The life cycle of the heartworm parasite

The heartworm does not travel alone: the buzzing ‘incubator’

Canine dirofilariasis (commonly known as ‘heartworm’) is caused by Dirofilaria immitis. The first known description of ‘something similar’ is attributed to Francesco Birago in 1626, referring to worms in hunting dogs. A tasty detail to open the article with a phrase like ‘this has been with us for 400 years’. But if I tell you the clinical truth, the main villain is not the adult worm: it is the mosquito, which acts as a biological vector (a living being that not only transports the parasite, but allows it to mature inside it until it becomes infectious). Understanding this completely changes the way we look at the disease: we are not talking about “bad luck” here, we are talking about a perfectly designed biological cycle… and, unfortunately, a very efficient one. And, from a practical point of view, this approach has a huge advantage for the owner: if we understand “how the parasite travels”, we can identify much more clearly where the risk accumulates on a daily basis (patios, terraces, walking times, trips) and what simple habits reduce it without having to live in alarm mode 🦟.

Heartworm disease in cats

In 758 cats in the metropolitan area, the overall seroprevalence was 11.47% (antibodies) and the antigen alone was 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.

Source: https://link.springer.com/article/10.1186/s13071-014-0506-6

The distribution of roles and the key point: without an ‘incubator’ mosquito, there is no transmission

The transmission of heartworm disease only occurs when several elements align, as if they were gears. The first is the reservoir, usually an infected dog that has microfilariae in its blood. These microfilariae are microscopic circulating larvae, a kind of parasitic “seed”: they do not jump from one animal to another on their own, nor do they have the ability to become a heart infection without completing intermediate stages. The second cog is the vector, and this is where the mosquito takes centre stage. From the owner’s point of view, this “division of roles” helps to focus the conversation with the vet: rather than asking “are there mosquitoes?”, it is usually more useful to describe how the dog lives (whether it sleeps outside, goes for walks at dawn, travels) and whether the date of the last parasite control and/or test is known, because this information allows the risk to be estimated accurately. And when you look at the numbers, you understand why lifestyle matters: in Spain, the published prevalence rates vary greatly depending on the area and microenvironment, from low figures such as 2.4% reported in a provincial study in Barcelona to historically very high outbreaks in areas linked to bodies of water and irrigation (for example, local prevalence rates of 35.8% and 26% described in the Ebro Delta according to an epidemiological summary). 

It is worth emphasising an idea that seems small but is actually enormous: the mosquito is not simply a taxi that “picks up parasites and drops them off somewhere else”. In reality, it functions as a biological incubator, because essential larval maturation takes place inside its organism. Technically, certain mosquitoes (depending on the region and ecological conditions) of genera such as Culex, Aedes or Anopheles can intervene in the cycle. But more important than the “surname” of the mosquito is what happens inside: if the parasite does not complete its development in the vector, there is no effective transmission. And this also has a very simple practical interpretation: you cannot always “eliminate mosquitoes from the world,” but you can reduce your points of contact with them, especially at predictable times and places, and that already lowers the probability of exposure. This dependence on the vector and the environment is so strong that, in models used in Europe, the viability of the cycle is directly linked to thermal thresholds and the average lifespan of the mosquito. 

Curiosity with real impact: “a bottle cap can be enough”

Environmental authorities warn that some mosquitoes can lay eggs in amounts of water as small as a bottle cap; therefore, domestic prevention is not about “emptying swimming pools,” but about eliminating micro-containers that go unnoticed. 

To put it simply: the microfilaria circulating in the dog’s blood is like an uncut key. The mosquito ‘cuts’ it (transforms it) and only then does the larva become an infectious form capable of initiating an infection in another dog. This detail explains why heartworm disease is so dependent on climate, season and mosquito density: it is not enough for there to be bites, there must also be a context that allows the parasite to develop within the vector. Therefore, when a client tells us, “My dog hardly ever scratches,” the professional response is usually: scratching is not a reliable indicator here; what matters is exposure to the vector and the viability of the cycle. And, as a practical method, it is usually a good idea for the owner to get into a habit that is as simple as it is useful: noting down any trips and changes in routine on their mobile phone (e.g. “since June, he has been sleeping on the terrace”), because months later, these details are invaluable for interpreting the risk. This “calendar” approach is based on very solid facts: for example, a seroepidemiological study in Madrid described a seroprevalence of 9.1%, with municipal peaks (e.g., 27.3% in Alcalá de Henares and 20.8% in Aranjuez) in areas linked to river basins, which illustrates very clearly the relationship between water + vector + exposure. 

