Heartworms, which are spread by mosquitoes, can only be transmitted if temperatures are ABOVE 60 degrees for 14 CONSECUTIVE days.

• Not just average temperatures: It’s not about the daily average temperature being 60°F; it’s about the ambient temperature being consistently above 57–60°F, especially at night, for heartworm larvae to develop inside the mosquito.
  
  
• Why 14 days? Heartworm development (Dirofilaria immitis) inside mosquitoes depends on something called the Heartworm Development Units (HDUs). It takes about 130 HDUs for larvae to reach the infective stage, and this happens only when temperatures are warm enough for long enough.
  
  
• Consistent warmth is required, particularly at night, for 14 days for heartworm transmission to become possible.