A popular herbal remedy can cut the risk of catching a cold by more than a half, according to a scientific assessment of the supposed health benefits of the echinacea plant.
Scientists reviewed 14 previous trials of echinacea to assess whether the herb really works against the common cold virus and concluded that it can reduce the risk of infection by 58 per cent. The researchers also found that echinacea remedies can reduce the time that a person, once infected, is affected by a cold virus by an average of 1.4 days - a statistically significant reduction.
Echinacea, a group of native North American plants, has long been considered to have medicinal properties but the latest study, published in the journal The Lancet Infectious Diseases, appears to justify its reputation when it comes to the common cold.
"Echinacea is one of the most commonly used herbal products, but controversy exists about its benefits in the prevention and treatment of the common cold," according to the scientists, led by Craig Coleman of the University of Connecticut School of Pharmacy in Hartford, Connecticut.
"An analysis of the current evidence in the [scientific] literature suggests that echinacea has a benefit in decreasing the incidence and duration of the common cold," they say.
However, the researchers stopped short of recommending the prescription of echinacea to prevent or treat the common cold until further research based on larger-scale trials can show which doses and preparations are the most effective.
Colds are caused by any one of more than 200 viruses that can infect the respiratory tract and echinacea contains several natural compounds that may have an effect on the body's immune system. It is therefore difficult to assess what it is in the herb that can prevent the symptoms of an attack by such a wide range of viruses. The scientists found, for instance, that some studies suggested that echinacea is less effective against rhinovirus, one of the most ubiquitous cold viruses. Only 35 per cent of people who were deliberately inoculated with rhinovirus gained some protection when given echinacea as a prophylactic.
"With more than 200 viruses capable of causing the common cold, echinacea could have a modest effect against rhinovirus but marked effects against other viruses," the scientists say.
The echinacea botanical group includes nine species of plants indigenous to North America and three in particular are widely used in traditional remedies, some dating back to the plains Indians.
There are more than 800 herbal products based on flowers, stems or roots of echinacea and scientists have some evidence to suggest that three chemical constituents of the plant - alkamides, chicoric acid and polysaccharides - may stimulate the immune system either on their own or in combination with other herbal ingredients. One of the 14 studies, for instance, found that echinacea taken with vitamin C can reduce the incidence of colds by 86 per cent, but the reasons were not known.
"The mechanisms of action underlying the proposed immunostimulatory effects of echinacea remain unclear," the scientists say.
Other scientists gave the findings a cautious welcome. Professor Ronald Eccles, the director of the Common Cold Centre at the University of Cardiff, said: "Harnessing the power of our own immune system to fight common infections with herbal medicines such as echinacea is now given more validity with this interesting scientific evaluation of past clinical trials."
Professor Ron Cutler of the University of East London said that people with impaired immune systems may benefit from taking echinacea ... but healthy people do not require long-term preventative use.
"There has also been the suggestion ... that continuous treatment with echinacea is not recommended - the benefits may only be effective for one or two weeks - and that after taking the agent for this time people should stop and give the immune system a week without the agent," Professor Cutler said. "The true benefits and how the agents work remains unclear and further better controlled ... clinical trials still have to be carried out," he said.
From plants to drugs
* Aspirin and willow:
The active ingredient of aspirin, salicylic acid, comes from willow bark. The trees have been used since ancient times to treat rheumatism and other aches and pains. Edward Stone, an Oxfordshire vicar, described in 1763 how he treated fever with powdered willow bark.
* Heart drugs and foxglove:
The foxglove plant, Digitalis purpurea, has been used since the 18th century to treat heart conditions because it seemed to have a calming effect on irregularities in the heart's rhythm. Scientists have since identified its active ingredient as digitoxin, or digoxin, and some heart drugs are based on these natural compounds.
* Malaria and artemesia:
The artemesia shrub has been used in Chinese medicine for thousands of years. It has since been shown to contain artemisinin, which is especially effective against drug-resistant forms of malaria. Its effect on cancer is the subject of ongoing trials.