Heartworm

The following article was written by Rachel Peeples.

Heartworms:

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 (oe 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 its 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.

All text and graphics are Copyright � by LabsR4U. Please do not copy any text or graphics without permission.