AHS recommends year-round administration of chemoprophylactic drugs to prevent heartworm disease, increase compliance and control pathogenic and/or zoonotic parasites.
..Aedes albopictus (Asian tiger mosquito), which was introduced into the southeastern United States in 1987, has now spread north approaching Canada and has extended past the Rocky Mountains to the west coast. This urban-dwelling mosquito is able to reproduce in small containers such as flower pots. In the northern half of the United States, urban sprawl has led to the formation of “heat islands”, as buildings and parking lots retain heat during the day and subsequently radiate it during the night. This can potentially create microenvironments that support development of heartworm larvae in mosquito vectors during colder months, thus lengthening the transmission season.
As these vectors expand their territory the number of unprotected animals infected will continue to increase. A pivotal prerequisite for heartworm transmission is a climate that provides adequate temperature and humidity to support a viable mosquito population, and sustains sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within this intermediate host. It has been shown under laboratory conditions in three mosquito species that maturation of larvae within mosquitoes ceases at temperatures below 57ºF (14ºC) and similar activity is expected in other mosquitoes capable of transmitting heartworms. Heartworm transmission does decrease in winter months but micro-environments commonly present in urban areas virtually ensure that the risk of heartworm transmission never reaches zero. Some species of mosquitoes overwinter as adults. While heartworm larval development in mosquitoes may cease in cool temperatures, development quickly resumes with subsequent warming.
The length of the heartworm transmission season in the temperate latitudes is critically dependent on the accumulation of sufficient heat to incubate larvae to the infective stage in the mosquito. The peak months for heartworm transmission in the Northern Hemisphere are usually July and August. Models predict that heartworm transmission in the continental USA is limited to six months or less above the 37th parallel, i.e., Virginia-North Carolina State line. Furthermore, predictive risk maps have been produced coupling these basic models with Geographic Information Systems (GIS) based on a thermal regimen and information about mosquito vectors. While these model-based predictions are academically appealing, they do not yet consider several potentially important factors, such as the influence of microclimate and the unique biological habits and adaptations of the numerous mosquito vectors on larval development. Once a reservoir of microfilaremic domestic and wild canids is established beyond the reach of veterinary care, the ubiquitous presence of one or more species of vector competent mosquitoes makes transmission possible and eradication becomes improbable.
