As mentioned in the lifecycle of heartworm part 1, in order to develop into an adult worm, the microfilariae which are the first larval stage of these parasites, must pass through the body of a mosquito and into the body of a new host dog before they can develop into adulthood.

They cannot become adults in the dog in which they were born.

There are a total of five larval stages in the heartworm, L1s are the microfilariae, the first stage. The subsequent stages are L2, L3, L4 and L5.

Once L1 larvae have passed into the body of the mosquito, the lifecycle of the worm is dependent on the mosquito staying alive long enough for the larvae to develop through the L2 stage and become L3s. This speed of this process is dependent on the temperature, faster in warmer weather, slower in cooler weather.

However, development cannot take place if the temperature goes below 57 degrees Fahrenheit and the time for development to be complete and the L3s ready to infect a new dog is a matter of a few weeks on average.

Heartworm Microfilaria In Dog Blood

Once the L3 larvae are transmitted to a new host, they do not pass directly into the new dog’s bloodstream, they are actually deposited on the surface of the skin in the mosquito’s saliva and provided that this does not evaporate before the larvae can swim into the body of the dog through the puncture wound of the mosquito bite, the dog will become infected as the L3 develops into the L4 stage within the dog’s skin.

L4s live in the dog’s skin for a few months before development into the L5 stage is complete. At this point, the larvae enter the bloodstream and find their way to the heart and pulmonary arteries.

By the time the larvae have been in the dog’s body for anywhere between five and seven months, the worm is fully mature and will mate and start the cycle all over again.

Find out more about heartworm prevention for dogs and heartworm treatment for dogs by visiting the pages on this site.