A promising vaccine candidate against leishmaniasis, a parasitic disease also known as kala-azar, developed by researchers at Ohio State University in the United States, is offering fresh hope to scientists and public health experts across the world.
For India, where visceral leishmaniasis has long afflicted some of the country's poorest communities, the development is being watched with particular interest.
If the vaccine succeeds in human clinical trials, it could become the first effective preventive vaccine against leishmaniasis, a neglected tropical disease transmitted through the bite of infected female sandflies.
More than one billion people across nearly 100 countries, including India, remain at risk of the disease, according to the World Health Organization (WHO).
India has made remarkable progress against kala-azar over the past decade, bringing down cases to historically low levels through sustained surveillance, early diagnosis, improved treatment and vector-control measures. Yet public health experts point out that elimination as a public health problem does not mean the disease has disappeared. The parasite continues to circulate in some endemic areas, and experts fear that any relaxation in surveillance could allow transmission to return.
It is against this backdrop that the new vaccine assumes significance. It has been developed by Dr. Abhay Satoskar, an Indian-born physician-scientist and professor of pathology at Ohio State University College of Medicine. His interest in the disease dates back more than three decades to his days as a surgical intern in India.
Dr. Satoskar has often recalled the death of a young boy who appeared to have suffered only a minor fall while playing. The real cause, however, was an enlarged spleen weakened by visceral leishmaniasis, which ruptured after the accident. The incident, he says, convinced him that preventing the disease would ultimately save more lives than treating its complications.
"An effective vaccine is indispensable for the elimination of leishmaniasis. More than 10 per cent of the world's population is at risk. When I began my career, it was largely considered a disease of developing countries. Today, it has become a global concern," Dr. Satoskar said in a review published in the New England Journal of Medicine.
Unlike earlier experimental vaccines, which relied on weakened parasites carrying antibiotic-resistant genes and therefore raised safety concerns, the new vaccine uses CRISPR gene-editing technology to remove a key gene from a skin-limited parasite. The modified parasite is designed to stimulate protective immunity without causing disease.
In pre-clinical animal studies, the vaccine showed complete protection. It has also received approval from the US Food and Drug Administration as an investigational new drug, allowing human clinical trials to begin. Initial studies are expected to be conducted in Brazil and Kenya, with plans to include a study site in the United States.
Researchers believe the vaccine represents an important scientific advance because no licensed human vaccine against leishmaniasis currently exists despite decades of research.
Leishmaniasis has traditionally been associated with tropical and subtropical regions of Asia, Africa, the Middle East and Latin America. However, researchers say climate change, environmental degradation, deforestation and increased human movement are altering the distribution of sandflies, allowing the disease to spread into newer regions.
"Sandflies do not recognise national borders," said Dr. Bradford McGwire, infectious disease physician and Professor at Ohio State University Wexner Medical Center. He noted that infected sandflies are increasingly being detected in parts of the United States bordering Mexico, while locally acquired infections have also begun to appear.
The WHO estimates that about six million people are currently living with leishmaniasis, and nearly one million new infections occur every year.
The disease occurs in three principal forms. Cutaneous leishmaniasis causes skin ulcers that may heal with permanent scars. Mucosal leishmaniasis affects the nose, mouth and throat, causing severe tissue destruction and disfigurement. Visceral leishmaniasis, the most severe form, attacks internal organs such as the spleen, liver and bone marrow and can be fatal without timely treatment.
For India, visceral leishmaniasis has historically posed the greatest challenge. Bihar has accounted for the overwhelming majority of reported cases, although Jharkhand, West Bengal and Uttar Pradesh have also remained endemic.
The country, however, has achieved one of the most successful public health interventions against the disease through the Kala-Azar Elimination Programme (KAEP).
In a review published in PLOS Neglected Tropical Diseases last year, Dr. Shyam Sundar, Professor of General Medicine at Banaras Hindu University and one of India's foremost kala-azar experts, described the programme as a major public health success built on scientific innovation, political commitment and coordinated implementation.
According to Dr. Sundar, India's success was driven not by vaccination but by a combination of early diagnosis, effective treatment and sustained vector control.
"The Kala-Azar Elimination Programme succeeded because it combined early diagnosis with effective treatment. The introduction of the rapid rK39 diagnostic test allowed early detection without relying on invasive and risky parasitological tests. At the same time, oral miltefosine and later single-dose liposomal amphotericin B (L-AmB) provided safer and more effective treatment options, replacing older therapies that were often toxic and less effective," he noted.
He observed that these advances fundamentally changed India's strategy against kala-azar.
"The new diagnostic and treatment tools not only improved patient outcomes but also reduced transmission within communities, leading to a rapid decline in the number of cases," he said.
However, Dr. Sundar cautioned that maintaining elimination presents a different set of challenges.
"The real challenge now is to sustain the gains and ensure that visceral leishmaniasis incidence remains below the elimination threshold. Maintaining elimination is often more difficult than achieving it," he wrote.
According to him, post-kala-azar dermal leishmaniasis (PKDL) and HIV-visceral leishmaniasis co-infection remain the biggest threats because both serve as reservoirs for the parasite.
"Patients with PKDL often delay or avoid treatment because the skin lesions are painless, while patients with HIV-VL co-infection frequently relapse until their immune status improves with effective antiretroviral therapy. These reservoirs can sustain transmission and trigger fresh outbreaks," he explained.
Dr. Sundar also pointed out that treatment for PKDL remains less than satisfactory and warned that declining disease transmission may gradually reduce community immunity, increasing vulnerability if the parasite re-emerges.
For this reason, he stressed that active case detection, surveillance for both visceral leishmaniasis and PKDL, prompt treatment, sustained indoor residual spraying and continued political commitment remain essential to prevent a resurgence.
Against this backdrop, researchers believe that a safe and effective vaccine could eventually complement—not replace—India's existing control strategy.
Even if the vaccine proves successful, experts say early diagnosis, prompt treatment, surveillance and vector control will continue to remain the backbone of India's kala-azar programme. However, vaccination could provide an additional layer of protection by preventing infections, reducing transmission and helping countries sustain elimination over the long term.
