The scientific name for heartworms is Dirofilaria immitus. These parasites are now common in pet and stray dogs in all American States - except Alaska. The warmer the climate and the more mosquitoes present, the more likely it is that your pet will catch them.

Some Facts About This Parasite:

The male worms are a few inches long and look like angel hair spaghetti. The female worms are much larger. They cause most of the damage Heartworms are transferred from dog to dog through the bite of an infected mosquito.

After a mosquito sucks larva heartworms up with blood from an infected dog; it rests for a period of time before these larva become infective. When that mosquito then bites another dog or the same dog, it transfers these microscopic larva as it bites. That is the only way a pet gets a new heartworm. During the next 6-7 months, the larval heartworms migrate slowly through the dogs body and arrive at the heart . There they mature into adult worms.

Heartworm disease is worst in warm areas of the World where mosquitoes are active all year long. The more mosquito bites your pet gets, the more the chance it will get infected. So dogs that spend a lot of time outside or in unscreened areas are much more likely to have heartworms than indoor pooches.

What Are The Signs Of Heartworms In My Dog?

Most dogs do not show any signs early in infection. The first sign of heartworm disease I often see is premature aging. Dogs with heartworms often gray prematurely about the muzzle and forelegs. Their coat looses luster and with time, their activity level decreases. Owners don't notice this because it occurs so gradually and many write it off as "just getting old". They just don't have that old"bounce in their step". This occurs much slower or not at all when a dog only has a few heartworms.

With time, a persistent, dry cough begins. This cough is most noticeable at night when the house is quite and the dog is resting or in a sitting position. This cough is due to three things: bronchitis that develops as pieces of dying worms become trapped in the lungs, fluid that accumulates in the lungs as the heart fails and the enlarged damaged heart pressing on the pet's wind pipe.

Later, the dog’s tummy assumes a pear-shaped, pot-bellied look as the dog’s liver enlarges and fluid accumulates in the abdomen. While these events are occurring, the dog’s heart and pulmonary arteries are enlarging due to mechanical obstruction of the worms, inflammation and damage to the heart valves.

How Do Heartworms Injure My Pet's Heart?

Not only do mature heartworms clog up the arteries leading to the lungs, as the heartworms grow, they irritate the lining of the network of blood vessels that lead from the heart to the lungs. This irritation partially blocks these arteries and makes the heart work harder to pump blood through the lungs to receive oxygen. The more heartworms are present, the harder the work becomes. Eventually, the heart begins to enlarge and fail due to the overwork.

How Do I Keep My Dog From Getting Heartworms?

Ordinary worm medicines do not kill heartworms. To prevent your pet from catching heartworms, you must give it a special preventive medication once a month. The three that are most commonly used are Ivermectin (Heartgard), Milbemycin oxime (Interceptor), and selamectin (Revolution).

All three of these medicines kill heartworms between the time that they enter your pet through a mosquito bite and the time they enter the heart. But they will not kill heartworms after they take up residence in the heart.

How Will My Veterinarian Find Out If My Pet Has Heartworms?

If you are lucky, your veterinarian checked your pet's blood for heartworms before its heart was damaged. Many veterinarians check their patient's blood for heartworms every year. The most accurate test checks for products that heartworms release in your pet's blood stream. This is called an ELISA heart worm antigen test and the most popular one is the Idexx SNAP test that your veterinarian performs while you wait. A simpler test is for your veterinarian to examine a drop of your pet's blood for heartworm larva. If they are there, your dog has mature heartworms. But if no larva are there, it may still have adult heartworms - so dogs that are negative on that test still need the ELISA test run.

All heartworm tests rely on finding substances that only mature heartworms produce or finding larval heartworms (microfilaria) produced by adult heartworms in your pet. It takes takes 5-7 months from the day your pet was bitten by an infected mosquito for these substances to appear. So there is no point in asking for these tests if your pet is less than 5-7 months old. If you missed giving heartworm preventative, the same 5-7 months have to pass before the tests have any value. But fortunately, ivermectin-containing products are known to "reach back" a few months destroying baby heartworms - even if you missed giving your pet its monthly pill.

