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Labsr4U Pawsitive Advice
Heartworm - Article 2
The following article was written by Rachel Peeples, DVM
Heartworms are transmitted by mosquitoes and
require the mosquito to complete their life cycle. A mosquito becomes
infected when it bites a heartworm infected animal that has microfilaria
(ie: baby heartworms) in its blood. The microfilaria undergo
metamorphosis in the mosquito to larvae. The larvae are then transmitted
to an animal by the mosquito when the mosquito bites the animal. The
larvae then travel through the tissues of the body to the major blood
vessels maturing along the way. When they reach the heart they develop
into adult heartworms. The adults mate and produce microfilaria which go
into the bloodstream and are infective to the next mosquito that bites
and thus the cycle begins again.
Heartworms live in the right side of the heart and extend out into the
pulmonary (or lung) vessels. They cause inflammation and irritation of
the vessels, heart and lungs; alterations to blood flow resulting in
decreased heart efficiency as well as turbulence which makes blood clots
likely to develop. Clinical signs vary with the number of worms and the
individual animals tolerance of them and the exertional demand on the
animal (a sled dog would tolerate less heartworms than a couch potato
because the sled dog needs ts heart to work at maximal efficiency).
Infected dogs with high worm burdens can develop a condition called
caval syndrome where there are so many worms the blood vessels become
completely blocked and, thus, no or very little blood flow. More common
signs are exercise intolerance, coughing, inappetance, and insufficient
blood delivery. Not all untreated dogs die but most will. There is a
very high risk especially as the worms mature and die of blood clots or
even pieces of decaying worms being thrown out of the heart and blocking
a major blood vessel. And dogs can be continually reinfected so they
never "clear" the infection like cats seem able to do.
Treatment is possible but is relatively hard on the dog. It is an
arsenic compound that though safer than previous treatments is still
toxic to the liver and kidneys which is why it is very important to make
sure these organs are functional before treating heartworms. Many dogs
survive heartworm treatment without complication but there are
significant numbers that do not. It is interesting to read the
alternative therapies -- I would like to see scientific proof of their
efficacy and decreased toxicity to the current treatment before using
them though. Just because it is "natural" does not mean it is same. Many
horses each year die from eating red maple leaves . . .
Prevention is the method of choice for dealing with heartworms. There
are daily preventatives - all of which are associated with acute liver
disease in some dogs. Monthly preventatives are of a class of drugs
called avermectins. Ivermectin is the first of this class used and it is
99.999% effective in preventing heartworms. At the dose in Heartguard it
does not kill microfilaria, roundworms, or hookworms (the pyrantel in
Heartguard plus kills the roundworms and hookworms). At higher doses it
does treat roundworms, hookworms, microfilaria, mange mites, ear mites,
whipworms, and some even propose fleas. Interceptor or milbemycin is the
second avermectin to be developed. At the dose in Interceptor it does
kill microfilaria, roundworms, hookworms, whipworms. Since it does kill
microfilaria, if given to a dog with microfilaria it can initiate a
anaphylactic (severe allergic) reaction and kill the dog. The newer
monthly preventative like Pro-Heart and Revolution are all avermectins.
ALL the avermectins are capable of causing neurologic problems if the
get into the brain. Collies and collie-derivatives are very susceptible
to (thru a difference in the blood-brain barrier) allowing avermectins
to cross into the brain. Ivermectin does this more readily than the
others but all will. Heartguard levels of ivermectin are not known to
cause problems in Collies and Colie-derivatives but higher doses can and
do. To my knowledge the sensitivity of collies and Collie-derivatives
has nothing to do with immune-mediated problems.
As far as using monthly preventatives to treat heartworms -- the
heartworm treatment available is much superior to using the preventative
because it kills them more uniformly, quickly, and minimizes the damage
the heartworms do. BUT if using the heartworm treatment is not an option
due to health, age, finances, etc. then putting them on preventative
keeps them from being reinfected and once the current heartworms die
(3-5 years) there will be no more heartworms. Very recent studies by Dr.
McCall at University of Georgia indicate (but have not proven yet to my
knowledge) that ivermectin actually cripples and kills the heartworms
over a 4-5 months period. None of the other preventatives studied showed
this effect. BUT this is again inferior to the heartworm treatment
because it allows the heartworms to for 4-5 months worth of damage. But
again if treatment is not an option then it is an alternative.
Using Ivomec -- many. many, many people have used it safely throughout
the years. But it IS not formulated for dogs and does contain some
inactive ingredients whose safety is not known in dogs. So do realize
using it is AT YOUR OWN RISK. The reason to avoid its use in Collies and
Collie-deriviative is that it is VERY concentrated being a large animal
product and it is very easy to overdose them to the point of toxicity.
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