Heartworm disease is what is called a vector borne disease. What this means is that a vector, in this case a mosquito, transmits or gives the disease to the animal through a blood meal.
When the mosquito is feeding on your dog or cats’ blood, they are simultaneously injecting an immature form of heartworm! It is caused by the nematode (small worm) parasite, Dirofilaria immitus. We abbreviate their name as D. immitus.
When these immature heartworms, or larvae enter the body they start to grow and mature. Depending on whether they enter a dog or cat, as to when the problems will start to occur.
Dogs are what is called the ‘definitive or natural’ host for heartworms, cats on the other hand are known as ‘aberrant’ hosts. Because of this, the disease behaves very differently in each of these host species.
Dogs and Heartworm Disease
Let’s first look at heartworm disease in dogs, the natural host. All canid species can become infected with heartworm disease, that means not only the neighborhood dogs, but the foxes, coyotes and wolves (yikes!). If you have mosquitos in your yard, you have heartworm disease!
As you can see this is a complex cycle, and can takes years before a toll is taken on the dog from this long-term infection.
In the beginning, a mosquito will bite an infected dog, while taking up their blood meal, they also take in immature heartworm larvae, these are called L1 larvae (the smallest babies).
These L1 larvae mature inside the mosquito until they become L3 larvae, at this time they migrate to the mouth of the mosquito, and when they bite another dog, they swim down into the victim dog’s blood stream.
After some swimming around and growing they become L5 larvae, this is the oldest larvae, and these migrate to the preferred home for mature heartworm, the pulmonary vessels (lungs) and heart vessels where they set up house, and start having babies, lots of babies, all L1 larvae.
All this growing and moving takes 6-7 months, during which time the host dog is unaffected by the larvae. As the adult heartworms grow, they start to cause problems. Imagine if you had worms who were the size of spaghetti inside your major blood vessels!
Early signs of disease include a persistent soft cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss.
As the disease progresses, dogs can develop heart failure, because the heart is having to work much harder to pump blood around and through all these worms having a party in their major blood vessels.
The owners may notice a swollen belly as fluid backs up into the abdomen. Dogs can also develop sudden blockages of blood flow due to large numbers of worms in the vessels. This is called caval syndrome, and is an emergency procedure. Treatment of caval syndrome involves surgical removal of this blockage of adult heartworms. I’ve seen up to 100 worms removed during this type of procedure!
This video link shows the doctor using a snare tool to physically remove the mass of heartworms from the jugular vessels.
Cats and Heartworm Disease
Remember cats are aberrant hosts, which means the worms aren’t happy living there, but will tough it out if they can. Fewer infective larvae survive to adulthood in cats, but cats can definitely be infected with heartworm if exposed to L3 larvae.
Once infected with L3 larvae from a mosquito bite, the natural resistance in a cat induces a shortened period of infection, and a decreased or absent presence of microfilaria, or the baby heartworms (L1). It is believed that some cats may be able to spontaneously rid themselves of an infection with a strong immune response.
If cats are able to spontaneously rid themselves of adult heartworms, why are we even worrying about infections in them? The primary reason being, that the damage to cats is caused by the migrating L3-L5 larvae throughout the cats’ body with the main portion of the damage occurring in the cat’s lungs. Very seldom do adult worms even make it to the major vessels in a cat.
The increased immune response seen in the cat begins approximately 100 days after the initial infected mosquito bite. The lungs respond to these L5 larvae arriving with a severe inflammatory response that can exhibit asthma-like symptoms. This inflammatory response causes non-functional lungs, and an acute respiratory distress syndrome, termed HARD. The immature larvae are also responsible for significant damage to the lungs.
The primary damage to the cat’s lungs occurs with the arrival of the immature L5 larvae. In cats, acute lung injury results in generalized respiratory failure. This disease in not simply an obstructive disease associated with blocking of blood flow, there is a significant acute inflammatory component that can be seen especially with adult heartworm deaths.
These cats may initially present with a history of coughing, dyspnea (difficulty breathing), and vomiting as their primary signs. Rapid and difficult breathing may be seen on exam. Additionally, you may note lethargy, anorexia and weight loss.
