Heartworm disease: The ‘heartworm’ and the mysterious cough

The cardiopulmonary thriller that begins with a bite

Some diseases make their presence known immediately: fever, diarrhoea, fatigue… and you think, “Something’s wrong here.” Then there’s heartworm disease (also called filariasis or “heartworm“), which is just the opposite: an infection that can take hold for months without showing any signs. The problem is that, by the time it finally manifests itself, it has often already done some of its dirty work in the cardiopulmonary system (heart + lungs). In veterinary medicine, we refer to it as a chronic cardiopulmonary parasitic disease, with progressive lesions in the pulmonary vasculature and an impact that can range from negligible to potentially fatal, especially if there is a high parasite load. And here it is worth backing this up with data: in a major epidemiological update in Spain (with 9,543 samples), the overall prevalence of Dirofilaria immitis in dogs was 6.47%, with areas of the Mediterranean coast ranging from 5–10%. This is not just “theory”: these are real percentages of positive dogs in our environment (which is why prevention is not a whim). (research results)

In this article, I explain, with a very clinical but easy-to-understand approach, how the disease progresses in stages, what signs usually appear at each stage, and why it is sometimes confused with bronchitis, asthma, or degenerative heart disease. The idea is that you can recognise patterns and, above all, understand why a simple test at the right time can change everything. In addition, you will find practical advice integrated into the text itself: small everyday actions that, when done right, greatly reduce the risk.

When the enemy travels in a mosquito: what really happens inside the dog

Heartworm disease is caused by a nematode (a “roundworm”) called Dirofilaria immitis. Infection does not occur through contact between dogs, licking, or the park itself: it occurs when an infected mosquito bites and deposits infective larvae. From a veterinary point of view, the mosquito acts as a biological vector: it is not just a “carrier”, but the organism essential for the parasite to complete part of its development before reaching the dog. In other words, if there are no competent mosquitoes, there is no effective transmission.

Here’s an important term: pre-patent period. It sounds very technical, but it’s simple: it’s the time between infection and when the disease can be detected using standard methods (for example, when there are detectable antigens or microfilariae in the blood). In heartworm disease, this period is usually several months (often around 6 months), and during that window, the dog may appear perfectly healthy. It is as if the parasite is ‘setting the stage’ behind the scenes. In clinical practice, this explains why annual screening is so strongly emphasised: because the absence of symptoms does not mean the absence of disease. (advice from official authorities)

During this process, the larvae mature and, over time, the adult parasites are found mainly in the pulmonary arteries and, in more severe infections, also in the right heart. And that’s when things get serious: it’s not just “a worm inside”, but a series of progressive lesions in the vessels and circulation. In a nutshell, the presence of the parasite induces endothelial inflammation (the endothelium is the “inner layer” of the vessels), promotes proliferative endarteritis (thickening and persistent irritation of the arterial wall) and triggers pulmonary vascular remodelling. Put simply, the blood vessels in the lungs become irritated, thickened and lose elasticity, like a hose that is narrowing and hardening on the inside. There are even technical infographics that summarise development and survival times: in dogs, adults can live for 5–7 years and microfilariae can persist for up to 3.5 years, which explains why an infection does not “go away on its own” quickly. (physiopathological data summarised)

At the same time, there is an invisible player that is increasingly mentioned in veterinary medicine: Wolbachia, a symbiotic bacterium that lives in association with these parasites. When the organism reacts to the parasite (or when the parasite dies), components related to Wolbachia can amplify the inflammatory response. In simple terms: there is a “bacterial companion” that can increase inflammation and, therefore, damage. This detail, which sounds academic, has practical implications: it helps to understand why the clinical management of the disease must be methodical and not improvised.