The cycle explained in detail: from dog to mosquito, from mosquito to dog (and the decisive role of temperature)

From dog to mosquito: the ‘collection moment’

When a mosquito bites an infected dog, it can ingest microfilariae while feeding on blood. This step is essential: without microfilariae available in the circulation, the mosquito does not ‘carry’ the biological material necessary to perpetuate the cycle. In practice, this helps to understand why some dogs, without showing any signs, can sustain transmission in a mosquito-infested environment: the circulation of microfilariae acts as the silent fuel for the problem. From a practical point of view for the client, there is a very direct lesson here: if you live with several dogs (or if you adopt a new one), it is not enough for “one to be fine”; it is advisable for the vet to assess the risk for the whole group, because a silent carrier can keep the wheel turning without anyone suspecting it.

Inside the mosquito: actual incubation (microfilaria → infective L3 larva)

This is where the event that changes everything takes place. Ingested microfilariae do not remain in “microfilaria mode”. They evolve and moult into larval stages that culminate in L3 larvae, considered the infective larvae. This process depends on environmental and physiological variables, but one stands out above all others: temperature. The higher the temperature within favourable ranges, the easier it is for larval development to be completed; at low temperatures, the process slows down or may not be completed. To put this idea into figures (and not just theory): larval development in mosquitoes is described as dependent on a threshold of 14 °C and the need to accumulate 130 “degree-days” above that threshold (often expressed as “heartworm development units”, HDUs), according to a classic analysis of extrinsic incubation (quantitative explanation of the thermal threshold).

“It’s not theory: D. immitis DNA has been found in mosquitoes… and not just a few.”

In an entomological surveillance study in Portugal, thousands of mosquitoes were captured and, after PCR screening, Dirofilaria immitis DNA was detected in multiple pools; the finding included results in the abdomen and also in the head+thorax, a relevant detail because it suggests presence in areas associated with transmission capacity (not just “ingested blood”).

This has direct implications for epidemiology: it is not only important “where” the dog lives, but also the microenvironment (humidity, available water, mosquito breeding areas) and the time of year. In simple terms: the mosquito needs to exist, but it also needs to be able to act as an incubator. And to incubate, it needs conditions. Translated into a “practical method” for the home: the more you prevent the immediate environment from offering breeding sites, the fewer adult mosquitoes you will have around. There is no need to turn your home into a laboratory: just check typical spots such as gutters, bins, forgotten outdoor drinking troughs or containers that accumulate water after watering from time to time; this simple routine reduces the mosquito “factory” around your dog. In addition, the “cooking” speed changes greatly with temperature: a recent article describes that the process can take approximately 16–20 days at 22 °C and accelerate to 8–10 days at 28–30 °C, which helps to understand why heat waves and long summers are not just a meteorological detail, but a direct biological factor. 

From mosquito to dog: entry through the skin and silent migration

Once the mosquito carries L3 larvae, when it bites again it can deposit them near the skin lesion of the bite, facilitating their entry into the dog’s skin. From there, the parasite begins a progressive migration through tissues, with successive moults, until it reaches the cardiopulmonary system months later. This time lag is crucial: the infection may be underway while the animal appears normal. Therefore, in heartworm disease, “waiting to see symptoms” is not a strategy, it is a biological trap. For the owner, a useful way to understand this is to realise that the key question is not “how is he today?”, but “what was he exposed to months ago?”. Therefore, if a dog has travelled or changed habits (more time in the garden, more nights outside), it is worth mentioning this even if it was some time ago: in diseases with a slow timeline, calendar memory is part of the diagnosis. And here is a figure that often surprises people: it is considered that it takes at least 6 months from infection for a test to become positive in many scenarios, which explains why checks are planned with a specific timeline.