On their own, none of these signs are consistent with only heartworm infections. Clinical signs and possibly chest radiographic are similar in cats with abbreviated heartworm infections and those with other causes of bronchial disease, such as feline asthma.
Unfortunately, may times the first sign of an infection is sudden death, this is a hard one to treat! We also see cats that fail to recover from anesthesia, which is especially hard on both the families and the veterinary staff.
There is no effective treatment for cats as with dogs, so it is even more important that they be on year-long preventatives! I don’t know about you, but there are a lot of things I have no control over when it comes to my pets, I’ll control what I can, and all my cats are on preventatives!!
As scary as this all sounds, the good news is we have several very effective preventatives D. immitus.
Preventatives come in three forms, either oral, given by mouth, topical that is applied to the skin, or as an injection given by your veterinarian. As with any medications using it correctly, and at the right time is important for effectiveness.
All of these medications work by eliminating the L3 larvae and preventing them from growing. Because the L3 larvae grow so quickly, our dosing interval is typically every 30 days for the oral and topicals, and every 6 months for the injectable, to be sure that we kill them when they’re little. If you miss this window of time, they can mature and start making babies!
What happens if you decide to play Russian roulette and not use heartworm preventative? The estimate is that 10% of the mosquitoes are infected with D. immitus. So, EVERY TIME your dog is bitten by a mosquito, it has a 1 in 10 chance of becoming infected. That is every time. When was the last time you go bitten by only 1 mosquito?
Or let’s say, you’re feeling lucky, and just want to see what’ll happen. For a dog that is infected with heartworm, but haven’t developed caval syndrome yet, there are effective treatment options, and depending on the size of your dog, the area you’re in, and how sick your dog is at the time of treatment.
Treatment can run, without complications, $1,000-$2,000!
Treatment consists of killing the adult worms in the major blood vessels, and allowing them to disintegrate and be removed by the ever-efficient white blood cells. There is no exit way for these worms other than disintegration and ingestion by white blood cells.
If a clump of dying worms lodges in another area of the body, blood flow can be cut off, and an emergency occurs. Dogs have to be on strict exercise restrictions for up to 6 months while these worms are slowly removed by the body.
Significant damage can also be done to the body by the presence of the worms, and there is no guarantee that these changes will go away with the worms.
And to make things even more interesting, while they’re undergoing treatment, if they get bitten by a D. immitus carrying mosquito, they can set up a new infection!! The medication that kills the adults (adulticide), does not prevent the L3 larvae from maturing, so preventatives must also be given during this treatment.
But, Fluffy never goes outside
Oh, but you have a totally indoor dog or cat, they’ll never get bitten, right? Unless you have a very special house, you get mosquitoes in it, and most likely your pet is in that house for a much longer time each day then you are.
Who do you think is more likely to get bitten? Right, your pet! Or they only go out for 5 minutes in the morning and night. I don’t know about you, but my mosquitoes can swarm and bite me in the 20 seconds it takes me to get to my car in the morning.
Check ups and blood tests
When getting preventatives, your veterinarian will require that your dog be tested yearly to make sure that they are negative for an infection before starting the preventatives.
Testing of cats, due to their immune response is trickier, and not usually done more than once. The current recommendation by the American Heartworm Society is that preventatives be given year-round, and that dogs be tested yearly.
The yearly testing is to ensure that a breakthrough infection doesn’t occur. No medication is 100%, and owner compliance doesn’t appear to be 100% either.
Depending on your needs, and how compliant your dog is as to what type of preventative your veterinarian will recommend. Whichever one you select, be sure to USE IT!!!
There have also been numerous infections seen in dogs who owners have purchased discounted heartworm preventatives through the internet. These medications may be off label, ineffective or illegal. Purchase your preventatives from your veterinarian or a licensed veterinary pharmacy with an established relationship with your veterinarian.
As you can see, heartworm disease is a preventable deadly disease. Make sure to follow your vets recommendations and protect your furry family member.
Ann Wortinger BIS, LVT, VTS (ECC, SAIM, Nutrition)
This website is NOT a substitute for veterinary care with a veterinarian. We recommend you follow the advice and treatment plan as prescribed by your veterinarian, and only after discussing anything found on this website with your veterinarian, with their approval, implementing advice found here.
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