Article published in the Whole Dog Journal, April 2006
Whether or not you choose to use conventional heartworm preventatives, there is always the possibility that you may one day find yourself with a heartworm-positive dog, and be faced with the decision of how to treat it. Unfortunately, there is no one right answer when it comes to heartworm treatment. Decisions must be tailored to the individual dog, taking into account both the extent of the heartworm infestation, and the dog’s overall age, activity level and health.
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 from a rescue organization. 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, 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.
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.
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. Aspirin may be prescribed to lower the risk of blood clots, though this is controversial. Remember that it’s dangerous to combine aspirin with any other nonsteroidal anti-inflammatory drug (NSAID) or with prednisone, and to give it only with food.
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 Immiticideinjections 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.
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, a Border Collie/Chow-mix in June 2000. A blood smear in the vet's office at the time of adoption was positive for microfilaria. “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.”
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.
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 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 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 Heartgard, either is preferable to leaving the dog untreated, or using unproven, alternative methods that may have no effect or even be harmful.
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.
WolbachiaAny dog that is currently infected with heartworms should be treated with doxycycline for 30 days. If the infected dog will be treated with Immiticide (fast-kill method), it is best to give the doxycycline prior to beginning Immiticide treatment, as this should make the treatment much safer, by greatly reducing the potential for embolism and allergic reaction to the death of the worms. There may also be benefit in continuing to give doxycycline during treatment. If anyone has dogs currently undergoing treatment with Immiticide, I would start doxycycline immediately, as it may be beneficial even if prior treatment was not done. Doxycycline should also be given to dogs that are being treated with monthly Heartgard (slow kill method) or any type of alternative heartworm treatment method, as it will weaken the heartworms, prevent them from reproducing, and reduce the chance of adverse effects caused by the heartworm infection itself, and by the worms dying. It appears unlikely that the Wolbachia parasite persists in the body after the heartworms have been cleared, though we do not know for certain at this time. To be safe, it may be best to give doxycycline for 30 days to any dogs that have completed heartworm treatment in the past, to clear any possible remaining Wolbachia. I was unable to find information on the recommended dosage of doxycycline for dogs with heartworm infection. Because Wolbachia is a rickettsial organism, similar to those that cause tick disease, it may be advisable to use the higher dose of doxycycline that is recommended for treatment of tick disease, which is 10 mg/kg, twice a day. However, the standard dosage, which is half that amount, may be fine. Veterinarians may contact Merial, the manufacturer of Immiticide, for more information on this topic, if needed. For links to more information, see my web site at http://www.dogaware.com/heartworm.html
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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 Pyrenes, 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 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.
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For More InformationHeartworm-positive dog requires tailored treatment AHS Treatment Guidelines (see Veterinary Information) |
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