How Often Should You Give Your Dog Heartworm Medicine?

In pets, mostly dogs, cats, and ferrets, heartworm disease can cause severe lung disease, heart failure, other organ damage, and death. The disease is caused by a parasitic worm called Dirofilaria immitis, which is spread by the bite of a mosquito. 

Dogs are the definitive hosts of the worms, as they mature into adults, reproduce, and produce offspring within the dog. 

Since the worms live inside a mosquito for a short period before becoming infective (capable of causing heartworm disease), the mosquito is the intermediate host. Infected animals suffer from heartworms since the adult worms live in their hearts, lungs, and the vessels that supply them with blood.    

Heartworm disease is most common along the Atlantic and Gulf coasts, from the Gulf of Mexico to New Jersey, as well as along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states. 

How often should I give my dog heartworm medicine?

The Centers for Disease Control and Prevention recommends annual testing for heartworm infection in dogs over 6 months old, and every 3-4 months thereafter until the animal reaches 2 years of age.

how to cure heartworm in dogs

This recommendation applies to areas where D. immitis is endemic. If your pet lives in an area free of heartworm, you may wait until he or she turns 2 years old before starting treatment.

If your pet tests positive for heartworm, we will provide you with information on how to treat him or her. You may also want to consult your veterinarian about the best way to prevent heartworm infection in your pet.

Heartworms in dogs: their lifecycle

A heartworm-infected dog releases its offspring, called microfilariae, into his bloodstream.  The microfilariae are transmitted to mosquitoes when they bite an infected dog.  Under the right environmental conditions, the microfilariae become infective larvae during the next 10 to 14 days.  Mosquitoes must pass through microfilariae before they can become infective larvae. 

If an infected mosquito bites another dog, the infective larvae are spread to the dog through the bite wound.  The infective larvae of a newly infected dog mature into adult heartworms after about 6 to 7 months.  The adult heartworms mate and release their offspring into the bloodstream of the dog, completing the life cycle.  Below is a diagram of how heartworms in dogs live.

Dogs cannot catch heartworm disease from being around infected dogs since it is not contagious.  Mosquitoes are the only source of transmission.

The lifespan of a heartworm inside a dog is between five and seven years.  The adult heartworm looks like strands of spaghetti, measuring about four to six inches in length for males and ten to twelve inches for females. 

Infectious dogs have a worm burden, which measures how many worms they contain.  In dogs, the average number of worms is 15, but that number can range from 1 to 250. 

Heartworm drugs and their risks

Neurotoxins are found in heartworm medications. The way they kill larvae is by paralyzing them. This is why your dog can also be hurt by them. If you look at the list of side effects below, you’ll see they often include neurological problems, such as ataxia, tremors, convulsions, or seizures. 

Those are just short-term results. Nobody knows the long-term risks of heartworm medication. They haven’t tested the effects of giving them to your dog for several months every year, for their whole lives.

Therefore, the most likely cause is adverse reactions that occur after taking heartworm medication.  The following are some reported side effects of common heartworm medications.

HEARTGARD And TriHeartPlus (ivermectin)
Symptoms include depression/lethargy, vomiting, anorexia, diarrhea, mydriasis, ataxia, staggering, convulsions, and hypersalivation.

INTERCEPTOR (milbemycin oxime)
All of the above plus weakness.

SENTINEL (milbemycin oxime)
Vomiting, depression/lethargy, pruritus, urticaria, diarrhea, anorexia, skin congestion, convulsions, hypersalivation, and weakness.

The topical parasiticide Revolution® (selamectin) for dogs and cats
You may experience vomiting, loose stools, diarrhea, anorexia, lethargy, salivation, tachypnea, muscle tremors, pruritis, urticaria, erythema, ataxia, and fever. Dogs have died and had seizures in some cases.

ProHeart 6 and Proheart 12
are injectable drugs that last six or twelve months. Anaphylactic reactions (severe allergic reactions) include: swelling of the face, hives, difficulty breathing, collapse; lethargy (sluggishness); lack of appetite; any change in activity level; seizures; vomiting and/or diarrhea (with or without blood); weight loss; pale gums, increased thirst or urination, weakness, bleeding, bruising; rare instances of death. 

ProHeart 6 was withdrawn from the market in 2004 due to deaths. It has since been reintroduced. They have now introduced ProHeart 12 as well! One of the scariest things about these injectables is that if your dog has a reaction, you can’t just stop giving it. Drugs can stay in the body for up to a year. 

Getting side effects from the medications will weaken your dog’s immune system as well. A weakened immune system makes him more susceptible to all diseases, including heartworms.

