But according to his official citation, supplied and signed by the local Zambian district council, he was respectability personified. "Mr Nyirendra is a true herbalist," this document proclaimed. "The medicine he is using is not dangerous, but for curing, and he is doing very fine."
Nevertheless, when he donned a long, blood-red cotton robe, and a muffin- shaped hat of the same colour, fringed with yellow, his appearance became somewhat sinister. Diagonally across his torso he slung several strings of beads. On his chest hung a small, beaded casket and in either hand he held a wildebeest tail - a hank of shiny black hair a foot long, bound with rings of white and crimson and with yellow beads at the root.
Thus equipped, he took up position in the middle of a small grass hut. On his right stood an assistant, holding a tattered Bible open in both hands, as if reading the lesson in church. I and my two companions squatted on the ground in a corner.
When the first patient of the day - a hefty young woman - came and stood in front of the n'ganja, he began to swish one of the wildebeest tails fast up and down with his left hand, holding the handle of the other to his right temple, while keeping his eyes tightly shut.
"He is calling the spirits," explained Sanford, our guide. Faster, hypnotically faster, went the swishing tail, up and down, up and down. Falsetto gibbering began to pour from the witch doctor's mouth. "Can you understand what he's saying?" I whispered. Sanford shook his head. "The spirits are speaking to him."
For several minutes the torrent of squeaky utterances continued, ascending higher and higher until it sounded like birds twittering. Occasionally the n'ganja held both tails in his left hand, and with his right made precise, clawing movements at the air. Then, for the benefit of the girl, the assistant began to interpret in the local language, and Sanford translated for us: "The trouble is in your stomach. It can be cured. I will make medicine to purge it. You must take the medicine I give you..."
In five minutes the consultation was finished. The patient handed over 5,000 kwachas - about pounds 1.50 - which, in her terms, was a considerable sum, and went off looking thoughtful. But it took the witch doctor some time to wind down from his trance, which he finally seemed to throw off by rolling his neck backwards and giving several loud grunts of "Oh!"
Then, on their knees, he and the assistant began sorting through a litter of sticks and roots, broken into short lengths, which lay on the rush matting at the back of the hut. As they pushed different kinds of wood into heaps, Sanford steadily translated their comments: "This bark, ground up and put in your porridge, is for the ladies, to keep their bodies warm for a man. It keeps men strong also. This one is for back pain, this one for the stomach."
Soon it became clear that the doctor had medicines not only for physical ailments, but for psychological ones as well. Besides normal illness, he could ward off spells put on people by a fiti, or sorcerer. "The mwanja creeper is to stop witchcrafts coming into your house; the babguru bush gets rid of spirits."
Outside, in the hot sunshine of the Zambian winter morning, it would have been easy to dismiss all this as mumbo-jumbo. But the point is that, in his own environment, the witch doctor's remedies work. With no recourse to Western doctors or medicine, people rely on him absolutely, and - whether through the chemical properties of his potions, or through auto-suggestion - many of them are cured of their ailments.
By Western standards, his village is desperately poor. The people live in grass huts, without electricity or sanitation, and their water has to be fetched from a stream two kilometres away. Their diet consists almost exclusively of maize, which they dry, grind and cook into a kind of porridge. Their circular, grass-walled stores, raised on stilts to keep out termites, are frequently raided by elephants. The nearest hospital is 20 miles away, but most people have no means of reaching it; and even though a few rural clinics have been established, they lack most basic equipment and drugs.
"We depend heavily on traditional doctors," said Joseph Mwanza, headmaster of the school in Mwizala, another village close by. "Without them, many of us wouldn't have reached the age of 25."
Parents are supposed to contribute two dollars per annum for each child attending his establishment, but in practice the families have so little money that this year only 60 of the 320 pupils have been paid for. "We are born in poverty and grow old in poverty," he said cheerfully, "but we can't get ashamed of it, because it's not a thing of our making."
One cause for optimism is that money from wildlife tourism in the South Luangwa National Park, whose eastern boundary is close by, has now started filtering through to local communities, instead of being creamed off by central government.
Mwizala has benefited not only from an official scheme, but also from the generosity of Robin and Jo Pope, who run tourist camps and walking safaris in the park.
The village, in consequence, has a supply of clean water from its own borehole - an incalculable benefit; but it is still looking for funds to buy modest extras such as mosquito nets for visitors to sleep under, and a volley-ball for the school.
For anyone caught up in arguments about rural conservation in Britain, it is a salutary experience to visit a country where rats, boiled whole and dried in the sun, are regarded as rare delicacies, and where the idea of set-aside - of paying farmers not to grow food - is not merely incomprehensible, but manifestly insane.