The sand fly: a tiny insect with a huge impact

Understanding its life cycle is the biggest battle against leishmaniasis

Canine leishmaniasis is not just another disease: it is a chronic, complex and potentially fatal condition that is endemic in large areas of southern Europe and the Mediterranean basin. Its transmission depends on an insect that is as discreet as it is decisive: the sand fly, a small blood-sucking insect whose biology determines, almost mathematically, the success or failure of any preventive strategy. And here it is worth putting some figures on the table: in Spain, prevalence rates vary greatly depending on the area, but overall, national estimates have been published of around 15.7% of dogs with evidence of infection/exposure, with large geographical differences. Globally, it is estimated that more than 12 million people are infected with leishmaniasis and that the disease is endemic in 99 countries, ranking among the ten most important neglected tropical diseases in the world. Although dogs are the main reservoir in the Mediterranean, these data remind us that we are talking about a zoonosis with a strong impact on human public health, not just “animal” health.

A recent study used ecological models and data on Phlebotomus perniciosus to construct a map of the risk of L. infantum infection in dogs throughout the Iberian Peninsula. They found that the areas currently at greatest risk are the south-west, the centre of the peninsula, the Mediterranean coast, the Balearic Islands and the Ebro basin, and that with climate change scenarios, the risk would increase in much of the territory, especially in the north, by up to 70% by 2060.

Regional studies have observed figures close to 19–20% in specific areas, and the most treacherous thing is that a significant proportion of these dogs show no clinical signs. Put simply, there may be infected dogs that appear perfectly healthy, yet still form part of the epidemiological problem.

Scientific note – Europe has an official vector fact sheet

The ECDC maintains a technical data sheet on sand flies with details on biology and measures (impregnated barriers, repellent strategies, etc.). It is useful for understanding that vector control is not just a matter of “collars or pipettes”, but a range of complementary tools.

For years, attempts have been made to combat leishmaniasis by focusing solely on the infected animal. However, the real key lies one step earlier, in understanding how the sand fly lives, when it acts and where it develops. Adjusting dog walks to avoid the insect’s peak activity hours, reducing night-time stays in damp gardens, or changing the location of resting areas are not just “common sense” measures: they are direct applications of knowledge about the vector. In terms of preventive medicine, this is pure gold 🧠: acting where transmission is cut off, not where we are already treating the consequences.

Sand flies are not synonymous with mosquitoes (and this difference matters)

Sand flies belong to the genus Phlebotomus, within the Psychodidae family. Although at first glance they may resemble mosquitoes, they are not, either in their anatomy or in their behaviour. They are significantly smaller, with a short and erratic flight pattern and, most disturbingly, completely silent, which explains why many bites go unnoticed even when the dog is only a few metres away from its owners.

Did you know? They are much smaller than mosquitoes

The CDC points out that sand flies that transmit Leishmania are usually about a quarter of the size of a mosquito (or even smaller) and, moreover, their bites can go unnoticed. This explains why many people underestimate their exposure: “if I didn’t see it, it didn’t bite me”… well, yes, it did.

Source: https://www.cdc.gov/leishmaniasis/about/index.html

This insect is crepuscular and nocturnal, with a marked preference for dark, humid and poorly ventilated environments. Only females feed on blood, as they need it to complete the maturation of their eggs. It is precisely during thishttps://www.cdc.gov/yellow-book/hcp/travel-associated-infections-diseases/leishmaniasis.htmly blood intake that they can inoculate the Leishmania infantum parasite into the dog, initiating an infection that can remain dormant for months or even years before becoming clinically apparent. In practice, this means that a dog can go through an entire season of risk “without knowing it”… and neither can we.

Sand flies are such poor flyers that public health guides, such as the American CDC, recommend that fans or intense ventilation can hinder their movement in bedrooms or rest areas. Therefore, keeping your dog indoors at dusk, preventing it from sleeping in patios or gardens on warm nights, and favouring well-ventilated spaces indirectly reduces the chances of contact with the vector. In houses with patios/gardens, sometimes a night-time fan near the sleeping area adds up to more than it seems. Another very effective household trick: if your dog’s bed is near an open door to a damp garden, move it a few metres inland during the sand fly season; that small distance can make a real difference in exposure. 

This is one of the great dangers of the disease: the apparent initial normality of the animal 🕰️, which can lead to a false sense of security if constant preventive measures are not maintained, even in apparently healthy dogs.

🧪 Interesting fact – Did you know?

Sand flies can bite several times in a single night and do not emit any audible buzzing. Many infections occur without anyone — human or dog — noticing anything, reinforcing the importance of continuous, non-reactive prevention.