A practical tip that fits here, and one that is often forgotten: prevention is not just about “giving a pill”. It is also about reducing exposure to mosquitoes with common sense measures, especially at dusk and dawn (when many mosquitoes are most active). Avoid long walks in damp areas at these times, install mosquito nets if possible, remove standing water from terraces or gardens (plant pot saucers, buckets, outdoor drinking troughs), and use specific repellents for dogs recommended by your vet (note: human products can be toxic to them). These are small details, but they add up. And, if you want a very specific piece of information that usually speaks for itself: the FDA itself describes the “average load” in infected dogs as around 15 worms, but it can range from 1 to 250; in other words, the disease can range from almost imperceptible to massive. (official facts and figures)

Phase 1 — ‘Nothing is happening here’… but it is already happening: the subclinical stage

In the first few months after the bite, you will most likely not see any symptoms. In veterinary terms, we say that the animal is in a subclinical (or asymptomatic) phase: the disease exists, but it does not yet “translate” into visible signs. In medical terms, there may already be initial alterations in pulmonary microcirculation and a discreet immune response, but the dog continues to live a normal life.

At this stage, diagnosis depends on testing. Two key words here are:

• Antigen: this is a molecular “fingerprint” of the parasite (especially adult females) that some rapid tests can detect in blood. Put simply, we are not looking for the worm directly, we are looking for its “signature”. Technically, many of these tests work like ELISA immunoassays (a technique that detects specific proteins), although they are available to users as “rapid tests”. In terms of diagnostic performance, reviews have estimated average sensitivities of around 78% and specificities of around 97% (this may vary depending on the test and context), which explains why other techniques are used to complement it in doubtful cases. (published diagnostic statistics)
• Microfilariae: these are the circulating larval forms that can appear in the blood and be detected with specific techniques. If they appear, they are very helpful, but their absence does not rule out infection 100% (there are situations in which there may be few parasites, only males, or a particular immune response). In consultation, the modified Knott test can be used, for example, which ‘concentrates’ the sample to better see these microfilariae. In other words, it is a way of increasing the probability of finding them if they are present in low numbers. And to understand how common “hidden infection” (without detectable microfilariae) is, there are prevalence studies where approximately 1 in 4 positives were hidden cases (≈ 26.5%), which fits perfectly with the idea that “be careful, a negative microfilariae test is not a free pass”. (field data on hidden infection)

The trap in this phase is psychological: since the dog is fine, there is no urgency. And that is precisely where the value of screening lies. Heartworm disease is one of those diseases where prevention and screening win hands down over “we’ll see”. In fact, many veterinary protocols recommend a combination of: (1) periodic testing, and (2) preventive prophylaxis with appropriate products, especially in areas where there are mosquitoes and circulation of the parasite. European guidelines emphasise that prevention using macrocyclic lactones (a family of preventive antiparasitics) should be administered monthly during the transmission season and, in addition, that certain repellents (e.g. permethrin formulations in dogs) can demonstrate a repellent effect against mosquitoes for weeks, adding an extra layer of protection. Translated: infection can be “blocked” on two fronts, internally (prevention) and externally (repellency), always with appropriate veterinary products. (official European recommendations)

A practical method for clients that works very well is to make prevention a routine: associate prophylaxis with the same day of a fixed habit (e.g., “first Sunday of every month”) and set a reminder on your mobile phone. This is not silly: intermittent forgetfulness is one of the most common causes of gaps in protection. And if you travel with your dog to warmer or more humid areas (or to regions with a higher presence of mosquitoes), discuss this with your vet: sometimes the seasonal risk changes and it is advisable to adjust the prevention schedule. Sometimes, too, the risk is not just “in your area”, but across the map: in Spain, expansion and heterogeneity due to climate and environmental factors has been documented, making prevention a sensible decision even for those who “have not seen cases nearby”… because the disease does not ask permission to appear. (documented epidemiological expansion)

Phase 2 — Dry cough, fatigue and signs that seem to be ‘something else’: the moderate clinical stage

Over time, the problem becomes visible. When the adult parasites establish themselves and the body reacts, signs begin to appear that usually point to the respiratory tract or heart, but in an insidious way (another useful technical term). Insidious means that it appears gradually, without a sudden onset, and therefore becomes normalised: ‘it must be age’, ‘he’s tired’, ‘he must have caught a cold’.