Occasionally, the results of another test makes veterinarians suspicious of heartworms. When the vet sees a particular pattern of blood vessels in the pet's lungs, combined with enlargement of the right side of the heart, heartworm disease is a very likely cause.

If your veterinarian does an echocardiogram of your pet's heart, the actual heartworms can sometimes be seen.

How Can I Get Rid Of Heartworms Once My Dog Has Them?

Your pet should begin taking ivermectin as soon as heartworms are diagnosed. This will kill any baby heartworms that have not yet reached the heart. But it will not cure your pet of the dangerous adult heartworms.

Unfortunately, there is only one approved drug available that will do that. It is called melarsomine dihydrochloride and it is marketed as Immiticide by Merial Pharmaceutical Company. It contains arsenic.

The drug can result in numerous side-effects and even an occasional death in advanced heartworm disease.

Before your veterinarian decides to give this medication, he/she will want to know if your pet is strong enough to survive the treatment. So they will run blood tests to see if your pet's liver and kidneys are still functioning normally. They will also x-ray it's chest to see how much heart damage has occurred. Pets with heartworms are often anemic as well. Based on the test results, your veterinarian may decide it is safer to try to stabilize the pet before treating the heartworms.

Three Immiticide injections are usually given. First a single injection to "soften up" the parasites. The drug is very irritating so the needle is inserted deep into the muscles of the back. After the injection, some of the heartworms die and begin breaking apart. There is nowhere for them to go other than downstream into the lungs. Your pet will be very sore for a few days due to the painful injection. Try not to touch the area or it may bite you. Ask your veterinarian for some pain control meds or permission to to give aspirin in case you dog needs them. You will be asked to maintain your pet in strict "bed rest" for a period of time after this injection.

If everything goes well, your dog will come back in one month for two additional Immiticide injections, given 24 hours apart. Strict rest is very important after the second and third injections.

After receiving Immiticide, watch your pet carefully. It is common for these pets to run a fever. It is also common for them to cough, be depressed, and eat less. Sometimes they will have nose bleeds. If any of these things occur call your veterinarian immediately. If anything more serious happens, such as weakness, difficult breathing, bloody cough or bluish gums, put the pet in the car and take him in immediately. The first 7 -10 days after heartworm treatment are the most dangerous. But your pet is not out of the woods for several weeks more. These side effects are happening because the dead worms are breaking up and being carried into the lungs where they cause severe inflammation. With time, the body absorbs them. The danger is that a big wad of dead worms and clots will plug up a major artery in the lungs (an embolism). Major embolisms can be fatal. The less active you pet is during this period, the less likely a fatal embolism will occur.

How Should I Care For My Dog After It Receives The Immiticide Injections?

The most critical time is the first few weeks after the Immiticide injections. This is the time the dog's body must deal with all the dead heartworms in its circulation. The most important thing during this period is to keep the dog as still as possible. This means as little exercise as possible. The best way to do this is to keep your pet in a cage (crate) in a cool, isolated, quiet area. Take the pet out on a leash frequently enough to relieve itself - but no more.

It is common for dogs to run a fever during this period. Be sure it gets plenty of water.

How Risky Is This Treatment For My Dog?

Here are the factors that determine how risky heartworm treatment will be for your dog:

1) The length of time your pet has been infected

It takes approximately 7 months from to time a mosquito bites your pet until an adult heartworm develops that begins to damage your pet's circulatory system. This damage occurs over an extended period of time. So young dogs (less than 2-3 years old) and recently infected dogs usually have considerably less circulatory damage - and less risk.

2) The Amount of Heart, Liver and Kidney Damage That Has Occurred

The degree of elevation in certain blood tests will tell your veterinarian the extent of any damage to the pet's liver and kidneys. An x-ray of your pet's chest, or perhaps a cardiac ultrasound will show if the pet's heart is damaged. We always hope that no damage is found. Any significant damage increases the risk of Immiticide treatment.

