Heartworms, heartworm disease and heartworm treatment in dogs

Heartworm transmission

Adult  heartworms, about six inches long, live in the heart and large blood vessels.  The adult male and female worms  produce thousands of microscopic baby worms.  These baby heartworms do not grow up in the dog where they were born.   (If they did, the dog would immediately die and that would be the end of the heartworms.)  Before baby worms can  develop further, they must be inside a mosquito.

Next, a mosquito comes along and bites the infected dog, sucking up baby heartworms.  This probably isn’t too good for the mosquito, but it is what the worms have been waiting for.  During the next month, the heartworm babies grow into heartworm teenagers, a stage partway between baby and adult.

Now, the mosquito bites another dog, infecting the new dog with teenage heartworms,  ready to develop into adults.  After six or seven more months, the life cycle is complete:  new adult male and female heartworms are busily producing thousands of baby heartworms.

Heartworms in the Northern Sierra foothills

The tree hole mosquito, which breeds in oak trees at our elevation, is very good at spreading heartworms.  It lives longer than most mosquitoes, takes repeated blood meals, and even seems to remember where it found food the last time.  Since it is likely to deliver teenage heartworms to the to the same dog they originally came from or to nearby dogs, we tend to see clusters of very severe heartworm infestation.   If you look out your window and see oak trees, you are in prime heartworm territory.

In 1972, when the heartworm problem in this part of the United States was first being recognized, 25% of all heartworm infections which were reported in the state of California that year were reported by our hospital.  Entomologists from U. C.  Berkley came to Placerville to catch mosquitoes for their studies.

We have a lot of heartworms here. Unprotected outdoor dogs will probably catch heartworms.  Indoor dogs going outside in the morning and evening will probably catch heartworms.  If you think your dog’s long hair will protect him, forget it.  Statistics show that long hair offers no protection whatever.

Canine heartworm disease

Symptoms

There are no symptoms at all until the disease is very advanced.   Then, the symptoms are those of congestive heart failure:  dull coat, lack of energy, coughing, difficulty breathing, perhaps fainting spells and an enlarged abdomen.  Waiting for symptoms to develop and then treating is not a realistic alternative to prevention.

Prevention

There are three different drugs used for once-a-month heartworm prevention, plus a new one that is applied to the skin and also kills fleas. We currently recommend a good-tasting monthly product called Interceptor because it is entirely internal and also controls the most important intestinal worms.  In cold climates, prevention medication is not necessary in the wintertime.  In Placerville, we have mosquitoes year ’round, and although we could theoretically skip the medication when it is sufficiently frigid, the weather is different every year.  Our recommendation is once a month, year ’round, permanently

Heartworm testing

No medication is perfect and none of us have perfect memories.  Dogs should be periodically retested.  Although the official  recommendation is once a year, we think this is excessive.   In situations where pet owners feel confident that the medication is being given regularly, testing every two years is adequate.

Heartworm treatment

Although heartworms can be fatal and treatment for the disease involves risk, the condition is nearly always curable. Treatment requires careful medical care and complete rest at home afterwards.

The first thing we will do is evaluate your dog’s condition, performing a physical examination, laboratory tests and chest x-rays to evaluate the condition of the heart and lungs. We might find other health problems that need attention first, or if the heartworm infestation is very severe, we might want to use a different treatment schedule.

Adult heartworms are about six inches long and live mostly inside the heart. Baby heartworms are microscopic and live within blood vessels throughout the body. Each stage must be treated separately. First we eliminate the adult worms by giving a series of injections spaced out over a two day period.

When we finish treatment, your dog’s heartworms will be dead or dying. That’s good, but the heart is still full of worms.  The worms gradually break into smaller and smaller pieces until the fragments are tiny enough for the body to eliminate them. The critical period is when worm fragments are small enough to disperse into the body but still large enough to plug small arteries in the lungs. Vigorous activity makes the heart pump faster, pushing bits of dead heartworm out into small blood vessels where they can cause trouble, so vigorous activity must be avoided.

Home care

You can help reduce the risk of complications by giving your dog aspirin. Start two weeks before the in-hospital treatment and keep giving it for a month afterwards. Dogs 20 to 45 lbs. get 1 baby aspirin once a day, dogs 50 to 90 lbs. get 2 baby aspirin once a day. One 325 mgm regular aspirin equals four baby aspirin. Give at meal time. It doesn’t really matter whether you use buffered aspirin or plain, but use aspirin, not Tylenol.

Your dog needs rest (indoors or on a leash) for five weeks. Dogs that are kept outdoors must have an enclosed kennel or other arrangement which prevents vigorous exercise. If you see any sign of illness such as poor appetite, depression or vomiting, there may be a problem. Check your dog’s temperature using an ordinary human rectal thermometer. Lubricate the thermometer with Vaseline or KY Jelly, insert halfway and read after two minutes. The morning temperature should be below 102.4. Dogs with a morning temperature higher than this should be examined. Early treatment will control most heartworm treatment complications.

After five weeks, the adult heartworms are gone but there are still thousands of baby heartworms in the bloodstream. The best way of eliminating these microscopic worms is by using Interceptor, the same drug used for heartworm prevention.

We will schedule your dog to spend a day with us about five weeks after the initial treatment. First we give a drug to reduce possible reactions.   Then, about half an hour later, the first dose of  Interceptor. A few dogs may still have a mild reaction, so we will want to watch your dog for the day. Drop him off between 8:00 and 10:00 in the morning and come back about 5:00 or 5:30. Give the next dose of Interceptor one month later, at home. There is no further concern about drug reactions.

Because the heart and lungs are not yet completely back to normal, it is a good idea to avoid heavy exercise like hunting or ball chasing for an additional eight weeks.

Follow up testing

Sometimes a few heartworms survive treatment. To detect these worms, we do a final test six months later. If any heartworms are still alive, the Immiticide injections must be repeated, which costs approximately half as much as the original treatment.

Summary

* Hospital treatment for adult worms (about 3 days)
* Rest at home for 5 weeks
* Hospital treatment for baby worms (1 day)
* No vigorous exercise for 8 more weeks
* Give Interceptor once a month, permanently
* After 6 months, test for surviving adult worms

Treatment cost

There are two different drugs used for treating heartworms:

Although other drugs have been used in the past, practically all veterinarians use an arsenic-containing drug called Immiticide - safer than what we used in the past but more expensive. It is given by injection deep into the muscles of the lower back. Unfortunately, Immiticide is extremely irritating. Painful even when injected properly, the least bit of Immiticide injected in the wrong place causes great pain, sometimes for days.  This is such a terrible problem that we went back to using the old drug for a while, even though Immiticide is a better drug in other ways. Since then, we have learned to give a potent pain killer / sedative before injecting Immiticide. This eliminates injection pain, and because our patients are now sleepy and comfortable, there is no struggle and practically no chance of accidentally getting any Immiticide where it might cause trouble afterwards.

With proper pre-treatment evaluation, the use of newer and safer drugs, diligent control of pain and attentive nursing care, we can make heartworm treatment safer and a lot less miserable for our patients than in the past. Unfortunately, this also makes treatment a bit more expensive. The total cost for routine heartworm treatment, including the pretreatment workup and post treatment follow up, is usually around $500 for a medium sized dog (23 - 44 lbs.) or $600 for a large dog (67-88 lbs.).  (Nov ‘02)

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