Current Vet

In this episode of Current Vet, Dr. Lottie talks about heartworm disease, caused by Dirofilaria immitis. She covers how these parasites infect and cause disease in the body, the different diagnostic tests available and what the recommended treatment protocol is. She also briefly talks about the fact that cats always present differently.

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Timeline:
00:00 Intro
01:17 Case
02:15 Aetiology & Pathogenesis
07:38 Clinical Signs
08:25 Diagnosis
11:43 Treatment
16:44 Prevention
19:53 Key Points
21:46 Outro

References

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make veterinary medicine simple

Intro

Hello, hello, and welcome back to Current Vet. If you're new here, this is the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and today we are talking about heartworm disease caused by Dirofilaria immitis. It's a classic condition that is always tested in exams, and it's common in clinical practice in different regions around the world.

So definitely one that we need to be aware of. This episode is the third episode in our month of dog-focused topics, and then next week we are finishing off this month talking about pyometra in dogs.
So tune into that to round off our dog month. I also do wanna remind you guys again about the really exciting new launch we have coming at the end of this summer. I promise I will have more information on this soon, but I'm so excited and there are some more teaser trailers and more info coming out on our social media at the moment.

So if you want to see more of what I'm talking about, then find us on Instagram or TikTok. Go and have a look, and sign up for the waitlist. I will leave the link for the waitlist in the show notes of this episode.

Case

Let's start with our case. You're halfway through your consults for the day and your next appointment is Buddy, a five-year-old Labrador retriever.

His owner mentions he's been coughing on and off for a few months. Nothing too dramatic, just a bit of a cough, usually after exercise, and he seems slightly less enthusiastic on his walks lately. He's still eating, he's still wagging his tail, but he's just not quite himself. On exam, Buddy is bright and responsive.
His temperature is normal. His mucus membranes are pink. You auscultate a mild right-sided systolic murmur and slightly harsh lung sounds. There is no ascites and no obvious respiratory distress.

You practice in an area with known heartworm prevalence, so it's one of your top differentials, and you do a point of care antigen test, and it comes back positive.

Aetiology & Pathogenesis

So what is heartworm disease? Heartworm disease or dirofilaria, dirofilariasis, I can never say that, refers to infection with the nematode, the Dirofilaria immitis nematode.

Adult worms live primarily in the pulmonary arteries, and in heavy infections, they can actually extend down into the right ventricle and right atrium of the heart. Transmission of this nematode requires a mosquito vector, which explains why the disease has a higher prevalence in those warm, humid regions, because that is where we're gonna find the mosquito vectors.

A bit of an overview of their life cycle is that the microfilaria circulate in the bloodstream of an infected dog. A mosquito then ingests those microfilaria while they take a blood meal. Inside the mosquito, they develop into infective L3 larvae over about two weeks, depending on the environmental temperature.

The mosquito then transmits these L3 larvae into a new host when they bite them. The larvae then molt into the L4 and L5 stages, and immature adults migrate through the tissues to reach the pulmonary arteries several months after infection. They then mature and begin producing microfilaria at around six to seven months post-infection, which is why this time period is known as the pre-patent period in infected animals.
This timing is actually really important to understand when it comes to diagnostic testing because antigen tests detect adult female worms. So testing too early after infection has occurred can give false negatives because those worms haven't actually matured into adults yet, so those adult antigens are not gonna be able to be detected.

I do just quickly wanna touch on infections in cats. So infection in cats is quite different to that in dogs. Many of these worms don't actually mature in cats, so we're getting a reaction to immature larvae. And worm burdens are typically much lower, but they can develop really severe respiratory disease, known as HART or heartworm-associated respiratory disease.

Essentially, these immature larvae in the cats are causing severe immune responses and massive lung damage, which then causes quite acute respiratory distress. We see coughing, wheezing, open mouth breathing, and severe dyspnea.

Despite the fact that they can actually have really low worm burdens, like sometimes only literally two or three worms.

Okay, so how do these worms actually cause disease? So adult worms in the pulmonary arteries cause endothelial damage, inflammation,

and increased pulmonary vascular resistance. So over time, this is gonna cause pulmonary hypertension as the heart pumps harder to try and overcome this increased pulmonary resistance. This then leads to right ventricular remodeling and potentially right-sided heart failure in more advanced cases.

Another thing to note is that Dirofilaria immitis can also carry Wolbachia bacteria, which then causes a concurrent infection when dogs are infected with the nematode. So the bacteria doesn't do that much, but it can contribute to the inflammation caused by the worms and just worsen disease.

So dogs can be staged depending on how severe the disease is, and these are according to the guidelines provided by the American Heartworm Society. So Class I or Stage I is asymptomatic or mild cough with minimal radiographic changes. Stage II is a mild to moderate cough, exercise intolerance, abnormal lung sounds, and some radiographic evidence of pulmonary artery enlargement.

And that is going to be because we see those adult worms residing inside the pulmonary artery. Class III is significant exercise intolerance. We also see dyspnea, weight loss, right-sided heart failure signs, so things like ascites, hepatomegaly, and jugular distention, and death may occur at this stage as well.