Four phases, one goal: perpetuating transmission

The sand fly’s life cycle is a fascinating example of adaptation to the environment, and at the same time its greatest weakness if we know how to interpret it correctly. It all begins when the female lays her eggs in moist, protected soil rich in decomposing organic matter. She does not lay them randomly: she seeks out stable, shady and undisturbed microenvironments, such as corners of gardens, piles of leaves, areas under flowerpots or cracks in walls and buildings.

The eggs hatch into larvae, which are saprophagous organisms, meaning they feed on organic debris. This larval stage is exclusively terrestrial and highly dependent on environmental humidity. Translated into concrete actions: removing accumulated leaves, avoiding improvised “composting corners”, keeping storage rooms and sheds as dry as possible, and reducing decaying plant matter near areas where the dog sleeps are measures that tackle the problem before the insect reaches adulthood. Even something as domestic as checking the perimeter of the house—cracks in walls, gaps under steps, corners with constant watering—can be more relevant than it seems, because these micro-refuges function as “nurseries” for the vector.

After several moults, the larva transforms into a pupa, an immobile but crucial stage, where the insect undergoes a complete reorganisation. Finally, the adult emerges, whose life is surprisingly short—between one and two weeks—but long enough to feed, reproduce, and transmit the disease. This brevity explains why the continued use of topical repellents or insecticide collars with proven effectiveness is so effective: it is not necessary to “kill all the sand flies on the planet”; it is enough to prevent them from completing one or two blood meals.

Here are some reassuring figures (because they are concrete): in controlled studies, a sustained-release deltamethrin collar has shown a 94–98% reduction in sand fly feeding on protected dogs for almost a full year (364 days). This does not mean “zero risk”, but it does mean a drastic reduction in the probability of an effective bite, which is exactly what we are interested in when we talk about vector transmission. In addition, recent studies have shown that cats can also become infected and act as a blood source for the vector, although dogs remain the main epidemiological reservoir.

Temperature and geographical expansion

The European Environment Agency summarises that sand flies thrive where temperatures exceed ~15°C for several months, and that climate change may facilitate expansion to higher latitudes/altitudes. This is not science fiction: it means longer seasons and shifting maps.

The duration of each phase is modulated by environmental factors, especially temperature and humidity, which explains the marked seasonality of sand flies: in Mediterranean climates, maximum activity occurs between May and October, with peaks in the warmer months, and expansion is favoured by rising temperatures, as detailed by the European Environment Agency platform. That is why a classic mistake is to ‘start late’: putting on the collar or starting to use repellent when it is already consistently hot or when cases have already started in the neighbourhood. Prevention works best if it is started early and maintained consistently throughout the risk season.

Where this invisible enemy lives… you may have it in your garden!

Sand flies do not need large natural spaces to thrive. In fact, they tend to develop in peri-urban and domestic environments: gardens, patios, stables, walls with cracks, sheds, damp storage rooms or areas with accumulations of leaves and plant debris. This makes the home itself a key point of prevention, where small structural modifications can make a significant difference. And, to put the phenomenon into perspective with some powerful epidemiological data: in the Mediterranean, it is estimated that there are millions of infected dogs, which means that the parasite is not rare; what is rare is that the vector does not ‘find’ opportunities when the environment is favourable.

Dogs that live or spend a lot of time outdoors, especially at dusk, are statistically more exposed. Raising beds and kennels slightly off the ground, preventing the animal from sleeping directly on damp surfaces and placing physical barriers such as mosquito nets in resting areas are simple measures that take advantage of a little-known characteristic of the insect: its flight is low and limited. A practical method at home, which is very easy to implement, is to create a ‘safe night-time area’: during the sand fly season, the dog sleeps indoors or in a space protected by fine mesh; if this is not possible, at least prevent it from sleeping in the garden or in a kennel attached to walls with cracks and dense vegetation.

Normal mosquito nets may not be enough

Health authorities warn that standard nets/mosquito nets may be ineffective because sand flies are so small that they can pass through certain meshes; very dense mesh or insecticide-treated nets are recommended in high-risk situations. Translation for the home: not all mosquito nets are created equal.

This is where prevention ceases to be abstract and becomes concrete and everyday, integrating naturally into the routine of the animal and its family.

Prevention is not improvisation: it is acting on the cycle

Effective prevention of leishmaniasis does not consist of a single miracle measure, but rather a combined strategy designed to interfere at critical moments in the sand fly’s cycle. Avoiding walks at dawn and dusk, using repellents with proven effectiveness (e.g. collars with deltamethrin or flumethrin, or pipettes with repellent effect according to veterinary guidelines), reducing environmental humidity and applying antiparasitics regularly are not isolated gestures, but coherent decisions based on applied biology.

