Frequently Asked Questions (FAQs)
Used with permission from the American Heartworm Society (website; http://www.heartwormsociety.org)
What is heartworm disease?
answer; Heartworm disease is one of the major health problems of dogs in the United States and throughout the temperate and tropical areas of the world. As well as being found in dogs and other species, it is now being found in cats in ever increasing numbers. The disease develops when a pet becomes infected with parasites called Dirofilaria immitis that are transmitted through the bite of an infected mosquito. Dogs may be infected by a few or up to several hundred heartworms. Cats are similarly infected although usually by only a few worms. Heartworm infection often leads to severe lung disease and heart failure and can damage other organs in the body as well.
Is only my dog at risk for heartworm infection? What about cats, ferrets or wildlife?
answer; Heartworm infection primarily affects dogs, but infection may also occur in cats. While cats may not be infected as frequently as dogs, diagnosis in cats is on the rise. Ferrets, as well as other mammals such as wolves, coyotes, foxes and even sea lions can be infected with heartworms. Outdoor pets are at greatest risk for infection, especially in regions of the world with high mosquito populations. However, even indoor pets become infected by heartworms as infected mosquitoes can get into houses. In addition the disease has been found throughout the country.
How big is my pet's risk for heartworm infection?
answer; Many things must be considered even if heartworms do not seem to be a problem in a local area. If owners travel with their pets to heartworm-endemic areas, pets will be at risk of infection. Heartworm disease is spreading to new regions of the country each year. Uncared-for dogs and certain wildlife such as coyotes, wolves and foxes can be carriers of heartworms. Mosquitoes blown great distances by the wind and the transportation of infected pets to different geographic locations all contribute to the spread of heartworm disease to areas that may have previously been considered heartworm-free. The best way for easy, safe prevention of heartworm infection is to administer a year-round heartworm preventive as directed by your veterinarian.
How is heartworm disease transmitted from one pet to another?
answer; Adult female heartworms living in an infected dog or other host release their young, called microfilaria, into the bloodstream. Mosquitoes become infected by the microfilaria while taking a blood meal from these infected animals. During the next 10 to 14 days, microfilaria mature to the infective larval stage within the mosquito. When the mosquito then bites another dog, cat or susceptible animal, the infective llarvae exit the mosquito's mouth parts and are deposited onto the surface of the animal's skin. The infective larvae can then actively enter the new host through the fresh bite wound.
Inside a new host, it takes a little more than six months for the infective larvae to mature into adult heartworms. Once mature, heartworms may live up to five to seven years, and because of their longevitygevity, each mosquito season can lead to an increasing number of worms in our pets.
What physical signs could my dog have?
answer; Heartworms may accumulate gradually over years, or quickly when conditions allow exposure to high numbers of mosquitoes carrying infective heartworm larvae. Clinical signs of disease may not be easily recognized in pets that have been recently infected or in those with low numbers of heartworms as they may not yet exhibit outward signs of disease. However, pets heavily infected with heartworms or those with chronic disease often show prominent clinical signs.
In dogs, signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss. As heartworm disease progresses, pets may develop heart failure commonly recognized by an accumulation of fluid in the abdomen giving the pet the appearance of a "swollen belly." Dogs infected with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life threatening form of cardiovascular collapse called " syndrome." Signs of caval syndrome include a sudden onset of labored breathing, pale gums and dark bloody or "coffee-colored" urine. Without prompt surgical removal of the heartworm blockage, few pets survive.
Can you tell me more about heartworm testing in dogs?
answer; Two common types of tests exist for diagnosing heartworm infection in dogs. Because adult heartworms release their young (microfilaria) directly into a dog's bloodstream, a relatively simple blood filter test can identify them. A positive test tells us adult worms are present. Positive means positive. Unfortunately, 15 to 20 percent of heartworm-positive dogs will not have "microfilaria" circulating in their bloodstream and a negative test will sometimes be falsely negative. The most accurate test for detecting heartworm infection in dogs is the antigen test. This test looks for the presence of small proteins released by adult female heartworms into the dog's bloodstream. A positive test tells us mature female worms are present. And, while false negative results are uncommon, they can occur if a pet has a "male-only" infection (since the test detects antigen from females), if only one or two worms are present, or if the female worms are immature.
