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Canine Heartworm Primer


The following Canine Heartworm Primer was written by Kathleen Cole (kwcole@awod.com) and is reproduced here with her permission. Kathleen is a vet tech in South Carolina and is involved with Second Time Around Aussie Rescue. Kathleen has a lot of wonderful information on her rescue site.


Canine Heartworm (Dirofilaria immitis) is a growing epidemic in the dog population of the United States. Each year regions not previously considered to be in "heartworm country" are being added to the maps of heartworm afflicted areas. It is now recommended that dogs through-out the nation be on year-round heartworm preventative to protect them from this deadly parasite. Although it has been reported in other animals as well, even man, dogs and other canids are the natural hosts through which the heartworm completes it's life-cycle.

Understanding of the life-cycle of the heartworm is necessary for it's proper prevention and treatment. It takes about 6-7 months for a heartworm to complete it's life cycle, although it may live for up to five years within the heart of the host dog. Heartworms grow between 4 to 12 inches in length, and as many as 250 adults may infest a dog's heart. The right side of the heart is where the mature adult heartworms live, absorbing nutrients from the blood flowing through the heart. Their presence interferes with efficient pumping of the heart, putting a burden on the heart and taking years off the dog's life, ultimately resulting in agonizing heart failure and death.

Mature males and females mate in the heart and the females produce live young called microfilariae, up to 5000 a day per female. These microfilariae circulate through the dog's bloodstream as well as travel through body and organ tissues where they can cause cumulative microscopic tissue damage. These microfilariae can live in the host for up to three years. When the intermediate host, a mosquito, bites an affected dog and takes a blood meal, the ingested microfilariae become infective larvae within 10-48 days. The larvae then travel to the mouthparts of the mosquito. When the mosquito bites another dog, the larvae are deposited upon the skin and then burrow into the dog. They undergo several changes in form (molts) during 3-4 months, finally becoming small immature adults. At that point the heartworms find and enter a vein, make their way to the heart, and become sexually mature. There they mate and produce microfilariae for transmission to another host.

Several brands of heartworm preventative are now commonly available through veterinarians. "Filaribits" uses diethylcarbamazine as the active ingredient. It is given daily as a chewable tablet that most dogs enjoy like a treat. It also comes in a "Plus" version that additionally prevents intestinal parasites such as roundworms, hookworms, and whipworms.

The brand "Heartguard" was the first once-a-month preventative, and uses ivermectin as the active ingredient. It also comes in a "Plus" version with intestinal parasite control. WARNING: Ivermectin has been reported to produce side-effects and deaths in some dogs in the collie family. Australian Shepherds are included in the collie family and several Aussies have been documented as suffering adverse Ivermectin reactions. Many Aussies have been given Heartguard preventative without any reactions, but there is a risk of deadly reaction, especially at high dosage levels when ivermectin can cross the blood-brain barrier. Please inform your veterinarian and all Aussie and collie-type owners of this risk (see Aussie Times Jan-Feb 1995 pgs 67-71). NEVER give ANY dog ivermectin livestock-wormer! Dogs of any breed can easily die from overdoses since proper doses for heartworm prevention in dogs are much less per pound of body weight than used to worm livestock!

Update February 2/2002 - Cause of Ivermectin sensitivity in Collies has been determined at the College of Veterinary Medicine, Washington State University...check out their website

Another monthly heartworm preventative, "Interceptor", uses milbemycin oxime as the active ingredient. This product also prevents intestinal parasites and is reported to produce fewer reactions in general than ivermectin and is generally regarded as safer to use for collie-type dogs (Aussies). This is also the active ingredient in the heartworm/flea combination product "Sentinel". The first topically applied heartworm preventative is "Revolution", containing selamectin. Given monthly, the product is applied to the skin, where it is absorbed into the body to prevent heartworms. This product also prevents fleas, ticks, sarcoptic mange and ear mites.

All preventatives are equally effective against heartworms as long as they are administered in the correct dosage for the dog's weight and at the proper interval (daily or monthly). Regardless of daily or monthly preventatives, it is imperative that the dosage be appropriate for the dog's weight and given at the correct time. Under-dosing or skipping doses allows larvae present in the dog to mature into adult heartworms.

Although the southern regions have long, perhaps continuous mosquitoes seasons, even dogs in northern and mountainous regions should be on heartworm preventative year-round. The preventatives kill heartworm larvae as they pass through their vulnerable periods (molts) through-out their 3-4 month larvae stage. That is why dogs should never be taken off preventative during winter months even though mosquitoes may not be present. Even in winter, the larvae picked up in late summer or fall are still in the dog and must be killed by the preventative during their susceptible periods. The daily or monthly preventative you give your dog today will kill the larvae he was infected with up to four months ago. Don't risk a dog's life to save a few dollars!