A very practical approach—one that tends to improve owner compliance—is to turn prevention into a “system” rather than a vague reminder: associate collar or pipette replacement with a fixed event (e.g., the first weekend of the month), check the condition of the collar (fit, integrity, contact with the skin), and reinforce environmental measures during hot periods. It also helps to remember that many preventive failures are not due to the product “not working,” but to human factors: poorly fitted collars, frequent bathing that reduces the effectiveness of topical products (depending on the product), forgetting to reapply, or intense exposure of the dog during peak vector activity times.

Your vet advises

A reference article in JAVMA as well as the OM itself emphasise the value of a

 combined approach to prevention in dogs: reducing transmission through both vector control (sprays, treated nets, personal/animal protection) and environmental management. This point reinforces something that many owners are grateful to hear: cleaning the environment and reducing shelters is not a “fad”, it is prevention with epidemiological logic. It is also the idea of an “antidote to human error”, namely that it does not depend on a single element, but on a system.

When these measures are maintained consistently, even in dogs that do not show symptoms, the probability of transmission is significantly reduced. Understanding the vector allows us to stop reacting to the disease and start anticipating it 💡.

📌 In summary

Leishmaniasis does not begin with a blood test, or even with a bite. It begins much earlier, with a small insect that thrives silently when we do not understand it. Knowing the life cycle of the sand fly allows us to transform prevention into a logical, consistent and effective strategy, capable of providing real and lasting protection for the health of our dogs. And when specific data is added — published prevalence rates that can be around 15% nationally, regional peaks close to 20%, and anti-bite measures with reductions in vector feeding of over 94% under experimental conditions — prevention ceases to be a “recommendation” and becomes a well-founded health decision.

Sources and recommended reading

Paulin, S., Dautel, H., Le Sueur, C., Hensel, P., Deleu, S., Lienard, E. (2018) – Laboratory evaluation of the anti-feeding effect for up to 12 months of a slow-release collar containing deltamethrin (Scalibor®) against the sand fly Phlebotomus perniciosus in dogs – Parasites & Vectors – Controlled experimental study demonstrating a 94–98% reduction in sand fly feeding on protected dogs over a period of up to 364 days – https://pmc.ncbi.nlm.nih.gov/articles/PMC6161463/

Gálvez, R., Montoya, A., Fontal, F., Martínez de Murguía, L., Miró, G. (2020) – Recent trends in Leishmania infantum infection in dogs in Spain. Part I: mapped seroprevalence and distribution of sand flies – Parasites & Vectors – Large-scale epidemiological analysis showing high regional variability and confirming the magnitude of the canine reservoir in the Mediterranean basin – https://pmc.ncbi.nlm.nih.gov/articles/PMC7171843/

Vélez, R., Ballart, C., Domenech, E., Abras, A., Fernández-Arévalo, A., Gállego, M. (2019) – Seroprevalence of Leishmania infantum infection in dogs in the province of Girona (northeast Spain) – Preventive Veterinary Medicine – Cross-sectional study estimating a seroprevalence of around 19.5% with a high proportion of asymptomatic infected dogs – https://www.sciencedirect.com/science/article/pii/S0167587718303209

Merino-Goyenechea, J., Álvarez-García, G., Siles-Lucas, M., Martínez-Carrasco, C., Ortega-Mora, L. M. (2025) – Molecular prevalence of Leishmania infantum infection from oral samples in dogs in north-western Spain – Pathogens – Study that collects and contextualises national estimates of canine prevalence at around 15.7%, highlighting geographical heterogeneity – https://www.mdpi.com/2076-0817/14/6/569

ESCCAP (European Scientific Counsel Companion Animal Parasites) (2016) – Control of vector-borne diseases in dogs and cats. Guide No. 5 – ESCCAP Europe – European reference document establishing evidence-based practical recommendations for the prevention of vector-borne diseases such as leishmaniasis – https://www.esccap.es/wp-content/uploads/2016/06/guia5_P31620-FINAL.pdf

World Health Organisation (2008) – Epidemiological profile of leishmaniasis in Spain – WHO – Leishmaniasis Country Profiles – Official report describing the endemic situation of human and animal leishmaniasis in Spain and its public health context – https://leishinfowho-cc55.es/wp-content/uploads/2022/07/pdfs/country-profiles/Spain/LEISHMANIASIS_CP_ESP_2008.pdf