HEARTWORMS: Disease in Dogs and Cats

GENERAL RULE:  It is much, much better to prevent than to treat.

HEARTWORM TRANSMISSION:

 Heartworms are about six inches long. They live mostly in the heart and the large blood vessels that bring blood from the lungs to the heart. Adult worms living in the heart produce thousands of microscopic baby worms (larvae), which circulate throughout the body.  These baby heartworms do not grow to adulthood in the dog where they were born. If they did, the dog would quickly die.  Before baby heartworms can develop further, they must live in a mosquito. 

 Mosquitoes will bite an infected dog, sucking up heartworm larvae. This is what the larvae have been waiting for. During the next month, the larvae develop 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 adulthood.  After six or seven more months the life cycle is complete:  new adult male and female heartworms are producing thousands of baby heartworms.

 Unprotected dogs living outdoors are most prone to catch heartworms.  Indoor dogs going outside in the morning and evening are also at risk. Having long hair offers the dog no protection at all.

 The dog has no symptoms at all until the disease is very advanced.   Then, the symptoms are:  dull coat, lack of energy, coughing, difficulty breathing, perhaps fainting spells and an enlarged abdomen.  Treatment is difficult, expensive and frequently not successful.  Prevention is the key to preserving a dog’s health.

 CANINE HEARTWORM PREVENTION:

 There are several different drugs used for heartworm prevention, all of them highly effective and easy to use.  You have your choice between a monthly good-tasting pill, or a monthly application of medicine to the skin.  In cold climates, prevention medication is unnecessary during the winter. Mosquitoes are not active then.  Dogs should be retested periodically.  Testing every two years is usually adequate.

 Heartworm treatment in dogs:

Although heartworms can be fatal, and treatment for the disease involves risk, the condition is usually curable.  Treatment may require  surgery, careful medical care and complete rest at home afterwards.  The expense of heartworm treatment pales compared to the cost of prevention.

Feline heartworm disease

Cats are not a natural host for the heartworms and are much less likely to get them.  The disease in cats is quite different.  Symptioms can be complicated and apparently illogical.

Heartworm symptoms in cats:

Usually, there are not symptoms.  When noted, symptoms resemble those of other more common problems.

Symptoms may include:

Vomiting

Wheezing,

Choking

Fainting spells

Hindquarter paralysis

Sudden unexplained death

Heartworm treatment in cats:

 We do not have a satisfactory drug for heartworm treatment in cats.   If we did, using it would be risky because cats do not deal well with dead heartworms. They sometimes absorb the dead worms successfully; they sometimes die from arterial obstruction or from allergic reaction to the dead worms.  So, if we could kill feline heartworms, it is not clear that we should.  As it stands now, the best treatment for feline heartworm disease is to start on prevention (so they don’t get more) and try to control the symptoms, if there are any. Heartworm testing in cats testing is much less reliable than in dogs. We do not ordinarily test cats that look good and act normal.

 Heartworm prevention in cats

 We urge everyone in our area to protect his or her cats from heartworm disease.  If your cat is entirely indoors and you don’t get mosquitoes in the house, she may not need heartworm protection, although a surprisingly high proportion of feline heartworm cases occur in “indoor” cats. Many Vets recommend Revolution, which controls heartworms, intestinal worms, fleas and ear mites and is easy to use. 

Revised: 12.31.08

jep