Science: Microbe on the Month: One hop ahead of the grim reaper: Bernard Dixon on the double-edged sword of myxomatosis

FORTY years ago, on the night of 11 August 1953, as a strong south-easterly wind was blowing from France to England, a 'temperature inversion' occurred over the Channel, the air becoming warmer from sea-level up to 500 metres.

It was almost certainly this meteorological combination that brought myxomatosis, the horrendous and lethal rabbit disease, to Britain for the first time.

Within a few months, the infection was spreading quickly and causing widespread consternation over the ugly and agonising way in which rabbits were dying, with swollen eyes and faces and grotesque, glutinous skin tumours.

The myxomatosis virus, introduced to France in 1952 to kill wild rabbits that were damaging crops, travelled rapidly from warren to warren. But it was also transmitted by Anopheles atroparvus mosquitoes, which breed in coastal marshes in France and England. They seem to have carried the virus over the Channel on that August night four decades ago.

The following morning, as the temperature inversion disappeared, the mosquitoes arrived in Bough Beech, near Edenbridge in Kent, where the first infected rabbits were observed in the middle of September. Tests completed a few weeks later confirmed that animals were succumbing to exactly the same strain of virus that had been released in France the previous year.

There are alternative ways in which myxomatosis may have reached the British Isles. The most obvious explanations are the introduction of infected rabbits and the deliberate infection of native animals, but there is no evidence to support these ideas.

Virus-infected rabbit fleas carried by birds are a remote possibility. But by far the most convincing explanation is that the mosquitoes were carried by the wind, as outlined by Robert Sellers, former director of the Animal Virus Research Institute at Pirbright in Surrey. A few years ago, he used Meteorological Office data to establish that the virus was most likely carried by mosquitoes from the departements of Nord, Pas de Calais and Somme. The distance was 120-160km (70-100 miles) and the journey time 6 1/2 to 8 1/2 hours.

Although myxomatosis arrived in the UK by accident, it had the same dramatic effect here as in those countries where the virus was deliberately introduced. After its overnight flight into Kent, it destroyed about 99 per cent of the rabbit population throughout the country. Subsequent events, however, provided lessons about employing 'biological control' - the use of naturally occurring microbes to combat agricultural pests.

When applied to insects, biological control has clear advantages over chemical pesticides, which may persist in the environment and have harmful side-effects. Yet its eventual success can be less conclusive than expected.

Our knowledge of myxomatosis dates from just before the turn of the century, when an Italian microbiologist, Guiseppi Sanarelli, established a public health laboratory in Montevideo, at the invitation of the Uruguayan government. The following year the rabbits he had taken with him (to prepare batches of serum containing antibodies against a broad range of infectious agents) died of a ghastly and extremely infectious disease. The infection, then unknown in Europe but presumably acquired from local rabbits, was myxomatosis.

In 1950, Australian agriculturalists released infected rabbits in the Murray Valley in an attempt to cull the European wild rabbit that had become a major pest. The splendid weather that summer and over the following two years provided excellent conditions for mosquitoes to breed and travel, and myxomatosis spread rapidly - with awesome consequences. Millions of rabbits - about four-fifths of those in south-eastern Australia - perished.

When myxomatosis was introduced to France, the epidemic developed at a similar furious pace, not only reaching the UK in little more than a year but also moving rapidly through Belgium, Luxembourg, Germany, Spain and the Netherlands. Yet by the early Seventies, Britain's rabbit population was increasing once more, and had again become an important factor in agricultural economics. In Australia, although most summers see a myxomatosis outbreak, the disease now has only minor significance in regulating rabbit numbers.

The principal explanation for these changes is a classical piece of Darwinism. In the midst of those massive epidemics, tiny numbers of rabbits displayed some measure of resistance to the virus. They survived, and as they bred from year to year, rabbit populations began to grow again.

Less predictable were corresponding changes in the virus. At first the deliberately released strains lost much of their virulence, as their hosts died off, and the resulting milder varieties killed far smaller proportions of rabbits. More recently, the virulence of the virus has increased in response to the high degree of resistance among animals that have survived.

Biological control can be catastrophically effective. But in the long term, its value may decline rapidly unless there is a sophisticated understanding from the outset of the interplay between a pest and its parasite, and their prospects of evolutionary change are much better understood.

Four decades after myxomatosis was unleashed on the rabbits of Europe, such a repugnant disease will probably never again be deployed for such a purpose. Or will it?

(Photograph omitted)