Heartworm, rapid tests and laboratory tests

How to catch Dirofilaria immitis without letting it escape 🕵️‍♂️🩸

Canine heartworm disease is much more than just a “parasite”: it is a cardiopulmonary infection caused by Dirofilaria immitis, transmitted by mosquitoes, which in certain contexts can develop over months with such subtle signs that they go unnoticed. In other words, the dog may “be fine” … until one day it is not. That’s why the real superpower here is not guessing, but diagnosing: combining rapid tools (clinical tests) with laboratory techniques and, when necessary, imaging tests to understand not only whether there is an infection, but also what impact it has on the body. And yes: the sooner it is detected, the more options we have to act safely and with a better prognosis. 🫀🫁 

To put it in real terms (because numbers bring us back down to earth): in a multicentre study in Spain that included 4,643 dogs sampled between September 2018 and February 2020 in 111 veterinary clinics, the prevalence of D. immitis antigen was 6.25% (95% CI: 5.59–6.98). In other words, it is not an exotic rarity, but a diagnosis that occurs frequently enough to justify serious screening. 

Detection in the clinic and laboratory: how fast, how useful, and what can go wrong

Canine heartworm disease (the famous “heartworm disease”) has an unpleasant talent: it can progress silently for months, while the dog seems to be doing great… until it is no longer doing great. Therefore, diagnosis is not an “extra”, but the turning point that changes the prognosis. Based on your draft and expanding on it with a more clinical and laboratory approach, here is a corrected, enriched version that is more than twice as long.

The best thing is that today we have fast, accurate and relatively accessible tools: antigen tests, microfilaria screening and confirmatory techniques such as PCR, as well as imaging tests to assess cardiopulmonary impact. The key is not to “run a test,” but to interpret the whole picture without falling into diagnostic traps (which do exist). And here’s a practical tip to keep in mind: if your dog travels to areas with more mosquitoes, changes residence, or has had lapses in prevention, don’t wait to “see symptoms”; the goal is to detect it before the heart and lungs pay the price. Furthermore, according to the AHS 2024 guidelines, the biological cycle is relatively long (7–9 months) and microfilaria infections can appear as early as 6 months, although “the usual” is between 7 and 9 months: this detail explains why testing too early can give a false sense of security. 

Antigen test: the rapid “radar” that searches for adult females (but does not read minds)

Antigen tests detect circulating proteins produced mainly by adult female Dirofilaria immitis in serum/plasma. They are popular in clinics for a reason: they are fast (minutes), practical and, under the right conditions, very reliable for screening. Even so, it is important to know exactly what they are “seeing” so as not to expect miracles: the test does not detect “the idea of filaria”, it detects circulating antigen from adult females, and this conditions false negatives. One nuance that provides a great deal of scientific reassurance: the current generation of antigen tests is considered to be almost 100% specific (i.e., a well-confirmed positive is usually truly positive), but there may be differences in sensitivity when the parasite load is low or the antigenemia (amount of circulating antigen) is very small. The latter sounds technical, but it is simple: little “signal” in the blood = easier for the test to miss it. 

However, it is important to understand its ‘blind spot’. There is a pre-patent window, i.e. an initial period after infection when the parasite has not yet matured and, although it is inside the body, it does not yet produce detectable levels of the antigen we are looking for: therefore, before approximately 6–8 months post-infection, a negative result may be biologically ‘normal’ and not a guarantee of absence. This figure (6–8 months) is described in the ESCCAP guidelines for antigen and in AHS 2024 for the overall chronology of the cycle. 

Furthermore, in infections with a low parasite load or with only males, there may not be enough antigen for the test to be positive. In fact, when we talk about very low loads, a very illustrative fact is that according to a diagnostic laboratory report (KSVDL), tests can detect dogs infected with 1–2 females only about 60–70% of the time. This number is not meant to scare you: it is to remind you that “negative” does not always mean “impossible”. 

