Holistic Care : Heartworm in Dogs : Calculating Risks

Temperatures need to be above 60 degrees for 30 consecutive days in order for larvae to survive. This is the best time to start heartworm prevention. Here's a temperature chart for most of the United States. For those of you in areas with cold winters, heartworm prevention may not be necessary for certain times of the year. By following a program of heartworm prevention only when needed, your animal is protected during periods of risk without getting unnecessary chemicals in its system.

Temperature Chart for Heartworm Prevention

The heartworm has 5 separate larval stages referred to simply as L1, L2, L3, L4, and L5. The L5 is actually the young adult stage. The fully mature adult is often referred to as L6. In addition, heartworm also has two separate cycles, which, combined, make up the total lifecycle of the heartworm. One cycle takes place in a mosquito, and the other inside a dog or cat.

When a mosquito bites a dog that is harboring microfilariae, the mosquito ingests the L1 larvae, or microfilariae. This can only happen if the dog is also harboring the L6, or mature adult heartworm, because the microfilariae are the offspring of the adult heartworm. These microfilariae can live for up two years in the dog’s blood without causing any harm. They must, however, be taken up by a mosquito in order to develop any further. If they are not, they will simply die of old age and be passed out of the system.

Once the mosquito ingests the microfilariae from the infected dog, the larvae must go through two stages of development, or molts, changing from L1 to L2 and from L2 to L3, while in the mosquitoe’s system before the mosquito can infect another dog. Once the L3 stage has been reached, the larvae migrate to the mosquito’s mouth. It is only the L3 larvae which are capable of infecting another dog. This mosquito cycle takes anywhere from two weeks to about a month depending on the weather. The warmer the weather, the faster the development. If the larvae haven’t made the final development by then, they never will because the mosquito dies of old age at about 35 days, and along with the mosquito, die the larvae.

The importance of temperature: While the larvae are developing in the mosquito, development continues only when the temperature is above 64 degrees F. Further, the temperature MUST remain above 57 degrees Fahrenheit at all times, day and night during the entire mosquito cycle. If at any time during the development into the L3 stage, the temperature drops below 57 F, the development is aborted and must start over. Remember, it is only the L3 larvae which are capable of infesting your dog.

Now let’s say that a mosquito has bitten an infested dog; and that the temperature has remained above 57 degrees F for a minimum of 14 days since that bite; and that the mosquito bites your dog. Still, your dog is not infested because the L3 larvae are deposited in a tiny droplet of mosquito saliva adjacent to the bite, not injected into your dog by the mosquito--as many would have us believe. Providing the humidity and temperature are such that the droplet does not evaporate before the they get the chance, the L3 larvae must swim through the saliva and into the hole left by the mosquito bite, thereby entering your dog’s system.

Once inside your dog, the L3 larvae must spend the next two weeks or so developing into the L4 larvae. During this period of time the larvae are living in the subcutaneous tissue just under the skin, not in the blood of the newly infected dog. The L4 will continue to live and develop in the subcutaneous tissue for the next two to three months, where they develop into the L5 stage. Once they make this development or molt into the L5 or young adult stage, they then leave the subcutaneous tissue and enter the blood stream. The L5 or young adults then migrate to the heart and pulmonary arteries where they make their final development into the L6 or mature, breeding, adult stage and attach to the tissue of the heart and pulmonary arteries. Once there, approximately 5 to 7 months after entering the dog’s body, they will mate. This mating produces the microfilariae.

If the dog is not re-infested with L3 larvae from another bite from another infected mosquito, the adult heartworm will die of old age in about 2 years. The microfilaria will also die a natural death unless taken up by a mosquito. The adult female mosquito, the only one which bites, usually lives little more than thirty days.