Heartworm disease- how to prevent it

It’s a jungle out there – there are many different brands of products on the market these days – all claiming to be effective in preventing this deadly disease. It’s tough to know which ones to use to protect your family, which is why we’ve compiled this information so you can understand the disease and what the best way for you to protect your family is.

heartworm protection sydney



heartworm prevention

There are two main kinds of products – monthly tablets and the yearly injection.

Monthly tablets:

Of these many brands, the vast majority contain the active ingredient milbemycin oxime. Why is this important information for you, a dog owner? Because, if you are one of the millions of owners who relies on monthly tablets for preventing heartworm, your dog likely isn’t actually protected against heartworm right now.

Why we don’t like the monthly tablets…

40% of dogs in Australia infected with heartworm are actually currently on monthly preventatives and the last 3 positive cases in dogs at Southern Cross Vet have been also on routine monthly tablets.

Remembering to give the tablets at the same time each month is difficult even with the most sophisticated calendar. The trouble with missing the dose by even a few hours, on the day the tablet is due, means that if your pet gets bitten by a  mosquito containing the heartworm parasite, then they will likely be infested.

It’s almost impossible to find a tablet that will cover ONLY heartworm, most tablets available contain drugs that also kill the other worms (intestinal worms), but the other worms only need to be treated every 3 months, so you could arguably be overdosing your pet by using these products.

The science may be against monthly treatments

the most concerning thing about monthly heartworm preventative lies not in simple human error, but the active ingredient itself.
How is it possible that 40% of dogs with heartworm were supposedly protected against the deadly parasite?
The answer is simple; the preventative in question must be ineffective.


A vast majority of the oral and spot-on formulations for heartworm prevention in Australia contain the active ingredient milbemycin oxime.
An intensive multi-centre efficacy study was conducted by Holstrom et al in 2014. What they found was greatly concerning. The milbemycin oxime (MBO) was not even detectable in the bloodstream of the subjects after 14 days. This means that these animals receiving just MBO may not have had sufficient amount of the drug in their system to protect them against heartworm!

Furthermore, there have been several recent studies in Australia, New Zealand and the United States revolving around milbemycin oxime and another antiparasitic, ivermectin. Both of these drugs are a part of a group of drugs known as macrolytic lactones. They are routinely used for successful endoparasite (gastrointestinal worm) prevention and treatment in both domestic animals and livestock. What were are concerned with, however, is their use as heartworm treatments.

A study conducted by Snyder et al in North Carolina in the United States is one of these recent studies. The group conducted blind trials on groups of fourteen dogs to assess the efficacy of milbemycin oxime and ivermectin against a control group.
The study was conducted by inoculating all of the dogs with live heartworms, keeping one group untreated as the control and giving the other dogs either milbemycin oxime or ivermectin thirty days after the inoculation.
After a four-month period, necropsies were performed to assess how many heartworms persisted in both the control group (wherein the dogs would rely on their natural defences alone to deal with the heartworms) and the two treatment groups.
Not surprisingly, the control group dogs all had considerable heartworm burdens at the four-month mark.

Unfortunately, of the two groups of dogs which received either milbemycin or ivermectin, TWO dogs actually had heartworm! This means that, in theory, nearly 10% of dogs receiving either of these products marketed as a heartworm preventative may have heartworms. This study concluded that the claims of 100% protection put forward by any product with these active ingredients were inaccurate and unsupported.

But this is just one study, right? Surely these results are inaccurate.

A similar study was conducted by Blagburn et all in 2016. In their study, the team divided forty dogs into eight groups of five. One group served as control animals (group 1), one group was given a tablet with milbemycin oxime and spinosad in it (group 2), another given a tablet with ivermectin and pyrantel (group 3), the third medicated group was given a selemectin spot-on (group 4) and the final medicated group was given a spot-on containing moxidectin and imidacloprid (group 5).

Again, each dog was inoculated with live heartworms and if they were in a medicated group, they received their treatments and then again on a monthly basis until the end of the study. At the four-month mark, as with the other study, all of the control dogs still had considerable heartworm burdens.

Of all four of the monthly treatment groups, only the moxidectin plus imidacloprid group had total elimination of heartworms in each patient. The other three monthly preventative groups had either whole adult worms or fragments of worms still circulating in the patient at the end of the study despite receiving monthly “preventatives”.

In fact, this study demonstrated an even lower efficacy than the first study mentioned! Group three, who was given the ivermectin-containing product, showed that 70% of the patients weren’t adequately protected, while the milbemycin group (group 2) showed that just over half of the animals in the group were actually protected fully against heartworm.