And then there is one of the clinical laboratory’s favourite traps: immune complexes. Put simply, sometimes the antigen is “kidnapped” and bound to the animal’s own antibodies (antigen-antibody complex) and the test “sees” less than is actually there. Here is a fact with significant clinical implications: an informative text from the American Heartworm Society on blocked antigen states that various studies estimate that between 6.0% and 38.7% of microfilaremia-positive samples may test negative for antigen. (Translation for humans: there are dogs with circulating microfilariae where the antigen test may fail). 

In selected cases, some laboratories apply immune complex dissociation techniques (using heat or chemical methods) to release antigen and improve detection, but this is not a universal solution and should not be used routinely without discretion, precisely because it can complicate interpretation in the presence of other filariae. And here is a key practical message (and very “guideline-like”): AHS 2024 states that routine heating of samples is NOT recommended for routine screening, although it may be available in reference laboratories for specific situations. 

In practice, for all of the above reasons, many guidelines recommend that annual screening be done with a very simple (and very powerful) logic: antigen + microfilariae, even if the dog is on preventive medication. Translated into real life for the owner: even if your dog is “up to date”, annual testing helps to detect failures due to forgetfulness, vomiting after taking the medication, incorrect dosages or periods of intense exposure to mosquitoes. In addition, AHS 2024 highlights something that clinicians often see: an animal can become infected from a single “oversight,” because missing or delaying a single dose is enough to lose protection at a time of high exposure. 

Mini revelation 🔍

A negative antigen test does not always mean “zero filariae”: it may be too early (before 6–8 months), there may only be males, or it may be an infection with a low detectable antigen. 

Microfilariae in the blood: when the parasite “sends its offspring into the bloodstream” (and the microscope smiles)

Microfilariae are circulating larvae that can appear in peripheral blood when the infection is established and reproduction is occurring. Detecting them is useful for three very specific reasons: first, it confirms infection and provides epidemiological information (a microfilaremic dog is a reservoir that can maintain the cycle via mosquitoes); second, it helps explain discordant results (e.g., microfilariae positive with antigen negative); and third, it guides therapeutic behaviour, because high microfilariae loads require extreme caution when initiating certain strategies. One detail that sometimes surprises owners is that microfilariae are not present in all infected dogs; these cases are called “occult”. And this statement is not debatable: it is explicitly stated in the CAPC (updated in 2026), indicating causes such as single-sex infections, elimination by monthly preventatives, or host immune response. 

An important practical detail for the client: when scheduling the extraction, it is advisable for the dog to be calm and well secured; it is not a “painful test” as such, but movement can make venipuncture difficult and prolong the process, and what we are looking for is speed and good sample quality.

In terms of techniques, the blood smear (extended drop) is a quick and inexpensive first look, although it may fall short if microfilaremia is low. That is why, when we really want to “not miss” cases, concentration techniques come into play. The modified Knott test is the classic method: it concentrates microfilariae and makes them much easier to observe under a microscope. Here is a useful concept for the public: “concentrating” literally means bringing together what was dispersed in the blood so that the eye (and the microscope) can detect it better. It also has a key clinical advantage: the Knott test helps to differentiate between D. immitis and other non-pathogenic (or differently relevant) filariae based on morphology and body measurements. There are also membrane filtration methods and other concentration techniques used in the laboratory, which are very useful when working with large volumes or seeking maximum sensitivity. 

⚠️ Note: seeing “microfilariae” does not always automatically mean Dirofilaria immitis. Therefore, when identification matters (and it almost always does), morphometry and/or molecular techniques such as PCR are used, especially if the animal comes from areas where species may coexist.

When the results don’t add up: hidden infection, discrepancies and high-precision confirmation

Hidden infection: when the parasite plays hide and seek 😼 (and you have to play better)

It is called a hidden infection when there are adults (sometimes enough to cause damage) but no detectable circulating microfilariae. This can occur due to immune responses, unisexual infections, previous preventive treatment, etc. In practical terms: you can have a dog that is antigen positive and microfilaria negative… and still be a real case. The opposite can also occur (microfilaria positive with antigen negative) in specific situations, which requires confirmation and, above all, not making “automatic” decisions based on a single result. 

