The most reliable way to protect your dog from heartworm disease is to administer a veterinarian-prescribed, FDA-approved heartworm preventative, at the recommended dose and interval, all year round. These medications are very safe at these dosages (yes, even for your Collie, Sheltie, Australian Shepherd, or other breed with a MDR1 mutation) and highly effective when given as directed.
There are about a dozen preventative drugs that are FDA-approved (see chart). All of them are made with a drug that will kill both developing heartworm larvae as well as other parasites; which other parasites are killed depends on the drug. Some of these products are given orally monthly, some are administered topically monthly, and others are given as a subcutaneous (under the skin) injection by a veterinarian, at either six-month or 12-month intervals. All of them each equally capable of protecting your dog from a heartworm infection – as long as you ensure they are given as directed, on schedule. You should use whichever drug you are most likely to reliably procure and deliver to your dog on schedule.
A further consideration may be to use whichever drug also addresses infestations of other parasites that may be a concern for your dog, or in your area. For example, if flea infestations have been a problem for your dog, using a heartworm preventative product that also kills tapeworms (which are transmitted to dogs via infected fleas) is a good idea. Dogs who have been rescued from hoarding situations or shelters may have been exposed to hookworms, roundworms, and whipworms (which are transmitted via feces from infected dogs) would benefit from the drugs that address those infections.
The Heartworm Life Cycle and How Preventatives Work
Adult heartworms are the most life-threatening of the nematodes (round worms) that infect dogs, causing severe disease and even death if not treated.
Heartworms are spread by mosquitoes. When a mosquito bites an infected dog, it ingests the tiny immature larvae (microfilariae) circulating in the infected dog’s blood stream. Over the next couple of weeks, these microfilariae mature to an infective larval stage inside the mosquito. When the now-infective mosquito bites another dog, it injects the larvae into that dog. These larvae continue to mature inside your dog as they start their journey from the point of the bite, through the body tissues, to their final destination as adult worms that reside in the dog’s pulmonary vessels and heart. This whole process takes about six months, at which time these adult worms start producing baby heartworms (microfilariae), which circulate in your dog’s bloodstream, waiting for the next mosquito to ingest them so they can leave home and infect another dog. And around it goes.
Most infected dogs end up with multiple worms, each measuring 10 to 12 inches in length, and living inside the dog’s heart and pulmonary vessels for up to seven years.
Heartworm preventatives work by killing the sensitive larval stages of the heartworm that are developing in your dog before they become adults. In other words, the preventatives don’t stop your dog from becoming infected; they kill the infection at the larval stage, before the larvae can go on to become reproducing adults. This is why preventatives must be given repeatedly at the recommended intervals, or a long-acting timed-release product used. If the larvae are allowed to mature past the sensitive, susceptible stage, the medications have little to no effect on the more mature stages. So if you miss a few doses, don’t be surprised if your dog tests positive next year, even if you got back on track with the prevention.
Natural Heartworm Prevention
Holistic veterinarians often make the case that vibrantly healthy animals are able to withstand an infection of heartworm larvae, injected into the dog by infected mosquitoes. They may further posit that the healthiest dogs are those who have been fed a home-prepared or fresh commercial diet, only lightly vaccinated, and not treated with any pesticides – and that the immune systems of these dogs will be able to eliminate the heartworm larvae (somehow). They may also recommend the use of herbal or homeopathic remedies to help the dog control a heartworm infection.
This is a great idea – but there is no scientific evidence, only anecdotal evidence from some of these practitioners, to support these claims. Studies have shown similar heartworm infection rates among wild wolves and coyotes – who eat “natural” diets and are unexposed to vaccines and pesticides – as in unprotected dogs.
How to Strengthen Heartworm Prevention
In addition to administering a heartworm preventative, you can up your game in the fight against heartworm infection for your dog by going after mosquitoes as well. According to the American Heartworm Society (AHS), concurrent use of an EPA-registered mosquito repellant like K-9 Advantix (Bayer/Elanco) or other ectoparasiticide effective against mosquitoes increases the overall efficacy of your heartworm prevention program. Eliminate or avoid any areas of standing water whenever possible, and avoid outdoor exercise with your dog during peak mosquito feeding times like dusk and dawn.
The AHS also recommends annual testing to include antigen (adult heartworms) and microfilaria testing. (Microfilaria are the first larval form of heartworms, born from a mature female heartworm after it mated with a mature male heartworm. They cannot develop further until they are consumed in a blood meal by a mosquito, but they can grow so numerous in the dog’s blood and small blood vessels that they block small blood vessels; they can also damage the lungs and liver.) At this time, most veterinary practices recommend only antigen testing, reserving microfilaria testing for those dogs who test positive for antigen, but there are several situations that can result in a negative antigen test even when a dog is infected with heartworms; the presence of microfilaria clinches a heartworm infection diagnosis. Puppies should be tested starting at 7 months of age, and annually thereafter.
