Saturday, September 17, 2005

What is plague

It was 229 years after the Great Plague dealt London such a terrible blow that the probable cause of the disease was discovered. In 1894, during an epidemic in Hong Kong, two rival research teams – one led by the Japanese scientist Shibasaburo Kitasato and the other by the Frenchman Alexandre Yersin, a former pupil of Louis Pasteur – isolated the bacillus Pasteurella pestis (now called Yersinia pestis) that is responsible for plague.
Further research showed that this was a disease of black rats and other rodents, spread by their fleas. When all the rats died, the fleas would frantically look for new hosts: human beings. The plague bacillus is extremely virulent. Laboratory mice die after being infected with just three bacilli – and fleas can disgorge up to 24,000 in one bite.
The different types of the plague
There are three human types of the disease.
Bubonic plague is the most common. A flea bite deposits the bacilli into the body's lymphatic system. As a result, the disease is characterised by 'buboes', large, inflamed and painful swellings in the lymph glands of the groin, armpits or neck, depending on where the flea bite occurred. Historically, 60% of all those infected died of bubonic plague.
In septicaemic plague, which is almost invariably fatal, the bacilli enter the bloodstream directly, rather than the lymphatic system where they might be contained. Like bubonic plague, the septicaemic variety is caused directly by flea bites.
Pneumonic plague is the most deadly: it is usually fatal and it does not require flea bites to spread. When the bacilli reach the lungs, severe pneumonia occurs, and the bacilli are present in the water droplets spread by coughs and on clothing. Thus it is highly contagious, especially in crowded, poorly ventilated buildings.
Death occurs quickly with pneumonic plague – within three or four days – and even faster with the septicaemic type: 24 hours. In all three, internal bleeding causes large bruises to appear on the skin – hence the plague's name in the 14th century, the Black Death.
It is widely believed that the Great Plague's memory lives on in the nursery rhyme 'Ring-a-ring o' roses': the 'roses' refer to the red spots that appear over the buboes, and 'A-tishoo! A-tishoo! We all fall down!' recalls the violent coughing and swift death that accompanies pneumonic plague.
A cure for the plague
In 1932, with sulphonamide drugs, there was finally an effective treatment for the plague. Yet, within 50 years, there were reports – persistent, though unconfirmed – that a strain of plague bacillus resistant to all medicinal and control measures had been developed for bacteriological warfare by the US army.
Plague is still endemic in parts of Asia and, surprisingly, in the western United States, where it is spread by squirrels, prairie dogs, chipmunks and other rodents and infected family pets. In 1992 (the latest figures), the World Health Organisation reported 108 deaths from 1,768 cases of bubonic plague worldwide. There is a vaccine, but because it takes several weeks to become effective, it is of little use in an epidemic.
A new theory?
In a new book – Biology of Plagues (Cambridge University Press, 2001, £65) – Susan Scott and Christopher J Duncan have challenged the belief that bubonic plague is caused by a bacillus carried by fleas travelling on black rats.
Scott researched the parish records of the small town of Penrith in Cumbria and discovered that, not only did plague reach this far outpost of England in the Middle Ages, but it had a definite spread from person to person within households. In addition, black rats would have not been present – they tend to congregate in areas of high human habitation, especially seaports, and in more temperate climes – and the only other likely animal carrier, the brown rat, did not arrive in Penrith until about 1720.
From this information, the two authors have concluded that the bubonic plague that affected Penrith was an infectious disease – that is, one spread from person to person – not a rat/flea-borne one. When they looked for a modern equivalent, they plumped for the Ebola and Marburg viruses, two of the most infectious and lethal viruses known today.


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