Typical symptoms at this stage are:

A dry, persistent cough, especially with exercise or excitement. Sometimes it is a cough that does not “sound like mucus”, but rather like irritation. In veterinary language, this may correspond to secondary bronchitis or irritation due to alterations in pulmonary circulation; it is sometimes accompanied by a faster or shallower breathing pattern.

An intolerance to exercise: shorter walks, stopping sooner, panting easily. This is related to changes in the pulmonary circuit and the progressive increase in pressure in the arteries of the lung.

Here comes an important concept: pulmonary hypertension. It is not general “high blood pressure” as in humans, but an increase in pressure within the pulmonary circulation. Translated: the right heart has to push blood into vessels that offer increasing resistance, and that wears it out. Over time, the right ventricle can become hypertrophied (thickened) and tricuspid insufficiency (a valve that does not close properly) can develop, which in consultation can translate into a murmur detectable with a stethoscope. In other words, the heart is working “against the tide” and some valves may begin to fail.

There may also be gradual weight loss or reduced performance, even with a normal appetite. And, as all of this is very similar to chronic bronchitis, asthma, tracheal collapse, or degenerative heart disease, the risk of confusion is real.

In clinical practice, the key is not to “guess” but to confirm. Because if you treat what you think is bronchitis and the cause was heartworm disease, not only will you not improve, you will lose valuable time. From a veterinary diagnostic point of view, in addition to blood tests, chest X-rays (to assess lung pattern and heart size), echocardiography (an “ultrasound” of the heart that allows you to see structure and function, and even, on occasion, images compatible with parasites), and tests that assess general health (complete blood count, biochemistry, urine). For the general public: these are tests that help to determine whether the heart and lungs are suffering, and to what degree. Veterinary reference manuals emphasise that antigen testing is the preferred method for routine screening and for confirming suspicions, precisely because of its good balance between practicality and diagnostic performance. (veterinary clinical reference)

A practical tip: if your dog is coughing or fatigued and you are waiting for an appointment, avoid ‘testing’ them with exercise to see if it ‘goes away’. In cases of suspected cardiopulmonary disease, the general recommendation is to reduce exertion until a clear diagnosis is made. And at home, you can observe a simple, very useful piece of information without obsessing over it: the respiratory rate at rest. When your dog is asleep or very calm, count its breaths for 30 seconds and multiply by 2. If you notice a sustained increase from its normal rate, this is a sign to mention it to your vet (it does not replace a diagnosis, but it provides real information). In addition, field studies often identify classic risk factors that help to “suspect earlier”: living in a rural environment, spending more time outdoors and certain age groups increase the likelihood of exposure, which is consistent with what we see in our practice. (risk factors described in studies)

Stage 3 — When the damage is already severe: advanced disease and cardiovascular involvement

If left untreated, heartworm disease can progress to a severe condition. Here we are no longer talking just about coughing and fatigue, but about a real impact on circulation and, in some cases, on survival.

At this stage, signs consistent with right-sided congestive heart failure appear. It sounds intimidating, but the idea is clear: the right side of the heart is unable to pump blood to the lungs properly, causing congestion and the body to ‘accumulate’ fluid where it shouldn’t. At the veterinary level, jugular distension, hepatomegaly (enlarged liver due to congestion) and, in chronic cases, loss of body condition can be observed.

A very representative sign is ascites, which is the accumulation of fluid in the abdomen. Sometimes the belly enlarges without the dog “gaining weight,” and apathy, laboured breathing, and weakness may be noticed.

Syncope (fainting) or collapse may also occur, especially with exertion or stress. Ultimately, this is the result of poor circulation.

