Leishmaniasis is the second deadliest parasitic infection, behind malaria, causing an estimated 700,000 to 1 million new cases and some 26,000 to 65,000 deaths annually. Now, if that information isn’t alarming enough, new data from an international team of investigators led by scientists at the Federal University of São Carlos and the Oswaldo Cruz Foundation shows that there is an emerging leishmania-like infection potentially being spread by mosquitoes in South America. Moreover, this parasite currently does not respond to treatment with standard leishmaniasis drugs.

Leishmaniasis is a parasitic disease found in parts of the tropics, subtropics, and southern Europe. It is classified as a neglected tropical disease and is transmitted by the bite of the phlebotomine sandflies. The two most common forms of leishmaniasis are cutaneous, which causes skin sores, and visceral, which affects several internal organs (usually spleen, liver, and bone marrow).

According to the World Health Organization, each year between 50,000 and 90,000 people become sick with visceral leishmaniasis (kala-azar), a form of the disease that attacks the internal organs and is fatal in more than 95% of cases if left untreated. During the last several decades, researchers have described rare cases of patients co-infected with both Leishmania and other groups of protozoan parasites that usually infect insects, including Crithidia. The current study of parasites isolated from a Brazilian patient confirms that Crithidia parasites also can infect people.

In the current study, a 63-year old patient initially sought treatment for the symptoms of visceral leishmaniasis, including weight loss, fever, anemia, and an enlarged liver and spleen. However, after eight months of standard leishmaniasis treatment, the patient’s symptoms had not improved. The patient developed widespread skin lesions with poorly defined edges (unlike the small lesions with well-defined edges that sometimes appear after treatment for visceral leishmaniasis) and ultimately died.

To determine the cause of disease, researchers cultured parasites taken from the patient’s bone marrow and skin lesions, sequenced their genomes and discovered that the parasites were not closely related to known disease-causing Leishmania parasites. Instead, the researchers found that they were more closely related to Crithidia fasciculata, a parasite that usually colonizes mosquitoes. To confirm that these Crithidia parasites could infect mammals, the researchers exposed mice to the parasites isolated from the patient, both intravenously and by injection into the skin, and found that both types of parasite-infected the liver. The parasites collected from the patient’s skin also caused skin lesions in the mice.

The study raises concerns that the Brazilian patient might not be an isolated case. If Crithidia infections represent an emerging infectious disease in people, there will be an urgent need to develop novel effective treatments, the researchers wrote. They expressed concern that the disease may be mosquito-borne because Anopheles and Culex mosquitoes can host the Crithidia parasite. More research will be needed to find other human cases, confirm the parasite’s range and host species, and discover potential treatments, the authors noted.