Disease

Disease

Usually a parasite (which causes a disease) will not kill its host. This would be considered as being a successful parasite. Those parasites that kill their host will kill themselves in the process. There is an example that demonstrates that a disease can virtually eliminate a population, perhaps an entire species. That example involves a rapidly reproducing rabbit into Australia where rabbits had never before existed.

Domestic rabbits were first introduced into Australia with the first fleet. They were imported on many subsequent occasions but did not become feral except in Tasmania. It was after Thomas Austin brought twenty four wild rabbits from England in 1859 and released them on his property in southern Victoria, that the rabbit became established on the mainland. There may have been other unpublicized releases in Victoria and South Australia at the same time, but Austin received the credit, or rather the blame, for the introduction of the rabbit to the mainland.

EUROPEAN RABBIT

The establishment of the rabbit was initially regarded as a great success for the sporting gentleman. In 1866, only 7 years after its introduction, 14,253 rabbits were shot for sport alone on Austin's property. This was Australia's first intimation at the amazing reproductive capability of the rabbit from which the saying "Breeding like rabbits" would work its way into the Australian lexicon.

Sometime in the 1850's a man was charged at the Colac (Victoria) Police Court with having shot a rabbit, the property of John Robertson of Glen Alvie. He was fined 10 pounds. A few years later, Robertson's son spent 5000 pounds a year in an attempt to control rabbits. By 1869 it was estimated that 2,033,000 rabbits had been destroyed on his property and that they were as thick as ever. This illustrates beautifully what happened so often in different parts of Australia. Domestic rabbits were initially highly prized and many attempts were made to establish them, until the inevitable invasion of the wild rabbit only a few years later. To control the rabbit numbers once the rabbit had invaded land was almost impossible.

The rabbit spread from Austin's property and from other release points in both Victoria and South Australia. The rabbit took 15 years to reach the NSW border, another 15 years to reach Queensland and another 10 to reach Western Australia and the Northern Territory. Numbers were such that the movement of rabbits across the landscape was refered to as " a grey blanket". Australia witnessed the fastest rate of spread of any colonising mammal anywhere in the world.

Throughout Australia, shooters and trappers were being hired as rabbits devastated crops and reduced the carrying capacity of the land dramatically. Many fences were erected to control the spread of the rabbits, yet these were mostly unsuccessful. Early fences were destroyed by wombats, rabbits, kangaroos, buried by sand drifts and because of the vast lengths of the fences, they were poorly maintained. Rabbits were sometimes stopped by fences, but in plague proportions, there were so many rabbits piled up by the fences, that the rabbits acted as a ladder for others that simply walked over the fence. Rabbits also will climb fences and they have been known to climb trees up to five meters. Many techniques of rabbit control have been used in attempts to lower the rabbit population eg; fencing, warren ripping, warren fumigation and poisoning. All of these techniques are expensive, time consuming, labor intensive and often have to be repeated over consecutive years to achieve low populations yet they are necessary if any attempt is to be made at rabbit control. Although historical attempts at rabbit proof fences were largely unsuccessful, netted fences can be highly effective if maintained and control measures are taken within the boundary. Eventually the rabbits were competing with cattle for food. Something had to be done.

A search was made for some biological control agent. It was discovered that in South America, a relatively benign disease called myxomatosis was found to infect cottain tail rabbits. The disease was caused by a virus and although the cotton tails were not affected, it proved lethal to the European Rabbit introduced into Australia. The disease is transmitted by mosquitos and fleas. It was first field tested in 1938 and successfully released in Australia in 1950, and that resulted in a estimated 99% mortality rate. However the rabbits began a comeback because they were resistant to the disease. The efficacy of myxomatosis declined in the 1950's but has remained more or less constant since then and myxomatosis still plays a major role in limiting rabbit numbers. Rabbits with some degree of genetic resistance were selected for and populations became more resistant. Myxomatosis now kills about 50% of susceptible rabbits which are infected. In many areas this degree of myxoma mortality is sufficient to keep rabbits at low numbers. As rabbits become more resistant stronger myxoma strains are selected for and thus transmissibility and percentage mortality remains about the same. Until the virulence of the myxoma virus reaches some biological limit myxomatosis can be expected to exert about the same level of control in the future as it does at present.

RABBIT INFECTED WITH MYXOMATOSIS

MYXOMATOSIS VIRUS

The big question for us is there a possibility for such a disasterous epidemic or plague that would kill 99% of all humans on earth? The rabbits in Australia all came from an original population of 25. That means that the original gene pool was small and genetic variability equally small. Such a population is genetically weak. But the human species is highly diverse genetically and it appears highly unlikely that such a virulent virus or bacterium could devestate such a high percentage of the human population. Even so, a 10% death toll would be considered catastrophic.

But can we say for certain that such a plague would never occur? As the human population rapidly increases and the world becomes more densly populated, it will be easier and easier for new virulent strains of viruses to evolve and be transmitted from person to person because of crowded conditions. In addition, people who are suffering from malnutrition are more suseptable to infection.

Harvard University scientists estimate that by the year 2000 over 24 million people worldwide will have full-blown AIDS, and an additional 100 million people will be infected with the HIV virus. However, AIDS is not the only modern plague to afflict mankind. Virtually unheard of among our ancestors, over 100 different kinds of cancer now kill over 5 million people every year. The depletion of the ozone layer, which blocks much of the sun's ultraviolet radiation, is thought to be responsible for a dramatic rise in melanoma, a deadly skin cancer. At the present time, the HIV virus is not easily spread compared to other sexually transmitted diseases such as gonorrhea. Most strains of gonorrhea can be cured by antibiotics, however there is now a mutliple drug resistant form that no known antibiotic can cure and it spreads rapidly.

Even though the HIV virus is not easily spread, some countries in Africa have a 43% infection rate among sexually active people! Furthermore there is no known cure and as of 2004, no known vaccination to prevent the HIV from spreading to unaffected people...and for most, it results in a miserable death.

The world recession, resulting in poverty and homelessness, has devastated the health care systems of many countries and diseases once thought banished by modern science have made an alarming comeback. Epidemics of typhoid, diphtheria, and even the Black Death, have afflicted areas of India and the former Soviet Union. Deadly new strains of Malaria, Tuberculosis and Cholera are becoming resistant to all known anti-biotics and are killing millions of people. Even the common bacteria that causes pneumonia, children's ear infections and many other every day diseases are evolving into forms untreatable by all known medicines, threatening a chilling post-antibiotic era where even the simplest infections could quickly escalate into fatal diseases. These bacteria are called multiple drug resistant (MDR).

It is not possible to say that a "doomsday" virus could never evolve from present day existing viruses (even those that cause mild medical distress such as the flu), for example a new form of the HIV virus or Ebola Zaire virus that could be spread airborne. Even more threatening is the possibility of a mutant highly virulent or drug resistant bacteria evolving, especially now that antibiotics are widely dispersed and abundantly used.

Again it must be emphasized, that as more and more people pack onto this planet, the greater the possibility for an extremely disasterous plague occuring. The more humane way of using artificial methods of birth control is a far better choice than to let nature solve the worlds population by means of severe and drastic ways.