And there is a word that is mentioned with respect in advanced heartworm disease: pulmonary thromboembolism. This means that “plugs” (clots and/or material associated with inflammation and parasites) form or move, obstructing the blood vessels in the lungs. In plain language: parts of the lung stop receiving blood properly, and this can suddenly destabilise the animal. In veterinary medicine, the risk of thromboembolism becomes particularly relevant during certain phases of treatment (when parasites die), which is why professional protocols place such emphasis on careful management and, above all, strict restriction of exercise during critical periods. In simple terms: running when the system is fragile increases the risk of complications.

In extreme cases, there is a condition known as caval syndrome, a critical situation associated with a heavy parasite load that acutely compromises circulation and can be fatal if not treated quickly. It is not the most common, but it is the most dramatic face of a disease that, paradoxically, began “without symptoms”. In this situation, very marked signs may appear (extreme weakness, pale mucous membranes, collapse), and it is considered a veterinary emergency.

Here is some clear and honest practical advice: if you notice rapid abdominal swelling, fainting, laboured breathing or sudden and severe weakness, do not wait until tomorrow. These are signs of a serious cardiopulmonary problem and require urgent veterinary assessment. 🚑

Mini anatomy lesson: why is it called “heartworm” if the lungs are also affected?

Because the adult parasite can be found in the heart (especially on the right side) when the load is significant, but much of the damage occurs in the pulmonary arteries. It is a cardiopulmonary problem, not exclusively cardiac. In more technical terms, the main involvement is usually the pulmonary vasculature, and the right heart suffers as a result of this increased resistance.

Think of it as a motorway (lung arteries) that is narrowing and becoming rigid due to chronic inflammation; the engine (right heart) has to work harder to push the blood. At first, it can cope. Then it becomes fatigued. And if the process continues, serious signs appear.

“Practical application”: when should you sound the alarm 🚨

If your dog coughs for no apparent reason, tires more easily, performs less well or shows signs of weakness, it doesn’t deserve a “it’ll pass” attitude. It deserves clinical judgement: ruling out the important stuff, starting with what is silent but serious. In consultation, we often talk about differential diagnosis, which means “making a list of possible causes and confirming them with tests” instead of sticking with the first hypothesis. Simply put: it’s not dramatising, it’s good medicine.

And here comes the reassuring part: in many cases, a test is the first step towards shedding light on the situation. It doesn’t hurt, it’s not complicated, and it can prevent the disease from being discovered when there is already advanced damage. And if the test is positive, your vet will decide (depending on the animal’s condition) what additional tests are necessary to stage the disease, i.e. to determine “where it is on the path”. This staging approach allows for safer and more tailored treatment.

As a practical tip, if you live in an area with mosquitoes or spend time in humid/coastal areas, ask your clinic about the most appropriate preventive schedule and products that not only prevent but also repel bites. And if you adopt a dog or do not know its preventive history, an initial test is a very sensible way to get off to a good start. In fact, North American and European guidelines insist that, even when prevention is done well, it is advisable to maintain control and testing strategies because the longevity of the parasite (years) means that “a bad season” can have long-lasting consequences. (preventive and educational criteria)

Conclusion: a discreet disease… that should not be underestimated

Heartworm disease is not obvious at first. It is patient. It takes its time. And that is precisely why we must beat it at its own game: early diagnosis, prevention and surveillance. Because when the signs appear, sometimes what we see is not the beginning… but the end of the “silent” phase.

If you take away one idea, let it be this: when faced with unexplained respiratory symptoms or fatigue, it is better to confirm than to assume. And if your vet suggests an annual test in a risk area, it is not “routine”: it is good preventive medicine, the kind that avoids nasty surprises. 🙂 In short, heartworm disease is a perfect example of why modern veterinary medicine insists so much on prevention: because treating it early is safer, more effective and often cheaper than treating it late.

