The Truth About Heartworms - Part 1

The Truth About Heartworms

By Dr. Jean Hofve, Veterinarian Advisor

For the past two years, veterinarians and pharmaceutical companies have teamed up in a marketing campaign to frighten pet guardians into giving year-round heartworm preventatives to both dogs and cats. The campaign has really ramped up this year. They say they're doing this to improve protection for individual pets, but we need to take a closer look to discover the truth.

How do pets get heartworms?

Heartworms are transmitted by mosquitoes. Tiny heartworm larvae, called microfilaria, circulate in the blood, and are sucked up by the bug when it feeds on an infected host animal; for heartworms, their natural host is the dog. Once inside the mosquito, the larvae must develop through more stages before they can infect another dog. For that to occur, outside temperatures must remain above 57 degrees F, day and night, for a minimum of 8 days. The warmer the temperature, the faster the larvae will mature. If the temperature drops below that critical level, larval development will stop; but the larvae don't die—development will re-start at the same point when the weather warms back up. Larvae reach their infective stage in 8 to 30 days (the latter being the entire lifespan of the average mosquito).
When an infected mosquito bites a dog or cat, the heartworm microfilaria are deposited on the skin, where they crawl into the bite wound and enter the bloodstream. Inside the body, they get ready to "settle down and raise a family." In dogs, the heartworm natural host, the larvae migrate to the heart and eventually develop into adult worms, reproduce, fill the blood with microfilaria, and pass it on to the next mosquito. The maturation process takes 6-7 months.

"Heartworms have been diagnosed even in cats who spend 100% of their time indoors."