Your veterinarian may have reason to suspect a negative test result to be inaccurate and might recommend re-testing using other methods. Chest X-rays and ultrasound evaluation can help identify heartworm disease and may be indicated.
How long does it take before heartworm infection can be detected by blood tests?
answer; It takes five to seven months from the time a dog is bitten by infected mosquitoes until a blood test can accurately detect the presence of adult worms.
My dog tested positive for heartworms. What does this mean?
answer; A positive microfilaria or antigen test indicates that your dog has adult heartworms in the heart and arteries of the lungs. Even if your dog is not showing any symptoms, there may already be damage to the heart, blood vessels and lungs. If not treated, this can lead to serious, even life-threatening disease. If treatment is started before symptoms are obvious, the chances are very good that your dog will not have any significant complications following treatment.
Do you need a prescription for heartworm preventive medication? If so, why?
answer; Yes, heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy. The U.S. Food and Drug Administration (FDA) labeling on heartworm preventives indicates that the medication is to be used by or on the order of a licensed veterinarian. This means a veterinarian must have a doctor-client-patient relationship in order to write a prescription. Typically, prior to prescribing a heartworm preventive, the veterinarian will perform a simple heartworm test to make sure your dog doesn't already have adult heartworms. It is not necessary to test very young puppies prior to starting preventives since it takes approximately six months for adult heartworms to develop to adulthood in a dog. If the pet is free of heartworms, prevention is prescribed. Giving preventives to dogs infected with heartworms can lead to rare but possibly severe reactions that could be harmful or even fatal to the dog.
There are many types of medications available for heartworm prevention. What is the difference between the daily and monthly tablets?
answer; Until the late 1980s, the only medication available for the prevention of heartworms had to be given daily. These products work by killing the microscopic heartworm larvae deposited by the mosquito, but must be given every single day to be most effective. This is because infective heartworm larvae quickly molt within two to three days into their fourth stage of development. This fourth stage can not be killed by the daily medication. Daily heartworm preventives have largely been replaced by monthly products and are no longer commercially available in the United States. Compounding pharmacies still formulate daily preventives on an "as needed" basis. An important note to keep in mind is that monthly medications are quickly eliminated from a pet's system and do not continue to work for 30 days. Instead they work "backwards" to eliminate the larvae the pet acquired the previous 30 days, in essence, "de-heartworming" our pets each month in many cases, these monthly preventives control other parasites too.
How do monthly heartworm preventives work?
answer; Fortunately, there are many very effective once-a-month heartworm preventives available today. Some are chewable tablets and others are topically-applied solutions. Monthly heartworm preventives, because of their ease of use and effectiveness, have become the popular choice for prevention of heartworm disease. Unlike the daily products of the past, these compounds are capable of killing developing heartworm larvae, and administering the preventive every month will effectively eliminate the chance of infection. Check with your veterinarian to see which product is right for your pet.
Why do dogs need to be blood tested before starting heartworm medication?
answer; Before starting a preventive program, all dogs should be tested for heartworms. Giving preventives to dogs that have adult heartworm infection can be harmful or even fatal to the pet.
Adult heartworms produce millions of microscopic "baby" heartworms (called microfilaria) into the bloodstream. When you give a monthly heartworm preventive to a dog with circulating microfilaria, this can cause the sudden death of microfilaria, triggering a shock-type reaction. Even if your dog does not have this type of reaction, heartworm preventives do not kill the adult heartworms (although they may shorten the worms' life expectancy). This means an infected dog will remain infected with adult heartworms.
Unfortunately, as long as a pet remains infected, heartworm disease will progress and damage the heart and lungs, which can lead to life threatening problems. Giving heartworm preventives to heartworm-positive dogs can mislead an owner into thinking everything is all right, while within a pet, heartworm disease is worsening.
