Could it be possible that the Black Death never disappeared from Australia and still lingers on unnoticed? A foolish question you might think since there has been no sign of plague cases in Australia since we were swept up in the last pandemic that dominated the world between 1890 and 1926 and which caused almost 1400 cases and 535 deaths in Australia.
Yet is it possible that plague might just have made a permanent home among Australian wildlife and persisted hidden away among native ground-living animals in remote parts of Eastern Australia?
Recent research in England has suggested that plague rather than disappearing after major epidemics simply retreated back into the habitat of ground-living rodents and lingered on unnoticed only to reactivate when warmer summers came along producing a surge in rodent numbers and their fleas. In such circumstances epizootics occasionally occurred and it was only then that humans became at risk. Extremely cold winters and heavy downpours of rain had the opposite affect causing a decline in the number of rodents and their fleas.
Most of the research into plague, however, suggests that while changes in weather conditions could and did influence the number and distribution of rodents and fleas its overall impact on major plague epidemics was not that important. Such research has always strongly argued that plague outbreaks among human populations in the developed world were the result of the disease being introduced to new locations via the medium of infected rats aboard ships which originated in parts of Asia where plague has been entrenched for centuries.
Certainly that seemed to be the case in Australia in the years from 1900 until 1909 when plague outbreaks were closely linked to infected shipboard rats passing the infection to wharf -side rats and from there to humans living close by. But is it that simple? Is there any evidence to suggest that plague might have established a permanent foothold among Australian native animals or indeed among urban rats?
Look at what happened in America, for example. Plague was introduced to San Francisco around the same time as it appeared in Sydney. From 1900 on the disease spread very quickly from infected urban rats to invade a wide range of indigenous ground- living rodents such as voles, deer mice, ground squirrels, prairie dogs and wood rats, and over the next 20 or so years became permanently entrenched in many parts of the Western USA.
Today, plague exists in permanent natural reservoirs among ground burrowing rodents across many of the Western States of the USA including California, New Mexico, Arizona, Colorado and Oregon. Significantly, plague among the animals in these reservoirs adapted perfectly to the life cycle and habits of their rodent hosts. Some of the rodent hosts develop immunity to the disease, others experience only a minor illness and in winter months the plague bacteria goes into hibernation along with its hosts only to reactivate when the weather warms.
Very occasionally an epizootic occurred such as in Kern and Tulare Counties in Nevada in March 1934 when thousands of squirrels and other rodents died as a huge outbreak of sylvatic plague swept over thousands of square kilometres. Not one human case was reported. Today such areas attract many campers and visitors and most seem unconcerned about plague. In fact a bumper sticker popular on some local cars actually read – LAND OF THE FLEA, HOME OF THE PLAGUE.
After 1909 plague disappeared in Australia only to reappear in 1921 in Sydney, Brisbane, Townsville and Cairns. Why did the disease disappear after 1909? Is it possible that plague simply went underground and lay unnoticed among domestic and native rodents until a combination of climatic, zoonotic and human factors produced another human epidemic in 1921? Or did plague totally disappear from Australia after 1909 and the 1921 resurgence simply represent a reintroduction from abroad? These are questions that remain unresolved to this day.
There is certainly evidence that plague established what was called temporary beachheads among Australian rural and bush rodent populations and it would appear that Australia's native animals were highly susceptible to plague. In 1902, for example, plague claimed a large number of native animals in Sydney's Zoological Gardens. Fifty-two animals died including wallabies, kangaroos, opossums and native cats as well a wide array of native birds.
Of some significance is the fact that plague also invaded some of Australia's rural wildlife. In the latter part of 1904, for example, plague broke away from its coastal urban focus when a major outbreak occurred among native rodents along the banks of the Clarence River near the town of Ulmarra. The area around Ulmarra was home to a rich variety of native wildlife including at least eight species of native rats and mice. Thousands of rats and mice are recorded as dying from plague in 1904 and the disease spread quickly along the river banks, through adjacent maize fields and nearby farms.
Outbreaks of plague also occurred in the agricultural areas around the towns of Lismore and Ballina. In all these cases it is more than likely that the disease was introduced to these areas via the river steamship which transported agricultural products, goods and food supplies between Brisbane, Sydney and the local areas.
In 1907 a major also epizootic raged among cane field rats in the sugar growing area of the Mossman district near Port Douglas in Queensland. This area supported a rich native rodent population with at least 12 species of native rats as well as cane, bush, swamp and field rats as well as native mice. Between January and May a major epizootic raged among such rats which eventually spread to involve to cane –cutters living in local camps. Little is known about whether the infection remained entrenched among local wildlife after 1907.
The penetration of plague into Australia's native wild life is more than just of passing interest. If what happened in the USA is any guide is it not possible that plague followed the same route in Australia and permanently established itself among some of our wildlife and has remained unnoticed for more than a century? To the best of the writer's knowledge there has never been any survey of the sera of Australian rodents to ascertain whether any plague antibodies are present.
At a time when plague is more geographically widespread than at any time in history and still remains an important threat to world health perhaps we should be more interested.