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Preventing Heartworms

By Michele Welton, Dog Trainer, Breed Selection Consultant, Author of 15 Dog Books


Heartworms are large (6 to 12 inches) worms that can find their way into the blood vessels around your dog's heart and lungs. A severe infestation can damage your dog's heart and lungs and result in death.

That's the bad news.

The good news is that heartworm is not the immediate death sentence that pharmaceutical companies and some veterinarians might have you believe. It doesn't strike your dog overnight and require immediate surgery, for example.

How a dog gets heartworms

First, an infected mosquito must bite your dog.

Heartworm is carried by mosquitoes. Actually, that's not quite true. A mosquito obviously can't carry a six-inch heartworm! What mosquitoes carry are tiny organisms called heartworm microfilaria  (sort of PRE-heartworms).

Mosquitoes aren't born with microfilaria. They ingest it if they bite another animal that has it. In some parts of the country, that doesn't happen very often because so many people keep their dogs on heartworm medication that there aren't many infected dogs left to bite.

But suppose a mosquito does happen upon an infected dog and ingests that dog's blood. The tiny heartworm organisms called microfilaria  enter the mosquito's body.

Second, inside the mosquito's body, the microfilaria must develop into larvae.

This can occur ONLY:

  • IF the weather stays warm enough (above 57 degrees Fahrenheit for 30 consecutive days and nights)
  • AND IF the mosquito lives long enough. If the temperature gets too cool, no larvae will develop. If the mosquito gets swatted, no larvae will develop.

So it takes perfect conditions and luck for microfilaria to become larvae. If they make it, they crawl into the mosquito's mouth parts and wait for their buggy host to inject them into another dog.

Finally, this mosquito, with its surviving larvae, must bite YOUR dog.

Bummer if it does. But still your dog is not yet infected.

When the mosquito bites, it injects a few larvae, but not quite into  your dog. The larvae are deposited in a drop of mosquito saliva adjacent  to the bite. The larvae must swim through the saliva (before it evaporates in the heat) and into the puncture hole left by the mosquito bite, thereby entering your dog's skin.

NOW your dog is considered heartworm-positive.

But he still doesn't have heartworm disease.

Heartworm larvae, you see, don't go directly to the heart. They spend a couple of months living under the skin, continuing to develop. Some of them will die from various causes.

But eventually a few larvae, perhaps 2 to 5, will find their way into your dog's bloodstream and then into the area around his heart and lungs. There they will mature into adult worms and begin producing their own microfilaria – their own "children" if you will.

At this point your dog is a source of infection for every other dog and cat in the neighborhood.

How is your dog a source of infection? The microfilaria is circulating in his bloodstream. If he gets bitten by a mosquito, the insect ingests your dog's microfilaria, then flies off toward some other (unlucky) dog.... and the whole cycle begins again, for that next dog.

So a heartworm-positive dog, whenever he goes outside where there are mosquitoes, is a hazard to every other dog in the area.

Meanwhile, the adult heartworms in your own dog's pulmonary system (if there are enough of them or if they lodge in a particularly sensitive place) will begin to irritate his arteries or constrict his heart passages. Eventually he'll start to cough.

NOW your dog has heartworm disease.

Your dog with heartworm disease needs treatment

This is major treatment (several hundred dollars) that will take two or three months to complete.

Your dog will need a seris of injections of a powerful drug, an injected form of arsenic. Then your dog must be kept inactive for several weeks so the dying worms don't break off from his heart all at once and form a mass that might block a major blood vessel.

So treatment for full-blown heartworm disease is both serious and expensive.

It would be better to prevent it, don't you agree?

The three keys to preventing heartworm

  1. Avoid mosquitoes.
  2. Have your dog tested annually (a simple blood test).
  3. Consider giving heartworm medication every month to kill any larvae that might have been injected by a mosquito.

Let's look at those three keys to preventing heartworm.

First, avoid mosquitoes

The best way to cut down on mosquitoes in your yard is to remove all sources of standing water.

Mosquitoes breed in standing water. Scout around your property and remove or alter anything that might collect water.

