Heartworms in Dogs

The scientific name for heartworms is Dirofilaria imitis. 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 larval heartworms up with blood from an infected dog; it rests for a period of time before these larvae become infective. When that mosquito then bites another dog or the same dog, it transfers these microscopic larvae 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 quiet 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 that are present, the harder the work becomes. Eventually, the right side of 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 every 30 days. The drugs that are most commonly used are Ivermectin (Heartgard), Milbemycin (Trifexis) Moxidectin (ProHeart 6 injection lasting 6 months) and selamectin (Revolution) for cats.

All of the oral medicines kill heartworm immature larvae accumulated in the pet's skin between the time that they enter your pet through a mosquito bite and 30 to 45 days earlier. That is the reason for giving the oral medicine every 30 days. These medications are only in the dog's system for a short time. But they will not kill heartworms after they take up residence in the heart.

ProHeart 6 is a slow dissolving injectable medication that provides a low level continuous amount of heartworm prevention. It is placed under the loose skin in front of the shoulder by a painless injection. With ProHeart 6, the infective heartworm larvae are killed immediately as they are deposited in the dog's skin by the mosquito. This is much more efficient way to prevent heartworm disease in dogs.

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 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 month or so destroying baby heartworms - even if you missed giving your pet its monthly pill on time.

Occasionally, the results of another test makes veterinarians suspicious of heartworms. On a chest xray, for example, 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.

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

Your pet should begin taking an antibiotic called Doxycycline as soon as heartworms are diagnosed.  Doxycycline is an antibiotic used for 1 month to kill a bacteria that lives in the adult heartworm, which weakens it.  In addition, a monthly heartworm preventative will be prescribed to prevent any new heartworms from surviving  in your dog .   Heartworm preventatives should be given for at least 2 months before heartworm treatment to make sure that all larvae in the dog's body will develop significantly to be affected by that drug used to kill the adult heartworms.

Unfortunately, there is only one approved drug available that will kill mature adult heartworms. It is called melarsomine dihydrochloride and it is marketed as Immiticide by Merial Pharmaceutical Company or a Diroban by Zoetus. It contains arsenic, which can be very toxic to the dog's liver.

Melarsomine 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  and lung 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.

Two or three injections are used for a full heartworm treatment. For early or very mild heartworm disease, two injections are given 24 hours apart in the hospital. For severe heartworm disease, three Melarsonamine injections are usually given. First a single injection to "soften up" the parasites and decrease the worm burden that blocks up the pulmonary artery and right ventricle is administered. The dog is observed in the hospital overnight. Then 30 days later, two injections are given 24 hours apart with continued strict rest.

Melarsomine 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 may 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

After receiving Melarsomine, watch your dog carefully. It is common for these dogs 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 dog in the car and take him in immediately. The first 7 -10 days after heartworm treatment are the most dangerous. But your dog 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 dog is during this period, the less likely a fatal embolism will occur.

After your Dog Receives The Melarsomine Injections.....

Most dogs that get heartworm treatments have no serious side effects if the proper aftercare is done!!

The most critical time is the first few weeks after the Melarsomine 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 dog 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 Melarsomine 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

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. If your dog receives a regular 6 month injection as a heartworm preventative 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.

Ron Hines DVM PhD

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