Heartworm Treatment in Dogs
Get the worms out: Treatment for heartworm infection is critical, though risky.
Article by Mary Straus, published in the Whole Dog Journal, April 2006
Also see these related articles:
- Heartworm Prevention in Dogs
- Shortage of Immiticide for Heartworm Treatment (WDJ April 2010 and October 2011)
- Update on Doxycycline and Heartworm Disease (WDJ August 2009)
There are a number of reasons why someone may have a dog that is heartworm-positive. One of the most common is adopting a dog with an unknown history. Especially in the South where heartworm is ubiquitous, most dogs that are not given heartworm preventative regularly will be test positive for heartworm.
There is also a small chance that your dog could become infected with heartworms if you choose to extend the time between doses of the heartworm preventative you give your dog beyond what is recommended on the label, if you give less than the recommended dose, or if your dog should vomit up the pill without your knowledge. Your dog has a much higher risk of becoming infected with heartworms if you choose not to use preventatives or if you use alternative methods that have not been shown to be effective.
It is important to understand that heartworm infections are not detectable until about six months after a dog has been bitten by a heartworm-infected mosquito. (This is why it's not possible to have a heartworm test performed monthly and give the preventative only if an infection is found.) Blood tests generally will not detect heartworms in a dog until the larvae have matured into adult worms, which takes about six months following initial infection. Symptoms, such as coughing, lethargy and difficulty breathing, will not show up until the infection is advanced.
Judy Wilds, of Corpus Christi, Texas, adopted her dog, Taffy (pictured at right), in September 2004. Taffy tested negative for heartworms when adopted, and was started on a regular heartworm preventative schedule. Six months later, a routine heartworm test showed that Taffy was heartworm-positive. “Thankfully, a holistic vet had suggested that I have her rechecked six months after her first test, as it was possible (though unlikely) that she could have heartworms,” says Judy. In this case, Taffy was obviously infected when she was adopted, but had not had the infection long enough for it to be detectable.
It's a good idea to run a heartworm test on an adopted dog with an unknown medical history immediately upon adoption and then again about six months later. The same would apply if there is a lapse in heartworm preventative dosing for more than two months during the heartworm "season."
If the dog has not been given heartworm preventative recently, then a test for circulating microfilariae can also be done, which will confirm the presence of adult, breeding heartworms in the body. However, a negative microfilariae test cannot be used to rule out heartworm infection for a number of reasons.
All heartworm preventatives kill the microfilariae that are in a dog's blood, so any dog who has been given heartworm preventatives recently may be negative for microfilariae but could still have a heartworm infection. There are also cases of single-sex heartworm infection where no microfilariae are produced. A population of all male or all female worms cannot produce microfilariae. And finally, , in any dog with a very low heartworm burden -- say, just a couple of worms -- the microfilariae may not be detected (this can also cause a false negative on the antigen test).
Once heartworm infection has been confirmed, additional tests should be done to try to determine how extensive the infestation is. Radiographs can reveal inflammation and damage to the arteries and the heart, and blood tests will show whether the liver and kidneys have been affected.
Heartworm infection is divided into four or five stages (depending on the model used), based on the severity of the infestation and the age and health of the dog.
Stage 1 (mild) consists of young, healthy dogs with no symptoms and minimal changes evident on X-rays.
Moderate (stage 2) infection will show heartworm disease that is evident on X-rays, but symptoms are minimal, mostly coughing. Stage 3 is a severe infection, with weight loss, coughing, difficulty breathing, more damage visible on X-rays, along with liver and/or kidney damage.
Stage 4 and 5 are considered critical, with the dog often collapsing in shock. These dogs will not survive ordinary heartworm treatment, and must have the worms surgically removed if they are to have any hope of survival.
The current Canine Guidelines for the treatment of heartworm recommend starting dogs on heartworm preventative immediately, regardless of whether you intend to proceed with Immiticide treatment. "The logic for this approach is to kill susceptible heartworm larvae and thus prevent re-infection of the dog, while allowing less susceptible juvenile worms, the opportunity to develop into more susceptible adult worms [Immiticide won't kill juvenile worms]. . . . Administration for two to three months should result in reduced antigenic mass [the part that causes a reaction], which in turn may reduce the severity of pulmonary thromboembolism [less chance of a reaction to the Immiticide treatment]."