How often should I have my dog tested for heartworm infection?
answer; Annual testing is recommended for several important reasons. First, many of us do not take our own medications as directed let alone medicate our pets. We're busy; we forget; we miss a dose here and there. Second, even if you never miss a dose there is nothing to prevent your dog from eating some grass and vomiting up the medication you just gave. Your pet would be without protection for an entire month. Third, if your pet accidentally became infected with heartworms, your veterinarian needs to detect it as soon as possible before irreversible heart and lung damage occur. Early detection and treatment are always best. Finally, some heartworm tests now come combined with tests that monitor other significant diseases (Lyme disease, Ehrlichiosis and Anaplasmosis). With annual testing, you know your pet is heartworm free and not infected with these serious tick-borne diseases, some of which could affect family members. The tests are quick and accurate and make sure your pet is free from infection. Annual testing provides peace of mind in knowing that your pet is free of heartworms, and should your pet be infected, it assures you of early diagnosis.
I heard that certain heartworm prevention medications will also protect against intestinal parasites. Is this true?
answer; Certain heartworm preventive products are also effective in removing specific intestinal (and external) parasites and are labeled for such uses. They either contain a single active ingredient that is effective against several parasites including developing heartworm or a combination of ingredients to achieve control of many different parasites. Such products have been tested and meet the same safety requirements as the heartworm-prevention-only products.
I heard that the heartworm prevention medication is toxic to certain breeds of dogs, particularly collies. Is this true?
answer; When given as prescribed, all of these medications are safe. It has been found, however, that some dogs are genetically predisposed to be more sensitive when doses dramatically exceed the recommended amount. Problems can occur when products designed for large animals (horses, cattle, pigs) are inappropriately used in dogs, or when dogs are dosed incorrectly. Heartworm preventives are safe for all breeds of dogs when used as directed.
Is a puppy born with immunity to heartworm disease?
answer; No, even nursing puppies are at risk for heartworm infection. Puppies of any age exposed to mosquitoes carrying infective larvae can become heartworm-infected, so it is important to begin prevention early. Puppies can be started on heartworm preventive as early as four to eight weeks of age, depending on the label recommendations of the preventive.
I live in Minnesota. How long should my dog be on heartworm prevention?
answer; For a variety of reasons, even in regions of the country where winters are cold, the American Heartworm Society is now recommending a year-round prevention program. Dogs have been diagnosed with heartworms in almost every county in Minnesota, and there are differences in the duration of the mosquito season from the north of the state and the south of the state. Year-round prevention is the safest, and is recommended.
I live in Arizona where it is very dry and there are very few mosquitoes. My vet says I should use monthly prevention. What should I do?
answer; There are different climates in Arizona, including micro-climates such as irrigated fields, backyard ponds and man-made golf courses, which affect the severity and duration of the mosquito season. We also know that areas can have heartworm infection in wild species such as coyotes, and these infected wild animals can be a source of infection to your dog or cat as well. Despite the fact that heartworm disease may not be diagnosed as often in Arizona as in some other states, it is definitely present.
The American Heartworm Society is now recommending year-round prevention, even in states like Arizona. And remember, if your dog or cat travels out of state with you or to another part of Arizona where mosquitoes are common, they may be at higher risk of exposure.
Are heartworms more common in certain areas of the United States?
answer; Heartworms have been found in all 50 states. Certain areas have a higher risk of heartworm. Some very high risk areas cover large regions, such as near the Atlantic and Gulf coasts and along river tributaries. Most states have "hot spots" where the heartworm infection rate is very high compared to other areas in the same state. Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas and presence of animal reservoirs (such as infected dogs or coyotes).
Is there an effective natural prevention for heartworm?
answer; No, there is no natural prevention for heartworms.
Is it OK to have surgery on a heartworm positive dog or cat?
answer; If surgery can be postponed, it is better to treat a dog for the heartworms first. If this isn't possible, your veterinarian can assess your dog and make appropriate recommendations. Since cat are not routinely treated for heartworm infection, your veterinarian can assess your cat and make appropriate recommendations.
How often should I have my cat tested for heartworms?
answer; Heartworm infection in cats is easily overlooked and harder to detect than in dogs. The preferred method for screening cats include, the use of both an antigen and an antibody test. Your veterinarian may also use X-rays or ultrasound to look for heartworm infection. Your cat should be tested prior to starting a preventive, and annually thereafter. Remember, since there is no approved treatment for heartworm infection in cats, prevention is critical.
