Don't forget to factor in the unpredictability of nature. We know that Culex tarsalis mosquitoes can live up to two months when temperatures hold at 77 F,9 but there is a good chance they can live longer at cooler temperatures. Near George Lake in Alberta, Canada, over half of
the overwintering Culex territans female mosquitoes studied survived more than 138 days at 23 F.10 These mosquitoes will continue to seek blood meals every time they are about to lay eggs, and if they are infected with
heartworms in October, they could still easily transmit the infection during an unseasonably warm December, like the one we
experienced this winter in Ithaca, N.Y. Such microclimate situations put dogs at risk all year long and are part of the rationale
that led to the recommendation in the capc guidelines that dogs receive preventives year-round. With locally acquired heartworm
transmission likely occurring in every state, it seems there is no good reason for a dog to be at risk in a nice November
or in a warm March. But there is no good reason for dogs to receive preventives all year; it is just not needed.
Perhaps we could take a lesson from our counterparts in human medicine. Human lymphatic filariasis, a disease caused by cousins
of the heartworm, is transmitted by mosquitoes that bite infected people. The disease induced—elephantiasis—is horrible. Small
thread-like worms live in the lymphatics and cause severe disfiguring and immobilizing disease in their human hosts. The World
Health Organization is leading the charge to eradicate these parasites through "mass drug administration strategies for disease
elimination."10 These mass treatments of populations have had a significant impact on the rate of infection in people and vectors.11 We can only hope that someday we will place similar pressure on the transmission of heartworms, reducing the occurrence
of the devastating disease that heartworms cause in dogs.
Dwight D. Bowman, MS, PhD, is a professor of parasitology at the Department of Microbiology and Immunology, College of Veterinary
Medicine, Cornell University, Ithaca, NY 14853.
1. Heartgard Plus. Heartworms are a serious threat. Available at:
2. National Cancer Institute. Cancer Trends Progress Report—2005 Update. Available at: http://progressreport.cancer.gov/.
3. Agency for Toxic Substances & Disease Registry. Available at:
4. Immiticide product insert. Duluth, Ga: Merial Limited.
5. U.S. Environmental Protection Agency. Arsenic in drinking water. Available at:
6. Rawlings CA, Losonsky JM, Schaub RG, et al. Postadulticide changes in Dirofilaria immitis-infected Beagles. Am J Vet Res 1983;44:8-15.
7. Levine ND. Nematode parasites of domestic animals and of man. Minneapolis, Minn: Burgess Publishing Company.
8. Munnell JF, Weldon JS, Lewis RE, et al. Intimal lesions of the pulmonary artery in dogs with experimental dirofilariasis.
Am J Vet Res 1980;41:1108-1112.
9. Mahmood F, Reisen WK, Chiles RE, et al. Western equine encephalomyelitis virus infection affects the life table characteristics
of Culex tarsalis (Diptera: Culicidae). J Med Entomol 2004;41:982-986.
10. Hudson JE. Cold hardiness of some adult mosquitoes in central Alberta. Can J Zool 1978;56:1697-1709.
11. World Health Organization. Lymphatic filariasis. Tropical disease research: progress 2003-2004. Seventeenth Programme Report of the UNICEF/UNDP/World Bank/WHO Special Programme for Research & Training in Tropical Diseases,