3) The Number of Heartworms Present

The damage done is proportional to the number of female worms present. Dogs with high heartworm numbers are at much greater risk.

4) The Size Of Your Dog

The diameter of the pulmonary arteries is much larger in big dogs than small dogs. So small dogs are at a greater risk than large dogs with the same number of heartworms.

5) The Sex Of The Heartworms

It is the female worms that do most of the damage. In some infections, the worms are mostly male. There is no chemical test that will tell what sex the worms are. However, a cardiac ultrasound can often see the heartworms and tell if they are the larger females or smaller males.

Is There An Alternative Treatment For My Pet?

Because of the expense of melarsomine treatment as well as the risks involved , some owners and humane groups decide to simply place heartworm-posative dogs that are not showing symptoms on ivermectin and doxycycline antibiotic. The hope is that this will weaken and shrivel the heartworms and improve the dog's general condition, while we waits for the heartworms to die naturally.

The normal life span of a heartworm is about 2 years - possibly shorter when the dog receives monthly or twice-a-month ivermectin. Ivermectin does not kill adult heartworms outright like Immiticide does. Heartworms must not like ivermectin, because they stop producing larva (microfilaria). But as long as your dog continues to test positive with the SNAP test, live worms are still present doing their damage in your pet's heart and lungs. If your veterinarian has determined that your pet already has significant heart and lung damage, then this form of slow ivermectin/ doxycycline treatment is not for you.

However, in December of 2008, the results of an a very important new study were published. In this study, dogs with heartworms were either treated with:

After 9 months, the remaining live heartworms were counted.

When there are only 5 dogs in a group and only one study, the resuts don't give hard and fast rules. But it appeared that giving just doxycyline did not really reduce the number of live worms. Giving just ivermectin probably reduced them slightly. But giving both ivomectin and doxycycline did significantly reduce the number of heartworms by the end of the 36 wk study. To run a scientific study of this type is a great accomplishment and the results are quite exciting. It is only through experiments like this that veterinarians discover new treatments that help save the lives of pets. But it will take a number of years before we know if these results stand the test of time.

What is Wolbachia?

Wolbachia is an organism that lives inside of heartworms. Some veterinarians now believe that Wolbachia is responsible for some of the blood clots and and malaise that occur during treatment. Because of this, more and more veterinarians are pre-treating dogs with an antibiotic, doxycycline, to destroy Wolbachia prior to administering Immiticide.

What Is Vena Caval Syndrome?

If a pet is bitten by an unusually high number of infected mosquitoes in a single season, enough heartworms can develop to suddenly plug up the the large vein that returns blood to the heart. This vein is called the vena cava. These dogs may suddenly collapse and they frequently pass dark brown-colored urine. Most do not survive. A few can be saved by emergency surgery during which a forceps is passed into the heart and vena cava and the worms manually extracted. Eighty to over one hundred worms can be found in the heart of a 40 pound dog with caval syndrome.

How Often Should My Dog Be Tested For Heartworms?

If you give your pet once-a-month heartworm preventative according to the instructions on the package, it should not get heartworms. So why do veterinarians suggest annual heartworm tests? There are several reasons why:

1)Your pet may have caught heartworms before you began giving the monthly preventative.

2)You may have accidentally missed some months.

3)Your pet may have spit out the medication.

4)Most State veterinary laws require a physical examination or heartworm test before a prescription for heartworm medication can be renewed.

What If My Dog Is Still Positive For Heartworms After Being Treated?

Young, female heartworms are the hardest to kill with Immiticide. So some may survive the treatment. If a follow up SNAP antigen test is still positive 6 months after the dog was treated, some living worms have survived.

The health of these dogs is usually much improved because the majority of the heartworms are now gone. It is rarely, if ever, necessary to retreat these dogs. The American Heartworm Society gives some guideline to help your and your veterinarian make the decision about retreatment in the section of their 2005 Guidelines entitled Confirmation of Adulticide Efficacy.