The best heartworm preventive medications for heartworm-infected dogs are Advantage Multi (moxidectin) and Heartgard (ivermectin).
See Update: Wolbachia below for information on using doxycycline to make all forms of heartworm treatment safer.
In each case, and indeed even if no treatment is done, there is a risk of the dog dying from a pulmonary embolism caused by worm die-off. In addition, there is risk while the worms are present of damage to the heart, the arteries, and the rest of the body, due to inflammation and immune reaction.
When deciding which method to use, you need to take into account the age of your dog, his level of activity, and the severity of the infestation.
New research has shown that it is also important to treat for Wolbachia, a parasite that lives symbiotically with the heartworms and may be responsible for much of the damage that they cause, both from inflammation and from pulmonary emboli, no matter what other form of treatment you decide to do. See below for more information.
New research indicates that giving doxycycline prior to treatment with Immiticide will make the treatment much safer.
See Update: Wolbachia below for more information.
Also see Shortage of Immiticide for Heartworm Treatment for what to do if you can't get Immiticide.
The dog must be kept confined and his physical exertion kept to an absolute minimum, in order to prevent pieces of the dead worms from being forced by a rapid heart rate and/or increased blood pressure into clogging the tiny blood vessels in his lungs, causing embolisms. This generally means that the dog must be kept crated or penned and allowed out to potty only on a leash.
Prednisone reduces inflammation and the risk of complications. Heartworm Treatment Review recommends giving " a strong anti-inflammatory dose of prednisone daily for the first seven days, then half that once daily for 7 days, then continue that lower dose once every 48 hours for 14 days."
According to the article, Are Heartworms Getting Worse?, a single injection of immiticide kills most of the male worms, but the female worms survive -- thus, a single injection kills 50% of the worms. Two injections 24 hours apart kill 90% of all worms. A split-dose schedule, where a single injection is given, followed by two injections 24 hours apart a month later, kills 98% of worms.
The article also talks about the fact that immiticide does not kill immature adult worms (L5 stage), which can later mature into adult worms after treatment. Note that Advantage Multi (moxidectin) has been found to kill immature adult worms up to five months old, making it the heartworm prevention treatment of choice for heartworm-infected dogs, or any dogs that have missed more than one heartworm prevention dose.
Heartgard (ivermectin) given monthly on a continuous basis for at least a year following immiticide treatment has some effect against these younger worms and will eventually kill them.
A safer protocol, sometimes called a “split-dose”, “staged-kill” or “three-dose” protocol, consists of giving one injection, waiting one month or more, then giving two more injections 24 hours apart. This has the benefit of reducing the worm burden by about 30 to 50 percent with the initial treatment, before the balance are killed by the second set of injections.
This protocol is more expensive, requiring three injections of the drug instead of two. Also, the dog must be kept strictly confined for a longer period of time. Nevertheless, this split-dose protocol over two months is recommended for dogs with heavy worm burdens or other health problems (Stage 3), and is also considered safer for dogs with a lower worm burden.Judy Wilds treated Taffy (described above) using the fast kill method with Immiticide. Although it is recommended to keep treated dogs in a crate to limit their exertion, Taffy was unused to being confined, and her vet was concerned that crating her could be dangerous, as excess stress and barking could cause lung problems. Wilds used a small outdoor pen for Taffy, instead.
“Taffy handled the heartworm treatment quite well. One evening, however, she gave me quite a scare when she began gagging. Later, I realized that this was related to her heartworm treatment,” Wilds says.
Some coughing or gagging is considered normal in dogs being treated for heartworm. However, it is important to understand that, whatever method of heartworm treatment is used, any respiratory difficulty in dogs receiving therapy should be considered a life-threatening emergency.
If the coughing/gagging is very heavy, seems uncontrollable, or causes the dog distress, get her to the vet right away. Vomiting or any bloody discharge combined with lethargy, fever and/or pale gums should also be considered an emergency, and the dog should be taken to the veterinary or emergency clinic immediately. Corticosteroids, fluids, and oxygen may be needed at this time to help the dog survive.
