The discovery of Leishmania infantum in Pintomyia fischeri challenges established understanding of visceral leishmaniasis transmission and calls for revised public health strategies.
In the intricate world of infectious diseases, sometimes the most significant discoveries come from noticing what's missing. For years, scientists battling visceral leishmaniasis in Brazil—a potentially fatal disease affecting humans and dogs—focused their attention on a single sand fly species: Lutzomyia longipalpis. This insect was considered the exclusive vector, responsible for transmitting the deadly Leishmania infantum parasite. But then came a puzzling observation: both canine and human cases began appearing in regions where Lu. longipalpis was completely absent.
The persistent spread of VL in areas without the primary vector Lutzomyia longipalpis suggested other sand flies were involved in transmission 2 .
This medical mystery set the stage for a remarkable scientific discovery that would change our understanding of how this dangerous disease spreads through Brazilian communities. Researchers turned their attention to an unexpected culprit: Pintomyia fischeri, a sand fly species previously not known to transmit visceral leishmaniasis 1 2 .
Visceral leishmaniasis (VL), also known as kala-azar, is the most severe form of leishmaniasis, caused by the protozoan parasite Leishmania infantum. Unlike cutaneous forms that affect the skin, VL attacks internal organs including the spleen, liver, and bone marrow. If untreated, it's fatal in over 90% of cases within two years of infection 4 .
The disease manifests through prolonged fever, substantial weight loss, fatigue, anemia, and dramatic enlargement of the spleen and liver. In Brazil, VL remains a significant public health challenge, with the country accounting for 93.5% of all reported VL cases in Latin America 7 .
Sand fly bites a dog or human, injecting infectious parasites
Parasites multiply inside the new host's cells
Different sand fly bites the infected host and ingests parasites
Parasites develop inside the sand fly's gut
Infected sand fly bites another host, continuing transmission
| Species | Primary Role | Geographical Distribution | Infection Rate with L. infantum |
|---|---|---|---|
| Lutzomyia longipalpis | Primary VL vector | Widely distributed in endemic areas | 9.8% (experimental infections) 2 |
| Pintomyia fischeri | Potential VL vector | Greater São Paulo region | 0.23-4.8% 1 2 |
| Migonemyia migonei | Potential VL vector | Multiple regions | Not found infected in studies 2 |
| Nyssomyia neivai | Potential VL vector | Southern Brazil | Recently detected with L. infantum |
The first clues emerged from the western part of Greater São Paulo, particularly in municipalities like Embu das Artes. Here, dogs and humans were testing positive for VL, yet intensive entomological surveys failed to find Lu. longipalpis 2 . Instead, researchers consistently found high populations of Pintomyia fischeri—comprising up to 89.5% of all sand flies collected in some areas 1 .
This distribution pattern raised a critical question: Could Pi. fischeri be responsible for transmission in these regions? To answer this, scientists needed to apply rigorous vector incrimination criteria, requiring evidence that this species:
Previous laboratory studies had already demonstrated that Pi. fischeri was susceptible to experimental infection with L. infantum, with infection rates of 4.8%—lower than Lu. longipalpis (9.8%) but still significant 2 .
The species also displayed attractive behaviors to dogs and had a survival rate long enough to transmit parasites. But the critical missing piece was evidence of natural infection in wild-caught specimens.
To confirm natural infection, researchers designed a comprehensive field study in the Embu das Artes municipality, a known VL focus without Lu. longipalpis. The study employed:
Monthly sampling from November 2018 to October 2019
Shannon and CDC light traps placed strategically
6:00 PM to 9:00 PM, aligning with sand fly peak activity
Detailed dissection and molecular analysis of captured specimens 1
The team collected 951 sand flies representing 10 different species. As expected, Pi. fischeri dominated the catches, representing 89.5% of all specimens. From these, 426 female Pi. fischeri were carefully dissected to examine their midguts for parasite infection 1 .
During dissections, researchers made the critical discovery: one female Pi. fischeri was found harboring live promastigotes (the insect-stage form of Leishmania) in its midgut. To confirm the identity of these parasites, they:
This represented the first definitive evidence of natural L. infantum infection in wild Pi. fischeri, with an infection rate of 0.23% in the dissected females.
| Collection Metric | Number | Percentage |
|---|---|---|
| Total sand flies collected | 951 | 100% |
| Male sand flies | 457 | 48.1% |
| Female sand flies | 494 | 51.9% |
| Pintomyia fischeri specimens | 851 | 89.5% |
| Female Pi. fischeri dissected | 426 | - |
| Naturally infected Pi. fischeri | 1 | 0.23% |
| Parameter | Lutzomyia longipalpis | Pintomyia fischeri | Nyssomyia neivai |
|---|---|---|---|
| Natural infection rate | Well-documented 2 | 0.23% 1 | 0.4% |
| Experimental infection rate | 9.8% 2 | 4.8% 2 | Not fully determined |
| Infective life expectancy (days) | 2.4 2 | 1.68 2 | Not specified |
| Attraction to dogs | High | Moderate 2 | Evidence in Southern Brazil |
| Proportion in collections where dominant | Varies by region | Up to 89.5% 1 | Up to 85.8% |
The discovery of Pi. fischeri as a potential VL vector was made possible through sophisticated research tools and methods:
Specialized traps that use light and sometimes heat or carbon dioxide to attract and capture sand flies for monitoring populations.
A molecular technique that amplifies specific DNA sequences, allowing researchers to identify minimal amounts of Leishmania genetic material in sand flies 1 .
A method that uses specific enzymes to cut DNA at precise sequences, creating unique patterns that distinguish Leishmania species from each other 1 .
Growing parasites obtained from sand fly guts in specialized media, allowing researchers to obtain sufficient material for further analysis 1 .
The confirmation that multiple sand fly species can transmit VL has profound implications for disease control in Brazil. Traditional VL surveillance and control programs have heavily focused on Lu. longipalpis, potentially missing significant transmission by other species.
As the study authors noted, Pi. fischeri displays several characteristics that make it an effective secondary vector: it's highly abundant in certain regions, susceptible to L. infantum infection, attracted to dog and human hosts, and has a survival rate sufficient to transmit parasites 2 .
This discovery helps explain why VL continues to spread into new territories despite control efforts targeting the primary vector. It also highlights the need for:
One promising approach involves using synthetic versions of sand fly sex-aggregation pheromones in "lure-and-kill" strategies. These methods attract sand flies to insecticide-treated areas, potentially controlling multiple species regardless of their specific host preferences 6 .
The detection of Leishmania infantum in Pintomyia fischeri represents more than just the identification of another insect that can carry a dangerous parasite. It underscores a fundamental principle in disease ecology: transmission systems are often more complex than they initially appear. As research continues, each new discovery adds another piece to the complex puzzle of disease transmission, ultimately bringing us closer to more effective and sustainable control strategies for visceral leishmaniasis and other vector-borne threats.