A worm's-eye view of the Incas

When the Spanish invaded South America many artefacts of the Inca people were destroyed. But now research into parasitic worms in fossilised human faeces is shedding light on the lifestyle of this ancient civilisation, says Kate Ravilious

The Inca people are usually remembered for introducing a sophisticated culture and civilised society to much of South America. In their heyday during the 15th century they were comparable to ancient Roman societies. Places such as Machu Picchu and Pisac are a visual reminder of their skill and precision in stone masonry, while the Inca Trail provides evidence of the roadways that they built, running the length and breadth of South America. And up in the Andes, the Inca terraces and irrigation channels, teetering along the hillsides, are still used to this day to farm the steep land. But now it turns out that the Inca people's lives were not as comfortable as they first appeared, and the marching progress of the Inca empire also carried a rather unwelcome gift.

Dr Calogero Santoro, an archaeologist at the Museo Arqueológico San Miguel de Azapa, University of Tarapacá, Chile and Professor Karl Reinhard, a pathoecologist from the University of Nebraska in Lincoln, USA, have spent the past 10 years carrying out some rather grim research. They have been analysing ancient faeces and delving into the intestines of ancient mummies. Now their results are showing how the change in lifestyle imposed by the invading Incas caused parasitic disease to increase.

Much of the history and many of the artefacts from the Inca empire were destroyed by the Spanish when they invaded South America in the 1530s. The Inca roads were torn to shreds by the heavy wagons and horses' hooves of the Spanish invaders; many of the stone buildings were destroyed in the belief that they were the work of the devil, and the carefully crafted golden statues of Inca gods were melted down. For this reason we are surprisingly ignorant about the Inca civilisation as there is little left for archaeologists to piece together how they lived.

Reinhard and Santoro have resorted to analysing something that the Spanish invaders didn't destroy – human faeces. They have excavated ancient settlements in the Lluta valley, near the modern city of Arica, Chile. This area lies near a river in the Atacama Desert region. The proximity of the river means that the area has been inhabited almost continuously for thousands of years, while the dry desert climate means that the remains are well preserved. As time went by the soft soil next to the river subsided and houses sunk slowly into the mud. Forgotten odds and ends from people's daily lives were trampled in and travelled downwards along with the building foundations. Over time the history of the Lluta valley people has been preserved, layer by layer, in the soil.

One of the most important aspects of the excavation for Santoro was to uncover the latrines in the ancient settlements and collect samples of fossilised human faeces. In the Lluta valley he unearthed the latrines and collected individual stools from two time periods corresponding to two soil horizons. The uppermost horizon dated back to the time of the Inca occupation, during the 15th century. Foundations of small towns were evident, with both public buildings and family homes clustered together. The older and deeper horizon dated back to between 1100AD and 1400AD. At this point, the Incas hadn't yet arrived and people lived a fairly simple life. Small settlements were unearthed that contained just a scattering of houses and sometimes a cemetery.

Back in his lab in Nebraska, Reinhard carefully resuscitated each individual stool with mild chemical solutions so that they could be analysed, just like modern clinical faecal samples. He then looked at the contents of each stool under a high-powered microscope. Among the bits of undigested corn and manioc, Reinhard could identify the larvae and eggs of different parasitic worms. The four most common parasitic worms that he saw were pinworm, fish tapeworm, hymenolepidid tapeworm and whipworm. These parasites all live in the human gut, gorging themselves on the food passing through and often damaging the person's intestine. They still exist today but advances in modern medicine mean that they are less troublesome than they were in the past.

By carefully counting the numbers of each type of parasite egg and larvae in each slide Reinhard noticed a dramatic change in parasite infection between the pre-Inca and Inca time-periods. The pre-Inca people appeared to have a bit of a problem with whipworm and hymenolepidid tapeworm. Around 7 per cent of the pre-Inca faeces that Reinhard analysed contained larvae or eggs from these parasitic worms. "The unfortunate individuals who had whipworm or hymenolepidid tapeworm might have had to endure a nagging stomach-ache, and in the more severe cases they might have suffered from toxicity from the parasites' metabolic by-products," says Reinhard. It certainly wasn't pleasant, but the level of infection was mild compared with what was to come later.

Once the Incas settled in the valley a serious infection of pinworm occurred, with almost a quarter of the population carrying the larvae. On top of this, the first signs of the fish tapeworm appeared in that region. The pinworm is a particularly repulsive worm that wriggles down the intestine during the night and comes out to lay its eggs around the anus. This causes intense itchiness and often disturbs the sleep of the victim. Meanwhile, the fish tapeworm produces unique symptoms relative to other tapeworms such as stomach-ache, weight loss and anaemia. If anyone was unlucky enough to have more than one of these parasites, they would have been very weak indeed. "It is unlikely that any treatments were available for these parasites as no evidence for the relevant medicinal plants and herbs were found during the excavations," says Reinhard. "People would have just had to live with the parasites and in small children this may have been fatal if compounded with other diseases."

But what was it about the Incas that the parasites found so tasty? Reinhard began to think about how each parasite is transmitted, and realised that the change in lifestyle associated with the Inca civilisation could be to blame for the increase in parasitic disease. Both the whipworm and the hymenolepidid tapeworm are passed on by faecal contamination of hands, food, water and crockery. In contrast, pinworm is spread by person to person contact, or through the air, while the fish tapeworm is contracted by eating poorly cooked fish. "The pre-Inca people had problems with diet and poor-hygiene related parasites, while the crowded Inca towns developed the additional problem of airborne parasites," says Reinhard.

During pre-Inca times, the Lluta valley had a scattering of small, simple hamlets and the people survived mainly by subsistence farming. When the Incas arrived they abolished homesteads and re-settled people into large towns and cities, concentrating the population in a few places. "Population crowding caused poor sanitation and hygiene, and allowed the pinworm to get a hold," says Reinhard. The Incas also encouraged trading between places, and diet became more varied. Fish was transported inland from the coast, while tuber crops such as potatoes were transported down from the highlands. "Increased trading made fish a more common part of the diet, and the fish tapeworm began to proliferate."

The Incas' highly developed civilisation was the root of their problems. Building towns and opening up trade routes brought many benefits but not without the drawback of spreading disease. And Reinhard's research indicates that the level of parasitic infection would have had a real weakening effect on the Inca population.

Reinhard is not just trying to put historical records straight when he studies these ancient diseases. He hopes to shed light on the evolution of pathogens and the origins of ancient plagues. The ancient faeces offer several new clues, and now Reinhard hopes to use molecular probes to search for DNA and see how diseases develop over time. The Spanish conquistadors may have melted down all the gold, but to Reinhard and Santoro human excrement has turned out to be the more valuable commodity.

This research is soon to be published in a special publication of the 'Memórias do Instituto Oswaldo Cruz'