And then Class IV is the most severe class, and it's known as Cavill syndrome.

And this is when worms completely obstruct blood flow. So we also see acute hemolysis, pigmenturia because of that hemolysis, hemoglobinuria, and shock. And this stage is an absolute emergency. So Buddy, based on our case, fits into around that Class II stage.

So dogs develop progressive vascular disease and cats typically develop inflammatory respiratory disease.

Clinical Signs

So what clinical signs are we going to see in infected dogs? We might see a chronic cough, exercise intolerance, dyspnea, weight loss if we have advanced or chronic cases, ascites if when we start to see that right-sided heart failure signs, and then syncope can occur if there is severe pulmonary hypertension.
As we said before, in cats, we're really going to be seeing that kind of coughing, dyspnea, respiratory distress. In acute cases, we might see collapse, , sudden death, or sometimes we can also see vomiting .

Diagnosis

Okay, so what are we going to do for diagnosis? There are a couple of different things that we can do. So first up is going to be antigen testing. We briefly touched on this before. So this test detects antigens produced by adult female worms. it's very specific. So positive tests are very reliable.

You can trust your positive results, but false negatives can occur for several different reasons. for example, we could have a low worm burden and the test may not detect any antigens. A male-only infection because then we don't have the females producing the antigens. It could be a pre-patent infection, which just means that the adult worms haven't yet matured, so they're not going to be detected.

Or we can have when antibodies produced by the host against the antigens bind and form immune complexes, which can then block the antigen being detected.

Heat treatment of serum samples can be considered to try and break down these immune complexes to try and improve the sensitivity of tests. But this process is not routinely recommended for heartworm testing in dogs. It is sometimes recommended in cats. The next test we've got is microfilarial testing.

A modified knot test detects circulating microfilaria, which is the immature form of that heartworm. It should be used to confirm or follow up any antigen tests that come back positive. And if the animal is antigen positive, but microfilaria negative, it's known as an occult infection.

We can then do some imaging. So thoracic radiographs may show those enlarged pulmonary arteries because remember, those worms are sitting inside the pulmonary arteries. We might see right ventricular enlargement, seen as a kind of reversed D heart shape on radiographs, and we may also see an interstitial or alveolar lung pattern.

Echocardiography could be used, and sometimes you can actually see the worms directly in some cases, but , this isn't guaranteed.

In cats, our antigen tests are gonna be unreliable because remember, a lot of those worms aren't reaching maturity, and we can have male-only infections as well. So there aren't gonna be many adult female worms to detect. The AHS, which is the American Heartworm Society, does recommend the heat treatment of serum samples to improve sensitivity of antigen tests in cats, just not really in dogs.

Antibody tests

can be used to detect host-produced antibodies against the worm, but they are gonna indicate exposure, not necessarily an active infection.

Microfilaremia, which is just the presence of microfilaria in the blood, is quite rare in cats, so testing for these microfilaria is not really that reliable for diagnosis. But we do have echocardiography and radiography, which can be useful for assessing the cardiac health and pulmonary health.

Treatment

Okay, let's talk about treatment. So the goal of treatment is to eliminate adult worms, clear the circulating microfilaria in the blood, and minimize complications associated with infection. So the current AHS recommended protocol combines three main components. We have a macrocyclic lactone preventative. We then have doxycycline, and then we have the three-dose melarsomine adulticide therapy. It's a bit of a mouthful, so let's go through all of these individually.

So the protocol starts with a preventative given right away, and that is the macrocyclic lactone. So we start an approved macrocyclic lactone at diagnosis even before adulticide injections are given. These drugs help to clear the circulating microfilaria and help to protect against new infection before we treat the adults that are currently present.

So we could use things like ivermectin, milbemycin, moxidectin, and selamectin are a couple of options there. And glucocorticoids and antihistamines can be given at the same time to reduce the risk of anaphylaxis and immune response caused by dying microfilaria in the body. It's also important to minimize activity and restrict exercise for all patients at this stage, as that can trigger respiratory distress.
We also give doxycycline for four weeks. We do this because of those Wolbachia bacteria. It's the endosymbiotic bacteria that are actually inside the Dirofilaria immitis nematodes, and it contributes to inflammation caused by the infection.

So doxycycline reduces those bacterial levels, helping to reduce inflammation, especially when the parasites are dying, and there is more immune response and more inflammation in the body anyway. Once the full doxycycline course has been given and the macrocyclic lactone drugs have been started, we then wait until day sixty to start adulticide therapy.

Okay, so let's talk about the three-dose melarsomine protocol. Before I get into this, remember that strict exercise restriction is gonna continue throughout treatment and into recovery. Even mild exercise and mild activity can increase the risk of thromboembolism when worms die, which is absolutely not what we want.
So exercise restriction is really key for these patients.

Okay, so at day sixty, we give one injection of melarsomine. We then wait for thirty days, continuing that exercise restriction, and you give one injection on day ninety, and then twenty-four hours later, you give the next injection, the third injection, on day ninety-one.