Immiticide (melarsomine) has a much lower risk of complications than its historical predecessor, another arsenic compound called Caparsolate (thiacetarsemide sodium). Unlike Caparsolate, Immiticide does not damage the liver and kidneys, and kills a higher percentage of worms, so that fewer treatments are needed.
However, the Immiticide injections will cause muscular pain and soreness for a few days. It's critical that the injections be done in an exacting fashion in order to minimize this effect. This includes changing needles after filling the syringe before injecting, choosing the site with care, putting pressure on the site after injection, and alternating sites for future injections.
Corticosteroids (e.g., dexamethasone) can be given at the same time to lessen the reaction. Combination painkiller and sedative drugs, such as xylazine, may also be used to reduce the pain of the injection. Pain medications, such as NSAIDs, are also often prescribed for a few days.
Christie Keith, a Scottish Deerhound breeder from Northern California, had two dogs who were heartworm positive. “Raven had some obvious clinical changes, and was showing some symptoms (coughing). Due to her hyperreactive immune system, I was concerned that the presence of the worms in her pulmonary arteries, with their known ability to stimulate autoimmune reactions, would further worsen her existing allergies. So I opted to do the conventional Immiticide heartworm treatment, with the thought that it would be best to get the worms out of her more quickly.”
Christie intended to treat Raven with the split-dose schedule, but a couple of weeks after the first dose, Raven, despite having been completely confined and kept from activity more strenuous than going out into a tiny potty yard on a leash, developed a series of pulmonary emboli (clots), and nearly died. "We rushed her to the ER and when we got there, my mom, who was with her in the back of the van, thought she had died. She was blue,” recalls Keith. Emergency treatment saved Raven’s life. Tests conducted after the first treatment revealed no trace of heartworms, so Raven didn't have to undergo another round of treatment.
New studies show that treatment with monthly Advantage Multi (moxidectin) or weekly Heartgard (ivermectin), combined with pulsed doxycycline, will speed up the process of killing the adult worms and lessen the danger they present.
See Update: Wolbachia below for more information.
Although this method is gentler than the use of Immiticide, the danger from the dying worms is still present, and for a much longer period. A recent Italian study showed that pet dogs (as opposed to the caged laboratory dogs this method had been tested on before) did get pulmonary emboli and some of the dogs died of it. The more active the dog, the higher the risk.
In addition, damage is being done to the arteries leading to the heart, and possibly to other areas of the body due to inflammation and immune response as long as the adult worms are present in the body. Most vets recommend using the fast kill method to treat heartworms, unless the dog’s health will not permit it or the owners cannot afford it. In those cases, or in the case of a very mild infection, the slow kill method may be used instead.
Janice Adams adopted Pepper (pictured at right), a Border Collie/Chow-mix in June 2000. A blood smear in the vet's office at the time of adoption was positive for microfilariae. “Pepper didn't seem to have any symptoms of heartworm disease, no coughing or shortness of breath, so, at the recommendation of my regular vet, I elected to start her on Heartgard monthly," says Adams.
"Pepper is a very calm dog, and didn't have any activity restrictions. I kept her on Heartgard monthly for about 18 months, then switched her to a low dose of Interceptor] on a 45-day schedule. We live in Florida, so I give heartworm preventatives year-round. All subsequent heartworm tests have been negative. She didn't seem to have any problems related to the heartworm disease or treatment.”
Christie Keith’s second heartworm-positive dog, Bran, had no radiographic changes and no allergies, so Keith opted not to do the Immiticide treatment on him. “I put Bran on Heartgard monthly. At the time, there was less research on the 'slow kill' with Heartgard than there is today, but there was enough that I felt confident it would be effective. And it was; within a few months or a year, he tested heartworm-negative.”
Veterinarians at the University of Florida announced in October 2011 that they are now offering a minimally invasive method of extracting heartworms through the jugular vein using specialized instruments. See UF veterinarians offer alternative treatment for severe heartworm cases or call the UF Small Animal Hospital at (352) 392-2235 for more information.