Heartworms Are Becoming Resistant To Meds
It might have come to your attention that heartworm medication is becoming less effective. As we use the drugs more and more, their effectiveness decreases. The number of dogs getting heartworm while on heartworm medication is increasing in the United States each year.

Do dogs need heartworm medicine all year round? In order to treat drug-resistant heartworms, AHS recommends year-round treatment. That doesn’t make sense.

It is becoming more difficult to treat heartworms, and the solution is to give more drugs? The problem would be worsened, not reduced if more drugs were given.

Dogs need protection against heartworms during mosquito season when mosquitoes might bite them and transmit the disease.

How is a Dog Tested for Heartworms?

Veterinarians use blood tests to detect heartworms in dogs. Antigen tests detect specific heartworm proteins, called antigens, which are produced by adult female heartworms and released into the bloodstream of dogs. 

It is usually possible to accurately detect infections with one or more adult female heartworms with antigen tests. About five months after a dog is bitten by an infected mosquito, heartworm proteins can be detected in its bloodstream.    

In another test, microfilariae are detected in the blood of dogs.  The presence of microfilariae in the bloodstream indicates that the dog is infected with adult heartworms (since only adult heartworms can mate and produce microfilariae). 

About six months after being bitten by an infected mosquito, microfilariae are detected in a dog’s bloodstream (because it takes about that long for the heartworm larvae to mature into adults that produce microfilariae).

When Should a Dog Be Tested for Heartworms?

A veterinarian may recommend testing a dog for heartworms at any time. However, most veterinarians prefer to wait until the dog is exposed to the risk of infection before testing.

This is because many dogs become infected with heartworms even though they appear healthy and do not show symptoms. Testing a dog for heartworms before exposing it to the risk of infection could result in unnecessary treatment and expense.

If a dog is going to be exposed to the risk of heartworm infection, then testing is recommended sooner rather than later. This is especially true for puppies, who are still growing and developing. If a puppy is born with heartworms, he or she must receive preventive treatment immediately.

The best way to prevent heartworms is through prevention. Preventing heartworms requires that a dog lives in areas where there are no mosquitoes.

The best way to protect a dog from heartworms is to keep him indoors during mosquito season.

What Is Heartworm Prevention?

Heartworm prevention is the process of protecting dogs from contracting heartworms. There are two types of heartworm prevention: preventive medications and vaccines. Both work well to reduce the chances of a dog contracting heartworms.

Preventive Medications

Preventive medications are available as pills, liquids, sprays, collars, and topical treatments. These products kill or prevent the development of heartworms in the intestines of dogs. They also help prevent heartworms from migrating up the lymphatic system and entering the circulatory system.

Preventive medications have been proven to effectively control heartworms. They are safe and effective when used properly.

However, these drugs are expensive, so many owners choose to delay or skip preventive medication. In addition, some people believe that preventive medications cause side effects such as vomiting, diarrhea, lethargy, weight loss, and skin rashes.


There are several different types of vaccines available. Some are administered orally; others need to be injected into the muscle. Vaccine manufacturers claim that their product protects against all stages of the life cycle of heartworms.

Vaccines contain antigens that stimulate the immune system to develop antibodies that attack and destroy heartworms. When given regularly, vaccines provide protection for approximately three years.

There are two major types of heartworm vaccine:

  • Live-attenuated vaccines – These vaccines use modified organisms that cannot reproduce but still trigger immunity.
  • Recombinant subunit vaccines – These vaccines use purified proteins derived from the surface coat of heartworms.

Both types of vaccines are highly effective and safe. They are easy to administer, require minimal monitoring, and are inexpensive compared to preventive medications.

What is the mode of transmission of heartworm disease between pets?

Mosquitoes play an essential role in the life cycle of heartworms. Microfilaria are tiny worms that are produced in the bloodstream by adult female heartworms that live in an infected dog, fox, coyote, or wolf.

Whenever a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which grow into “infectious stage” larvae over a period of 10 to 14 days.

When the infected mosquito bites another dog, cat, or susceptible wild animal, the infective larvae are deposited on the animal’s skin and enter its body through the mosquito’s bite wound. Heartworm larvae develop into sexually mature adult heartworms after six months in a new host.

Heartworms can live up to seven years as adults in dogs and two to three years as adults in cats. Considering the longevity of these worms, an infected pet may develop an increasing number of worms with each mosquito season.

How to Treat Heartworm in Dogs?

The most common treatment for heartworm infection in dogs is the monthly administration of a combination drug called ivermectin (Ivomec). This drug kills microfilariae, preventing them from developing into adult worms. It also prevents the production of eggs by adult heartworms.

