Rain and disease

Excessive rain not only causes floods with their often widespread devastation, but may also assist in the spread of disease. Wet seasons produce large areas of stagnant water, which can become breeding grounds for mosquitos. Though Australia is now free of many diseases spread by insects, malaria was a problem in the first 30 years of the century, causing from 20 to 50 deaths a year in northern Australia. Subsequent preventive measures have largely eliminated malaria as a serious threat during the second half of the century, but other diseases have taken their toll.

In 1916 and 1917 exceptionally heavy rain drenched eastern Australia, and during 1917, 114 cases of a new disease, called Australian 'X' disease, were reported in southeastern Australia. Victims typically suffered fever, vomiting, nausea, diarrhoea and dizziness, and after a few days, brain dysfunction, confusion, drowsiness, convulsions and fits. Death, or long-lasting incapacity, often followed. Another 67 cases were reported in 1918. The disease then went into apparent remission, but re-emerged after the very wet year of 1950 (48 cases and 19 deaths in 1951). The causative virus was then isolated and the disease named Murray Valley Encephalitis (MVE). The wet year of 1974 precipitated 58 cases, with 13 deaths. Clearly, MVE outbreaks are one of the potential down-sides to wet years over eastern Australia. However the association between rainfall and MVE is far from perfect, and not all wet seasons have triggered outbreaks of the disease.

There are times, however, when the climate-related spread of disease can have a positive side. A recurrent theme this century (and the late 19th) has been that, during dry periods in inland areas, rabbits have competed with both native and farm animals for what little forage was available, with disastrous results. The mosquito-borne myxomatosis virus was introduced to attack this problem, but early attempts in the 1930s and 1940s to spread the disease among the rabbit population proved ineffective. It later became apparent that the attempt failed because, during this dry era, the standing water necessary for mosquitos to breed was largely absent. In fact the first successful proliferation of the disease occurred in the summer of 1950/51 along the Murray River, with another rapid spread the following summer. Both outbreaks coincided with wet years, with extensive areas of stagnant water. Again, although wet conditions appear necessary for the spread of the disease, subsequent wet years were not always years of rapid spread. Perhaps lower rabbit population density was a factor - with fewer rabbits it is harder for the disease to spread. Nevertheless, this episode illustrates the importance of climate in the spread and control of pests.