Cat Heartworm Disease And Prevention

Heartworm disease in cats is deadly but preventible. For cats there is no heartworm treatment to rid the infected feline of the parasite, as there is in dogs. Feline heartworm prevention is easy and affordable.

Just as with dogs, cats get heartworms from mosquitoes, which act as an intermediate host of the parasite.

Many people are under the misconception that their cats can’t get heartworms because they never go outside. Actually, researchers have discovered that heartworms occur at about the same rate in indoor cats as outdoor cats. The reason is that the mosquito that transmits heartworms best likes the indoor environment. Therefore, indoor cats are getting “curb service” for their heartworm infestations.

Once a cat becomes a host for Dirofilaria immitis, the scientific name for heartworms, a cascade of events is begun. These are cardiopulmonary events, meaning both the heart and lungs are involved.

As you read, keep in mind that average cat heartworm infestation is one to three adult worms. Cats typically don’t have the massive heartworm burdens dogs do.

Here is what happens on the “cardio,” or heart side. As with dogs, the young adult heartworms try to make their way to the right ventricle of the heart, but are pushed into the pulmonary arteries by the high volume of “used” blood that is sent to the lungs. Instantly the cat’s body recognizes these intruders as foreign, and begins to react to them. The immune system is put on alert, producing both cells and protein complexes in an attempt to eradicate the parasite. The physical pounding of the worm against the wall of the artery causes it to react to protect itself: the artery wall swells, thickens, becomes scarred and becomes infiltrated by some of the products the immune system has made, all attempts at self-preservation. Radiographs and ultrasounds will show pulmonary artery thickening.

On the “pulmonary,” or lung side, the breathing system is not happy, either. Fluid immediately pours into the alveoli, tiny sacs in the lungs where gas exchange takes place. The volume of fluid determines whether the victim breathes with difficulty or proceeds to drown on his own fluids. The immune system mediates most of this reaction, and many immune system cells arrive in an attempt to deal with the foreign body reaction. These cells only make matters worse, exacerbating the buildup of fluid, and resulting in inflammation.

By now your kitty is coughing. In the earliest stages these patients may be treated symptomatically for respiratory tract infection or feline asthma. Symptomatic therapy may actually help, as antibiotics will control the abnormal bacteria growing in the fluid in the lungs, and antiinflammatories used for asthma will reduce the immune system response, resulting in temporary relief. When the medications end, however, the damage resumes, along with the clinical signs.

As the disease progresses your kitty is ill more and more often, and symptomatic medications help less and less. When your pet’s doctor begins to suspect there may be more going on than feline asthma alone, he takes a radiograph of your cat’s chest. He discovers pulmonary arteries that are several times their normal size. There is fluid in the lungs, which explains why he is having so much difficulty breathing. Scar tissue is beginning to replace the delicate tissue of the alveoli, preventing them from participating in gas exchange. Now, oxygen can’t get in, and carbon dioxide and other waste gasses can’t get out.

The only good thing about this feline heartworm disease scenario is that you, as pet owner, have an opportunity to prepare for what eventually will happen: death of the heartworm’s host, your kitty.

However, this is the less-commonly-seen scenario. More commonly in cats infested with heartworms, sudden death is seen. Here’s how it goes: Kitty is bitten by mosquito. Months later adult heartworms reach the heart and pulmonary arteries. Kitty tolerates the presence of the heartworms, unlike the kitty above, whose cardiopulmonary system overreacted to them. Kitty seems to be as fat and healthy as any cat could be, until, one day, you come home for lunch and find him dead in the middle of the living room. No sign of a struggle or bodily injury, just a little blood-tinged fluid coming from his mouth and nose.

You rush him to your veterinarian. “Please help Roscoe! I came home for lunch and he’s not breathing and he’s bleeding from his mouth and nose.”

Examination reveals the bad news: Roscoe, indeed, has no heartbeat and no breath sounds.

“Doctor, what killed Roscoe? Our doors were all locked and our alarm system was on. the house was secure from the time I left for work until I got home for lunch. Roscoe doesn’t have a cat door, so he’s been inside all morning.”

“I’m sorry, Mrs. Jones,” her veterinarian replies. “From the outside, there are no apparent injuries or abnormalities. A post-mortem examination, a necropsy, possibly with blood and urine tests would be required to find the cause, and there are no guarantees that we would find the cause. Do you want us to proceed with the necropsy?”

“Yes, please. My husband and I just won’t be able to rest until we know what took him.”

Hours into the necropsy your pet’s doctor finds the culprit. In the outer reaches of the smallest branches of the pulmonary arteries he finds a single adult male heartworm.  The heartworm has been dead just a little bit longer than his host/victim.

How did one heartworm cause the death of a robust kitty?

Quite easily, actually. Some cats with heartworms are like the one in the first picture we painted, tolerating their heartworms poorly, constantly reacting to them and causing chronic illness until, eventually, the disease process overwhelms him.

In the second scenario, the cat’s body was very tolerant of his one heartworm, as long as it was alive. It was heartworm death that triggered a massive and sudden reaction from the body, filling the lungs with fluid, drowning the cat, and soon stopping the heart.

So, you see why we didn’t give a heartworm-killing treatment to the first kitty, as we would have done with a dog. It is the death of the adult heartworm that causes the intense reaction and takes its host with it.

Now you understand why we veterinarians are constantly after our clients to administer heartworm preventive to their cats every single month, year-round. Regardless of which one you use, and you can find a variety of choices by clicking this link, they are easy to use and quite affordable.

Think of feline heartworm preventive as inexpensive insurance against a disease that is nearly impossible to treat and is almost always fatal.


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