UNM biologists partner with PAHO to confirm elimination of parasitic disease

Schistosomiasis (shis-tō-sō-my-uh-sis) is one of the world's most prevalent neglected tropical diseases, currently infecting over 200 million people, mostly residing in Africa. This disease, caused by parasitic worms living in the human blood, has also affected communities in several Caribbean Islands since it was introduced during the transatlantic slave trade, circa 1500s.

For most of a decade, a team of University of New Mexico biology researchers affiliated with the Department of Biology, UNM's Center for Evolutionary and Theoretical Immunology (CETI), and the Museum of Southwestern Biology has partnered with the Pan American Health Organization (PAHO), which is world's oldest international public health agency and is the specialized health agency of the Inter-American System, to confirm the disease's elimination status. The team's specific purpose is to assess the continued presence of the vector snails on the islands and its infection status with the parasite involved.

UNM biologists Martina Laidemitt and Eric 'Sam' Loker have been collaborating on this project together since 2017, when Laidemitt was a doctoral student. They have traveled and worked with health workers on several islands, including Antigua, Montserrat, St. Lucia, St. Kitts and Nevis.

Their efforts will play a crucial role in supporting the data needed to declare the elimination of schistosomiasis, marking a major public health milestone for the region.

"A lot of people came together at the right time to take on this in-depth and timely project," said Loker. "I feel very fortunate to be involved when elimination truly appears to be attainable."

When the parasite was introduced to the islands, it found a welcoming host in an indigenous freshwater snail species Biomphalaria glabrata, which allowed the parasite to persist over time. The disease is transmitted to humans through skin contact with contaminated water, making it highly transmissible.

The parasite involved, Schistosoma mansoni, passes its eggs in the feces of infected people. If the eggs enter freshwater, they hatch, and swimming larvae emerge that seek out and infect particular kinds of freshwater snails, where they develop further, eventually producing numerous forms called cercariae. Cercariae then enter the water and penetrate human skin when people contact it while bathing or washing clothes. Cercariae develop into adult worms living in blood vessels in the liver and around the intestine.

According to the CDC website, early symptoms of schistosomiasis are an itchy rash. Within one to two months, an infected person may experience fever, chills, cough and muscle aches. If left untreated, schistosomiasis might eventually result in severe, potentially fatal liver and intestinal damage. Heavy, sometimes fatal infections were once a common occurrence on several islands, including Puerto Rico.

Concerted efforts on many fronts, improved living standards and some unforeseen developments have led to steep declines in the prevalence of schistosomiasis in the Caribbean region, so much so that its continued presence in the area has been doubted.

Laidemitt and Loker's current project is in the U.S., where they are working with health officials in Puerto Rico to gather and analyze samples relevant to decisions on confirming elimination.

"A lot of people came together at the right time to take on this in-depth and timely project. I feel very fortunate to be involved when elimination truly appears to be attainable."

-- Eric 'Sam' Loker

They have sampled over 100 freshwater sites in 5 different regions in Puerto Rico alone, many known to have once harbored the carrier snail and have not found evidence for its continued presence on the island.

"Puerto Rico once had an infection rate as high as 56% in some districts," said Laidemitt. "Through improving the island's water infrastructure, public health officials were able to make significant strides in reducing that percentage."

The vector snail species has sharply declined throughout the region, likely due to the introduction of multiple invasive snail species that are now widespread, but do not serve as suitable hosts for schistosome parasites.

In addition to collecting samples, Laidemitt and Loker worked with the PAHO officials to identify parasite DNA in freshwater samples, an approach that has also failed to find evidence of the vector snails or their involvement in transmission.

Besides sampling and running analyses, Laidemitt and Loker have held training sessions with local public health staff on each island.

"One aspect that has been at the forefront is the capacity-building component, training local offices to share expertise and develop local knowledge to ensure surveillance can continue," said Laidemitt.

These training sessions have also enhanced regional cooperation and improved knowledge of how to eliminate global neglected tropical diseases, leading to the formation of teams on the islands prepared to provide long-term health security.

To confirm elimination, the WHO review team will look for four factors:

1. Zero autochthonous cases, or cases that originate from within the area, in people, in the last 5 years
2. Evidence that transmission of the disease has been interrupted by monitoring the presence and infection status of the vector snails needed for transmission.
3. Proof that surveillance systems are in place and
4. Documentation submitted for international review.

"PAHO is currently preparing dossiers from each island's research and analysis reports to submit to WHO for scrutiny by world experts," Loker said. "This review group will make the judgment if the disease has been completely eliminated as a public health problem."

To learn more about Schistosomiasis, visit PAHO's website.