To sum it up in clinical terms: transmission occurs in minutes, but the disease develops over months. And, as a very specific piece of advice, there is one thing that helps a lot in the consultation: make a note of the approximate date of the last trip, change of address or walking routine; this information reduces doubts, speeds up decisions and avoids misinterpretations.

Real risk and practical consequences: why transmission is also decided in the backyard

If heartworm disease were just a “countryside problem,” it would be easy. But in reality, the risk can arise in everyday life: a watered patio, a garden with standing water, a neighbourhood with green areas, a terrace where the dog sleeps in summer, or even that classic saucer under the plant pot that becomes a spa for mosquito larvae. The point is not to dramatise, but to understand that the vector adapts very well and that transmission depends on microhabitats. In many environments, mosquitoes increase their activity at dawn and dusk, which coincides with frequent walking or outdoor rest times. Therefore, a very realistic practical tip for many clients is to adjust small details without “disrupting their lives”: for example, bringing forward or delaying a short walk to avoid peak mosquito activity when possible, or having the dog rest indoors during those times if it usually sleeps on the terrace. And when it comes to homes with outdoor areas, a simple but effective measure is to reduce any standing water (even if it is only a small amount), because a mosquito does not need a swimming pool: sometimes a forgotten container is enough. This logic explains why, even within the same province, very different figures can coexist: in a review of Spain, for example, 0.85% was reported as the provincial average in Tarragona, compared to much higher local prevalences in specific areas with favourable conditions, and high figures have also been reported in Mediterranean areas, such as 18% in Alicante, 9% in Murcia and 39% in Ibiza, in studies cited in European reviews.

Your vet advises: “A weekly anti-mosquito routine at home really reduces exposure.”

Once a week, carry out a mini-inspection: plant pot saucers, buckets, gutters, outdoor drinking troughs, fountains, “forgotten” containers. Eliminating micro-accumulations of water reduces the local density of mosquitoes and, therefore, the probability of transmission. If you wish, Eurovet can provide you with a checklist tailored to your home (flat with terrace, villa, housing estate, vegetable garden, etc.).

The “travel” factor also comes into play here: a dog may live in a moderate-risk area and, when travelling to an area that is more favourable to mosquitoes, be exposed to significant risk. And as the timeline is slow, the owner may associate the problem with “something recent” when, in reality, the infection was sown months earlier. This is one of the reasons why preventive medicine insists so much on anticipation: in vector-borne diseases, the typical mistake is to act when the parasite has already had time to organise itself. As a practical tool for travelling clients, a simple rule works very well: if you are going to travel with your dog to humid or warm areas during mosquito season, it is advisable to notify your vet in advance to assess the individual risk; it is not a matter of fear, but of planning. And, as a precaution, it is always important to remember a domestic detail that seems obvious but avoids problems: not all repellents are suitable for all species; some products suitable for dogs can be dangerous for cats, so any repellent measure should be chosen with veterinary advice if both species live together at home 🐶🐱. In Spain and Portugal, monthly risk maps are also being published that show how potential exposure can extend over much of the year in certain areas, reinforcing the idea that ‘mosquito season’ does not always mean ‘just two months’.

Sources and recommended reading

Montoya-Alonso, J. A., Carretón, E., Simón, L., González-Miguel, J., García-Guasch, L., Morchón, R., Simón, F. (2015) – Prevalence of Dirofilaria immitis in dogs from Barcelona: Validation of a geospatial prediction model – Veterinary Parasitology – Epidemiological study in Barcelona that validates a geospatial model and reports an overall prevalence (2.4%) associated with geo-environmental factors. https://pubmed.ncbi.nlm.nih.gov/26162560/ 