 Is the yearly injection safe?

Most people think it must be a pretty big dose of the drug to last a whole year and therefore dangerous. While it is a whole year’s supply of the drug, it is a super-slow release with the medicine diffusing into the body each day, so in actual fact, it’s a lesser hit to the body than the monthly tablets even.

The biggest benefit of the injection is that it is only required to be given once per year in adulthood and can tie into the annual kennel cough injection, so you go to your vet only once a year. At this appointment you can pick up an annual pack to cover all the other bugs like fleas, ticks and intestinal worms.

We send you text reminders at Southern Cross Vet to let you know when your next annual injection is due



mosquito heartworm
 Proper name

Heartworm disease’s fancy name is ‘cardiopulmonary dirofilariosis’.

How long can one grow?

Heartworms can grow up to 35cm and live in all four chambers of the heart, the lungs and blood vessels

How does my pet catch it?

It is a mosquito-borne parasitic disease and it affects both dogs and cats, as well as other domesticated species and wildlife. These parasites circulate in the bloodstream and are sucked up by mosquitoes during a bite. Within the mosquito, the parasites turn into larvae, which are injected into the bloodstream of an unprotected animal.

All it takes is one bite by an infected mosquito for your pet to catch it

How do heartworms cause suffering + death?
  • Blood clots lead to strokes
  • Their growth can form a matt-like structure clogging up the heart leading to failure
  • Some animals are allergic and can have an anaphylactic reaction to their presence
Symptoms of heartworm disease

Heartworm symptoms in dogs change, as heartworms grow in number and size the functioning of the heart and lungs become affected. The worms cause an inflammatory response in the vessels that they live in. Eventually, heartworm disease leads to heart failure. Large numbers of worms can also lead to a blockage of the vessels of the heart resulting in sudden death.

Symptoms of heartworm disease range from vague signs to serious life-threatening signs of illness:

  • Coughing
  • Exercise intolerance
  • Weakness
  • Collapse, pale gums and breathing difficulties
  • Sudden death
 The trouble with diagnosing heartworm

There is a blood test available but heartworm can’t be detected in the blood until the worms have been there for six months.

This means that, without effective heartworm prevention, heartworms may be silently (and dangerously) brewing in your pet with no signs for up to five years. At this point there may be nothing that can treat your pet.

Heartworms cause disease by growing and living in the heart and lungs of our pets. They cause damage to the blood vessels, lungs and the heart itself. In some cases, the migration of the heartworm can rupture blood vessels or release blood clots which can cause a stroke, heart attack and a painful death.

When should I have my dog tested for heartworm disease:
  • If your dog is over 7 months old and has never been on prevention
  • If you have missed an oral heartworm preventative- you must wait 6 months after the missed dose before testing.



Overall, it is incredibly important that you are aware of the active ingredients being used in any of the preventatives that you give your dog. It is especially important in this case to consider the efficacy of these products, as more and more independent studies are being released demonstrating less than ideal results when it comes to monthly heartworm treatments.

The Proheart yearly heartworm injection contains the active ingredient moxidectin in a slow release form, which therefore covers the pet for an entire year rather than just one month. Even better still, the Proheart injection has what is called a “reachback” period, meaning that even if you are a couple of months late for your appointment, your dog will still be covered against heartworm.

So, not only does the annual injection mean fewer vet visits and fewer tablets forced down throats, but it also means greater protection for your dog against heartworm. In fact, the Proheart annual injection is the only heartworm preventative product available with a proven 100% efficacy rate in terms of both prevention and treatment.

If you want to discuss change heartworm products with a veterinarian, please contact us to



Sheldon Rubin, the 2007-2010 president of the American Heartworm Society, answers some questions about heartworm disease:

Q: How do dogs get heartworms?

A: Only by the bite of an infected mosquito. There’s no other way dogs get heartworms. And there’s no way to tell if a mosquito is infected. That’s why prevention is so important.

Heartworm disease has been reported in all 50 states. And the bite of just one mosquito infected with the heartworm larvae will give your dog heartworm disease.

Heartworm disease has not only spread throughout the United States, but it’s also now found in areas where veterinarians used to say “Oh, we don’t have heartworm disease.” Areas like Oregon, California, Arizona, and desert areas — where irrigation and building are allowing mosquitoes to survive. And if you have mosquitoes and you have animals, you’re going to have heartworms. It’s just that simple.

It takes about seven months, once a dog is bitten by an infected mosquito, for the larvae to mature into adult heartworms. They then lodge in the heart, lungs, and surrounding blood vessels and begin reproducing. Adult worms can grow up to 12 inches in length, can live 5-7 years, and a dog can have as many as 250 worms in its system.

