A wealthy Vietnamese woman sits at a roadside cafe and prepares a dish for her own consumption. She is grinding up rhino horn. After a few minutes, water is added and she drinks the mixture out of a shot glass as a cure for her kidney stones. The woman paid several thousand US dollars for the piece of horn. The rhino died.

The image is one of a series taken by South African photographer Brent Stirton, who won an award at the 2012 Veolia Environnement Wildlife Photographer of the Year exhibition, which opened at the Natural History Museum London on Friday. Entitled "Deadly Medicine: Rhinos", the six photographs in his award-winning portfolio depict the impact of poaching on the world, from game rangers in action to end users of rhino horn who believe it to be a medicine.

Poaching has become a major part of organised global crime and so well organised that it threatens to wipe out some of the world's five remaining species of rhinos. Two of them, the Javan and the Sumatran, have been reduced to populations of only a few dozen each. Populations of the other three – black, white and one-horned rhinos – are also threatened.

"The illegal trade in wildlife is now the third largest criminal industry in the world and rhino poaching plays a key role," Stirton told the Observer.

"It is driven by the growing affluence in Asian countries, particularly Vietnam and China, where people have believed for centuries that rhino horn can cure many illnesses, including cancer. The belief is part of their culture and deep rooted, and therefore very difficult to dislodge."

The Wildlife Photographer of the Year exhibition is now in its 48th year and has earned a reputation for its superb images that glorify nature. However, new categories have been introduced in recent years to allow photographers to feature more uncomfortable truths about humanity's impact on the planet. One winner last year captured the devastating effect of the Gulf of Mexico oil spill on wildlife, for example. This year's exhibition includes a chilling account of the plight of rhinos. Driven by soaring prices for rhino horn, the number of animals slaughtered has soared.

In 2007 only 13 rhinos were killed by poachers; last year that figure rose to 448 – more than in any previous year. This year a total of 455 have been slaughtered so far, it was revealed last week by the government of South Africa, where 90% of all rhinos live. Most of these animals – 272 of the 455 – were killed in the country's Kruger National Park and were white rhinos, the least threatened species. It is believed there are around 20,000 left in the wild.

Stirton visited parks across South Africa to take his images, which include one of a vet's assistant holding up the horn of a tranquillised white rhino from a game farm outside Klerksdorp. Its horns have been removed to save it from poachers. "Cutting off a horn can be an effective way of making a rhino safe from hunters," said Richard Sabin, a wildlife expert at the Natural History Museum. "The animal no longer has value. On the other hand, a horn has only to grow back a few centimetres before it becomes lucrative enough to make it worthwhile for a poacher to go back and kill a rhino. That is a reflection of the enormous prices that rhino horn can now command."

However, it is the image of the Vietnamese woman drinking rhino horn liquor that provides the most striking image in Stirton's portfolio. Her enjoyment at her task contrasts vividly with the blood shed to obtain that piece of horn. Getting the picture took considerable ingenuity.

"I posed as a Canadian who was trying to set up his own illegal market in my home country," said Stirton. "I persuaded the dealer, who is in the background, to let me see someone preparing it so that I would know what to do to set up my own market when I got back to Canada. She is grinding the horn in a dish specially made for the purpose. It has a rhino symbol on the side and has a rough base on which to grind the horn into powder. After a few minutes, water was added and she swallowed it. The woman told me she took it daily for her general health. The interesting thing is that rhino horn is an illegal substance in Vietnam. But both the woman and her dealer had no fear of the police and happily ground the rhino horn in full view of the street. The dealer told me he paid $1,500 a month to the right people so he could sell rhino horn with impunity."

The rising slaughter of the world's endangered rhinos is particularly ironic given the makeup of their horns, added Sabin. "Their horns are made of keratin. That is the stuff from which hair, fingernails and toenails are made. In other words, chewing your own fingernails would do you as much as good as taking powdered rhino horn, and at least that would avoid the slaughter of rhinos."

The Veolia Environnement Wildlife Photographer of the Year is at the Natural History Museum until 3 March 2013, before embarking on a global tour.