Montoya-Alonso, J. A., Morchón, R., García-Rodríguez, S. N., Falcón-Cordón, Y., Costa-Rodríguez, N., Matos, J. I., Rodríguez Escolar, I., Carretón Gómez, E. (2022) – Expansion of Canine Heartworm in Spain – Animals (Basel) – National epidemiological study (9,543 samples) estimating an overall prevalence of 6.47% and linking positivity to climatic and lifestyle factors – https://pmc.ncbi.nlm.nih.gov/articles/PMC9137490/

• Vieira, A. L., Vieira, M. J., Oliveira, J. M., Simões, A. R., Diez-Baños, P., Gestal, J. (2014) – Prevalence of canine heartworm (Dirofilaria immitis) disease in dogs of central Portugal – Parasite – Prevalence and risk factor study (304 dogs, 2009–2011) discussing the role of asymptomatic animals and epidemiological risk – https://pmc.ncbi.nlm.nih.gov/articles/PMC3927308/
• Rohrbach, B. W., Patton, S. (2013) – Effects of diagnostic test accuracy and treatment efficacy on the occurrence of suspected failure of heartworm prophylaxis in dogs – Journal of Veterinary Internal Medicine – Review/modelling with diagnostic performance figures (mean sensitivity 78.2% and specificity 97.3% in antigen testing) and their impact on false positives and “suspected failures” – https://pubmed.ncbi.nlm.nih.gov/23663198/
• ESCCAP (European Scientific Counsel Companion Animal Parasites). (2025) – ESCCAP Guideline 01 (Seventh Edition – June 2025): Worm Control in Dogs and Cats – ESCCAP – Evidence-based technical guide for helminth control in dogs and cats (includes Dirofilaria immitis and control/prevention measures) – https://www.animalshealth.es/fileuploads/user/PDF/2025/Junio/guia-esccap-2025-control-Parasitos-Perros-Gatos.pdf
• U.S. Food and Drug Administration (FDA), Centre for Veterinary Medicine. (2022) – Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease – FDA (Animal Health Literacy) – Official informational resource with specific figures (e.g., parasite load range 1–250; longevity 5–7 years in dogs) and basics of testing and prevention – https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease
• Zoetis Services LLC. (2023) – Heartworm Lifecycle Infographic (ZPC-02970) – Zoetis – Infographic on the biological cycle and “diagnostic blind spot”; includes development and survival times (adults 5–7 years; microfilariae up to 3.5) – https://www.zoetisus.com/content/_assets/docs/PDFs/ZPN/Heartworm-Lifecycle-Infographic-ZPC-02970.pdf
• American Heartworm Society (AHS). (n.d.) – Heartworm Basics – American Heartworm Society – Official educational page for owners (transmission, clinical signs, prevention and testing; includes the “Think 12” approach) – https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics
• Ames, M. K. (2025) – Heartworm Disease in Dogs, Cats, and Ferrets (Dirofilariasis) – MSD Veterinary Manual (Professional Version) – Clinical and pathophysiological review (endarteritis, pulmonary hypertension, diagnosis, treatment, and prevention); revised/updated April 2025 – https://www.msdvetmanual.com/circulatory-system/heartworm-disease/heartworm-disease-in-dogs-cats-and-ferrets

15 unpublished anecdotes (to be inserted into the article)

(1–5) Results of veterinary scientific studies (with data)

