Heartworm Disease

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The PetStuff Online Newsletter
Volume 1 Issue 1  August 13, 1999

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Produced by Dr Dan

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In this first issue we will begin to learn what works and what doesn't work as far as e-mailing images. I want you to see what is happening. "A picture is worth 1000 words" but it is prohibited by download speeds. So here we go...

"Heartworm Disease in Dogs and Cats"

How do our dogs and cats get heartworms?
Look at the image. The adult heartworm lives in the right chamber of the heart and pulmonary artery which routes blood through the lungs where carbon dioxide is removed and oxygen is added to the red blood cells. I have actually seen hearts so chucked full of worms that only a trickle of blood can get through. This all begins with a single mosquito bite! The mosquito bites an infected dog and picks up some microfilaria from the blood meal. The microfilaria live about two weeks in the salivary glands of the mosquito. Now is that a tiny worm or what? When the mosquito bites a dog or cat it punches a little hole with some pinchers. It then releases some saliva that prevents the blood from clotting. As the mosquito sits back sipping at the blood some microfilaria find their way into the wound. The microfilaria then begin to migrate through the tissue following veins to the heart. As the larva migrate they molt and continue growing in size ( since insects have a rigid exterior they must molt or shed the exterior to grow in size). This process takes about six months to reach adult size in the heart and begin giving birth to offspring, the microfilaria. The process then can continue. Note: The offspring cannot grow to adults without coming out of the animal and spending time in the mosquito at cooler temperatures. In other words, the mosquito is necessary. If it released one microfilaria into the wound only one adult heartworm would lodge in the body of the pet. The more times the pet is stung by infected mosquitos, the more heart worms they can aquire.

Heartworms in the Cat
Is this just a bunch of crap for veterinarians to make more money? Nope, sorry it is a deadly disease! Unfortunately cats have been dying of heartworm disease for years. We just haven't known about it. Due to the size of the heart a single adult heartworm can cause major problems. Until about 12 months ago there was NO heartworm test sensitive enough to detect the presence of heartworms. Now we can detect that "single" worm and diagnosis is being made; we are beginning to understand the implications of the heartworm in the cat. There are some differences between dogs and cats: #1 Usually the adult heartworm does not give birth to microfilaria while in the cat (In other words the cat must get heartworm from a mosquito that has fed from a dog with heartworms - Wouldn't you know it!). #2 Cats are not as well adapted to having heartworms in their bodies as dogs. For this reason the cat body tries to wall off the heartworm as it is traveling through the body as a molting larva. While their body tries to stop the foreign heartworm they may form cysts in the brain, kidneys, liver, etc. This is the reason that in a cat heartworms can cause many different disease conditions other than just heart failure. We can see circling, seizures, kidney and liver failure, lethargy or sudden unexplained death in the cat with heartworms. In the dog it is generally a slow buildup of heart failure. #3 Cats cannot be treated for heartworms. At the present time there is no treatment for adult heartworms in the cat. The cat is sensitive to drugs. If we use dog treatment protocols the cat tends to throw blood clots which plug blood vessels and kill the patient. 70% of cats treated die during treatment. So what do we do? As mentioned above, cats are not as well adapted to heartworms as dogs. If you control the symptoms of heartworms with differing medical treatments, the adult heartworm will die off on its own in two or three years in the cat. We can't treat so the answer is to prevent them from ever getting in!

A Canine Heart with Heartworms

How do we detect heartworms in the dog and cat?
There are several tests based on two principals.
The oldest principal is where we look for the offspring (microfilaria) in the blood. A few drops of blood are drawn from a vein in the front leg and then passed through a fine filter which traps any microfilaria passed through it. A drop of stain is applied to the filter which outlines the microfilaria and it is viewed under a microscope at from 100X to 400X magnification. If a Once-A-Day heartworm preventative with diethylcarbamazine in it is to be given this test must be used (If microfilaria are present and DEC is given it can cause a fatal anaphylactic reaction to occur). This test was great before we started using the Once-A-Month heartworm preventatives which can kill the microfilaria and prevent this type of test from detecting the presence of adult heartworms in the heart. A better method had to be developed.

Today we must use what is referred to as an "Occult Heartworm Test" As above we first draw a drop or two of blood. The blood is then processed in a detection kit. This kit actually detects antibodies to the adult heartworms by causing a color change in the media of the test. (Antibodies are proteins produced by the dog or cat's immune system in attempting to control the heartworm from within). Since this test does not rely on actually finding microfilaria, it is much more sensitive for detecting adult worms present in the heart. In the old days the microfilaria test would yield a false negative if the adult heartworm wasn't producing microfilaria as in all female or all male infections. Yes they can happen! Also for some crazy reason microfilaria do not circulate in the blood during the day hours as much as at night. God only knows where they hang out during the daytime. Aren't parasites strange?

How do we treat heartworms?
As mentioned above, in cats we cannot treat heartworms without a 70% treatment fatality rate. We must wait it out and control symptoms with drugs such as steroids, antibiotics, etc.

In the dog we can treat heartworms but it is very risky and expensive. The tests listed above do not tell us an all important consideration - how many adult heartwroms are present. This can alter the outcome of treatment.

