The spread of the Zika virus across tropical Southeast Asia is likely to be significantly under-reported as local health authorities fail to conduct adequate screening, regional experts said on Monday.
International travel hub Singapore confirmed 41 locally transmitted cases of the mosquito-borne virus on Sunday and said it anticipated more, raising fears about how quickly it could disperse throughout the region.
Other parts of Southeast Asia have reported dozens of cases of Zika, which in Brazil has been linked to thousands of suspected cases of microcephaly, a rare birth defect, but there are fears the true numbers are masked.
"Zika is under-reported and under-diagnosed," Khin Myint, head of the emerging virus research unit at Indonesia's government-funded Eijkman Institute, told Reuters. "We find many cases are not presented in hospitals because it's a relatively mild disease with mild symptoms and people are not going to doctors."
The World Health Organization (WHO) lists Indonesia, Thailand, the Philippines and Vietnam as countries with "possible endemic transmission or evidence of local mosquito-borne Zika infections in 2016."
But the Jakarta-based Eijkman Institute, the main body testing for Zika in Indonesia, said it tested only 1,000 people in the past year - a tiny number in Southeast Asia's most populous country. It found just one positive case, despite reports that Zika is prevalent in the country.
Thailand has recorded the highest number of cases in the region at almost 100 infected people across 10 provinces this year.
"Thailand's surveys have not been thorough enough," said Kriengsak Limkittikul, assistant professor at the Department of Tropical Medicine at Mahidol University in Bangkok, noting that people without symptoms are often not tested. "Screening is inadequate in other countries in the region, too, where health authorities are ill-equipped to test," he said.
Vietnam has recorded three cases of Zika infection, all locally-transmitted, while Cambodia has reported seven cases.
Hong Kong confirmed its first case of Zika on Friday, in a woman who had traveled to the Caribbean.
A major difficulty is that while Zika can cause mild fever, a rash and red eyes, an estimated 80 percent of people infected have no symptoms.
There is no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya and is transmitted by mosquito. There have also been a small number of cases of sexual transmission reported in the United States and elsewhere.
Some countries in the region began ramping up protective measures following the outbreak in Singapore.
Muhamad Subuh, a senior Indonesian health ministry official, said authorities are "in the process of stepping up health checks at main airports and ports, including in Batam," the island closest to Singapore.
Indonesia introduced thermal scanners at airports targeting arrivals from Singapore over the weekend, and plans to roll them out to ports. Authorities are also handing out printed warnings to all incoming passengers, noting Zika has an incubation period of around 10 days before presenting any symptoms.
Malaysian Health Minister Subramaniam Sathasivam said travelers who enter Johor Bahru, where up to 200,000 people commute daily to and from Singapore, will go through thermal screening at border checkpoints. People crossing in private vehicles will not be scanned but handed a pamphlet detailing the symptoms and asking them to report to authorities if they develop those.
Malaysia is also stepping up vector control mechanisms including fogging and larvicidal spraying in Johor Bharu.
"What we can do as a country comes down to how well we control our vectors, and at this point of time, dengue is still a bigger problem than Zika because people can die from dengue," Subramaniam said.
Other countries were not responding so quickly.
In Thailand, the Department of Disease Control said it was screening athletes returning from the Olympic Games in Brazil, but otherwise not changing its prevention methods. Vietnam and Cambodia made no immediate changes to their border controls.
"Different countries have different economic and political situations and it's not that they don't want to screen the virus, but they don't have the resources and capacity to do it at this stage," said Jasper Fuk-Woo Chan, a clinical assistant professor at the Carol Yu Centre for Infection at the University of Hong Kong.