Tighter infection controls in hospitals have contributed to a significant drop in Middle East Respiratory Syndrome (MERS) cases in Saudi Arabia, but a senior health official warns that sporadic primary cases will still pop up.

There is no cure or vaccine for MERS — a severe respiratory disease which causes cough, fever, breathing problems and can lead to pneumonia and kidney failure.

Researchers have not yet traced the source of a mysterious camel virus, that has killed at least 346 people in the Kingdom.

The lack of scientific evidence about how camels contract the virus, which causes MERS, echoes wider concerns about the threat posed to human health by animal-borne pathogens, including the Ebola virus.

“Coming into close contact with the nasal secretions of camels is a major risk factor,” said Tariq Madani, head of the scientific advisory board of the Health Ministry’s Command and Control Center (CCC) set up in June to handle the outbreak.

Yet studies of both camels and people infected with MERS in Saudi Arabia have given preliminary results that are helping authorities curb the disease’s spread, according to the scientist overseeing the work.

“Until now the camels we have examined have proven to be negative — and this is really very unexpected,” Madani was quoted as saying in a Reuters report.

“The main transmission is actually human to human,” Madani said.

MERS was first identified in humans in 2012 and is caused by a coronavirus from the same viral family as the one that caused a deadly outbreak of SARS in China in 2003.

Some 808 people in Saudi Arabia are known to have been infected with it since it was discovered, and 346 of them have died.

Madani said analysis showed most people were infected in hospitals.

“We found out that 97 percent of the cases were health care associated,” he said.

“And 3 percent of them were primary cases who probably acquired the infection from contact with camels,” Madani said in the report.

Asked whether this suggested the camels were being infected within Saudi Arabia’s borders, Madani said it was too early to reach conclusions, but said tests on imported camels will continue alongside studies of wild animals such as bats and monkeys that may also be harboring the virus.

Meanwhile, studies of 12 camels with which MERS patients had had contact showed six had the virus circulating in their noses, but tests of their milk and urine were virus-free — suggesting the risk to humans from drinking camel milk is far lower than from direct contact with nasal secretions.

Another study found that among a group of 36 workers in a camel slaughterhouse, 58 percent had MERS antibodies — indicating they had been infected with it at some point.

“None of them recalls having a severe infection or pneumonia,” Madani said.

“And this indicates that you get some sort of immunity as a result of repeated exposure to camels and the virus.”