The current emphasis on heartworm prevention reflects the dependable protection provided by the monthly administered macrolide endectocides. This article reviews the prerequisites for heartworm transmission and the importance of daily temperature as a limiting factor in determining the seasonality of the transmission period. The practice of some veterinarians to continuously prescribe monthly chemoprophylaxis exaggerates the actual risk of heartworm transmission in most parts of the country and unnecessarily increases the cost of protection to their clients. Guidelines are provided for making an objective, conservative estimate of the earliest and latest dates for administering monthly chemoprophylaxis; and the use of seasonal projections for other clinical applications such as timing and interpretation of heartworm testing are discussed.