1. Spain, but in the cat version: in a huge sample of 6,588 cats, the “picture” changes depending on the type of test: 0.5% positive for antigen (more “confirmed” infection) versus 9.4% seropositive for antibodies (exposure). And what is most striking for the owner: only 5.8% received regular prophylaxis… in other words, the majority were “without an umbrella” in the middle of a storm.
2. Source: https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2022.900371/full
3. Barcelona and the myth of the “city cat“: in 758 cats in the metropolitan area, the overall seroprevalence was 11.47% (antibodies) and the antigen only 0.26%. The twist: indoor-only cats also tested positive (7.05%), and outdoor cats skyrocketed (21.2%). In addition, of those who were seropositive, 24.1% had respiratory signs… the typical “feline asthma” that is sometimes not asthma.
4. Source: https://link.springer.com/article/10.1186/s13071-014-0506-6
5. The “failed infection” that leaves a scar: in an experimental model of HARD (Heartworm-Associated Respiratory Disease), a group of cats had no adult worms at necropsy… yet still showed severe lung lesions when immature worms reached the lung (approx. 70–90 days post-infection). Uncomfortable detail: they could not be clearly differentiated from other cats by X-rays, antibodies, blood count or BAL at different times. Translation for the public: in cats, the clinical picture can be misleading even if there are “no adult worms”.
6. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC5688437/
7. Large-scale PCR: when DNA speaks: in a molecular survey (qPCR) of 2,334 samples in the US, D. immitis DNA was detected in 6.3% of dogs (68/1080) and 0.3% of cats (4/1254). A very curious figure: the average number of copies (16S rRNA) in dogs was around 1,809,604 per 200 μl of blood, while in positive cats they found a single copy in each (a sign of very low loads or “borderline” detection).
8. Source: https://link.springer.com/article/10.1186/s13071-022-05459-5
9. Resistance: the day prevention ceased to be “automatic”: a classic study on resistance to macrocyclic lactones mentions the escalation of reports of “lack of efficacy” communicated to the FDA: from several hundred (1999–2000) to almost 1,000 (2000–2001) and more than 1,500 (2002–2003). Even more disturbing, they were able to confirm the heritability of a resistant isolate despite challenges with ivermectin (challenge dose of 24 μg/kg monthly for 3 months). For the owner, “doing it right” (schedule + monitoring) matters more than ever.
10. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC4228187/

(6–8) “Your vet advises” (Eurovet) — practical, integrable, backed up

6. Your vet advises (Eurovet): “Annual testing even if you already use prevention“. It seems redundant, but it is not: CAPC emphasises that all dogs, even those on preventive treatment, should be tested annually for antigen and microfilariae; and in areas of high exposure, even twice a year. This fits perfectly into the article as a preventive “golden rule” for absent-minded owners or frequent travellers.
7. Source: https://capcvet.org/guidelines/heartworm/
8. Your vet advises (Eurovet): “In cats, the diagnosis is made by accumulating clues.” A key point for clients: in felines, many tests may fall short, so it makes sense to combine medical history + imaging (X-ray/ultrasound) + serology, depending on the case. There are current clinical reviews that explain why feline diagnosis is a “puzzle” (and why it is advisable to seek prompt treatment if there is coughing, episodic vomiting or unusual breathing).
9. Source: https://todaysveterinarypractice.com/wp-content/uploads/sites/4/2024/06/TVP-2024-0708_Feline-Heartworm.pdf
10. Your vet advises (Eurovet): “Treatment = protocol + real rest“. For the public, a forceful and useful phrase: if a dog is undergoing treatment, “relative rest” does not mean a long walk “because it gets sad”. The AHS guideline summaries emphasise a protocol-based approach (including the three-dose adulticide regimen with melarsomine 2.5 mg/kg IM and mosquito control measures). This is the perfect place to remember that success depends as much on medication as it does on management at home.
11. Source: https://ufl.pb.unizin.org/app/uploads/sites/51/2021/02/2024-AHS-Canine-Heartworm-Guidelines-Summary.pdf

(9–15) “Did you know?” / scientific note / curiosity / historical fact (7 pieces)