What are the signs of heartworm disease in cats?
answer; Signs of heartworm disease in cats can be very subtle and misleading. Signs may include coughing, asthma-like attacks, intermittent vomiting, lack of appetite, or weight loss. Occasionally ataxia (difficulty walking), seizures, fluid accumulation in the abdomen (ascites) and syncope (fainting) have been reported as well. Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.
My cat goes outside daily. Should I put her on heartworm preventive medication?
answer; The prevalence of feline heartworm infection parallels that found in the local dog population, but at a somewhat lower rate. If heartworms are found in dogs in your area, cats are also at risk and should be placed on preventive medication. The products that prevent heartworm disease also remove intestinal parasites that are potentially contagious to family members. Intestinal parasites are a year-round problem and require year-round treatment and prevention.
My cat never goes outside. Should I put her on heartworm preventive medication?
answer; Whether living indoors or outdoors, cats can be exposed to mosquitoes carrying heartworm disease. While the outdoor cat is more likely to be bitten by a higher number of mosquitoes, mosquitoes do get into our homes and are capable of biting our indoor pets. It is important to realize that a single heartworm can cause severe consequences. Multiple studies have reported a significant number of heartworm infections in cats living exclusively indoors. If heartworms are found in dogs in your area, cats are also at risk and should be placed on preventive medication.
Can heartworm larvae spread to kittens through the mother's bloodstream?
answer; No. Heartworm larvae are transmitted from one animal to another only by the bite of a mosquito carrying the infective stage of the larvae.
What is the treatment for heartworm disease in dogs?
answer; If a dog is infected with heartworms, the treatment needs to kill the adult and immature worms. Currently, only one product is approved by the FDA for this purpose (Immiticide®- melarsomine hydrochloride). It is given by deep injection into muscle. A series of injections are given, either over a 24-hour period or two treatment periods, one month apart. While treatment may be administered on an outpatient basis, hospitalization for the procedure is often recommended. Other medications may be given at the time of treatment depending on the stage of heartworm disease.
Why is the treatment of heartworm infection so expensive?
answer; The expense of heartworm treatment is affected by multiple factors. Office visits, pre-treatment tests, hospitalization, and drug costs all contribute to the cost of controlling this most difficult parasite.
My vet said my dog has heartworm disease and is in the fourth stage. What does this mean?
answer; The severity of heartworm disease can be described as one of four stages, or classes. Determining which stage of the disease your dog is in helps the veterinarian assess the best course of treatment for the dog and can also help predict the chances of a successful, uncomplicated treatment. The least severe is stage, or Class One. Unfortunately, the fourth stage, or Class Four disease is the most severe and the most difficult to treat. These patients should be stabilized prior to treatment, and they carry the most guarded prognosis. Some cases may require a surgical procedure to remove the worms from the heart and major blood vessel bringing blood into the heart before any further treatment can be attempted. You should discuss your dog's condition and prognosis with your veterinarian immediately.
What causes the death of a dog due to heartworm disease?
answer; Heartworm disease may cause a combination of medical problems within the same dog, including heart, lung, kidney and liver disease. The worms are found in the right side of the heart, and in the major vessels that bring blood to and from the right chambers of the heart. The worms cause inflammation of the blood vessels and can block blood flow leading to pulmonary thrombosis (clots in the lungs) and heart failure. Heartworm disease can also lead to liver or kidney failure, causing death by one or a combination of these problems.
Does the age of a dog have anything to do with better success in treatment of heartworm disease?
answer; The age of the dog is just one factor to consider when treating a dog for heartworms. Your veterinarian may also consider overall health, severity of clinical signs and laboratory test results. Older dogs with long-term heartworm infections may have damaged lungs, hearts, livers, and kidneys that could potentially complicate heartworm treatment. Fortunately, if a dog has moderately healthy organs and is exercise-restricted, he should have a successful treatment regardless of age.