To put a numerical range on this phenomenon (because ‘hidden’ sounds anecdotal and it is not): in a technical review and discussion on diagnosis (focusing on antigen and immune complex dissociation), it is noted that hidden infections can account for approximately 10% to 67% of infected dogs, depending on the context (population, use of preventatives, diagnostic methods, and timing of sampling). This range is cited in the literature reviewed in a technical article (MDPI, 2023). Even more striking: it is mentioned that in a study using a highly sensitive antigen format, around 31% of infected dogs did not initially show detectable antigen; of these negatives, a large proportion corresponded to “hidden” infections that could be revealed after immune complex dissociation under controlled conditions. (This does not mean “always heat,” it means “understand why it sometimes fails”). 

Practical application 🧠

Not all positive results indicate the same severity: a positive antigen test indicates infection with at least one adult female, but the clinical risk depends on the parasite load, duration of infection, and degree of cardiopulmonary involvement (which is why imaging and examination are so important). In addition, a fact little known to the general public: in dogs, adults can live up to 7 years, which explains why letting the parasite “coexist” with the heart is not a harmless option. 

This is where practical advice for owners becomes really valuable: if your dog has lived or travelled in areas with lots of mosquitoes, or if there were months of “irregular prevention”, tell your vet even if it seems like a minor detail. This information changes the reading of an early negative result and avoids false reassurance. And if you would like a numerical example close to home: in a study in Madrid, involving 1,716 dogs, a canine prevalence of around 3% (52/1716) was reported, with higher rates in specific municipalities influenced by rivers (e.g. 10% in Aranjuez). This illustrates how the risk can be very “local”, even within the same province. 

What if the results ‘don’t add up’? A mini clinical algorithm without drama

Instead of fighting with the test (or with the universe), it is best to follow a logical approach. If antigen and microfilariae are positive, the diagnosis is very solid and the next step is no longer to “confirm” but to stage and plan management. If the antigen is positive but there are no microfilariae, consider hidden infection and remember that many guidelines recommend confirming with a second antigen test from another manufacturer and/or expanding with complementary tests as appropriate. This recommendation (confirmation before treatment) is highlighted in AHS 2024: all positives should be confirmed by microfilariae or a second different antigen test before starting therapy. 

If the antigen is negative and microfilariae appear, do not ignore it: it may be a technical discrepancy, a different species, or a case where the antigen is not well detected; remember the range cited by AHS for positive microfilaremia with negative antigen (6.0–38.7%) in certain studies. 

And if both are negative but clinical or epidemiological suspicion is high, biology takes precedence: consider the time window (it makes no sense to “sentence” before 6–8 months for antigen) and repeat at the appropriate time, or refer to a laboratory for selective strategies, with the very clear idea that routine heating is not recommended as standard screening. 

A practical method for clients, integrated into everyday life, is simple: keep a record of preventatives (date and product) and note trips or stays in humid areas/riverbanks/orchards during mosquito season. This is not bureaucracy; it is clinical context that allows the vet to decide whether a test should be repeated 6–7 months after relevant exposure or after resuming prevention. And it is not a whim: AHS 2024 emphasises that even a single delay or missed dose can allow infection. 

PCR: the genetic “fingerprint” (ideal for confirming, typing and resolving doubts)

PCR (polymerase chain reaction) detects parasite DNA in the blood. Its great advantage is that it allows infection to be confirmed when there are doubts or discrepancies and, above all, to differentiate species (e.g., D. immitis vs. other filariae), which is key in areas where filariae coexist or when the initial finding was microfilariae without a clear antigen. In layman’s terms, if the antigen is an “alarm” and the microscope is a “photo”, PCR is the “fingerprint”. This utility (PCR to differentiate species from isolated microfilariae) is mentioned in ESCCAP Guideline 5. 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 only one copy in each (a sign of very low loads or “borderline” detection).

