I can vaguely remember the early 1950s, when polio was a dreaded disease in the Republic. Soon afterwards, state-wide vaccination against polio, or poliomyelitis, effectively protected us against the condition. Vaccination has proved so successful that, in 1988, the World Health Organisation embarked on an ambitious campaign to rid the world of the polio virus. That campaign is nearing completion, but many fear its success might have terrible consequences.
Poliomyelitis has plagued mankind since the time of the ancient Egyptians. It is caused by the polio virus, a member of a group known as enteroviruses that colonise the lining of the digestive tract. A virus consists of a small amount of genetic material enclosed in a protein shell. The virus does no harm in the intestine and is usually inactivated by the immune system before it can enter the bloodstream and cause damage.
The polio virus also has a strong affinity for nerves in the spinal cord and in the brain. Occasionally, perhaps due to an inadequate immune response, the virus breaks into the blood, from where it can get to the brain or through muscle tissue to the nerves, where it subverts the normal functioning of these cells. The disease is characterised by flu-like symptoms, muscle spasms and paralysis that is sometimes permanent.
Polio was not a major public-health problem until the 20th century. It usually spreads via contaminated water, and throughout history almost every child was infected shortly after birth, thereby acquiring permanent immunity against the disease. Only the very odd child who somehow avoided early infection was subsequently at risk.
Public water supplies were improved early in the 20th century, depriving babies of automatic early immunisation. Public water supplies are prone to occasional contamination by faecal matter carrying the virus, and in this manner the disease would break out here and there, striking hard and without predictability. By the early 1950s, polio killed or crippled thousands of children and young adults each year in the United States.
In 1955, Jonas Salk announced that he had developed a successful vaccine against polio. Vaccinations induce an effective immune response without causing the disease. Salk's vaccination was an injection of the dead polio virus. It was largely supplanted in the early 1960s by an oral vaccine developed by Albert Sabin. This vaccine consisted of live but weakened virus. By the mid-1960s, thanks to vaccination, polio had lost its grip in the US and Europe, but it remained a problem in many less industrialised nations.
Sabin's vaccine had several advantages over Salk's, being cheaper to produce and easier to administer. It does have one disadvantage, however. The oral polio virus remains capable of mutating - spontaneously changing its genetic information - into a form capable of inducing paralysis. This happens infrequently and usually causes no harm, as the mutated form is not very potent. Vaccine-associated paralysis occurs in one case out of 2.4 million, however. Consequently, on January 1st last year, the US switched from oral vaccine to a new version of Salk's dead, injected vaccine, which cannot switch to a new disease-causing form.
The World Health Organisation's plan to eliminate the polio virus from nature depends on a prolonged campaign of worldwide immunisation. Since the programme began in 1988, the number of annual new cases has fallen by 80 per cent. The target date for a polio-free world was December 31st last year, but that target was not met. It is particularly difficult to effect immunisation programmes in some war-torn Third World countries.
After the world has been declared polio-free for a number of years, the plan is to cease immunisation, which presently costs $1.5 billion a year. This money would be freed up to tackle other diseases, such as tuberculosis and malaria.
The World Health Organisation's plan has a threefold thrust: eliminate the polio virus from the wild; gather all laboratory research stocks, storing them in high-security containment facilities; and destroy the stocks. The organisation has been inspired by a previous "success" with smallpox, but the smallpox experience is not reassuring. A worldwide immunisation programme drove the smallpox virus from nature by 1980, and vaccinations against the disease have ended since. It was planned to gather together all laboratory research stocks of the virus and destroy them by 1993.
By 1991, however, it was discovered that the former Soviet Union had for decades secretly been growing tons of smallpox virus, as well as developing systems to spray it into the atmosphere, as a devastating biological weapon. It is believed that stocks of the virus are now in the hands of terrorists and several rogue states. Consequently, destroying the remaining laboratory stocks made little sense, and smallpox remains with us, although the natural spread of the disease has been halted.
The elimination of the polio virus from its natural niches would be a public-health triumph. It would be unwise to stop immunisation for the foreseeable future, however. In a vaccine-free world, the polio virus could be developed into a powerful biological weapon. The virus is very small, with a correspondingly small amount of genetic information. This means that if developers of biological weapons couldn't lay their hands on a stock of the virus, they could easily synthesise it from scratch, using modern technology.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC