Estimation of the onset of exposure risk for Dirofilaria immitis (heartworm) in selected cities in Ontario from 1996 to 2023.

Background: Dirofilaria immitis (causative agent of heartworm disease) is a mosquito-borne parasite that is endemic in canids internationally and can cause serious disease in dogs.

Objective: The objective of this study was to predict the earliest time for infective D. immitis exposure risk in multiple cities in Ontario. In addition, the potential impact of an increase in the daily mean temperature was evaluated.

Animals and procedure: The earliest date of D. immitis exposure risk for dogs within a calendar year was estimated for multiple cities in Ontario, based on the time to accumulation of 130 heartworm development units (HDUs).

Results: The earliest date of accumulation of 130 HDUs between 1996 and 2023 was May 25 (Windsor in 2012), and the latest was August 7 (Thunder Bay in 2004, Tobermory in 2014). The median date ranged from June 7 (Windsor) to July 18 (Thunder Bay). There were no significant changes in onset date over the study period for any location (all Q > 0.12).When 1°C was added to the mean daily temperature, the date until 130 HDUs was reached decreased by a median of 4.5 d for Windsor (range: 2 to 12 d), 7 d for Sault Ste. Marie (range: 4 to 17 d), and 6 d for Kitchener-Waterloo (range: 3 to 11 d). This resulted in an earliest date of May 19 for Windsor, June 8 for Kitchener-Waterloo, and June 18 for Sault Ste. Marie.

Conclusion and clinical relevance: Described data suggest that current recommendations for heartworm prophylaxis in Ontario still provide abundant coverage for the estimated onset of risk, even when factors that could result in an earlier onset are considered.