Eliminating Heartworm
Should
Dogs Be Given Medicine?
Studies show that all 50 US
states have reported cases of heartworm infection,
a condition that can affect all dogs
regardless of sex, age or habitat. The highest incidence
for dogs not taking preventive medicine rises to a
high of 45%, including areas from the Gulf of Mexico
to New Jersey, while some areas record rates of 5%
and below for the canine heartworm disease.
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The difference is due mainly to mosquito, environmental
and dog population factors, although all dogs in affected
regions are still seen as at-risk animals that need to be
monitored regularly by veterinarians and covered by prevention
programs.
Dogs become infected when they are bitten by mosquitoes
with infective heartworm larvae. The infection is transmitted,
with the larvae eventually growing into adult female and
male worms that live not only in the heart, but also in
the lungs and related blood vessels. The offspring, called
microfilariae, are released by the female heartworm into
the bloodstream. Practically all experimentally infected
dogs were found to have adult worms, with up to 250 worms
possible for one dog.
More adverse changes to the lungs and heart are observed
for dogs with a higher number of the worms. The infections
later lead to inflammation that affects the lungs and surrounding
arteries. The heart is thus pressured by the increased workload,
becoming enlarged and weakening and eventually, congestive
heart failure kills the dog. The heartworms can also be
found in the caudal vena cava - the main vein between the
liver and the heart - where they can cause liver failure
syndrome.
The main purpose of treating infected dogs is to kill both
offspring and adult worms using a microfilaricide and adulticide
respectively. A crucial condition of treatment: minimizing
any adverse side effect due to the drugs used and a tolerable
level of complications due to the dying heartworms.
Dogs without or with mild signs have shown significant
success following treatment, while those exhibiting more
severe signs also have successful treatment, but are more
prone to complications and death.
The US Food and Drug Administration has already approved
an organic arsenical compound, melarsomine dihydrochloride,
to kill adult heartworms. Dogs that will use this therapy
are required to undergo an extensive pretreatment evaluation
and must remain in the hospital during treatment.
This FDA-approved drug, which has proven to be less toxic
and more safe and effective than its predecessors, is given
through intramuscular injection into a dog's lumber muscles.
However, a key post-treatment concern is severe pulmonary
thromboembolism, in which lesions in the lung arteries and
capillaries and dead heartworms obstruct blood flowing through
the pulmonary arteries.
More severe lesions and a higher number of dead worms cause
greater obstruction, with cough, fever and hemoptysis appearing
as symptoms. Dogs showing these clinical signs will have
to be strictly limited in terms of treatment and exercise
and given corticosteroids to reduce inflammation.
Although total elimination of adult heartworms may not
be possible, dogs have shown clinical improvement following
adulticide therapy. A common follow-up to the therapy is
heartworm antigen testing: the antigen will not be detected
four months after adulticide treatment if all or a very
small number of the parasites survived.
Dogs found to be antigen-positive post-adulticide may have
the treatment repeated, but only following an extensive
case review. One option is to resume arsenical use with
ivermectin or other preventive, a combination expected to
eventually kill all surviving worms.
Meanwhile, selamectin, ivermectin, and moxidectin are available
as treatments for microfilariae. These macrocyclic lactone
anthelmintics are present in regular heartworm preventives
as active ingredients, although they do not have FDA approval
as microfilaricides.
Although these drugs do not have regulatory clearance,
they are still popular as treatments because of the absence
of approved drugs to eliminate microfilariae. Thus, dogs
using these ML anthelmintics must remain in the hospital
after treatment to be monitored for any potential side effect
due to the rapid death of the offspring worms. Use of ML
preventives is expected to eventually eliminate all microfilariae
in six to nine months' time.
Thus, dog owners should thoroughly weigh the risk between
heartworm treatment and the partial or full recovery of
their pets. Whatever decision is made, the owners should
always be pro-active in ensuring the best health care for
their dogs.
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