Montoya-Alonso, J. A., Morchón, R., Falcón-Cordón, Y., Falcón-Cordón, S., Simón, F., Carretón, E. (2017) – Prevalence of heartworm in dogs and cats in Madrid, Spain – Parasites & Vectors – Seroprevalence study in dogs and cats in the province of Madrid (includes data on dogs and cats and discussion of urban factors). https://pubmed.ncbi.nlm.nih.gov/28747221/ 

Cuervo, P. F., Fantozzi, M. C., Di Cataldo, S., Cringoli, G., Mera y Sierra, R., Rinaldi, L. (2013) – Analysis of climate and extrinsic incubation of Dirofilaria immitis in southern South America – Geospatial Health – Describes the thermal model (threshold 14 °C and 130 degree-days/HDUs) applied to extrinsic incubation in mosquitoes (conceptual basis for risk models). https://pubmed.ncbi.nlm.nih.gov/24258893/ 

Infante González-Mohino, E., Rodríguez-Escolar, I., Balmori-de la Puente, A., Collado-Cuadrado, M., Carretón, E., Montoya-Alonso, J. A., Morchón, R. (2025) – Monthly analysis of the current risk of heartworm transmission in Portugal and Spain through ecological niche modelling as a control measure – Current Research in Parasitology & Vector-Borne Diseases – Monthly risk modelling (ENM + parasite generations) for Spain and Portugal with maps useful for guiding prevention and surveillance. https://www.sciencedirect.com/science/article/pii/S2667114X25000901 

Rodríguez-Escolar, I., Balmori-de la Puente, A., Infante González-Mohino, E., Collado-Cuadrado, M., Carretón, E., Montoya-Alonso, J. A., Morchón, R. (2025) – Assessment of the monthly risk of dirofilariasis infection in Europe and its projection to 2100 under climate change from a One Health perspective – Parasites & Vectors – Annual/monthly risk models in Europe and climate projection to 2100 (One Health), integrating vector suitability and number of generations. https://pubmed.ncbi.nlm.nih.gov/41310784/ 

Morchón, R., Carretón, E., González-Miguel, J., Mellado-Hernández, I. (2012) – Heartworm Disease (Dirofilaria immitis) and Their Vectors in Europe – New Distribution Trends – Frontiers in Physiology – Review of distribution in Europe, vectors and expansion trends (includes discussion of climatic factors and animal movement). https://pmc.ncbi.nlm.nih.gov/articles/PMC3372948/ 

Diosdado, A., Simón Martín, F., Morchón García, R., Montoya Alonso, J. A., Carretón Gómez, E., González Miguel, J. (2016) – Current status of the distribution of animal and human dirofilariasis in Spain and Portugal – Argos (Zaragoza) – Informative-professional summary of distribution and local factors (irrigation, coast, islands) with prevalence data and zoonotic considerations. https://accedacris.ulpgc.es/handle/10553/73780 

Simón-Martín, F., Montoya-Alonso, J. A. (Eds.), Morchón, R., González-Miguel, J., Méndez, J., Carretón Gómez, E. (2012) – Animal and human dirofilariasis: A growing disease – Veterinary Profession – Monograph/guide in Spanish (USAL–ULPGC) on the biology, epidemiology and “zoonotic mosaic” view of dirofilariasis. https://accedacris.ulpgc.es/bitstream/10553/57460/1/Dirofilariosis.pdf 

American Heartworm Society (n.d.) – Heartworm Basics – American Heartworm Society (Resources for owners) – Practical and educational resource for understanding the essentials of the disease, the role of mosquitoes and the basics of control. https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics 

Companion Animal Parasite Council (CAPC) (2026) – Heartworm (Guidelines) – CAPC – Updated practical recommendations (diagnosis, prevention, and clinical considerations) for veterinarians/owners. https://capcvet.org/guidelines/heartworm/ 

American Heartworm Society (AHS) (2024) – Canine Heartworm Guidelines 2024 – American Heartworm Society (Clinical guidelines) – Comprehensive guidance document (prevention, diagnosis and management) updated and used as a clinical reference. https://d3ft8sckhnqim2.cloudfront.net/images/AHS_Canine_Guidelinesweb04APR2024.pdf