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Canine Heartworm Disease
by Frances
Smith, DVM
Heartworm disease in
dogs is caused by a parasite called Dirofilaria immitis.
This disease can be found all over the United States, and in many
other places around the world. Infections have been documented in
other species as well, including the cat, wolf, coyote, bobcat,
jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse,
orangutan, gibbon, sea lion, and even man.
The life cycle of the heartworm includes the mosquito. When an
infected animal that has circulating baby heartworms (microfilaria)
is bitten by a mosquito, the blood that the mosquito gets out of
that animal contains the baby heartworms. These baby heartworms
then develop inside the mosquito for at least 14 days to become
infective larvae. When the mosquito containing these infective
larvae bite an uninfected dog, it injects those infective larvae
into the dog, thus infecting it. The time it takes from this
mosquito bite to the time when the dog has adult heartworms that are
able to produce microfilaria is about 6-7 months. The adult
heartworms can live in the dog for about 5-7 years.
The adult heartworms live in the heart and large blood vessels
around the heart. These worms can be 12-31 cm (4 ˝ to 12 inches)
long and 0.7 to 1.3 mm wide. Dogs can carry 1 – 250 of these worms
at one time. They cause an inflammatory response that over time can
result in heart failure and lung problems. The symptoms vary from
no symptoms at all to coughing, exercise intolerance, difficulty
breathing, fainting, coughing up blood, jaundice, fluid in the
abdomen (ascites), and death. The severity of the symptoms is
related to how many worms the dog carries, the amount of time
infected, and the severity of damage caused by the worms.
Heartworms can infect other areas of the body such as the eye,
abdominal cavity, blood vessels of the brain, and spinal cord, but
this is not common.
Heartworm disease is diagnosed most commonly by blood tests done in
your veterinarian’s office. One type of test looks for the presence
of the microfilaria in the blood. Another is what is commonly
called the occult heartworm test. This is another blood test that
checks for the antigens produced by the heartworm. It is generally
more accurate that looking for microfilaria because the number of
microfilaria can change depending on the time of day and time of
year. Other tests such as radiographs (x-rays) of the chest, blood
work, and ultrasounds of the chest can help support a diagnosis of
heartworm disease and determine the severity of the changes that the
heartworms have caused.
The treatment for heartworm disease involves killing the adult worms
with one drug and the microfilaria with a second drug. Most dogs
with no symptoms or mild symptoms are treated with a high rate of
success and few complications. Dogs who are showing more symptoms
can still be treated, but the whole process may take longer and the
incidence of complications is higher. A few dogs are so ill or have
other diseases present so that treatment is extremely difficult and
sometimes not recommended.
There are two drugs approved for the
adulticide phase of heartworm treatment. The older drug,
Thiacetarsamide sodium, is rarely used today. The newer drug,
Melarsomine dihydrochloride, or Immiticide ®
(produced by Merial) is safer and more efficacious than the older
drug. It is given by deep injections in the lumbar muscles in the
dog’s back. After this phase of treatment it is very important to
keep the dog quiet for the next few weeks to prevent severe
pulmonary thromboembolism. If this occurs the dog will have fever,
cough, and may cough up blood. These dogs need additional treatment
of VERY strict rest and corticosteroids to reduce the inflammatory
response. Three to six weeks after the Immiticide is given, a
microfilaricide is given. This time allows the adult heartworms to
die and the body to clean them away, and for no new microfilaria to
be produced. The drugs used most commonly are ivermectin or
milbemycin. The dog is usually kept in the hospital or clinic for
at least 8 hours after the drug is administered to watch for adverse
effects from the rapid kill of the microfilaria. Three weeks after
this, the dog is tested for the presence of microfilaria, and if
none are seen, the treatment is complete and the dog begins
preventative treatment. By 16 weeks post treatment, an antigen test
should also be negative. It is possible to greatly improve clinical
symptoms of heartworm disease without completely killing all of the
adult worms and repeat treatment in dogs that show a persistence of
microfilaria or the antigen should be considered on a case-by-case
basis.
Preventing heartworm disease is much safer and more economical than
treating the disease. There are several drugs approved for
prevention of heartworm disease in dogs. Diethylcarbamazine and the
macrolide anthelmintics (ivermectin, milbemycin, moxidectin, and
selamectin) are the drugs in use today. The biggest advantage of
the macrolide anthelmintics is that they can be given monthly. For
most of the United States, especially here in Texas, it is
recommended that these drugs be given year round. If possible, dogs
should be started on these preventatives as young puppies and they
shou ld
be continued throughout the dog’s life. If you have a dog that is
older than 6 months old, it must be tested BEFORE you begin giving
the preventative. If you get a dog that already has heartworms, it
needs to go through the treatment process before preventative
medication can be given.
-Diethylcarbmazine is the oldest
preventative drug and was our mainstay for many years. It is
reliable, safe, and works very well, but it must be given daily to
be effective. It is available in many formulations including
chewable tablets with an intestinal worm medication added (Filaribits
Plus
®
made by Pfizer).
-Ivermectin, more commonly known as
Heartgard
®
or Heartgard Plus
®
(made by Merial), was the first of the
macrolide anthelmintics to be approved for use as a heartworm
preventative. It is effective in blocking development of larvae
that infected the dog as long as two month prior to treatment. The
“Plus” version also contains an ingredient that treats hookworm and
roundworm intestinal parasites.
-Milbemycin, also known as Interceptor
®
(made by Novartis) is similar to ivermectin, but it also controls
roundworm, hookworm, and whipworm intestinal parasites without the
addition of another ingredient. This one is also safe to use in
collies that are sensitive to ivermectin.
-Moxidectin or ProHeart
®
(made by Fort Dodge) is a very effective preventative but does not
kill microfilaria at the preventative dose. It can be given as an
oral pill monthly or in the newer formulation, ProHeart6 ® ,
as an injection every 6 months. The convenience of every 6 month
dosing is very appealing to many people, but the first dose must be
given after the dog is 6 months old and at a fairly stable weight.
-Selamectin, or Revolution
®
(made by Pfizer) is the newest drug to be approved for heartworm
prevention in dogs. It is administered topically and also is
effective against fleas, ear mites, and sarcoptic mange. It also
helps control the American dog tick.
The best way to
choose a heartworm preventative medication for your dog is to talk
to your veterinarian. He or she knows your dog and the conditions
of your area and can make the best recommendations. These
medications are purchased by prescription only. A yearly negative
heartworm test is usually required to refill the prescription and
will be necessary wherever you choose to have the prescription
filled.
The old saying that an ounce of prevention is worth a pound of cure
is very, very true with heartworm disease in dogs. It is much
easier and economical to prevent this disease than to treat it. The
drugs available today for prevention are safe and easily
administered. Keep your dog healthy by preventing this life
threatening disease!
Frances Smith, DVM
March, 2003
The North Texas Boston Terrier Club Rescue tests all the dogs that
come through our program for heartworms. The dogs that test
negative are put on heartworm prevention and the ones that test
positive are treated before they are placed in a new home. It is
required that all dogs adopted through NTBTC Rescue be maintained on
heartworm preventative medication throughout their lives.
Please note: The information provided here is meant to supplement
that provided by your veterinarian. Nothing can replace a complete
history and physical exam performed by your veterinarian.
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