Ivermectin is usually combined with diethylcarbamazine citrate (DEC), a drug that destroys microfilariae and adult heartworms. DEC is only partially effective at killing adult worms, however, and must be repeated every 30 days until there are no more adult worms present.

Ivermectin is relatively safe and well tolerated in dogs. The most common side effect is itching around the eyes and mouth. Other possible side effects include nausea, vomiting, diarrhea, liver toxicity, and allergic reactions.

If you suspect your dog has heartworm, contact your veterinarian immediately. Your vet will perform a complete physical examination, including a thorough history and laboratory tests, to determine if your dog needs treatment. If necessary, he or she will prescribe one of the drugs listed above.

The risk of heartworm infection in my pet is how significant?

Even if heartworms don’t seem to be an issue in your area, many factors must be considered. Heartworm disease may be more prevalent in your community than you realize—or you may unknowingly travel with your pet to an area where heartworms are more prevalent.

Every year, heartworms spread to new regions across the country. Several animals, including coyotes, wolves, and foxes, can carry heartworms, including stray and neglected dogs.

Heartworm disease is spread by mosquitoes carried by the wind as well as the relocation of pets infected with heartworms to previously uninfected areas (this happened after Hurricane Katrina when 250,000 pets, many of them infected with heartworms, were “adopted” and shipped across the country).

All 50 states have been diagnosed with heartworm disease, and risk factors are impossible to predict. Infection rates within communities vary dramatically from year to year due to a variety of factors, including climate variations and the presence of wildlife carriers. As infected mosquitoes can enter homes, both outdoor and indoor pets are at risk.

Accordingly, the American Heartworm Society recommends that you “think 12”: 1) have your pet tested for heartworm every 12 months, and 2) give your pet heartworm preventive 12 months a year.

Heartworm disease in dogs has certain symptoms.

The severity of heartworm disease is influenced by how many worms are living inside the dog (worm burden), how long the dog has been infected, and how the dog’s body is responding to the presence of the heartworms. 

Symptoms of the disease and the time when they first appear are also affected by the activity level of the dog. 

In dogs with low worm burdens, those that have recently been infected, or those who are not very active, heartworm disease may not be apparent.  Symptoms of heartworm disease are often obvious in dogs with heavy worm burdens, who have been infected for a long time, or who are very active. 

Heartworm disease can be classified into four stages.  As the class increases, the disease gets worse and the symptoms become more obvious.

An occasional cough or no symptoms are classified as Class 1.

An occasional cough and fatigue after moderate activity are symptoms of class 2.

A sickly appearance, a persistent cough, and fatigue after mild activity are symptoms of class 3.  Symptoms of heart failure may also be present. On chest x-rays, heart and lung changes are typically seen in class 2 and class 3 heartworm disease.

Caval syndrome is class 4.  A large mass of worms physically blocks the blood flow back to the heart due to the heavy worm burden.  The only way to treat Caval Syndrome is through quick surgical removal of the heartworms.  The surgery is risky, and most dogs with caval syndrome die even after surgery. 

Heartworm disease does not always lead to caval syndrome in dogs.  Leaving heartworm disease untreated, however, will cause the dog’s heart, lungs, liver, and kidneys to deteriorate, eventually resulting in death.

Heartworm disease in cats is different?

Cats have a different form of heartworm infection than dogs.  Cats’ heartworms live for a shorter period of time (only 2 to 4 years on average) and do not mature into adults as often.  There are fewer heartworms in cats than in dogs. It is very rare to find heartworms in a cat. Due to its relatively small body size, a cat with only a few worms is still considered heavily infected.

In cats, infective larvae mature into adult heartworms after 7 to 8 months and produce microfilariae.  Approximately one month longer than in dogs.  Cats rarely have microfilariae in their bloodstreams. 

There is only 20 percent of cats with heartworm disease have microfilariae in their blood, compared to 80 to 90 percent of dogs.  In cats, microfilariae are inconsistent and short-lived in the bloodstream.   

Heartworm infections in cats are harder to detect than in dogs.  Generally, veterinarians use two types of blood tests in combination to check a cat for heartworms. Positive test results may not necessarily indicate an active infection, while negative test results do not necessarily rule out heartworm infection. 

X-rays and an ultrasound of the heart are some of the tests veterinarians use to determine if a cat has heartworm disease, in addition to the results of both blood tests.

If my cat tests positive for heartworms, what should I do?

Cats can also be infected with heartworms, just like dogs. Diseases can differ in their nature and in how they are diagnosed and treated, however. The cat is not an ideal host for heartworms, which is why some infections heal on their own, although these infections can damage the respiratory system.

