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Heartworm Disease
What is a
heartworm?
Heartworms are worms that live
in the heart and large vessels heading from the heart into the lungs
(pulmonary arteries.) These worms can grow
up to 14" in length. A dog moderately infested with heartworms will
have approximately 25-50 adult heartworms living in and near the heart.
An adult heartworms life span is about 5 years during which time it may
mate and create baby heartworms (called microfilaria) with the other
heartworms living with it. Baby heartworms are released into the bloodstream
and circulate around the body.
How is heartworm
transmitted?
When a mosquito bites an infected dog it sucks up baby heartworms along
with its blood meal. When it goes to feed again on an uninfected dog
it deposits those baby heartworms onto the uninfected dog. These worms
migrate through the body gradually developing until they end up in the
heart and pulmonary arteries as adult worms. It takes approximately 6
months for the heartworm to develop into an adult once it is deposited
on the dog.
What are the
symptoms of heartworm infection in my dog?
Often
dogs are asymptomatic (meaning that they are infected, but are not showing
symptoms of the disease.) Sometimes a chronic (long-term) soft cough
is noted. Your dog may tire easily, have difficulty breathing and may
even develop congestive heart failure. Secondary problems can also occur
in the immune system as a response from the proteins of the worms. These
include destruction of red blood cells (anemia), platelets (bleeding),
kidney disease, joint disease, and Lupus.
How is heartworm
infection diagnosed?
One of the easiest
ways to detect heartworms is by a blood test that checks for the presence
of adult female worms. Most heartworm infections in dogs are 'co-ed,'
so this test is usually very accurate. It only detects adult worms so
if the dog has recently been infested and there are no adult worms present
the test will be negative even if the dog does have baby stages of heartworm
present in its body. There are other blood tests available that test
for the presence of baby heartworms. We do not perform these tests routinely.
Heartworm infestations can also be diagnosed by the presence of changes
on radiographs (x-rays), especially in more advanced disease. Heartworms
can also be seen when ultrasounding the heart of an infected dog.
How is heartworm
infection treated?
Baby heartworms can
be killed with regular preventative therapy. Adult heartworms are killed
with deep intramuscular injections of an arsenic derivative called Melarsomine
Dihydrochloride. Killing a large number of worms all at once can lead
to inflammation and clotting problems that in a worst case scenario could
be life threatening. The most common side effect with this treatment
is muscle pain and inflammation in the area of the injection. Treatment
requires at least two injections. The treated dog should be kept from
exercising during the time surrounding treatment.
Do I have
to worry about heartworm infection in my dog in Southern California?
There
were 30 reported cases of canine heartworm infection in Orange County
in 2001. One study (done in 1996-1998) reported an incidence of infection
in 1 in every 250 dogs in LA County; the incidence of heartworm in Southern
California has been increasing steadily each year. The same study found
that travel out of LA County was not necessary for infection. Even more
striking was that the study reported that 50% of the infected dogs were
described by their owners to be "always indoors." The bottom
line is that heartworm is present in Southern California and even mostly
indoor dogs are at risk of becoming infested.
How do I prevent
my dog from getting heartworms?
There
are several kinds of heartworm preventatives currently available; among
these are Heartgard, Proheart, Interceptor, Sentinel, and Revolution.
At Orange Villa Veterinary Hospital we carry Heartgard, a monthly chewable
tablet.
Our recommendations at Orange Villa Veterinary Hospital are:
- Every dog should be on heartworm preventative. It is possible for your dog to contract heartworms in Southern California even if it is an indoor only dog. Heartworm disease is very easy to prevent and, once contracted, the treatment is potentially dangerous to your dog.
- Before starting on a preventative it is important to test and make sure that your dog has not already contracted heartworms. This is not necessary in puppies under 6 months as they are too young to have an adult heartworm infestation. In puppies that are started on heartworm prevention before they are 6 months of age, we recommend testing for heartworms at their one-year booster appointment to insure they are not infected. If there was a lapse in the scheduled treatment time we recommend a heartworm test 6 months past the last time they could have contracted heartworms.
- If you are using the Heartgard tablet you may go to the Merial website (http://us.merial.com/reg_9_1.asp) and sign up for e-mail reminders to help you remember to give the monthly tablet.
- As of September 3rd, 2004 ProHeart 6, the injectable 6-month heartworm preventative was recalled by the FDA due to safety concerns. It is not currently available.
i This study was conducted by Dr. Jerry Theis at the U.C. Davis Veterinary School and the results were outlined on VeterinaryPartner.com.
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