Canine Heartworm Disease

Canine heartworm disease is caused by nematode called Dirofilaria immitis. The vector (the thing that spreads the disease) is the common mosquito.

A dog that is harboring adult heartworm is capable of infecting the mosquitoes in the area. In the dog, the adult heartworms mate and produce very small larvae that circulate in the bloodstream. These are called microfilaria. A mosquito that ingests a blood meal (a mosquito bite) ingests some of these microfilaria with the blood. The microfilaria mature to what is called a stage 3 larvae in the mosquito's salivary glands. When the mosquito bites another dog, these stage 3 larvae pass to the dog in the mosquito's saliva and enter through the puncture wound. They migrate through the tissues and arrive in the pulmonary veins where they mature and reach a size of 4-5" in length. The migration and maturation can take up to 6 months.

Over the years the detection, treatment and prevention of heartworm disease has gone through several changes. To detect the microfilaria, one of the earliest methods was called the Modified Knott's Test. One cc of blood was mixed with 9cc of a 2% formaldehyde solution and centrifuged. The sediment was mixed with stain and examined under a microscope for microfilaria. The next improvement involved the use of a micropore filter.  One cc of blood was mixed with a lysing solution that broke down the blood cells and then passed through a filter in a special holder. The filter was then removed, placed on a slide, stained with a special stain and examined under the microscope. In both these methods, the microfilaria were stained and easily seen. Another of the first methods was a direct smear. A drop of blood was placed on a slide and mixed with a drop of water and examined under the microscope. The microfilaria could be seen by their thrashing amid the red blood cells.

All of the above methods were effective but were labor intensive. These methods were also complicated by the possible presence of another filarial worm. Dipetalonema reconditum. The adult worms live in the tissues and produce microfilaria that also circulate in the bloodstream. It is impossible to differentiate between the two microfilaria using the direct method. When using the Mod Knott's or the filter method, the two could be differentiated by the shape of the tail.

Finally, techology came to the rescue. The current "high tech" methods detect antigens that are produced by the microfilaria. The tests are accurate and specific for Dirofilaria immitis.

The treatment of heartworm disease has always used an organic arsenical. The old standard has always been thiarsetamide sodium (brand name Caparsolate). The standard regimen calls for 4 IV injections given at 12 hour intervals.

Just recently, a new form of arsenical has been released. The drug is melarsamide (brand name Immicide). It is given by IM injection into the lumbar muscle. The protocol calls for 2 injections 24 hours apart.

Finally, we come to the evolution of the preventative drugs. First there was the daily pill, diethylcarbamazine(DEC). This drug also can be used to treat intestinal worms but at a higher per pound dose. As heartworm preventative it is given at a dose of 3-5mg/lb of body weight. Effective, but has to be given every day.

Next improvement came with DEC as a chewable form. Tasted pretty good so you didn't have to fight with most dogs. Some dogs still wouldn't eat it.

Now along comes ivermectin. This drug is technically a macrolid antibiotic and had been used for quite a while as a wormer in large animals. It proved to be effective against the developing/migrating larvae at a dose of 1.4-2.8 micrograms/lb of body weight when given once a month. It first came out as a plain pill that had to be "forced" down your dog. Still a pain but at least it was only once a month.

It was mixed with some good tasting stuff and came out as a chewable chunk(tablet) that dogs liked. You gave your dog a treat once a month. Probably doesn't get much better than this. The next improvement involved mixing in some other wormers and you had something that helped control intestinal worms too.

The ivermectin preventatives are marketed under the brand name "Heartgard". Shortly after Heartgard came out, another company developed and started marketing a different once a month pill, using a different drug, called "Interceptor".

The latest innovation is a topical drug that is applied once a month called "Revolution". It is marketed as the best thing since sliced bread and maybe it is, I don't know at this point. It is still brand new and doesn't have a track record. I tend to be a little conservative with new drugs until they prove themselves.