Practical limitations: it is not always available in-house, usually requires an external laboratory, and its performance depends on load, sample and biological timing. That is why it is mainly used as a confirmatory test or to resolve “rare” cases, rather than as a first step in mass testing.

From confirmation to understanding severity: imaging, smart scheduling, and resources for serious monitoring

Beyond “yes/no”: X-ray, echocardiography and the art of staging

Once you suspect or confirm filariasis, you are not only interested in knowing whether the patient is infected: you are interested in the damage and the risk. The main involvement is cardiopulmonary, and here imaging becomes purely clinical. Chest X-ray allows you to see patterns compatible with pulmonary vascular changes and indirect signs of pulmonary hypertension; it is a very useful window for estimating impact and evolution. Echocardiography assesses cardiac repercussions, pressures, flow and, in some cases, allows structures compatible with parasites or haemodynamic changes to be visualised; it is especially valuable for staging and planning treatment/monitoring. Simply put, the laboratory tells you “there is an invader,” and the image tells you “how much damage it has done and how dangerous it is to move/wait/treat.” This recommendation (X-ray and ultrasound to stage severity) is included in ESCCAP Guideline 5 and reinforced in the clinical management logic described in AHS 2024. 

Your vet advises you

In cats, the diagnostic strategy is not to ‘copy and paste’ that used for dogs, because detectable antigenemia can develop approximately 5.5 to 8 months post-infection if adult females are present. Furthermore, it is emphasised that a negative antigen result does not rule out suspicion due to the particular biology of cats.

The tests have nuances (low parasite load, aborted infections, different chronology) and, in felines, many tests may fall short. So it makes sense to combine clinical history + imaging (X-ray/ultrasound) + serology, depending on the case.

Therefore, before drawing conclusions based on a single result, at Eurovet we evaluate the combination of tests and the clinical/environmental context, following guidelines that take these particularities into account.

Here is another practical tip with real impact: if your vet recommends limiting exercise while the study is being completed, it is not “excessive caution”. In heartworm disease, exercise can increase cardiopulmonary demands and aggravate signs in already compromised animals; the responsible thing to do is to let the complete diagnosis set the pace. And this has an objective basis: AHS explains that there is a clear correlation between activity level, severity and risk of complications during management/treatment. 

When it makes sense to test: smart timing (so you don’t shoot blindly)

A common mistake is to test too early “just in case” and blindly trust a negative result. If the dog has been recently exposed (travel, adoption, change of area, intense mosquito season), the test may be negative due to biology, not actual absence. That is why many recommendations suggest annual screening combining antigen + microfilariae; and this is not just a nice phrase: AHS 2024 explicitly indicates annual screening with both tests in dogs older than 7 months, and insists that a “NAD” (no antigen detectable) result does not confirm absence, but only indicates that this method did not detect antigen at that time. 

In some areas or profiles of intense exposure, testing twice a year is even considered, but the basic logic remains the same: test at the right time and with a combination of tests.

Your vet advises: “Annual testing even if you already use prevention“.

It seems redundant, but it is not: CAPC emphasises that all dogs in endemic areas, 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.

A practical method for owners, integrated without “lists” or complications, is to treat it as a health routine: just as you check your dog’s dental health or weight, you check its heartworm status. And if your dog vomits shortly after being given an oral preventive, let your vet know: you may need to repeat the dose or adjust your strategy, and this detail may explain an unexpected positive result in a “supposedly protected” animal. In addition, here is some advice based on “guidelines”: AHS recommends reducing exposure to mosquitoes (environmental control and, when possible, avoiding feeding peaks) and points out that the use of repellents/ectoparasiticides can increase the overall effectiveness of the preventive programme by acting on the vector. 

Suggested keywords (SEO, without being heavy-handed): heartworm test, heartworm diagnosis, Dirofilaria immitis, antigen test, microfilariae, modified Knott’s test, canine heartworm PCR, chest X-ray, echocardiography.

