Canine Heartworm Disease
Once considered a
parasite of
southern climates, the heartworm (Dirofilaria immitis) is now
recognized
as a major, global pest affecting dogs, cats, wolves, coyotes, foxes,
and some
other animals. From its discovery in dogs more than a century ago and
the
documentation in cats in the 1920s, researchers have devised diagnostic
tests,
preventives and treatments, but the disease has spread to all 50
states.
According to the Heartworm Society, the highest infection rates occur
in dogs (not
maintained on heartworm preventive) within 150 miles of the Atlantic
and Gulf
coasts and along the
To jog the concern of
clients,
veterinary clinics may display a preserved heart infected with
heartworm in a
jar and hang posters about heartworm life cycles in examining rooms,
but seeing
is not necessarily believing; although clients can view the infested
heart
loaded with long, spaghetti-like worms every time they visit, many
gamble that
their dogs will never be bitten by an infected mosquito.
Heartworm prevention is
simple. It
involves a blood draw to determine whether the parasite is present and
regular
dosing with preventive medication. Heartworm infestation is dangerous;
untreated dogs die and treated dogs go through weeks of discomfort
while the
worms are killed and expelled from their bodies.
Parasites go through
several life
stages before emergence as adults and often need at least two hosts to
complete
the cycle. In heartworms, a mosquito serves as the intermediate host
for the
larval stage of the worm, also known as the microfilariae. The mosquito
ingests
the larva when it bites an infected dog and deposits its cargo in an
uninfected
dog when seeking another blood meal. The microfilariae burrow into the
dog and
undergo several changes to reach adult form, then travel to the right
side of
the heart through a vein and await the opportunity to reproduce. Adult
heartworms
can reach 12 inches in length and can remain in the dog’s heart for
several
years.
Dogs can have some
microfilariae
in their blood and worms in their lungs without manifesting the
disease. Once
the number of worms exceeds a certain number based on the size and
activity
level of the dog, however, the adult worms move to the heart and
symptoms begin
to occur. Very active dogs may experience symptoms with lower numbers
of worms
than couch-potato dogs.
The time lag between the
initial
infestation of microfilariae and reproduction by adult worms living in
the
heart is six-to-seven months in dogs.
Female heartworms bear
live young
– thousands of them in a day. These young – the microfilariae –
circulate in
the bloodstream for as long as three years, waiting to hitch a ride in
a
bloodsucking mosquito. They undergo changes in the mosquito that
prepare them
to infect a dog, and they transfer back to the original host species
the next
time the mosquito bites. The process of change in the mosquito takes
about 10
days in warm climates, but can take six weeks in colder temperatures.
The worms grow and
multiply,
infesting the chambers on the right side of the heart and the arteries
in the
lungs. They can also lodge in the veins of the liver and the veins
entering the
heart. The first sign of heartworm infestation may not manifest for a
year
after infection, and even then the soft cough that increases with
exercise may
be dismissed as unimportant by the owner. But the cough worsens and the
dog may
actually faint from exertion; he tires easily, is weak and listless,
loses
weight and condition, and may cough up blood. Breathing becomes more
difficult
as the disease progresses. The progression is traumatic: the dog’s
quality of
life diminishes drastically and he can no longer retrieve a Frisbee or
take a
long walk in the park without respiratory distress. Congestive heart
failure
ensues, and the once-active, outgoing pet is in grave danger.
Heartworm disease is
present on
every continent except Antarctica. It
occurs where these four factors are found:
Dogs are considered the
definitive
host for the parasite; even though the disease is not
transmitted
directly from one dog to another, untreated dogs provide a stable
reservoir for
the disease. Mosquitoes of several different species are the vectors
(intermediate host for the developing microfilariae). Development of
the
microfilariae in the mosquito requires a temperature at or above 80
degrees
Fahrenheit for about two weeks. No larval development takes place in
the
mosquito below 57 degrees F.
Heartworms can be
detected by
blood test. The filtration test finds microfilariae in the blood; the
occult
test locates adult worms in the heart. Many veterinarians prefer to do
both
tests as the absence of microfilariae in the blood does not necessarily
mean
that there are no adult worms in the heart. Both tests are done with a
single
blood draw, preferably in the early spring before daily temperatures
warm above
57 degrees F.
Radiographs (X-rays) can
also
detect the presence of adult heartworms in the heart and lungs.
If a blood test or the
onset of
symptoms alert owner and veterinarian to the presence of this
devastating
parasite, treatment is possible and successful if the disease has not
progressed too far. The first step is to evaluate the dog and treat any
secondary problems of heart failure or liver or kidney insufficiency so
that he
can withstand the treatment. The next step is to kill the adult worms
with an
arsenic compound. Veterinarians now have access to a Immiticide, a new
compound
that has fewer side effects than the previous drug and is safer for
dogs with
more severe infestations.
The treatment is
administered in
two doses, one dose given on two consecutive days, followed by several weeks of
inactivity to
give the dog’s system a chance to absorb the dead worms. Too much
physical exertion can
cause the
dead worms to dislodge, travel to the lungs, and cause death.
Depending on the severity of the infestation, three-to-four weeks after the administration of the initial treatment to kill the adult worms, a second treatment is administered to kill the microfilariae. Your vet can go over the treatment protocol with you and once it has been finished, a heartworm test should be performed 6 months from the treatment date.
Surgical removal of the
adult
heartworms is possible and may be indicated in advanced cases with
heart
involvement.
Preventive doses come in
oral and
topical versions and are only available from a veterinarian.
Diethylcarbamazine
(DEC) is given daily. Ivermectin (Heartguard), and Milbmycin (Interceptor) are given monthly.
Selamectin (Revolution) is a new preventive applied topically on a
monthly basis. Some of these drugs also kill other parasitic
worms, and Revolution also acts against fleas, ticks, and mites.
Moxidectin (ProHeart® and ProHeart® 6) is a potent preventive compound available in a six month sustained release injection administered by veterinarians or a monthly oral dosage form, however FDA took it off the market 1-2 years ago based on a study they did surrounding the number of poor reactions seen in too many animals.
Many veterinarians
recommend and
many owners use a year-round heartworm prevention program to guard
against the
occasional mosquito flying about in areas with mild winters. If Fido has already had his yearly check-up,
call your veterinarian to schedule a heartworm check.
Even if your dog is on heartworm preventative year-round, nothing is a sure
thing, and it's always a good idea to have your dog tested for heartworm on an
annual basis, especially in areas where there is a high infestation rate.