Why is testing important even if your dog is on preventative and you are diligent in giving the appropriate dose, on time, all year round? Mainly because there are numerous ways that a preventative program can fail. Heartworms have been shown to develop resistance to the current drugs used to control them (see below); dogs can vomit up their prevention without this being noticed by their owner. If, for some reason beyond your control your preventative program fails, and your dog becomes infected with adult heartworms, the annual screening heartworm test will catch this infection at an early stage, allowing you to treat the infection before organic damage is done to your dog’s heart and lungs. If you never screen for heartworm disease, it will be found only when your dog starts showing signs of heart failure – and then it’s too late for a good outcome.
Drug Resistance Is a Concern
Even though it seems like there are many options to choose from for heartworm prevention, all of the FDA-approved preventatives (ivermectin, selamectin, milbemycin, moxidectin) belong to the same drug class, macrocyclic lactones – which means we have basically been treating dogs with the same heartworm preventative since 1987 when Heartgard first came out, giving the parasites plenty of time to evolve and develop resistance.
Another factor that may contribute to increased populations of drug- resistant heartworms is that once a drug is approved for use as an effective heartworm preventative, there are no guidelines in place for withdrawing that approval if resistance is documented. Hopefully this issue will be addressed along with changing the standards for getting a new heartworm preventative FDA approved.
Why aren’t there more and different classes of drugs for heartworm prevention in the United States? It may be because the FDA standards for drug approval are so stringent and difficult to achieve that it discourages drug makers from pursuing discovery of new medications and seeking approval. Fortunately, the FDA is currently working on altering its approach to heartworm preventative approval in response to the growing concern of resistance to the macrocyclic lactones. Hopefully, this means we will start seeing newer, better medications on the horizon.
Why the Heartworm Threat Is Growing
There are a number of factors responsible for the fact that heartworm infections are increasing in prevalence in dogs (and even cats) in this country and others. These include climate and environmental changes, increased movement of dogs across the country, and the growing threat of parasite resistance to heartworm preventatives.
For heartworms to thrive and maintain survival of their species, an ample number of hosts (domestic dogs or wild canines) and vectors (mosquitoes) are necessary. Factors promoting increased numbers of both include:
- Climate change: Increasing global temperatures and humidity favors mosquito population growth, as does increased incidence of hurricanes, flooding, and other wet weather phenomena.
- Environmental changes: As more and more undeveloped land is taken over for human habitation, wildlife habitats are infringed upon, forcing wild canids to move closer to areas inhabited by humans and domestic dogs, increasing the numbers of heartworm host reservoirs obviously not on preventatives. Construction frequently alters natural land drainage, creating standing water reservoirs which promote mosquito population growth.
In addition, the popularity and success of rescue and foster programs, saving unwanted and homeless dogs from euthanasia, has exponentially increased the movement of dogs across state lines. Ideally, these dogs would be tested and treated for heartworm infections before being transported, but this is not always possible. Additionally, many are tested only for adult heartworms, not microfilaria, so while technically negative for adult heartworms these dogs may still be infective to mosquitoes in their new locales. This contributes to the geographic spread of heartworm disease across the country.
And finally, parasite resistance to the currently available FDA-approved heartworm preventatives has been documented in certain areas of the United States, especially in the Southeastern states. This is a huge concern, not dissimilar to the concerns surrounding increasing numbers of antibiotic-resistant bacteria in human medicine. As resistance grows, without newer and better medications we have no defense against disease.
Resistance happens for a number of reasons. Evolution, genetic mutation, and survival of the fittest are well known reasons. Parasites evolve and genes mutate, sometimes making the organism more resistant to medication. When this happens, only the weakest, most vulnerable parasites are killed, while the genetically stronger, resistant individuals survive and multiply.
Poor compliance with recommended drug dosing and intervals is another culprit in the promotion of drug resistance. If you skip doses or give a lower-than-recommended dose of heartworm preventative medication, again, only the weakest, most vulnerable parasites are killed, leaving the strong to survive and multiply.
Don’t Get Complacent
It’s easy to become complacent about heartworm disease in dogs. You know about it, you dutifully buy preventative and give it to your dog, you have you have your dog tested every year, and the test is always negative. This can lull you into feeling like heartworm disease is no big deal. But it is a big deal. And there are factors at play right now that are setting the stage for it to get a whole lot worse (see sidebar, “Why the Heartworm Threat Is Growing,” above). So, don’t get complacent and slip up on your dog’s heartworm prevention. The damage caused by an infection, and the cost and potential side effects of treating an infection, are exponentially worse than the hassle of preventing one.