When we start treatment, we need to first access the condition of the liver and lungs. X-rays are taken and blood tests performed. If all is well treatment is begun. Either an intravenous or intramuscular drug is given for two days in a row to begin killing the adult heartworms. What we want is a SLOW kill. A sudden killing of the adult heartworm would cause them to release and flow downstream into tiny blood vessels (arterioles and capillaries) and block blood flow causing the death of the patient. With a slow kill, the defensive cells of the patients body (macrophages) have a chance to go in and eat up the remains of the adult worms. After two days of hospitalization the pet goes home. During the next six weeks the dog must not get excited at all! If the heart races the worm debris can break loose and plug vessels. So it's a pretty boring life for the dog over six weeks of time. I have actually had certain clients box their dogs up in plywood cages so they see nothing ( they lived). During this time many cases of pneumonia can occur due to the worms dying in the interior of the lungs. These infections are treated with antibiotics and supportive care.

After the initial six weeks the dog is returned to the hospital for microfilaria treatment. This is very simple. An oral medication is given and the dog can go home. Now we might think it is all over and everything is fine. Unfortunately, we have to bring the dog back two weeks later and check again for microfilaria. What would it mean if we discovered microfilaria when we tested? You guessed it! There must still be an adult heartworm present to give the birth to microfilaria. The adult treatment did not kill all the heartworms and the whole process must start over again. We hate it but it happens once-in-awhile.

How do we prevent heartworms?
Since treatment is risky or even fatal - Prevention is the best course of action. If you are in an area with mosquitos your dogs and cats should be taking heartworm preventative. All 50 states have heartworms! The only place there aren't any mosquitos is high up in the mountains.

There are two types of heartworm preventative medications: The daily and the monthly. The difference is based on what stage of the migrating larva they kill. As mentioned above and shown in red on the "Heartworm Life Cycle Image" heartworm preventatives kill the larva as they migrate through the cat or dogs system heading towards the heart. The monthly preventative back-kills any larva that may have found their way into the pet's body during the previous 30 to 45 days; the daily preventative back-kills the larva that may have entered in the previous one or two days. Prevention is as simple as that. A tasty dehydrated meat chew with the medication in it or a pill, you make the choice. But remember... No heartworm preventative will kill adult heartworms.

Why do vet's want to test once a year even if your pet took heartworm preventative all year long?
Is it because we are money hungry? Nope! It is our job to make sure your pet never gets this horrible parasite. Several things may happen the day your pet gets it's preventative: Maybe that day he/she has the diarrhea and the medication just rips through the intestinal tract without allowing ample time for absorption. Or maybe "Bowser" decides to have a little grass salad after you let him out in the morning and up comes the grass and medication. In those two circumstances your dog or cat could get Heartworms Disease and become seriously ill or die. It is our responsibility to prevent such occurances if at all possible. That's why we ask to check your pet once a year for the presence of adult heartworms. It's kind of like changing the oil on your car on a regular basis. If you don't you may be ok, but chances are your engine may not last as long. In addition Heartgard (the brand of heartworm chew we sell) guarantees it's product if an annual heartworm test is performed and the preventative is given. If you do this and your dog or cat gets "ANY" worm Meriel the producer pays for the diagnostics and treatment. Isn't that great?

Do indoor only pets need heartworm preventative?
Actually, indoor only cats get heartworm disease more often than outdoor cats. That's strange but consider this. Outdoor cats and dogs build up a tiny amount of natural immunity by being exposed to heartworm microfilara everyday. Indoor animals do not have any of this natural immunity and in their case it only takes a single bite and they are infected. There is no natural immunity to stop a larva as it enters the system.

Signs of Heartworm Disease - There may be none at all but sudden death!


Cats Any or in combination: Cough, vomiting, breathing difficulties, sluggishness, circling, sudden death.


Dogs Any or in combination: Cough, breathing difficulties, sluggishness, exercise intolerance, sudden death.

In many cases sudden death is the first and only sign! Prevention is the best course of action...

A Little Pet Humor in Closing

Cat Rules

Most Cats think that this strange-looking plant is Food of the Gods and better even than tuna. There are some, however, who are sadly deprived of the ability to enjoy this wonderful treat and will look upon others with a mixture of confusion and disgust at the utter lack of Dignity of those partaking. If you are one of the latter, please skip to paragraph two.

Catnip is available in two forms--in the wild as an odd-looking plant that grows in delightfully fragrant, though often flattened, patches, and from the humans in a concentrated dried form. Unfortunately, the humans know our weakness for catnip and will try to hold it out from us, often employing some ingenious methods to do so. If the humans are careless enough to leave any catnip within reach, it is imperative to get it no matter what you have to tear apart to do so. Otherwise the humans will use it to attempt to coerce us to do things which otherwise would be beneath us.

The greatest hazard of catnip is that it causes those Cats under it's influence to utterly lose their Dignity. They roll around foolishly, purr at maximum volume, tear around the house at top speed, and do other things no sane Cat would be caught doing. Do attempt to control youself, especially if your humans have a video camera and are prone to using it.

Bad Dog

A list of phrases dog owners should get their naughty pets to write on the Blackboard 100 times...

I recognise that (XXXX) has a right to exist:
The human's shoes; the human's cats; children; couch cushions; the aquarium; 3rd grade art projects (even if they are made of macaroni shells); the other (dogs); the TV remote control; the human's little humans; the bath mitt; Rolling Stone Magazine; large patterns on wallpaper; any throw rug in the entire house; the child's Barbie doll (especially the day after Christmas).


Images used in this publication taken from Hill's Atlas of Clinical Anatomy, Published by Veterinary Medicine Publishing Company, Inc.  A publication used by veterinarians to teach their clients about their dogs and cats in sickness and health.

Have a good week

Dr Dan