A cat’s immune system is also affected by heartworms in the circulatory system, leading to symptoms such as coughing, wheezing, and difficulty breathing.

Cats may even have heartworms that migrate to the brain, the eye, and the spinal cord. When the adult worms die in a cat’s body, severe complications such as blood clots and lung inflammation may result.

Here’s what you can expect if your cat tests positive for heartworms:


Dogs with worms in their heart or lungs may have 30 or more, but cats usually have six or fewer and may even have just one or two. In dogs, the severity of heartworm disease is proportional to the number of worms, while in cats, just one or two worms can cause serious illness. A diagnosis can be complicated, requiring a physical exam, an X-ray, a complete blood count, and several types of blood tests. An ultrasound may also be necessary.


Drug therapy for heartworm infection in cats is not approved, and the drug used for dogs is not safe for cats. Even so, cats with heartworm disease can often be treated with good veterinary care. Your cat’s health will be stabilized and a long-term management plan will be established.

Monitor your cat

Despite spontaneous clearing of heartworms in heartworm-positive cats, the damage caused by them may be permanent. Chest X-rays every 6 to 12 months may be recommended if your cat does not show signs of respiratory distress but worms have been detected in the lungs. Small doses of prednisolone may be administered to help reduce inflammation if mild symptoms are noted.

Provide veterinary care

If the disease is severe, additional support may be required. A veterinarian may recommend hospitalization in order to provide therapy, such as intravenous fluids, drugs to treat lung and heart symptoms, antibiotics, and general nursing care. Heartworms can sometimes be surgically removed.

Maintain prevention

Cats that have developed heartworm disease have demonstrated that they are susceptible to heartworm infection, which is true of both outdoor and indoor cats. You should give your cat monthly heartworm preventives, which are available as spot-ons and pills. If an infected mosquito bites your cat again, preventive measures prevent new infections.

If my dog tests positive for heartworms, what happens?

Dog owners do not want to hear that their dogs have heartworm, but the good news is that most infected dogs can be successfully treated.

It is important to stabilize your dog if he is showing signs of disease, then kill all adult and immature worms as effectively as possible while minimizing side effects.

If your dog tests positive, the following may occur:

Confirm the diagnosis 

When a dog tests positive on an antigen test, a second, and different, a test should be conducted. The heartworm treatment regimen is both costly and complex, so your veterinarian will want to make sure the treatment is absolutely necessary.

Restrict exercise

If your dog is accustomed to being active, it might be difficult to adhere to this requirement. Physical activity must be restricted as soon as the diagnosis is confirmed since physical exertion increases the rate at which the heartworms cause damage to the heart and lungs. If the symptoms are severe, your dog should have less activity.

Stabilize your dog’s disease

It may be necessary to stabilize your dog’s condition before heartworm treatment can begin. It can take several months for a dog to recover from heartworm disease or another serious condition.

Administer treatment

After your veterinarian determines your dog is stable enough to undergo heartworm treatment, he or she will prescribe a treatment protocol that includes several steps. Guidelines are available from the American Heartworm Society for developing this plan.

Dogs with no symptoms or mild signs of heartworm disease, such as coughing or exercising too much, have a high success rate with treatment.

Treatment for more severe diseases can also be successful, but complications are more likely. As heartworm disease progresses, the severity of symptoms does not always correlate with the number of worms, and dogs with many worms may not display symptoms for some time.

Test (and prevent) for success

Your veterinarian will perform a heartworm test approximately 9 months after treatment is complete to confirm that all heartworms have been eliminated. For the rest of your dog’s life, you will want to administer heartworm prevention year-round so that he does not contract heartworm disease again.

If my ferret tests positive for heartworms, what should I do?

However, ferrets are extremely susceptible to heartworms. The nature of the disease as well as how it is diagnosed and treated differently.

Ferrets are highly susceptible to heartworms. Ferrets’ immune systems are also affected by heartworms in the circulatory system, causing symptoms such as coughing, wheezing, and difficulty breathing, even sudden death.

Other symptoms of ferrets include fluid in the lungs, decreased appetite, weight loss, paralysis of the hind legs, or an enlarged abdomen. When ferrets have heartworm disease, they often have bilirubinuria (dark urine).

If your ferret tests positive for heartworm, here are some things you can expect:


One ferret has been found to have up to 14 heartworms, but even ferrets with one worm can suffer serious health complications. An accurate diagnosis can be difficult, requiring a physical exam, an X-ray or ultrasound exam, a complete blood count, and several types of blood tests.


