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What To Do When Your Dog Has


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Heartworms In 2018 - Diagnosis,Treatment,Prevention

For Some Non-heartworm causes of coughing in dogs, please go here

Ron Hines DVM PhD

Lots of my articles are plagiarized and altered on the web to market products and services. There are never ads running or anything for sale with my real articles. Try to stay with the ones that begin with http://www.2ndchance.info/ in the URL box or find all my articles at ACC.htm.

The scientific name for heartworms are Dirofilaria immitus. These parasites are now common in pet and stray dogs in all American States - except Alaska. They, or their cousins, Dirofilaria repens, are found World-wide where ever mosquitoes are common. The warmer the climate and the more mosquitoes present, the more likely it is that your pet will catch them.

Some Facts About Heartworms

Male heartworms 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. In areas where heartworms are common, a few summers of repeated mosquito bites to dogs not receiving preventative medications can result in enormous numbers of adult worms in the pet's heart. If not offensive to you, you can view one of those hearts here.

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 four that are most commonly used are Ivermectin (Heartgard), Milbemycin oxime (Interceptor), and selamectin (Revolution) and imidacloprid-moxidectin (Advantage Multi® aka aka Advocate Spot On®). Some of these products are off-patent and available in generic form, others come and go. (Do not rely on the product label images in my article to still be correct. Carefully read the one that came with the product you personally bought)

All four 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 efficiently or rapidly kill heartworms at these doses after the worms take up residence in your pet's heart.

What About Natural and Homeopathic Remedies And Preventatives That I Read About On The Internet?

Clients write me to ask if they should trust their pet's health to non-regulated herbs and herbal combinations peddled over the internet, that contain things like geranium extracts, garlic, black walnut, artemisia, wormwood, clove buds, ginger root, turmeric, spearmint, hawthorn berry, capsicum, lavender and tea tree extracts. Please do not do that. At best, your pet will be unprotected against heartworms. At worst, some of these extracts will damage your pet's health. Both artemisia and black walnut shells can be toxic (ref 1, 2,). Administration of a single dose of 1.75mg/pound of juglone, the active ingredient in black walnut shells can kill a mouse (ref) as can 22.5 mg/pound of alpha-thujone, the active ingredient in wormwood/artemisia. Use black walnut extracts to dye your hair or stain your furniture, thujone-free wormwood to flavor your vermouth and spices to flavor your food - not to prevent heartworms in the animals you love.

How Will My Veterinarian Check 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 commonly used one is the Idexx SNAP test that your veterinarian performs while you wait. A simpler test (regular or modified knotts test) is for your veterinarian to examine a spun drop of your pet's blood for heartworm larva. If they are there, your dog has at least 1 mature male and female heartworm. But if no larva are there, it could still have adult heartworms (immature ones, single-sex infections or parasites inhibited from reproduction) - so dogs that are negative on the Knotts test still need to have the ELISA test run. The only advantage of the Knotts technique is that it is rapid and inexpensive and, (only when positive) sufficient to make a heartworm diagnosis.

All heartworm in-office tests rely on finding substances that only mature female heartworms produce or on 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 a month or so destroying baby heartworms - even if you missed giving your pet its monthly pill.(ref)

Occasionally, the results of another blood test, a radiograph (x-ray) or ultrasound examination 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 often comes to mind.

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

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

I begin pets on ivermectin as soon as heartworms are diagnosed. This may kill immature heartworms that have not yet reached the heart as well as most larval heartworms circulating in the pet's blood. But it will not immediately cure your pet of the dangerous adult heartworms that are obstructing its heart circulation and damaging it's lungs.

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.

As of August of 2011, and sporadically before that, Immiticide became unavailable in the United States. For more information about this crisis, go here. When it returns to the US market, the following paragraphs will again be applicable. For now, your pet will have to be treated with an avermectin (most likely ivermectin). My thoughts on that are in this color font in a box, farther down this article.

Immiticide/melasomine 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 the vet 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 by addressing some of these side issues before treating the heartworms.

Three Immiticide injections are usually given. Like a boxer in the ring, the first single injection is an attempt to "soften up" the parasites and kill as many as possible, and see how the dog handles the drug. Immiticide is very irritating, so the needle is inserted deep into the muscles of your pet's back. After the injection, some of the heartworms will die and begin breaking apart. The danger now is that there is nowhere for these dying parasites to go, other than downstream into the lungs. Your pet might also be quite sore and stiff for a few days due to the painful injection. Try not to touch the sensitive injection area because it hurts; and the pet might even bite you. Ask your veterinarian for some pain control meds 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.

God-willing, everything will go well, your dog will come back to the vet in one month for two additional Immiticide injections, given 24 hours apart. Strict rest is still very important after the second and third injections.

After receiving Immiticide, watch your pet very carefully. It is common for these dogs to run a fever. It is also common for them to cough, to be depressed, and to eat less. Sometimes they will have nose bleeds as well. 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 to the Doc immediately.