Q: Can people get heartworms from their dogs?

A: It can only be passed on by mosquitoes. It’s a specific parasite that only affects dogs and cats and ferrets and other mammals. In rare cases, heartworms have infected people, but it does not complete its life cycle. The heartworm will migrate to the lung and cause a round lesion that looks like a tumor. But these are very rare cases.

Q: If one of my dogs has heartworms, can he give it to my other dogs?

A: No. Again, the only way heartworms are transmitted is through the bite of an infected mosquito. And even if an uninfected mosquito bit your infected dog, and then bit your uninfected dog the same night, he wouldn’t transmit the parasite from one dog to the other. That’s because when a mosquito bites an infected animal, the heartworm needs to undergo an incubation period in the mosquito before the mosquito can infect other animals.

Q: Is it OK to adopt a dog with heartworms?

A: It’s a very common problem in animal shelters today, and public shelters rarely have the money to treat heartworm disease. It’s perfectly acceptable to adopt a dog with heartworms, but you have to be dedicated to having the disease treated appropriately, because it’s a horrible disease that can lead to a dog’s death if left untreated.

Q: How can I prevent my dogs from getting heartworms?

A: For less than the cost of going to Starbucks for a weekly coffee, you can prevent heartworm disease in your dog. There are monthly pills, monthly topicals that you put on the skin, and there’s also a six-month injectable product. The damage that’s done to the dog and the cost of the treatment is way more than the cost to prevent heartworm disease. A year’s supply of heartworm preventative will cost about $35 to $80, depending on a dog’s weight.

Q: What are the symptoms of heartworm infestations in dogs?

A: Initially, there are no symptoms. But as more and more worms crowd the heart and lungs, most dogs will develop a cough. As it progresses, they won’t be able to exercise as much as before; they’ll become winded easier. With severe heartworm disease, we can hear abnormal lung sounds, dogs can pass out from the loss of blood to the brain, and they can retain fluids. Eventually, most dogs will die if the worms are not treated.

Q: Once my dog has heartworms, what’s the treatment? How much will it cost?

A: There are a few drug options for treatment and all are injectable. The dog is given two or three injections that will kill the adult heartworms in the blood vessels of the heart.

The safest way to treat heartworms includes an extensive pre-treatment workup, including X-rays, blood work, and all the tests needed to establish how serious the infection is. Then the dog is given the injections. With all the prep work, it can run up to $5,000.

Q: Why do I have to keep my dog quiet during the several months he’s being treated for heartworms?

A: After treatment, the worms begin to die. And as they die, they break up into pieces, which can cause a blockage of the pulmonary vessels and cause death. That’s why dogs have to be kept quiet during the treatment and then for several months afterward. Studies have shown that most of the dogs that die after heartworm treatment do so because the owners let them exercise. It’s not due to the drug itself.

Q: If my dog is diagnosed with heartworms, can I just give him his monthly preventative instead of having him go through treatment? Won’t that kill his heartworms?

A: Studies have shown that if you use ivermectin, the common preventative, on a monthly basis in a dog with heartworm disease, after about two years you’ll kill off most of the dog’s young heartworms. The problem is, in the meantime, all of those heartworms are doing permanent damage to the heart and blood vessels.

But if there’s no way someone can afford the actual treatment, at least using the preventative on a monthly basis could be a lesser alternative.

Q: Can I skip giving my dog his preventative during colder months, when there aren’t any mosquitoes?

A: The American Heartworm Society recommends year-round heartworm prevention. One reason is, there’s already a serious problem with people forgetting to give their dogs the heartworm preventatives. It’s a universal problem. Now if you use it year-round, and you miss a month, your dog will probably still be protected. But if you miss more than one or two months your dog could become infected.

The other reason not to stop is that many of the preventatives today also include an intestinal parasite control for roundworms, whipworms, or tapeworms. You want your dog to be protected against those at all times.

Q: If I don’t treat my dog with heartworms, will he “outgrow” his heartworms?

A: No. He stands a good chance of dying from the disease.

Q: I’ve heard the treatment for heartworms can be dangerous. Are there any newer, safer alternatives?

A: We used to use plain arsenic to treat it, which had many side effects. What we use now is a safer product with fewer side effects. It’s a safe product if used correctly.

Q: If my dog gets heartworms, and is treated for them, can he get them again?

A: Yes, he can get them again. That’s why prevention is so important.


Book online now or call on 95160234 to book your pet in for a heartworm injection.