Reliable sources and references (to support clinical decisions): American Heartworm Society guidelines and materials (annual antigen and microfilaria screening, complex interpretation, confirmation before treatment, and mosquito/vector recommendations), ESCCAP guidelines (timeline for detection and usefulness of PCR and imaging), CAPC resources (reasons for occult infection), and technical literature on false negatives/blocked antigen and sensitivity at low loads. 

Sources and recommended reading

Nelson, C. T., McCall, J. W., Moorhead, A., Starkey, L., Ames, M. (2024) – Highlights of the 2024 AHS Canine Guidelines – American Heartworm Society – Summary (updated and peer-reviewed) of canine guidelines with key recommendations for diagnosis, prevention, and treatment – https://ufl.pb.unizin.org/app/uploads/sites/51/2021/02/2024-AHS-Canine-Heartworm-Guidelines-Summary.pdf 

Companion Animal Parasite Council (n.d.) – Heartworm – CAPC Guidelines – Practical guide with sections on epidemiology, diagnosis (including interpretation), prevention and treatment – https://capcvet.org/guidelines/heartworm/ 

ESCCAP (2019) – Control of Vector-Borne Diseases in Dogs and Cats (Modular Guide Series 5) – ESCCAP – Modular guide to vector-borne diseases (including cardiopulmonary disease caused by Dirofilaria immitis) with a focus on diagnosis and control measures – https://www.esccap.org/uploads/docs/3gtgj4y0_1058_ESCCAP_MG5__English_v13.pdf 

FECAVA Working Group on Canine Vector-Borne Diseases (2019) – Canine heartworm disease: Dirofilaria immitis – FECAVA – Summary clinical fact sheet (when to suspect, how to confirm, test interpretation and management), produced in collaboration with ESCCAP and ESDA – https://www.fecava.org/wp-content/uploads/2019/10/CVBD_DirofilariaImmitis_2018-web.pdf 

Montoya-Alonso, J. A., Morchón, R., Costa-Rodríguez, N., Matos, J. I., Falcón-Cordón, Y., Carretón, E. (2020) – Current Distribution of Selected Vector-Borne Diseases in Dogs in Spain – Frontiers in Veterinary Science – Multicentre study (4,643 dogs; 111 clinics) with national prevalences, including D. immitis – https://pmc.ncbi.nlm.nih.gov/articles/PMC7643126/ 

Montoya-Alonso, J. A., Morchón, R., Falcón-Cordón, Y., Falcón-Cordón, S., Simón, F., Carretón, E. (2017) – Prevalence of heartworm in dogs and cats of Madrid, Spain – Parasites & Vectors – Regional study (1,716 dogs; 531 cats) with prevalence, distribution and epidemiological variables – https://d-nb.info/1140970224/34 

Dryden, M. W. (2017) – Canine Heartworm Antigen Tests: (How accurate are they?) – Kansas State University Veterinary Diagnostic Laboratory (KSVDL), Diagnostic Insights – Applied review of antigen test performance, false negatives and pre-analytical considerations – https://www.ksvdl.org/resources/news/diagnostic_insights/january2017/canine-heartworm-antigen-tests.html 

American Heartworm Society (2018) – Blocked Antigen Causes False-Negative Heartworm Test Results – American Heartworm Society (In the News) – Clinical note on “blocked antigen” and published estimates of false negatives in antigen testing – https://heartwormsociety.org/in-the-news/501-blocked-antigen-causes-false-negative-heartworm-test-results 

Gruntmeir, J. M., Ward, H., Blagburn, B. L. (2023) – Improved Antigen Detection of Male-Only Dirofilaria immitis Infections in Canine Serum after Heat Treatment for Immune Complex Dissociation – Parasitologia (MDPI) – Work on immune complex dissociation through heat treatment to improve antigen detection in low-load or unisexual infections – https://www.mdpi.com/2673-6772/3/1/10