WARNING: Never give a dog heartworm preventative unless it had been tested and found to be clear of heartworms. Giving a heartworm-positive dog any preventative could lead to serious reactions including death by anaphylaxic shock. Always have a new dog checked for heartworms by your veterinarian BEFORE giving any type of heartworm preventative, regardless of what any former owner or shelter may tell you about the dog's heartworm status. Remember that with people and their pets moving all over the country, you might have heartworm positive dogs in your area regardless of where you live!

Always have dogs checked annually for heartworms, no matter how regularly and accurately preventative has been given. Some dogs gain weight without owners noticing, and when owners fail to adjust the preventative dosage for current weight, dogs are under-dosed. Sometimes people forget to give a dose here and there. Sometimes dogs spit out their pill when nobody looks. And sometimes dogs get sick and throw up their preventative. These situations can allow heartworm larvae to become adults. Annual heartworm checks are important!

There are three main types of heartworm tests in use today, with differing degrees of accuracy. All of them test for the presence of mature adult heartworms. Therefor it is pointless to test any dog less than six months of age, the minimum time for adult heartworms to develop. Simply put a puppy or dog less than 6 months old on preventative and have them tested for heartworms at a year of age.

The cheapest and least accurate heartworm test infers the presence of heartworms by examining a single drop of the dog's blood for microfilariae under a microscope. If microfilariae are found, the dog definitely has adult heartworms. However, this test may give a false negative up to 25% of the time for several reasons. Some dogs may only have adult heartworms of a single sex, therefor producing no microfilariae, or the adults are not yet sexually mature. Some dogs make antibodies that destroy any microfilariae the adult heartworms produce. Also, a single drop of blood may simply be too small a sample to detect microfilariae in low concentrations. This test should only be considered a screening tool, and currently is usually used only at animal shelters to identify dogs that definitely have adult heartworms. A negative result of the single-drop test still requires a more accurate test before declaring the dog heartworm clear and putting it on preventative.

A more accurate test for adult heartworms is commonly known as the di-fil test, where a larger sample of blood is examined for microfilariae. Several ccs of blood are drawn and mixed with a special solution, then passed through a filter material which allows blood cells to pass but traps any microfilariae present in the sample. The filter is stained and examined under a microscope for microfilariae. This test is much more accurate than the single drop test in detecting circulating microfilariae due to the larger sample size, but some false negatives can still occur due to strong antibody reaction against microfilariae, or single-sex or immature adult heartworm populations.

The most accurate heartworm tests available to date involve testing the blood sample for the presence of heartworm antigens. This test is called an antigen or occult ("hidden") test. It is also the most expensive, so many shelters can not afford to administer this test. When taking in a dog of unknown or questionable preventative history, request that your veterinarian use this test to confirm any shelter screening. Because monthly heartworm preventatives kill any microfilariae produced by present adult females and sterilize their reproductive capacity, this is the only test to use on a dog which has been on monthly preventatives. A negative test result is a definite sign that the dog has no adult heartworms present at the time of the test. Occasional false positives can occur due to the blood chemistry of some individual dogs, but test manufactures generally will run accurate follow-up confirmation tests at no charge to rule these out. Therefor, if you have a dog who does test positive with the antigen test, request a confirmation test to make certain. You don't want to put a dog unnecessarily through the risks of heartworm treatment.

At present there is no way to detect any developing larvae or sexually immature heartworms that may be in a dog's system. This is why there are cases where a dog may test negative for heartworms even with the antigen test, be given preventative at the correct dosage and schedule, and months later test positive for adult heartworms. Immature adults will not show up on any test, and are immune to preventatives. A later test will find these as mature adults, so it is suggested that all dogs older than 6 months be re-checked for heartworms about 6 months after first being put on preventative.

As long as the dog is on the proper dosage of preventative, no more larvae from subsequent mosquito bites will develop into adults. Monthly preventatives also kill microfilariae and sterilize the female adults so they no longer produce microfilariae. Under these circumstances where it is thought the number of adults is small and there are no circulating microfilariae to cause additional damage, some owners and vets may choose to avoid putting a dog through risky heartworm treatment and allow the adult worms present to die on their own over several years. These dogs are kept on preventative so that no new heartworms develop. This choice should only be made under advisement with your veterinarian. Remember that all program dogs must be heartworm clear to be adopted out. This choice is NOT an option for Second Time Around Aussie Rescue, Inc. program dogs.

