A North Korean defector who was shot as he crossed the border was carrying an "enormous number" of parasites in his intestines, his doctors have said, including some never before seen by a respected South Korean surgeon who operated on him.

The unnamed soldier was left critically injured on Monday after fleeing North Korea via the Joint Security Area (JSA) in the demilitarized zone (DMZ) under a hail of bullets fired by his former comrades in the military.

An estimated 40 rounds were fired by four soldiers as the defector ran for the South after his jeep got stuck in a ditch trying to reach the Military Demarcation Line (MDL), which bisects the two countries inside the JSA.

The man was left with five bullet wounds to his abdomen, shoulders and elbow, the Korea Herald reports.

Lee Guk-jong, who has operated twice on the soldier at the Gyeonggi Suwon Ajou University Hospital in Gyeonggi Province, expressed shock over finding dozens of unusual parasites in the man’s stomach.

“We are struggling with treatment as we found a large number of parasites in the soldier’s stomach, invading and eating into the wounded areas,” Mr Lee was quoted as saying by the Korea Biomedical Review.

The longest parasite found measured 10 inches, Mr Lee told journalists.

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"I have been doing surgery for more than 20 years, but I have not seen such parasites,” the surgeon added. “I will not be able to find them in (South) Korea."

"The parasites lay 200,000 eggs a day.”

Seong Min, a professor at Dankook University Medical School, has previously led studies into the parasites carried by some North Koreans. He told the Review: “I don’t know what is happening in North Korea, but I found many parasites when examining other defectors.

“In one case, we found 30 types of roundworms in a female defector. The parasite infection problem seems to be serious even if it does not represent the entire North Korean population.”

The soldier, who remains unconscious, was shown on surveillance footage being chased across the JSA on Monday, according to South Korea Joint Chiefs of Staff.

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The footage was described as being "like it was out of an action movie". It has not been made public.

Minutes later the soldier was found collapsed and bleeding heavily in bushes 50 metres south of the MDL. A South Korea battalion commander dragged him to safety, the Korea Herald reported.

The soldier is the first North Korean to defect to the South through the JSA in 10 years. According to a South Korean politician the man is in his twenties and held the rank of staff sergeant.