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. Continue reading →
Heartworm infection is acquired from the bites of mosquitoes that have picked up the larval form of the worms when they have bitten another infected dog.
These larvae are carried in the saliva of the mosquitoes and are passed to another dog when they bite it. This dog then becomes the new host for the worms to develop into adulthood.
In order for heart worms to survive in any geographic area, a very specific set of citeria have to be met.
First and foremost, mosquito species that can carry the larval stage of the worms must already be present in the area. Not all mosquitoes can carry the infection.
The larvae must be able to continue developing inside the mosquito before the mosquito takes its next meal and the transmission of the parasite to a new host can take place.
For this to occur, the weather must be warm enough and the parasites will not continue to develop at temperatures below 57 degrees Fahrenheit.
There must be dogs in the area already infected with adult worms and carrying the larvae so that the mosquitoes can carry them to a new host when they bite. Of course, this could also be wild dogs, coyotes or wolves, the parasites do not limit themselves to domestic animals.
Microfilaria In a Blood Sample
There must be dogs in the area that are vulnerable to infection with the parasite (i.e. dogs that are not on a preventative heartworm treatment schedule.
The adult worms when they mate, produce live baby worms called microfilariae and these circulate in the bloodstream for up to two years, just waiting to be picked up when a mosquito takes a meal. After two years in the bloodstream, if they have not been carried to a new host, they die naturally.
In order to mature into adults, these microfilarieae must be transferred to a new host, they cannot ever mature into adulthood within the same dog as their own parent worms. Take a look at the video at the foot of this page, it shows a live microfilaria in a blood sample from an infected dog.
This is one of the reasons why a program of preventative treatment for dogs is vital, to kill the microfilariae so that they cannot be transmitted to another dog via the mosquito bites. Dog heartworm medicine for the prevention of heartworm is effective and safe. Find out more on the relevant pages of this site.
The Latin name for the heartworm parasite is Dirofilaria immitis and the adult worm can be quite large, some as much as 35 centimetres in length.
The adult worm makes its home in the heart which is how it gets its name but in actual fact, it prefers to live in the pulmonary arteries where they get their nourishment from the blood as it pumps past on the way to the lungs.
The presence of worms in the arteries causes inflammation which, in turn can cause the blood to clot and cause secondary problems.
There can be as many as one hundred worms living in an infected dog but the infection load is medically termed as heavy in a smaller dog of around 40 pounds in weight if only twenty-five worms are present. At this point major problems start to occur as there are too many to fit into the arteries and they start to fill up the chamber in the heart which pumps blood into the dog’s lungs. This chamber is called the right ventricle and the problem for the dog at this point is that the heart cannot pump properly at this point and so there is less blood being pumped to the lung and it follows that there is less oxygenated blood to be pumped around the dog’s body. There is an amazing video on the heartworm treatment for dogs page which shows a successful operation on a dog that had 86 heartworms removed!
If the infection gets so bad that more than fifty worms are present this chamber of the heart is completely full and so the worms start filling up the chamber which receives blood from the dog’s body. This chamber is called the atrium. Untreated, the worm infestation keeps getting worse until the point is reached where almost no blood can be pumped by the heart.
The actual point at which this happens, depends to a degree on the size of the dog but either way, once this point is reached, unfortunately most dogs die from the infestation.
Effective heartworm prevention for dogs is available and an annual blood test to find out if juvenile worms (microfiliariae) will help your vet to advise you on the best course of treatment so that heartworm infection never gets to this heartbreaking stage.
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I am publishing an entry from the Dog Forum, of which I am a member with the authoring member’s permission as I feel that what she had to say is important to anyone who may find out that the dog they own, (maybe having rescued or adopted it), has heartworm infection.
I am publishing her post in its entirety:-
Dog Forum Post
I am sharing my story so that other people who find themselves with a dog with heartworm can benefit from what I’ve learned.
I have a 4-year-old Jack Russell named Spot that we rescued from a shelter after fostering him for 8 months. After adopting Spot we took him into the vet for a check up and discovered he has heartworm. He is completely asymptomatic, we would never have known he was sick if the blood work didn’t say so. We did a ton of research on the internet and saw several forum posts where people said that they just gave their dog the preventative medicine instead of doing the full treatment. The treatment for heartworm is very dangerous (it can be as bad as the actual disease if not worse) not to mention costly.
We told our vet what we read online about giving dogs the preventative medicine only and we were told that although that will keep him from developing more worms and spreading it to other dogs (via mosquitoes), the existing worms will continue to do damage. The doctor recommended we do a chest x-ray that would cost $130, the heartworm treatment that would cost $580, and put him on antibiotics that cost $70. Continue reading →
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.