Once you have ascertained that a dog definitely has heartworms, treatment to kill the heartworms is the next step. An organic form of arsenic is used to kill adult heartworms, in an amount designed to be strong enough to affect the heartworms but not strong enough to kill the dog. Because the liver is the organ that detoxifies poisons from the dog's system, it is important to make certain that the dog's liver is healthy enough to handle the job. Before liver functions were tested, fully 50% of all dogs treated for heartworms died of liver failure or arsenic poisoning. Even without problems it may take up to a year for the liver to fully recover from the treatment. Since removing anesthesia from a dog's system is an additional burden on a liver recovering from heartworm treatment, some vets recommend that all elective surgeries such as spay/neuters be done at least 6 weeks prior to any heartworm treatment, and avoid elective surgeries if possible for about a year after treatment. Other vets may reverse the order (fearing heart complications if they do surgery on heartworm-positive dogs), but allowing time between procedures for the liver to recover is very important.

The health of a dog's liver is determined by running a CBC & Chemistry blood test to look for elevated liver enzymes. (The dog may also be x-rayed to assess the condition of other organs.) If the blood results show elevated liver enzymes, the treatment should not be given until the liver can be made healthier. A common method of restoring the liver is to give a course of antibiotics (such as Delta Albaplex) plus a daily dose of Kayro Syrup. After a month the blood is checked again and if liver enzymes are normal the treatment can begin. If antibiotics and Kayro Syrup have not brought the liver enzymes down far enough and your dog has microfilariae present, your veterinarian may consider killing the microfilariae as an aid to the liver's recovery. Typically this is done with a dose of ivermectin under observation at the office, so that any side-effects can be promptly treated, and a blood test is run again in three weeks. Whenever the liver enzymes in the blood test within normal ranges, the treatment to kill the adult heartworms can begin.

To begin the conventional treatment to kill adult heartworms, four injections of organic arsenic (Caparsolate) are given intravenously over two to four days, depending on the schedule your veterinarian prefers. Alternatively, a product called Immiticide is administered in two intramuscular injections over two days, and is reputed to be safer for dogs with existing liver problems. Immiticide is becoming the product of choice and soon Caparsolate may no longer be available despite it's lower cost.

Several days after treatment the heartworms will begin to die, starting with the smallest ones. As they die the worms may wriggle about and cause the dog some discomfort and nausea, so monitor the dog closely and consult your veterinarian regarding anything out of the ordinary for your dog. As the heartworms die they are flushed out of the heart and into the lungs, where they lodge in small blood vessels. Any excitement or physical exertion which increases the heart rate can send clumps of dying heartworms into the lungs where they can block off the pulmonary artery, resulting in death of the dog. Therefor, the dog is to be kept quiet and confined for the six weeks following treatment, as the heartworms die and are individually pushed into the lungs. There the dog's white blood cells take up to a year to totally break down and digest the bodies of the dead heartworms.

After six weeks of rest and confinement while the adult heartworms die off, the dog is re-checked for microfilariae. If any are found the dog is given a treatment to kill them, usually a dose of ivermectin, and stays a day at the veterinarian's office for observation where any side effects can be immediately treated. Three weeks later the dog is checked again to see if microfilariae are still present. A second dose of ivermectin may be give. Whenever the dog no longer has microfilariae, it may be started on heartworm preventative.

Because heartworm antigens continue to circulate in the dog's bloodstream for up to four months after all heartworms have been killed, the antigen tests will produce positive results for that same length of time. It is suggested that you wait about six months before running any antigen test on a treated dog. If after six months past treatment the antigen test is positive, do not be alarmed. Studies have shown that with about 10-20 % of heartworm treated dogs, one to four of the largest female heartworms manage to survive the treatment. It is inadvisable to consider retreating the dog due to the stress on the dog's liver and the fact that the worms will likely require a stronger arsenic dose than the dog might be able to withstand. It is recommended that these dogs be maintained on monthly preventative so that microfilariae are not produced by these few surviving females, which will die on their on within a few years without causing noticeable harm to the dog.

If at any time you feel that your local veterinarian may not have the latest information or experience regarding heartworm prevention, testing, or treatment (especially if you live in an area not considered prone to heartworms) don't hesitate to call around to other vet offices to ask about their experience with diagnosing and treating heartworms. Consider calling your closest veterinary school to see who they might refer in your area. Don't forget that our Rescue Reps are valuable information resources, especially those of us in the southern regions prone to heartworms.

Hopefully, this primer has given you a lot of basic heartworm information and answered most of your questions on the topic. Should you ever have to treat a dog for heartworms, this information should help you better understand and communicate with your veterinarian about the procedures. Remember that even if you feel heartworms are not a problem in your area that you need to worry about, your next rescue Aussie could have lived in Florida before his previous owners moved to your town. You just never know, so be careful, and good luck!


Health Warnings (information for all canines)

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