The vaccine, developed by a team at the University of Melbourne, Australia, targets a tapeworm called Taenia solium. The adult form, caught from eating undercooked pork, grows only inside the human intestine, stealing nutrients from the host.
This can be debilitating. But far worse is what can happen if people swallow any of the microscopic eggs shed in vast numbers in the faeces of those carrying the adult form. The eggs hatch in the intestine and the immature parasites, known as oncospheres, burrow through the gut wall. They then migrate through the body and lodge themselves in muscles, the eyes and the brain, causing a disease called cysticercosis.
Its symptoms vary depending on the location and number of what are called cysticerci. Many people have no serious symptoms, but some go blind, become confused, suffer difficulty with balance or have epileptic seizures. Heavy infections can kill, often as a result of inflammation after the parasites die. The parasite has been virtually eliminated in developed countries, but even in the US there are around 1000 cases each year, mainly among immigrants. In central and South America, Asia and Africa, millions of people have symptoms of the disease.
While humans are the main host for T. solium, pigs are the intermediate host, and an essential step in the tapeworm's life cycle. If a pig eats the eggs then, as in humans, the eggs hatch in the intestine, burrow through the gut wall and travel to muscles, where they form cysticerci. The infection rarely causes symptoms in pigs.
Only cysticerci- not eggs- can develop into the adult form, so if you prevent infection in pigs, you can break the life cycle, says team member Charles Gauci. The Australian group has created a vaccine based on two proteins found on the oncosphere. T he vaccine triggers the production of antibodies by the immune system that cause oncospheres to burst.
Small-scale trials, in which eggs isolated from adult tapeworms were fed to up to 30 pigs, have already been conducted in Mexico, Peru, Cameroon and Honduras. The vaccine provided between 99.5 and 100 per cent protection in every trial. The Melbourne researchers, together with collaborators in Lima, Peru, now have plans for larger field trials in which the pigs will be allowed to forage as normal, they reported at a conference on parasitology in Melbourne last week. At the moment, two vaccinations about one month apart provide several months of immunity. The team's aim is to provide lifelong immunity with one or two shots, though they say the vaccine will still be beneficial even it has to be given yearly.
There is no reason why the vaccine would not work in people too. But safety issues make it much more costly to develop a human vaccine, and it should not be necessary, says Gauci. "An effective control programme would involve treating people with existing drugs to remove tapeworm from a population and preventing the reinfection of the human population by vaccinating pigs," he says.