My dog has heartworms. My vet started her on monthly prevention before he started treatment. Is this OK?
answer; Although not labeled to be used in this manner, beginning a heartworm preventive prior to performing a heartworm treatment is a very common and acceptable practice. However, it is important that your veterinarian assesses the severity of the disease and chooses the proper preventive accordingly. By starting the prevention program you are ensuring that your dog will not get a new heartworm infection while being treated for the existing heartworm disease. Furthermore, you are helping to keep your dog from being a source of heartworm larvae (microfilaria) for mosquitoes to pick up and eventually infect other dogs. Your veterinarian's approach may also make the treatment of the existing infection more effective and is in agreement with the recommendations of the American Heartworm Society.
My dog has heartworms. My veterinarian recommends a series of injections, hospitalization, pain medications and lab work to safely treat my pet. Is there any other treatment available?
answer; Your veterinarian is recommending what is best. Only melarsomine dihydrochloride (Immiticide®), which is given by intramuscular injection, is approved by the FDA for the treatment of heartworm infection in dogs. Although there are some risks associated with its use, most adult worms die quickly and may be eliminated within one to three months. Cage rest or at least limited exercise during this period helps control excessive injury to the heart and lungs and thus reduce this risk.
The American Heartworm Society recognizes the benefits of administering a macrocyclic lactone preventive to an infected dog for one to several months prior to melarsomine treatment when the clinical presentation does not demand immediate intervention, but the Society does not recommend the long-term continuous administration of these monthly heartworm preventives as an alternative to conventional adulticidal therapy.
My dog was treated for heartworm four months ago and his heartworm test is still positive. What does this mean?
answer; After treating a dog with adulticidal injections, adult worms may continue to die for more than a month following this treatment. Heartworm antigen testing is the most reliable method of confirming that all of the adult heartworms have been eliminated. Although many dogs are antigen negative 16 weeks after adulticidal treatment, it can take longer for the antigen to be completely cleared from some dogs. Additionally, even though the adulticide is highly effective, a single course of treatment may not completely clear all dogs of infection. Heartworm antigen testing is the most reliable method of confirming if the heartworm treatment completely killed all the adult heartworms. Consequently, in most cases, a dog that is still antigen positive at four months should be rechecked two to three months later before determining whether there are still adult heartworms remaining, and a second treatment course may be required.
My veterinarian wants to treat my dog a second time. What should I do?
answer; Since adult worms may continue to die for more than a month following heartworm treatment, it can take four or more months for the remnants of the dead worms to be cleared from the dog and for your dog's antigen test to turn negative. If it's been at least six months since your dog was treated, and your dog remains heartworm-antigen positive, this usually indicates that not all of the adult worms were killed with the treatment, and a second round of therapy may be indicated.
I have been given a choice of treatments for heartworms. One is to just give monthly heartworm prevention and the other is a drug called Immiticide®. My vet said the monthly treatment is safer. What should I do?
answer; There is only one approved treatment for heartworm disease in dogs, and that is Immiticide® (melarsomine hydrochloride). Long-term administration of heartworm preventives to heartworm-infected dogs should not be considered a treatment, but rather an alternative to treatment. To discuss this more thoroughly, we must understand each of these considerations, and in either case, a veterinarian should oversee treatment.
Immiticide kills virtually all adult worms within a few days. As with any living creature, the dead heartworms begin to quickly decompose and fragment into smaller pieces. As blood flows through the heart and pulmonary (lung) arteries, the dead and fragmented worms move farther into the vessels of the lungs where they can cause inflammation and obstruct the normal flow of blood. Very small fragments may disappear within a few days, but larger pieces may remain for a prolonged period of time. The reduced blood flow coupled with inflammatory changes in the lungs can lead to loss of appetite, fever, coughing and respiratory difficulty. In situations where heartworm disease is advanced, resultant damage to the vessels and lungs can possibly even lead to death. Although most dogs treated with Immiticide® progress through treatment with little complication, even the smallest possibility that treatment could lead to death does give reason for concern.
