Tick activity decreases significantly below this point, but any species can survive cold weather by going dormant and hiding out in leaf piles or under snow.

Source: ¹The Tick Lifecycle – Lyme & Tick-Borne Disease Testing & Statistics.
https://www.ticklab.org/blog/2020/12/01/the-tick-lifecycle

Flea eggs can survive cold temperatures longer than expected.^2,3 While freezing outdoor temperatures can reduce survival, fleas and flea eggs may continue development indoors.

Sources: ²Dryden MW. Biology of fleas of dogs and cats. Compend Contin Educ Pract Vet. 1993;15(4):569-577

³Companion Animal Parasite Council. (n.d.). Trichuris vulpis: Prepatent period and environmental factors. https://capcvet.org/guidelines/trichuris-vulpis

Many parasites, including roundworms, are more hardy than pet owners believe. They are capable of surviving months in cold temperatures, infecting pets once ingested.^3-5

Sources: ³Companion Animal Parasite Council. (n.d.). Trichuris vulpis: Prepatent period and environmental factors. https://capcvet.org/guidelines/trichuris-vulpis

⁴Parasite Prevalence Maps. Companion Animal Parasite Council. Published 2025. Accessed August 1, 2025. https://capcvet.org/maps/#/2025/january/roundworm/dog/united-states

⁵Companion Animal Parasite Council. (n.d.). Ascarid: Prepatent period and environmental factors. https://capcvet.org/guidelines/ascarid

This is the minimum temperature for heartworm larvae to mature inside mosquitoes, making the time between seasons more dangerous than many pet owners realize.

Source: ⁶Cornell University College of Veterinary Medicine. Heartworm Disease Infographic. Cornell University; [date unknown]. Accessed July 31, 2025. https://www.vet.cornell.edu/sites/default/files/Heartworm%20Disease%20Infographic%20Final_1.pdf

While heartworm disease transmission decreases in winter months, factors like some species of mosquitoes overwintering and microclimates in urban areas mean the threat remains.⁷

Source: ⁷American Heartworm Society. Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. Updated November 2024. Accessed August 1, 2025. https://d3ft8sckhnqim2.cloudfront.net/images/AHS_Canine_Guidelinesweb22NOV2024.pdf