So-called alternative methods to kill heartworms, such as Paratox, are no safer than conventional drugs, since they rely on the exact same action -- they kill the larvae/worms in the bloodstream. It is the death of the worms that causes the greatest danger to dogs during treatment.
In addition, no studies have been done to show that alternative treatments are effective. If these treatments do have any effect, they would be comparable to the slow kill method, with the same drawback of continued damage to the body while the worms remain present.
Finally, some of the herbs used to treat heartworm are considered dangerous and may be toxic in the amounts used to try to kill the worms.
The current Canine Guidelines for the treatment of heartworm in dogs no longer recommend using high doses of Heartgard (ivermectin) to remove microfilariae. "It is both unnecessary and dangerous to use livestock preparations [high doses] of these drugs to achieve higher doses for the purpose of achieving more rapid results."
Normal monthly preventative doses of Interceptor (0.5 mg/kg) or high doses of Heartgard (50 mcg/kg, approximately eight times the preventative dose) will eliminate most microfilariae immediately. Normal monthly doses of Advantage Multi, Heartgard or Revolution will also work for this task, but more slowly, over a period of several months.Because heartworm treatments can cause an allergic reaction due to the death of the worms and the microfilariae, veterinarians will usually give dogs Benadryl and/or corticosteroids prior to each treatment. Monthly Advantage Multi (moxidectin) or Heartgard (ivermectin), at preventative doses, should not be strong enough to cause this kind of reaction, although it's a good idea to stay home the day you give it to your dog.
If Interceptor (milbemycin oxime) at normal doses, or Heartgard at high doses, is used to kill microfilariae following heartworm treatment, anaphylactic shock can occur, especially in dogs with high microfilariae counts. This treatment is best done at the veterinarian’s office under close observation for any adverse reaction.
Treatment for heartworm infection is one area where conventional veterinary medicine offers valuable options. Whether you elect to do the fast-kill method using Immiticide, or the slow-kill method using monthly Advantage Multi or Heartgard, either is preferable to leaving the dog untreated, or using unproven, alternative methods that may have no effect or even be harmful. Giving doxycycline for four weeks (repeated yearly as long as the dog is still infected) can also reduce the risk of both heartworms and their treatment, no matter what method is used.
This is also a situation where steroids and antibiotics can be lifesavers, when used judiciously during treatment.
The decision of how to treat a heartworm-infected dog is not an easy one, and is best made after consulting with your veterinarian regarding the safest method to use for your dog.
Now, new studies published in late 2008 clearly indicate that treatment with a combination of weekly ivermectin and daily doxycycline given intermittently will sterilize the heartworms, prevent the dog from being infective to other dogs, speed up the death of the worms prior to (or in place of) Immiticide treatment, limit inflammation and damage caused by the worms presence, and reduce the chance of serious adverse reaction from Immiticide treatment. All of these effects are greater when the two drugs are used together than when either is given alone.
Update: A study published in 2015 showed that Advantage Multi (topical heartworm preventive medication using moxidectin) eliminated adult worms in 8 of 11 heartworm-infected dogs in just six months. It appears to work even better than Heartgard (ivermectin) against adult worms, as well as being safe to give to heartworm-infected dogs (will not cause anaphylaxis by killing too many microfilariae at once). In addition, Advantage Multi has been found to kill older heartworm larvae that are unaffected by ivermectin or Immiticide (see next Update below). Advantage Multi is also effective against fleas. More info:
Evaluation of the Adulticidal Efficacy of Imidacloprid 10 %/Moxidectin 2.5 % (w/v) Spot-on (Advocate®, Advantage® Multi) against Dirofilaria repens in Experimentally Infected Dogs.
Update: A combination of Advantage Multi heartworm preventive medication given monthly plus Doxycycline given for 30 days initially (10 mg/kg twice a day) has been found to kill immature worms that were three to five months old at the time treatment was started. This is significant because prior to this study, there was no known treatment that would kill these immature worms (other heartworm preventive medications generally kill larva that are up to about two months old, while Immiticide is effective only against adult worms six months of age or older). This information can be used to treat dogs who have missed heartworm preventive medication for up to five months. It may also indicate that Advantage Multi could be a good choice to use for the first few months of any heartworm treatment, since it would eliminate these older larvae before they become adult worms (when combined with doxycycline). See Experimental Dirofilaria immitis infection in dogs: effects of doxycycline and Advantage Multi® administration on immature adult parasites for more information.
