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Heartworm Care Guide

Heartworm Treatment Aftercare

Caveat: The following has been developed to help our foster homes understand and feel more confident about taking their foster dogs through heartworm treatment. It is written by a veterinarian, but is not designed to substitute for consultation with, nor the advice of, your own veterinarian. Please contact your veterinarian with questions or to develop a treatment plan for your own dog.

It is a virtual certainty that anyone who is involved in rescue work long enough will have to care for a heartworm-positive dog. An estimated one-third to one-half of adult dogs rescued in the South are found to have heartworms.

Our article on Heartworm Disease explains the life cycle of the parasite causing this condition, how the dog acquires heartworms, and how it affects the dog. This article will describe how heartworms are diagnosed and treated and the special care the owner or rescuer will need to provide to the patient during treatment.

It is our policy to test all dogs for heartworms as soon as they enter our program. The test involves drawing a small blood sample, and it can normally be run in-house in the veterinary clinic with results available in just a few minutes. These antigen “SNAP” tests are very accurate. They are testing for a substance called an antigen that is produced by female heartworms and is present in the dog’s blood. Although rare, it is possible that a dog infected with only male heartworms or with very small numbers of young female heartworms will test negative even though it is infected.

The other part of the testing puzzle is the time factor. Because the heartworm undergoes a long, complicated life cycle that begins with a mosquito bite to the dog, it takes a number of months (six to seven months from the time of the bite) before the heartworms reach a stage that can be detected by the antigen test. This fact is the part that seems difficult for some pet owners to understand. If their dog got bitten by a mosquito and infected with heartworm microfilariae in May, and we tested the dog in September for heartworms, the test will be negative. However, the dog is harboring heartworms that are just not old enough yet to be detected. If we start this dog on heartworm preventative now, it will not be able to protect the dog against the heartworms that are already in his body. It will keep him from getting any “new” heartworms, but the ones that are already there are growing up to maturity and getting ready to breed and reproduce inside the dog’s heart and blood vessels. The next time this dog has a heartworm test, it will most likely be positive. That is why we recommend retesting our rescued dogs about six months after adoption or heartworm treatment.

It should also be determined whether or not the dog has microfilariae (the microscopic prelarval stage or “baby heartworms” that the pregnant female heartworm delivers into the dog’s bloodstream). Something as simple as examining a drop of blood on a slide under the microscope can often reveal the wiggling baby heartworms, although there is a better chance of identifying them using a concentration technique such as a filter test.

At some point in the treatment regimen, the dog will be started on heartworm preventative, which works by killing larval stages before they can grow into adults. Different veterinarians and clinics follow different protocols, some starting prevention as soon as the dog is diagnosed and others waiting until after adulticide treatment. A dog that is heavily infested with microfilariae should be watched carefully the first time he is given heartworm preventative. Ivermectin appears to be a safer preventative in a heavily infested dog, as it causes a slower kill of the microfilariae; brands of ivermectin include Heartgard, Iverhart, and Triheart. Milbemycin (sold as Interceptor, Sentinel and Trifexis) is a more potent microfilaricide and in a dog that is loaded with baby heartworms, there is a risk, although small, that the dog could have a shock type reaction due to a sudden kill-off of microfilariae. In such cases, the dog is probably best monitored at the veterinary clinic for the day when first given a milbemycin preventative; the dog may be pretreated with medications to prevent a reaction.

The killing of the adult heartworms, or adulticide therapy, is accomplished through a series of injections of a medication called melarsomine dihydrochloride (Immiticide). These injections are given by deep intramuscular injection into the lumbar (lower back) muscles of the dog. As was discussed in our information on Heartworm Disease, the current recommendation is to treat all dogs with a split-dose regimen of one injection, followed in about four weeks with two more injections 24 hours apart. If the dog does poorly after the first injection and has a hard time recovering, it is possible to wait as long as three months before the second set of injections. The patient is normally hospitalized at least between the two injections for the second round of treatment. Some veterinarians like to keep the dog for a day or two after treatment as well.