The drug used to treat heartworm infections in dogs is not safe for ferrets, and there is no approved drug therapy for heartworm infection in ferrets. Veterinary care can often help ferrets with heartworm disease. Stabilizing your pet and determining a long-term management plan are the objectives.

Monitor your ferret

Infected ferrets usually show clinical signs of heartworm disease. A chest X-ray every 6 to 12 months may be recommended if worms have been detected in the lungs. To reduce inflammation, small doses of prednisolone can be administered if mild symptoms are observed.

Provide veterinary care

Additional support may be necessary if the condition is severe. If you require therapy, such as intravenous fluids, your veterinarian may recommend hospitalization
Patients receive fluids, lungs and heart medications, antibiotics, and general nursing care. In rare cases, heartworms may be surgically removed.

Maintain prevention 

Ferrets are very susceptible to heartworm disease and infection can have devastating effects. Indoor and outdoor ferrets are at risk and should be on monthly preventive medication for life. If an infected mosquito bites your ferrets again, prevention prevents new infections from developing.

What should I know about heartworm testing?

Heartworm disease is a serious, progressive condition. Pets that are diagnosed and treated early have a higher chance of recovering. When a dog, cat, or ferret is infected with heartworms, there are few or no early symptoms, so a heartworm test administered by a veterinarian is important for detecting their presence.

A small blood sample is all that is needed for the test, and it detects the presence of heartworm proteins. Heartworm tests are processed in some veterinarians’ hospitals, while others send them to a diagnostic laboratory. Both methods yield quick results. The results may warrant further testing.

Can dogs be treated for heartworm disease?

Melarsomine dihydrochloride (trade names Immiticide and Diroban) is an arsenic-containing drug approved by the FDA to kill adult heartworms in dogs. Dogs with stabilized class 1, 2, and 3 heartworm disease are treated with this drug by deep injection into the back muscles.

FDA-approved Advantage Multi for Dogs (imidacloprid and moxidectin) gets rid of microfilariae in the dog’s bloodstream. Advantage Multi for Dogs is applied directly to the dog’s skin.

A dog’s heartworm treatment is not easy on the owner’s wallet or on the dog.  The treatment can potentially be toxic to the dog’s body, causing life-threatening blood clots in the dog’s lungs.  The treatment is expensive since it requires multiple visits to the veterinarian, blood tests, x-rays, hospitalization, and injections.    

Prevention is the best treatment!

FDA-approved products are available to prevent heartworms in dogs.  Veterinarian prescriptions are required.  In most cases, the products are applied topically to the skin or are taken orally every month.  Chewable tablets are available as well as non-chewable tablets. 

It is injected under the skin every 6 or 12 months, and only a veterinarian can administer it.  Many commercial heartworm preventives contain ingredients that also work against intestinal worms (such as roundworms, hookworms, fleas, and ticks). 

Preventing all year long is the best option!  Decide which preventive is best for your dog with the help of his veterinarian. 

Heartworm disease in cats is possible?

Despite not being as susceptible to infection as dogs, cats can also contract heartworms after being bitten by an infected mosquito.  Because cats do not thrive inside their bodies, they are not natural hosts for heartworms.  Cats living indoors or outdoors are at risk for heartworm disease.

Is Treating Heartworm Painful?

Yes, it is painful. The pain is caused by the heartworms trying to get out of the dog’s body. They push on the wall of the heart and lungs, causing inflammation and swelling. This causes the heart and lungs to work harder than normal to pump blood around the body. If left untreated, this condition can result in organ damage and ultimately death.

Do senior dogs need heartworm medication?

All dogs over 6 years old should receive heartworm prevention. Older dogs are at greater risk of developing severe complications if they do not take medications regularly. In addition, older dogs are less likely to develop immunity to the heartworm parasite, which means they could contract heartworm again later in life.

Is it necessary to give my dog heartworm medicine every month?

Preventatives for heartworm are usually given as a monthly dose and come in a variety of forms. Among these are monthly pills that you can hide in his food as well as topical treatments that you apply to his skin to keep mosquitoes away.

For how long can my dog go without heartworm medication?

When a dog goes more than six weeks without heartworm prevention, the dog is at risk of infection. Heartworm prevention can cause a shock-like adverse reaction in dogs with mature infections (in which heartworms are mating and producing larvae in the bloodstream).

Is heartworm medicine necessary during the winter?

More and more dog owners are asking us if we recommend continuing heartworm prevention during the winter months as winter descends upon New England in full force. Yes, heartworm prevention is recommended during the winter months. Heartworm prevention should be year-round according to the American Heartworm Society.  


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