperatures. Such conditions are conducive to rapid and explosive surges in mosquito populations and the frequency of diseases they transmit. Increased temperatures also enhance the longevity of adult vector populations, while humidity and rainfall have been shown to influence vector behavior and survival