Once the dog has had his injection(s) and is allowed to go back home, it is absolutely imperative that his exercise is severely restricted for four weeks after both the first and the second treatment (in other words, a total of at least eight weeks). This means no running, jumping, playing, getting overly excited, overheated, or stressed out. He should go outside only for eliminations; if he tends to run wild when outside, he must be taken out on a leash even in his own yard. Although some veterinarians say the dog must be crated, this can be left up to the owner or rescuer in most cases. If the dog is comfortable in his crate and does not cry or bark or try to get out, the crate is the place to be, especially when the owner/rescuer is not home. However, if the crate is stressful for the dog, other arrangements will have to be made. Many dogs are calm enough that they will simply rest when the owner is not home.

The reason for the exercise restriction is that after adulticide treatment, the heartworms are dying and are literally breaking up inside the dog’s heart and blood vessels. This, along with the typical inflammatory reaction in the blood vessels of the lungs that is caused by the heartworms, can cause clots (emboli) to form in the vessels. To some extent, all heartworm patients will have a slight degree of thromboembolism after adulticide therapy, but in most cases it is minor and may cause a little coughing and gagging, beginning a few days after treatment. The owner/rescuer should not be surprised to note some lethargy and loss of appetite for the first week or two after treatment. Also, the dog will probably be sore over the lower back where the injections were given. Some dogs are very painful and will resent being touched in that area.

If the above symptoms are all that are seen, it should be possible to “baby” the patient along with lots of TLC; trying to coax the reluctant eater with canned food, broth or mixed vegetables mixed into dry food; providing a soft, quiet, comfortable place to rest; and being very careful about the sore back when lifting or touching the dog. Of course, the aforementioned exercise restrictions are also in place.

The caretaker should be watching for symptoms of more serious complications. Such symptoms would include a pronounced listlessness or lethargy, total refusal to eat, a persistent cough especially if it involves the production of mucus or blood, a change in the color of the dog’s normally pink gums (to pale, white, bluish, or a deep red), vomiting, or fever (normal rectal temperature is about 99.5 to 102.5 F). Rapid labored breathing and rapid heart rate (feel for a pulse in the chest behind the front leg or in the groin area) are also causes for concern. If any of these signs are seen, or if the rescuer has any concerns or questions, the treating veterinarian and one of the HSS coordinators should be contacted immediately. The vast majority of our dogs do not exhibit any of these more severe signs and come through treatment without complications.

The post-heartworm-treatment dog will most likely be sent home with medication. The owner/rescuer may be given prednisone, which is a steroid, to administer in case of coughing. With its anti-inflammatory effects, prednisone can help with the soft-tissue soreness from the injections as well. If the dog is particularly sore, additional pain medication may be sent. Prednisone should not be administered with any type of non-steroidal anti-inflammatory drug (these include Rimadyl, Deramaxx, Previcox, Metacam, and aspirin). Other medications may be called for depending on symptoms demonstrated by the patient.

Before heartworm treatment is even begun, the patient may be placed on an antibiotic called doxycycline for up to several weeks. This relatively new component of the protocol resulted from the finding that a bacteria called Wolbachia lives within the heartworm in a symbiotic (mutually beneficial) relationship. Doxycycline kills the Wolbachia, which in turn makes the heartworms easier to kill with the Immiticide and results in less lung damage as they die off. In cases where heartworm treatment must be postponed, the dog may be kept on doxycycline and Heartgard for months.

As was mentioned previously, a heartworm antigen test is recommended about six months after heartworm treatment, to be sure that the dog is clear of adult heartworms. The dog will, of course, have been on preventative for several months at least. Thereafter, a yearly heartworm test is recommended even if there is never a gap in the administration of preventative. Owners often question why this yearly test is necessary, but the fact of the matter is that all of us sometimes forget or are late giving medication (even to ourselves, let alone our pets). It is also possible that the dog later vomited up the pill, or never swallowed it and later spit it out. It is a simple blood test, normally timed with the dog’s yearly checkup, and well worth it for the peace of mind it affords.

Foster dogs are provided their monthly heartworm preventative by HSS. Once a dog is adopted, we require all adoptive homes to keep that pet on monthly heartworm preventative. It is such an easy thing to do, compared to the treatment that is described above. All of us want to avoid our beloved dogs having to undergo the difficulties and discomfort of heartworm treatment.

With thanks to Nancy Wilhelm, DVM for her generous contribution. Used with permission.