The first 7 -10 days after heartworm treatment are the most dangerous. But your pet is really 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 a severe inflammation. With time, the pet's body will absorb 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 (emboli) can be fatal. The less active you pet is during this period, the less likely a fatal embolism will occur. Some veterinarians still suggest small amounts of aspirin during this period. But it use has fallen out of favor with veterinarians who study the disease.

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.

Because it is common for dogs to run a fever during this period. Keep the house cool and 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 pet :

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, not right away. So young dogs (less than 2-3 years old) and recently infected dogs usually have considerably less circulatory damage - and less risk unless they have overwhelmingly high numbers of heartworms.

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 the degree of heart damage, if any. We always hope that no damage is found. Any significant damage increases the risks 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. Other than with an overwhelming, sudden exposure to mosquitoes, the number of heartworms present in your pet likely to be most related to the number of months or years that the pet lived without monthly heartworm preventatives. Of course, dogs that live indoors in screened homes are at considerably lower risk of mosquito bites.

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. However, I see many more cases of heart worm disease in large breeds. But I associate that with their spending more time out-of-doors.

5) The Sex Of The Heartworms

It is the larger female worms that do most of the damage. In some infections, the worms are mostly the smaller males. I know of no chemical test that will tell what sex the worms are. However, a cardiac ultrasound can often see the heartworms and might 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 was 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 usually stop producing larva (microfilaria) as well. But as long as your dog continues to test positive with a test like the SNAP test, live worms are still present and 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.

Remember, we don't have Immiticide right now . So your veterinarian is quite likely to attempt the ivermectin/doxycycline/corticosteroid method even if that would not normally be his first choice.

Ivermectin is an avemectin (macrocyclic lactone). The Heartworm Society recently mailed out their thoughts on how this should be done. You can read them here.

However, their recommendations ignore some things we all already know about avermectins....

They are often absorbed best when given with a fatty meal (ref ) In the case of ivermectin products, they can be 2.6 times better absorbed when taken with a fatty meal. (ref)

Blood levels of ivermectin products are likely to stay high (therapeutic) when given more frequently than once a month (ref)

Moxidectin products are likely to work best of all against adult heartworms (until Immiticide is again available) because they stays in the body longer than ivermectin (ref). There are two approved forms of moxidectin available (1 & 2). But I personally feel more comfortable using a form designed to be given orally (ref). (That is not an FDA-Approved use for Cydectin, Equest, or Quest Gel in the United States) All moxidectin products can be toxic when given in too high a dose. There might also be issues in using this product this way in collie-type dogs.

The amount of body fat that the dog possesses is likely to affect the dynamics of the avermectin in their body (ref).

Another point is that topically applied products are more subject to failures because they are harder to apply correctly than an oral medication. (the drug needs to be on the skin, not the hair)

Discuss these points with your veterinarian before deciding what to do. Remember, certain breeds are avermectin-sensitive (ref).

This "slow kill" or "soft kill" method relies allot on the results of a 2008 study. You can read that study here. We now also know that different "strains" of heartworms are affected differently by similar concentrations of ivermectin (ref) . Something not known in 2008. This is what was found in the 2008 study in which the dogs received the medications for 9 months:



What Each Group Of 5 Dogs Received

Percent Reduction In Heartworms After 36 weeks

Ivermectin Orally every week for 36 weeks
Doxycycline (at specific times )
Ivermectin + doxycycline, orally + 3 Immiticide injections
3 Immiticide injections
Ivermectin + doxycycline, orally
No Treatment


The number of dogs in the study was small. And one can not set hard or fast rules based on a single study. However a similar 2008 study obtained similar positive results. (ref)

You can see from the results that the most effective treatment was a combination of ivermectin and the antibiotic, doxycycline. Currently, the Heartworm society recommends that heartwom positive dogs receive 5mg/pound body weight twice a day in addition to their heartworm preventative. Please read their full recommendations here. Their suggested doxycycline dose is high. Not all dogs can handle that high a dose without vomiting but most can. I know of no reason why the dose can not be divided further and given more frequently than twice a day as long as the total daily dose remains the same.

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 and that it, in some way, protects the worm. Because of this, more and more veterinarians are pre-treating dogs with doxycycline, to destroy Wolbachia prior to administering Immiticide.

What Is 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. (You can read the controversy over that here). 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 For The Adult Parasites ?

Young, female heartworms are the hardest to kill with Immiticide. So some may survive the treatment. If a follow up blood 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 dogs that still have a few residual worms 6 months after "adultcide" treatment. Should you contemplate re-treatment, you have to understand that killing the very last worm will not improve the heart damage that has occurred (if any). I usually suggest that my clients put those dogs on their monthly heartworm preventative (preferably one that contains moxidectin or selamectin) along with a year of pulsed (intermittent) doxycycline administration.

The best and most thorough recent article on heartworms and their treatment was written in 2009. Since the writing, discoveries have been made and their preferred treatment medication (melarsomine dihydrochloride) has become unavailable. But you can read their excellent article here.