Since February, Yom Nob has been in the Ta Sanh Health Center observing samples taken from malaria patients in six nearby villages. The laboratory officer has twenty years of experience working with malaria, but he was seeing something new.

Rapid diagnostic tests in the area were equipped to detect more common kinds of malaria-causing parasites – Plasmodium falciparum and Plasmodium vivax. But the tests were unable to detect malaria in some of the district’s patients, even though they exhibited all the symptoms of the illness.

“We have never seen this in our villages before,” Yom Nob told VOA Khmer last month.

After further analysis, Yom Nob realized the malaria was likely caused by a rare malaria-causing parasite, sending his samples and findings to the Institut Pasteur du Cambodge in Phnom Penh.

Last month, the government and Institut Pasteur du Cambodge sent a team to Samlout district and collected at least 250 samples from villagers in Daun Troek, according to Yom Nob. Hundreds of other samples were taken from villagers in the neighboring villages of Phnom Rai, Chhak Roka, Srae Andoung, Ou Tontuem, and Ou Chrab.

VOA Khmer spoke to health workers who said the team was investigating if six malaria cases from Chhak Roka village were linked to Plasmodium knowlesi, a relatively rare parasite that causes malaria in Southeast Asia, especially Malaysia and parts of Thailand.

The malaria caused is similar to Plasmodium falciparum and can be treated with existing medications. According to research on the subject, the hosts for this parasite are likely macaques and baboons, which is then transmitted to humans through mosquitoes.

Huy Rekol, director of the Cambodia National Malaria Center, confirmed that a team had taken blood samples from villagers in Samlout district and captured mosquitoes to test for the presence of Plasmodium knowlesi. He added that investigations were ongoing, suggesting that preliminary indications suggest it might not be the rare malaria parasite.

“We have studied the case. We will release an official report...in the next two weeks,” Huy Rekol said.

“We will finish isolating the virus, and we’ll update [you] later. But the preliminary result is negative,” he added.

Benoit Witkowski, head of the Malaria Molecular Epidemiology Unit at Institut Pasteur, did not want to comment on the results of the investigation. He said Institut Pasteur was awaiting the results of the investigation, and that institute was only providing technical assistance.

He did add that even if Plasmodium knowlesi were to be detected it was not drug resistant.

“Existing data in literature suggests that P. knowlesi is very susceptible to available treatments and to my knowledge no drug resistance exists so far,” Benoit Witkowski said.

This would not be the first time Plasmodium knowlesi was detected in Cambodia. There were two confirmed infections of the parasite-causing malaria in Pailin province, according to a study of 1,475 patients, conducted between 2007 and 2010 by a researcher at Pasteur Institut.

At the time, the study recommended further wide-scale studies on the subject and research into whether simian species had been imported from overseas with the specific parasite or if it was already circulating within the country.

According to the World Health Orgnization’s 2019 World Malaria report, Cambodia reported no malaria-related deaths for the first time ever in 2018. As was the case with other countries in the Greater Mekong Subregion, malaria cases in Cambodia reportedly fell by 76 percent between 2010 and 2018, and malaria deaths fell by 95 percent over the same period.

Back in Samlout district, Tet Vichet has been a village malaria worker in the district’s Chhak Roka village for ten years. This is one of the six villages where samples were taken to test for Plasmodium knowlesi.

He said there were likely six potential cases of malaria linked to this specific parasite, adding that he had heard from other health workers that they were seeing similar cases as well.

He met Teak Nang, 25, and Sao Lyheng, 14, who fell seriously ill with malaria, around Khmer New Year in April. Tet Vichet said their symptoms evolved fast and were more severe than normal.

“I did the rapid diagnostic test on him, I couldn’t detect malaria,” referring to another villager who possibly had malaria from Plasmodium knowlesi. “If he had come one night later, he would have died.”

Samlout is located along Thai border, and north of the Cardamom Mountains, which is the country’s last remaining tropical forests.

In Cambodia, malaria is mostly prevalent in habitation near forested areas and is known as occupational disease, which means it affects specific high-risk groups, such as forestry workers and migrant populations who move to live and work in forested areas.

Teak Nang is a 25-year-old father of two, who grows corn and soybean in Samlout’s Chhak Roka village. He said the crop had not done very well this season, forcing him to work for people who wanted him to clear land in the forest.

Similarly, Sao Lyeng has been at home since March when the government closed all schools on account of the novel coronavirus pandemic, which has so far recorded 141 positive cases in the country.

Unable to go to school, he has been helping his neighbors harvest malva nuts from the forest, which is used in traditional medicine and foods.

Both Teak Nang and Sao Lyheng said they were working in Phnom Tro Ngol, an area around four kilometers from the village. The two are neighbors as well.

Despite likely having a relatively rare form of malaria, Sao Lyheng was not worried about the disease at the time, adding that the novel coronavirus was a bigger concern at the time.

“I had never thought I could have malaria,” Sao Lyheng said. “I was more worried about the coronavirus.”