The use of heartworm preventives in heartworm-positive dogs approaches the disease from a different angle. Preventives stop the further accumulation of heartworms in our pets and research has shown that some heartworm preventives will shorten the life span of young adult heartworms from the typical five or more years to two to three years. Unfortunately, the adult heartworms residing in the heart and lungs continue to live. By giving a heartworm preventive to infected dogs, we hope to keep a pet from acquiring more heartworms, and as heartworms die within the 2 to 3 year time period, we hope many pets will eventually be free of disease. Unfortunately, because infection often persists for several years, heartworm disease can continue to progress. As the heartworms die, similar complications can occur just as with the Immiticide® treatment, and because a normal pet may become symptomatic during this long wait, complications can still lead to death.
Most important, the use of long-term, continuous heartworm prevention should not be considered a substitute for conventional treatment of heartworms. While complications can occur during Immiticide® treatment, in asymptomatic dogs, less than 1 percent will develop significant complications.
Can heartworm larvae in an infected mother dog spread to the puppies through her bloodstream?
answer; Yes, however these infections are of little consequence to the puppies. The stage of heartworm circulating in the mother's blood are the microfilaria, the first larval stage of heartworms. The microfilaria can not develop into adult heartworms until they first pass through the mosquito. Only through the mosquito bite can a pet become infected with heartworms. Any microfilaria transmitted to the unborn puppy will be eliminated when the puppy is placed on heartworm prevention.
Is there a vaccine for heartworm disease?
answer; At this time, there is not a commercially available vaccine for the prevention of heartworm disease in dogs or cats. However, there are research scientists looking at this possibility. There are a variety of pharmaceutical products administered either orally or topically that effectively prevent the development of heartworm disease in dogs and cats.
Can ferrets get heartworm disease?
answer; Ferrets are highly susceptible to heartworm infections and can suffer the effects of heartworm disease, similar to dogs and cats.
How long after the expiration date on the heartworm package is the drug good for?
answer; As with all drugs or pharmaceutical products, heartworm preventives should be used before the expiration date on the package. The expiration date is established by a series of tests mandated by the U.S. Food and Drug Administration (or relevant international agency) to provide assurance that the potency and effectiveness of the product justifies the claims and uses on the label and that no significant deterioration of the product has taken place. It is unwise to use expired product, as it is impossible to predict if it will be effective or safe.
Is heartworm contagious from dog to dog?
answer; No. Heartworms can be transmitted from animal to animal only by the bite of a mosquito carrying the infective stage of the larvae. Heartworms release live young (microfilaria) directly into the bloodstream of a dog. When a mosquito takes a blood meal from an infected pet, it may become infected by several microfilariae. The larvae then develop into an infective stage within the mosquito. As the mosquito bites another susceptible dog or cat, the infective larvae can be left behind to cause infection. The life cycle of the heartworm requires the mosquito as an "intermediate host." Without the mosquito, heartworms can not be transmitted.
Can people get heartworm disease from a mosquito bite?
answer; Yes, it is possible. Fortunately, people are not natural hosts for heartworms. However, there have been cases in which the larvae migrate to the person's heart and lung arteries. In these cases, the larvae died before they matured into adult worms and the person's body produced "scar tissue" around the worm. This can look like a spot on the lungs on an X-ray.
According to medical reports, most people (over 50 percent) infected with a heartworm or two never had symptoms, but some people (33 percent) had chest pains, some (20 percent) had a cough, and fewer (15 percent) had a fever. About 1 percent experienced spitting up blood or difficulty breathing. Most people that are diagnosed with heartworms are "smokers." When the physician sees the white spot (called a "coin lesion") on the chest X-ray, they are also concerned that it might be primary lung cancer. Additional testing or even surgery (to remove the suspect spots) may occur. It is after surgical removal and microscopic evaluation of the spot that some of these cases have been identified as heartworm, rather than cancer. Unfortunately, there is no reliable diagnostic test for heartworm disease in humans.
Can children get heartworm disease by playing with and being licked by a dog with heartworm disease?
answer; No. Heartworm disease is transmitted only by the bite of a mosquito that is carrying the infective stage of the parasite.
I have missed two months of heartworm prevention for my dog. Should I worry?
answer; Yes, you should worry. You need to consult your veterinarian, and immediately start your dog back on monthly preventive and retest in seven months. The reason for testing seven months later is that heartworms must be approximately seven months old before the infection can be diagnosed.