Merchants plying the ancient Silk Road between China and the Mediterranean moved more than gold, fabrics, spices and tea -- they also exported gut parasites, researchers said Friday.

It has long been theorised that the Silk Road helped spread bubonic plague, leprosy, anthrax and other infectious diseases between East Asia, the Middle East and Europe -- though concrete archaeological evidence has been scant.

But now analysis of the contents of an ancient latrine along the route has revealed evidence that traders 2,000 years ago did indeed spread disease.

The team from Britain and China examined faeces preserved on wood and bamboo sticks wrapped in cloth -- the toilet paper of their day -- that were excavated in 1992 at the Xuanquanzhi pit stop in north-west China.

Unearthed from a latrine dating back to 111 BC, during China's Han Dynasty, and which was still in use in 109 AD, seven samples yielded eggs from four types of parasite: roundworm, whipworm, tapeworm and Chinese liver fluke, the researchers wrote in the Journal of Archaeological Science: Reports.

The fluke, a parasite that causes pain, diarrhoea, jaundice and liver cancer, needs wet, marshy areas to complete a life cycle, whereas Dunhuang is in an arid area on the edge of the desert.

"The liver fluke could not have been endemic in this dry region," said a statement from Cambridge University, whose researchers took part in the study.

"In fact, based on the current prevalence of the Chinese liver fluke, it's closest endemic area to the latrine's location in Dunhuang (in north-west China) is around 1,500 kilometres (930 miles) away, and the species is most common in Guandong Province -- some 2,000 km from Dunhuang."

Xuanquanzhi in Dunhuang was a popular stopping place for merchants, explorers, soldiers and government officials.

"Finding evidence for this species (liver fluke) in the latrine indicates that a traveller had come here from a region of China with plenty of water, where the parasite was endemic," said study co-author Piers Mitchell.

"This proves for the first time that travellers along the Silk Road really were responsible for the spread of infectious disease along this route in the past."

The Silk Road is so called for perhaps the most famous commodity that crossed its inter-connected network of trade routes criss-crossing Eurasia.