See Important New Information Regarding Heartworm Treatment and Doxycycline for more information.
Rescue groups, especially those located in the South, often deal with large numbers of heartworm-positive dogs. Darryl Bitter is the dog coordinator for the Frisco Humane Society, an all-breed rescue near Dallas, Texas. She estimates that 30 percent of the adult dogs that they rescue are heartworm-positive. A colleague who does Boxer rescue in the same area says that 75 percent of their rescues test positive for heartworms.
“In the last 18 months we have treated 20 dogs who were heartworm-positive," says Bitter. The dogs were treated with Immiticide, either the standard two injections in two days, or the split-dose schedule two, four or six weeks apart, depending on the health of the dog. The dogs were confined to a crate for a month following each injection, and taken outside only on leash. Prednisone and antibiotics were given during the treatment.
“Four of these dogs had bad reactions to the shedding of the worms, coughing blood and unable to breathe," says Bitter. "Typically these reactions take place two to three weeks after the injection. All four dogs were hospitalized and placed in oxygen tents for at least two days. Immediate treatment is required as soon as the dog has this reaction, otherwise she could die."
Two of the four bad experiences she's witnessed stand out particularly, says Bitter. “Bear, a Great Pyrenees, had a particularly bad reaction. His foster person came home to find pools of blood in his crate and Bear was coughing blood. Fortunately she was able to rush him to the vet and he survived after a week in hospital. The other, Dusty, a small Heeler mix, just collapsed as her foster was taking her outside on the leash. She was rushed to the vet as well and spent three days in hospital. Fortunately, she also recovered.”
Juli Thompson, who lives in Central Florida, has put two dogs thru heartworm treatment. The first dog, Oakley, was a year old when adopted. She was treated with Immiticide using the split-dose schedule, one month apart. She was kept crated and walked on leash, and recovered without a problem.
Thompson's other dog, Bo, was five or six years old at the time of his adoption, and had a more severe infestation. “He was coughing at any exertion. He went through the same treatment and then had to spend almost two months being hand-walked and low-key because the vet was very worried about clots. He survived and is still alive and very happy. He still coughs after heavy exercise due to damage done to his heart.”
Another issue that may come up with rescue dogs is when to schedule spay/neuter surgery if they are heartworm positive. Heartworm infection increases the risk of anesthesia. Although there is less risk with low worm burdens, it is usually considered best to treat the dogs for heartworm first, before proceeding with surgery six weeks or more following the final heartworm treatment.
“Dogs with pulmonary arteries blocked by adult heartworms are at greater risk for anesthetic death at the time of surgery or during recovery. The adult worms are effected by the anesthetic and will plug up distal branches of the pulmonary artery particularly the right branch,” says J. Theis, DVM, a specialist in heartworm from the University of California at Davis.
Whenever heartworm treatment is delayed, for whatever reason, it is usually a good idea to start the dog on monthly Advantage Multi or Heartgard in the meantime, which will help destroy the microfilariae and weaken the adult worms. Because there is a small chance of an adverse reaction to this treatment, it should only be done on a day when you are home to observe the dog and seek treatment if necessary. Do not use Interceptor (milbemycin oxime), which is much more dangerous to a dog with heartworms, due to its greater efficacy against microfilariae.
AHS Treatment Guidelines (see Veterinary Information)
Posts from a vet about heartworms, and about some of the considerations to take into account when deciding whether to use the slow-kill method of heartworm treatment:
Are Heartworms Getting Worse?: A seminar in 6 parts
More Heartworm Questions
Could I just give preventive medicine, instead of treating my dog for her heartworms?
Also see Heartworm Disease in Dogs: Prevention and Treatment for updated information.
You can contact me if you have any comments, but I regret to say that I can no longer respond to questions about individual dogs. See my Contact page for more information. My name is Mary Straus and you can email me at either or