This malarial poison is distinct from that which produces yellow fever in America, and is so far unlike it as it is not contagious.  The theory is that the poison is produced below the surface by decaying vegetable matter in low and dank parts during the more inactive but still warm and sunny winter season and during the hot months preceding the summer rainfall.  Upon the first rains the malarial poison escapes through the then softened crust in the shape of vapoury miasms.  This happens during the night, after the surface of the earth has been cooled off.  Those miasms are dissipated or neutralised by the action of the sun.  The dewy grass retains the poison until it is thoroughly dried to the root.  All surface water is liable to that poisonous impregnation.  Malarial manifestations occur all over South Africa, but in progressive degrees of virulence with the advance to warmer latitudes, and with the descent from the high table-lands to the coast levels.  On the Transvaal high veldt, for example, a mild form is developed which, in midsummer, to a small extent, affects and kills sheep.  It is called blaauwtong, and does not affect horses.  Descending further, this danger to sheep increases and begins earlier.  Below 5,000 feet altitude in the Transvaal the summer season is dangerous to sheep, and horses and mules are subject to horse sickness; whilst lower still the same malaria attains sufficient virulence to attack human beings, and becomes very deadly upon levels nearing the coast.  Komati poort, the frontier railway station already mentioned, is dreaded as a still worse death-trap than even Delagoa Bay, where it is very unsafe, say, from December to end of April.  The season of horse sickness terminates upon the appearance of the first sharp frost in May.  The safeguards for human beings consist in avoidance at night and early morning of low-lying localities, or such elevated places even which are subject to be invaded by miasmatic emanations produced on and wafted from dangerous lower levels.  Drink no unboiled water except that from deep wells or rain-water; maintain careful and moderate diet, active habits, but avoiding extreme exertions and excitements; a very sparing use of alcoholic drinks, preferably taken with the regular meals, is admissible.

Donkeys, horned cattle, and goats are exempt from malarial risks.

For horses and mules no certain remedy appears as yet to be known.  The best research, on behalf of the Transvaal Government, by specially requisitioned French bacteriologists, assisted by that famous microbe-hunter, Dr. Theiler (Dr. Theiler is the Transvaal veterinary surgeon and chief of the Medical Laboratory, Pretoria, a noted Swiss savant, who, with the aid of the said French experts, discovered the rinderpest inoculation remedy), has failed to find the bacillus of horse sickness.  Barely five per cent, of the horses attacked recover, and about ten per cent, of mules.  These are then called salted, and are immune from horse sickness; they can after that be safely used in the worst localities, and are correspondingly more valuable.  They are, however, liable periodically to light after-attacks, when it is safer to exempt them from work for a day, or for a few hours at least.