  • Cover trash barrels.
  • Turn over empty buckets, flower pots, and wheelbarrows.
  • Turn over kiddie pools when not in use.
  • Clean gutters of debris so water drains properly.
  • Fill in (or drain) low areas that form puddles.
  • Fill in hollow tree stumps that catch water.
  • Dispose of old tires (drill holes in the bottom of a tire swing).
  • Change the water in bird baths frequently.
  • A garden pond should include a waterfall or fountain to keep the water moving. Healthy ponds attract dragonflies and frogs, both of which devour mosquitoes and larvae.

Next, prune brushy growth, where mosquitoes like to sleep.

Next, repair your screens and make your house mosquito-tight. Consider using an indoor mosquito light such as the Katchy.

Finally, keep your dog INDOORS when mosquitoes are most active.

Second, have your dog tested annually

A blood test for heartworm should be done about one month before mosquitoes start appearing in your area.

Since it takes a few months for the various stages of heartworm to be detectable by the blood test, you're actually testing in the spring  to see if your dog was bitten by an infected mosquito last fall.

Third, consider giving monthly heartworm medication

If your dog's annual heartworm test is negative, you might decide to keep it that way by giving him a monthly drug that kills heartworm larvae if there are any in his bloodstream.

The trade-off, of course, is that you're giving the drug (an insecticide, a poison) for nothing if your dog isn't infected. That's the trade-off you make when you use heartworm medication.

Most vets believe that medication is an absolute necessity everywhere in the US. Other vets believe that medication is only necessary in areas with a lot of heartworm cases or in dogs who spend a lot of time outdoors. A few vets believe that long-term use of heartworm medication can be harmful.

I don't have a strong opinion on this. In the end, only you can decide whether you're more concerned about the risks of heartworm or the risks of ongoing medication.

Which heartworm medication is best?

I prefer Heartguard Plus. It has the longest track record, so we know exactly what to expect from it.

Unfortunately, the Plus  means the company has added another drug to kill roundworms and hookworms. The vast majority of dogs don't need that, so it's simply an unnecessary additional poison.

Other heartworm medications include Sentinel, Trifexis, and Revolution, none of which I feel comfortable with. Too many side effects, and too many other drugs mixed in.

Caution: A few breeds may react adversely to the drug (ivermectin) that's used in Heartgard Plus.

If your dog is a collie-type breed or cross, he might have inherited a mutation  in the otherwise normal gene mdr1 (which stands for multi-drug resistance, which means the dog can't metabolize certain drugs, such as ivermectin).

  • 70% of Collies (Rough and Smooth) have inherited the mutation in this gene.
  • 50% of Australian Shepherds have inherited it.
  • 10-15% of Shelties and German Shepherds
  • About 5% of Border Collies and Old English Sheepdogs

If your dog is one of those breeds (or a cross or mix of those breeds), you should TEST him to find out if he has this mutation. Your vet can either swab the inside of the dog's cheek or draw a blood sample, then send it to the Washington State University Veterinary School.

When should you give heartworm medication?

Your medication schedule should follow the life cycle of mosquitoes in your area.

The American Heartworm Society maintains a heartworm incidence map of where heartworm is most prevalent in the US.

There are a number of online maps that show you when mosquito season starts and ends in different US states. Do a Google search for mosquito seasons in US  or mosquito season in [yourState]

There's also an excellent map that suggests, based on the mosquito season in your area, the first day you might want to start heartworm medication, and the last day you might want to give it.

With this schedule, the goal is to protect your dog with medication during the periods of risk without giving him unnecessary drugs during the periods of little to no risk.

Mind you, following this schedule does not guarantee that your dog will not become infected with heartworms! It's simply a helpful tool that tries to balance the risks on both sides. The ultimate decision is up to you.

Michele Welton with BuffyAbout the author: Michele Welton has over 40 years of experience as a Dog Trainer, Dog Breed Consultant, and founder of three Dog Training Centers. An expert researcher and author of 15 books about dogs, she loves helping people choose, train, and care for their dogs.

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