Human leishmaniasis in Morocco: a systematic review and meta-analysis — I. Species distribution

In the arid landscapes and mountainous highlands of Morocco, a microscopic parasite called Leishmania causes a disease that leaves painful skin sores or, in some cases, attacks the internal organs. This illness, known as leishmaniasis, is not caused by a single enemy but by three distinct species of the parasite, each with its own lifestyle and home. One species thrives in the dry, pre-Saharan south, carried by a specific sandfly that feeds on rodents. Another lives in the semi-arid hills, transmitted by a different sandfly that bites humans directly. The third species inhabits the humid north, where it spreads from dogs to people. Because these three enemies live in different places and rely on different animals, the way to stop them is completely different. To control the rodent-borne type, you must manage the rodents; to stop the human-borne type, you must treat the people; to fight the dog-borne type, you must monitor the dogs. Knowing exactly which parasite is present in a specific town is therefore the first, most critical step in saving lives.

For decades, health officials have known the broad geography of these parasites in Morocco, but they have lacked a precise, quantitative picture of how many cases are caused by each species. A new systematic review, which gathered and analyzed every available laboratory study from the country, sought to answer this question by counting the actual parasites found in human patients. The researchers did not rely on guesses based on where a patient lived or what their sore looked like; they looked only at cases where the parasite had been identified in a lab using genetic or chemical tests. They found that while the overall numbers suggest one species is the most common, the reality on the ground is far more complex. The study reveals that the type of parasite present is determined entirely by the local environment, not by the country as a whole, and that the data needed to guide national health policy is still dangerously thin.

The team behind this work, based at Mohammed V University of Rabat, set out to compile a complete list of every human parasite sample from Morocco that had been identified to the species level. They scoured medical databases, university libraries, and national archives of unpublished theses, looking for studies that reported the number of patients and the specific type of Leishmania found in their samples. They were strict in their criteria: if a study simply assumed the parasite type based on the location or the appearance of the skin lesion, it was excluded. They only included studies where scientists had actually tested the samples in a lab. After filtering through hundreds of records, they identified sixteen studies that met these standards, covering 870 human isolates from twenty-two different provinces.

When the researchers pooled all these numbers together, the results showed a mix of the three species. Roughly 57 percent of the identified samples were L. tropica, about 22 percent were L. major, and the remaining 9 percent were L. infantum. However, these average numbers tell a misleading story. The data was not evenly distributed; instead, it was clustered in extreme patterns. In some specific towns, the researchers found that every single patient had the same type of parasite. In the southern province of Zagora, all 103 patients tested had L. major. In the town of Chichaoua, all 62 patients had L. tropica. In the seven northern provinces, the vast majority of cases were caused by L. infantum, a species usually associated with internal organ disease, which appeared in the skin of patients in a way that was far more common than previously thought.

The study highlights a crucial insight: the parasite found in a patient is a property of their specific local focus, not of Morocco as a whole. The variation between different towns was so great that calculating a single national average is not useful for health planning. If a doctor in Zagora sees a patient, they can be almost certain it is L. major; if a doctor in Chichaoua sees a patient, it is almost certainly L. tropica. The national average of 57 percent for L. tropica does not apply to either of these places. The researchers found that the differences between towns were so large that a new study in any given province could plausibly find almost any combination of species, making the national average a poor predictor for any specific location.

One of the most significant findings concerns the northern part of the country. For a long time, L. infantum was considered primarily a cause of severe internal disease, with skin infections seen only as rare exceptions. However, the data from the northern provinces showed that this species was responsible for the majority of skin lesions in that region. This changes the approach to control, as it means that in the north, the primary reservoir for the disease is the dog, requiring a different strategy than in the south where rodents are the key. The study also noted that in some provinces, the dominant species seemed to shift over time. In the province of Errachidia, a study from 2010 found only L. major, while a study from 2014 found mostly L. tropica. This suggests that the landscape of the disease can change within just a few years, further complicating long-term planning.

Despite the clarity of these local patterns, the review exposed a major gap in the country's knowledge base. Morocco reports thousands of leishmaniasis cases every year, yet the review found only 870 cases that had been genetically identified in the entire published literature over nearly two decades. A large portion of the country's medical research consists of university theses, and the researchers examined thirty-six of these in full text. None of them contained laboratory identification of the parasite species; they simply assumed the type based on the location. This means that for most of Morocco, there is no hard data on which parasite is circulating. The map of the disease is filled with large blank areas where the species is unknown.

The authors conclude that while the study successfully gathered every available piece of laboratory evidence, the resulting national averages should not be used to make decisions for specific provinces. The diversity of the disease is too high, and the data too sparse, to rely on a single number. Instead, the most valuable outcome of this work is a map showing exactly where identification has been done and where it has not. To move forward, the researchers suggest that the country does not need a massive new research program, but rather a simple change in routine reporting. If the species of the parasite were recorded as a standard part of the disease notification system, and if a small number of sentinel provinces were monitored annually, the country could build a reliable, up-to-date picture of the disease. Until then, health officials must recognize that the enemy they are fighting changes from one town to the next, and that the old assumption that geography alone can identify the parasite is no longer sufficient.