TREATMENT OF INFECTION
TREATMENT OF INFECTION
It has been said that the treatment of heartworm infection is something of an art. There are several strategies that can be used depending on the dog's medical condition including the option of not treating at all. The important concept to realize is that very harsh arsenic based drugs are necessary to kill adult heartworms and that treating for heartworm infection is neither simple nor safe in itself. Let us review some of the dangers and options in clearing the body of this parasite.
Prior to therapy, the heartworm patient is assessed and rated for risk into one of four categories. Important factors include: how many worms are thought to be present based upon the tests performed, the size of the dog, the age of the dog, concurrent health factors, severity of the heart disease, and the degree to which exercise can be restricted in the recovery period. Some hospitals use computerized formulas to categorize heartworm infected patients. The categories into which patients are grouped are as follows:
Chest radiograph from a moderately affected dog
Chest radiograph from a severely affected dog
To view the physical removal of adult heartworms from the jugular vein of a dog with caval syndrome, click here:
After knowing what Class the patient fits in, treatment can be determined. The dog has three groups of heartworms in his or her body:
NOW THAT WE HAVE STAGED OUR HEARTWORM INFECTION, LET'S WORK ON GETTING RID OF IT
The goal is to get rid of all the worms with as little inflammatory reaction as possible. To achieve this, treatment is approaches in stages with the goal being to minimize the number of adult worms to be killed and having the patient as healthy as possible going into what is called "adulticide" treatment.
STABILIZATION FIRST: Expect strict exercise restriction to be needed after adulticide therapy but moderately affected dogs will need restriction from the first. If heart failure is present, this will need to be controlled as well. Some dogs will need anti-inflammatory doses of steroids to control the inflammation that stems from the presence of worms. This is often done concurrently with the second step as described below.
KILLING THE MICROFILARIA AND MIGRATING WORMS
The next step in treatment is clearing the migrating immature worms. Heartworm treatment is a project that lasts at least a couple of months. We do not want immature worms to mature in that time frame. Since our goal is to have fewer adult worms when the time comes to kill the adult worms, it behooves us to kill the younger worms right off the bat.
Happily the microfilariae, L3, and L4 larvae can all be killed by monthly ivermectin based heartworm preventive products (i.e. Heartgard®, Triheart® etc.). The milbemycin based products (Sentinel®, Trifexis®, Interceptor®, etc.) will also do the same job but will kill the microfilariae much faster which can create circulatory shock if there are large numbers of microfilariae dying all at one time. The newer products such as the selamectin and moxidectin products do not clear microfilaria well enough to be used in the treatment of an active infection so right now the ivermectin based products seem to be the best for this use. The American Heartworm Society recommends 1-3 months of a preventive prior to treating the adult worms. How long one chooses to wait depends on how urgent the dog’s need is to remove the adult worms. After all, it is the adult worms that cause heartworm disease, not the immature worms addressed by the preventives.
KILLING THE ADULT WORMS
In the 2 dose protocol, the dog comes back for a second injection the next day on the opposite side of the lower back. In the 3 dose protocol, the dog comes back one month later for 2 doses 24 hours apart (the first dose representing an introductory treatment to kill some of the more sensitive worms.) Keep in mind, too many worms dying at once creates circulatory shock. The benefit of the 3 dose protocol (sometimes called the "split dose" protocol) is that the first injection serves to kill off any older or weaker worms without killing off the stronger, younger ones. When the two consecutive doses occur one month later, there will be fewer worms dying at once.
After treatment, the patient must be strictly confined for one month following the final treatment. No walks, no running around. The dog must live the indoor life. The reason for this is that embolism to some degree is inevitable and it is important to minimize embolism-related problems. Exercise increases heart rate and oxygen demand and we need the heart to rest during this recovery period.
If any of these occur, report them to the vet as soon as possible. The most critical time is 7-10 days following a melarsomine treatment but they can occur anytime in the following month.
IVERMECTIN ONLY (THE "SLOW KILL" METHOD)
Melarsomine treatment is expensive and often out of reach for rescue groups, shelters, and many individuals. If the dog is stable (Class I) one option is to simply leave the dog on an ivermectin based preventive. This option has led to a great deal of misconception about the ability of ivermectin to kill adult heartworms. Let us lay the rumors to rest now:
This means that if one opts to treat a heartworm positive dog with an ivermectin heartworm preventive only, one can expect the dog to remain heartworm positive for as long as 2 years and the heartworm disease will be progressing during that 2 years. This is not good for the dog but certainly beats getting no treatment of any kind.
There is another aspect to this treatment which is important to mention and that is the development of resistant heartworm strains. Using heartworm preventives long term in dogs with adult heartworms, leads to the production of heartworm preventive resistant microfiliaria. In other words, this "slow kill" method of treating heartworm infection leads to strains of heartworm that cannot be stopped by any of the heartworm preventives presently available on the market. Heartworm preventive resistance has now been documented in some of the heartworm strains of the Mississippi river delta area and inappropriate use of heartworm preventives is felt to be the cause. Previously, the "slow kill" method of heartworm treatment was deemed acceptable for dogs in the Class I (early) stage of heart disease but this has changed given confirmation of resistant strains. The Companion Animal Parasite Council now recommends that the "slow kill" protocol using heartworm preventives NEVER be used regardless of whether the patient is in Class I stage of infection or not. Heartworm infection should be dealt with definitively and promptly so as to preserve the effectiveness of the medications we have.
Page last updated: 6/6/2016