Knowing the phlebotomes: biology, ecology and health role

 

 

Biology and ecology

 

Sand flies  are small (<3.5 mm), short-range (<300-500 m) and silent insects. They have a pale yellow color similar to that of sand (sand fly) and the thorax, abdomen and wings are covered with thick hair.

 

All sand flies are generally active during the twilight and night hours, while during the day they shun sunlight by hiding in cool and humid environments such as stables, chicken coops, cellars, homes, cracks in walls, rocks and soil. They live both in domestic, pre-domestic and wild environments. They are absent only in New Zealand and the Pacific Islands. Their altitudinal distribution goes from below sea level, in the Dead Sea, up to 3300 m a.s.l. in Afghanistan.

In Italy the maximum limit was recorded in Sicily in the Madonie at 1200 m and in Filetto (AQ) at 1070 m a.s.l.

 

No colonization of new territories by passive transport from distant regions has ever been reported. On the other hand, a slow colonization of contiguous territories is conceivable when environmental conditions become favorable.

 

The life cycle is divided into an embryonic egg stage (~ 10 days) followed by 4 larval stages (~ 25-35 days) and a pupal stage (~ 10 days). The duration is closely linked to climatic factors: in temperate zones like ours, the cycle lasts 45-60 days, with at least two complete development cycles per reproductive season (June-September).

 

As with mosquitoes, only females are bloodsucking. Males are glyciphages (they feed on sugary vegetable juices.

 

The sting is annoying because through the indentations of the stinging apparatus they tear the dermal capillaries to form a small collection of blood and at the same time secrete saliva to prevent blood clotting. This technique is distinguished from more specialized bloodsucking such as culicides, which, on the other hand, introduce the stinging apparatus directly into the capillary causing minor skin damage. Female phlebotomists have no host preference and can sting any warm-blooded vertebrate (only Sergentomyia feeds mainly on reptiles).

 

Mating takes place after taking the blood meal or in the presence of a host on whom to carry it out. Subsequently, the eggs are laid and this is not accidental, but the females recognize the ecological niches guided first by the chemical and physical constituents of the substrate, then by the pheromones present on the eggs of the same species already placed in that place.

Each female can produce 50 to 100 at a time.

 

The larvae are terricolous, unlike those of mosquitoes which are aquatic, and develop in almost complete darkness, in the presence of high relative humidity (~ 90%) and constant temperature (24-30°C) on soils rich in organic decaying material, which they feed on.

Survival during the cold season occurs thanks to the diapause larvae in the Paleartic species and to the egg stage in temperate climates.

 

 

Health

 

Out of more than 800 species of sand flies described, about 100 are confirmed or suspected vectors of leishmaniasis. Italy is endemic for 8 species belonging to two genera, Phlebotomus and Sergentomyia, of which seven belonging to the genus Phlebotomus (P. perniciosus, P. perfiliewi, P. neglectus, P. ariasi, P. papatasi, P. sergenti and P. mascitti) while the second genus is represented only by the species Sergentomyia minuta which, feeding on cold-blooded animals (mainly geckos and lizards), is of no importance from a health point of view, but can constitute an important ecological indicator for the presence of sandflies in a territory (Table 1).

The most widespread and most abundant species is certainly P. perniciosus, which is of considerable epidemiological interest, being an established vector of visceral leishmaniasis in both humans and dogs.

 

 

Both for the biological characteristics and the habitat, these insects do not have a uniform presence on the Italian territory. In fact, since the early 1990s there has been an expansion from the Tyrrhenian, Ionic and central-southern Adriatic sides of the peninsula and from the islands, to the central-northern Adriatic side and in many pre-Alpine and pre-Apennine hilly areas. At the origin of this phenomenon there seem to be several concomitant factors: the increase in the canine population, stray dogs and the changed climatic-environmental conditions that have allowed the vectors to colonize new areas.

 

Sand flies can transmit VIRUS (Sand flies or three-day fever with the Naples and Sicily viruses; benign summer meningitis with the Toscana virus); BACTERIA (Bartonellosis, not present in Italy) and PROTOZOI (Leishmaniasis).

In most cases, arbovirosis occurs asymptomatically or with flu-like symptoms, and only sometimes can they give rise to acute infections of the central nervous system. The Toscana virus is the most represented in Italy and from 1 January to 30 September 2020, the national surveillance system of the National Health Institute confirmed 36 cases of neuro-invasive infection, all autochthonous, no deaths, which occurred in the Regions of Emilia-Romagna, Tuscany, Lazio, Sicily and 2 cases in the Marche Region.

 

The health importance of sand flies is related to the transmission of leishmaniasis, a vast complex of diseases caused by protozoa of the genus Leishmania. In Italy, leishmaniasis occurs in 3 different forms, all due to the same parasite Leishmania infantum: 1. human visceral leishmaniasis (serious and fatal if not treated); 2. sporadic cutaneous leishmaniasis in humans (benign, self-limiting form); 3. canine leishmaniasis.

 

The transmission of leishmaniasis does not occur directly from dog to dog or from dog to man but through the sting of the phlebotomist, which represents the vector, and the dog in an anthropized environment, which represents the reservoir. In other contexts, wild canids (fox, wolf) can also play a role of reservoir, although in Italy it is to be considered not very relevant, while the role of the cat is still to be clarified.

 

The cycle begins with the phlebotomist's blood meal on an infected dog and the ingestion of the aflagellate form of the protozoan. In the digestive tract of the vector it multiplies and takes on the flagellate form which goes back into the pharynx and forms a plug that will be expelled at the time of a new puncture, thus causing infection. Transmission occurs mainly during the summer months due to the seasonality of the vector.

Most human infections affect immunocompetent individuals; furthermore, human to human transmission is possible by blood transfusions or through contaminated syringes.

 

 

 

Further reading

 

ECDC (European Center for Disease Prevention and Control) site to view maps on the distribution of the various species of Phlebotomus, even if they do not represent the official data of the countries concerned.

 

 

Author:  Dr.ssa Maria Gabriella Pistilli

 

 

This note was drawn up in implementation of the DG / ASUR Resolution no. 734/2016 as part of the AV5-2020 risk communication.

 

 

 

Source: VesA Marche ; Redazione VeSA

 

 

Categories: Animal Health , Leishmaniosi

 

 

This article was translated by Francesca Dall’Acqua  (Istituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise)

f.dallacqua@izs.it