So this staged approach is gonna kill the majority of adult worms with the lowest risk of complications,
And is currently the preferred protocol. Once we've completed the adulticide therapy, we're gonna retest with the antigen and potentially the microfilaria test if needed at around six months post-treatment. Two consecutively negative tests indicate that the, the adult worms have been cleared.

And then we can continue to test yearly and make sure we're using those preventatives, those macrocyclic lactones at regular intervals as dogs can be reinfected even if they have been infected previously and treated and have been cleared of infection.

Just to briefly touch on the feline treatment recommendations, cats are very different. Basically, they're always different. The AHS guidelines for cats is that melastamine adulticide therapy is not, not, not recommended. Melastamine is toxic to cats. Also remember though that we don't actually really get that many adult worms in the cat infection.

So it wouldn't be useful for targeting adults that aren't there anyway. Instead, management really focuses on supportive care. Again, monthly preventatives, these macrocyclic lactones are essential. They reduce the severity of future infections and help to prevent new ones occurring.

We're also going to give a short course of corticosteroids that helps to control the inflammation, especially during these acute respiratory distress episodes.

We're then also going to give oxygen therapy, potentially bronchodilators to support respiration and ongoing anti-inflammatory support as well.

Prevention

Okay. So prevention should be year-round in both cats and dogs, regardless of a perceived seasonal risk associated with when mosquitoes are more active.

Because transmission depends on this mosquito exposure and mosquito seasons are becoming longer and less predictable globally due to climate change. So even in regions previously considered low risk, we are now actually seeing cases. So year-round prevention is definitely recommended.

So prevention basically relies on, again, those macrocyclic lactones. We're gonna have the ivermectin, the milbemycin, moxidectin, selamectin, and these drugs do not kill adult heartworms. That's important to remember. They're eliminating the larval stages that have been previously acquired , if they've been acquired since the previous treatment.

So they're essentially a kind of resetting the clock every month because it means that the microfilaria are never given enough time to fully mature into adults. If a dose is missed, larvae can progress beyond the point where they are susceptible to macrocyclic lactones. So this means that they can then mature even if you suddenly remember and think you're making up for that missed dose, it may be too late. The AHS guidelines currently recommend, as we said, year-round prevention, annual antigen testing, even in dogs on preventatives, just to make sure that there haven't been any missed doses, your dog secretly spat out the pill when you weren't looking, any of that nonsense.

It means that we're keeping on top of it and know what we're dealing with. And they also recommend testing before starting prevention in dogs over seven months old.

There have been some documented cases of reduced susceptibility or resistance in heartworm to macrocyclic lactones, but the majority of prevention failures are still caused by missed doses or improper administration, so current guidelines are continuing to recommend these macrocyclic lactones as the key method of prevention.

And then compliance and proper dosing, are the most important factors here

Just to briefly touch on cats again. Because there is no safe treatment and no safe adulticide protocol for cats, prevention is gonna be even more important. So these macrocyclic lactones are safe, they're effective, and they're really good for preventing infection. And again, we're gonna make sure we're doing year-round prevention, for all cats in endemic or at-risk areas.

And we're obviously gonna include indoor cats in this because indoor cats are still exposed to mosquitoes.
If you do wanna do some further reading on this topic, go and visit the American Heartworm Society website. They have really great resources and good explanations of everything we have discussed in this episode.

Key Points

So here are the key things that you need to know about heartworm before you go. Heartworm disease is caused by Dirofilaria immitis nematodes, which are transmitted by mosquito vectors.

The adult worms live in the pulmonary arteries and cause progressive pulmonary vascular inflammation, pulmonary hypertension, and then potentially leading to right-sided heart failure. Dogs are staged from Class I to IV based on the severity of the clinical signs and then also radiographic findings.

Diagnosis in dogs relies on antigen testing and can be supported by microfilarial testing. Imaging is great for staging and also assessing the severity of the effects on the cardiopulmonary system. Treatment follows the AHS protocol, so lactone prevention, four weeks of doxycycline, strict exercise restriction, and then the three-dose melarsomine protocol.

Very severe cases or caval syndrome may require surgical removal of the worms, but this is rare.
Macrocyclic lactones are key for prevention, and they target the larval stages of the nematode. Year-round prevention and annual testing is recommended for all animals living in high-risk areas. And then also remember that cats are different. They're always different.

Worm burdens are typically lower, and they cause inflammatory lung disease known as heartworm-associated respiratory disease. And then the adulticides, so the melarsomine, are not recommended because they are toxic, so treatment focuses on supportive care.

Outro

And that is heartworm. It's definitely a big topic, but breaking it down and also following the really great guidelines provided by the AHS, understanding the differences between cats and dogs is all really key.
Thank you so much for listening to this episode. If it helped subscribe, like, share the podcast with someone else who needs to hear this, and if there are any topics that you want to hear next, find us on Instagram and TikTok at Veterinary Vista and let us know what it is. See you next time