9. Medical fact (Gran Canaria, “natural laboratory”): in a historical comparison, it was reported that the prevalence in dogs in 1994 was 67.02% and in 2020 15.81%; in cats, seroprevalence went from 33.03% (2010) to 17.20% (2020); and in humans, incidence fell from 18.66% (2008) to 8.27% (2020). It’s a brutal block for a section like “prevention works, but the risk doesn’t go away.”
10. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC11273938/
11. Curiosity with numbers (cat shelters, USA): in a study of 2,232 shelter cats, the national prevalence of D. immitis antigen was 0.55% (and in the northeast it reached 1.29% in the reported subsample). In addition, they collected ectoparasites: 1,120 (mostly fleas). This is a good example to highlight that shelters are epidemiological sentinels.
12. Source: https://www.mdpi.com/2673-6772/4/4/29
13. Historical fact (1626, the first ‘spoiler’): the first known description of ‘something similar to Dirofilaria’ is attributed to Francesco Birago in 1626, referring to worms in hunting dogs. An interesting detail to open the article with a sentence such as ‘this has been with us for 400 years’.
14. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC3901479/
15. Scientific note (risk maps, Europe by month): it is no longer just a question of “endemic area yes/no”. Recent studies have constructed monthly transmission risk maps for Europe using eco-computing tools, precisely to adapt prevention according to season and region. Ideal for a paragraph such as: “the calendar is not the same for everyone”.
16. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC12751232/
17. Iberian curiosity (the tiger mosquito on the radar): in Spain, there has been discussion about the role of Aedes albopictus as a potential vector for animal filariae, including Dirofilaria immitis. This piece fits in with “ecology rules”: if the vectors change, the risk maps change.
18. Source: https://scielo.isciii.es/scielo.php?pid=S1135-57272012000400002&script=sci_arttext
19. “Not everything is immitis” (Europe, feline heartworm disease): European guidelines for cats remind us that D. immitis (cardiopulmonary) and D. repens (subcutaneous) coexist in Europe and that D. repens is considered an emerging zoonosis in several areas. It is perfect for a “do not confuse species” box.
20. Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC11132531/
21. One Health fact (humans, Europe): a scientific document cites that in 2012, 33 cases of human pulmonary dirofilariasis were reported in Europe (with the caveat that there is likely to be underdiagnosis). A powerful conclusion to emphasise that this is not just a “dog issue”.
22. Source: https://www.cambridge.org/core/services/aop-cambridge-core/content/view/4BD6846037DCEBCA641DA750A3725D02/S0950268819001687a.pdf/exposure_of_humans_to_the_zoonotic_nematode_dirofilaria_immitis_in_northern_portugal.pdf

3) “In cats, sometimes the parasite does not even need to reach ‘adulthood’ to cause disease.” 🐱

A controlled study described that cats infected with immature adults can develop HARD (Heartworm-Associated Respiratory Disease), a respiratory condition associated with an inflammatory response even if a ‘classic’ adult infection is not established. Therefore, in cats, transmission and lung damage can go hand in hand even with low parasite loads or aborted infections. 

4) “The bacteria that live inside the parasite also influence the damage: Wolbachia“

In an experimental study with 20 infected beagles, different treatments were evaluated for their influence on lung pathology, highlighting the role of Wolbachia (bacterial endosymbiont of the nematode) in inflammation. This concept helps explain why, in modern filarial medicine, we sometimes talk not only about the worm, but also about its “internal microbiology”. 

8) Your vet advises (Eurovet): “In cats, the diagnostic strategy is not to ‘copy and paste’ that of dogs.”

In cats, tests have nuances (low parasite load, aborted infections, different chronology). Therefore, before drawing conclusions from a single result, at Eurovet we evaluate the combination of tests and the clinical/environmental context, following guidelines that take these particularities into account. 

12) Medical data “One Health”: in humans, it can simulate a suspicious pulmonary nodule

Although humans are usually accidental hosts (and the parasite does not complete its cycle properly), there have been reports of pulmonary nodules (“coin lesions”) associated with Dirofilaria, which can be confused with tumour pathology in images if not contextualised.

14) Did you know? In cats, the antigen may appear late and a ‘negative’ result does not always rule out infection

In felines, it has been reported that detectable antigenemia can develop approximately 5.5 to 8 months post-infection if adult females are present; furthermore, it is emphasised that an antigen-free result does not rule out suspicion due to the particular biology of cats.