TO SOME DISEASES OF STOCK 
889 
but at every subsequent bite. So far as diseases of animals 
are concerned, the outstanding example of this is that of 
trypanosomiasis or tsetse-fly disease ; in man, we have malaria, 
elephantiasis, and sleeping sickness — which last-named is 
essentially tsetse- fly disease. 
There is a group of disease, both in man and animals, which 
is believed to be conveyed by Bittern ; but research is not quite 
sufficiently advanced (chiefly owing to present inability to 
identify the microbe) to dogmatise as to whether transmission 
is mechanical or cyclical. Three days’ sickness of cattle is 
presumed to be carried by midges. Horse-sickness may perhaps 
be conveyed by midges, gnats, or mosquitoes. Sand-fly fever 
of man, and yellow fever of the same subject, are instances of 
this class ; and blue-tongue in sheep — a disease with marked 
affinity to horse-sickness — is a further example of a condition 
believed to be insect-borne. 
In some cases the connection between the insect and 
disease is proved, e.g. yellow fewer ; in others — as, for example, 
horse-sickness — it is largely circumstantial. It is known that 
the stabling of sick and healthy horses together is harmless, 
that the disease is not infectious; and by a process of elimination 
one is reduced to apparently incontestible evidence that some 
nocturnal flying ‘ dudu ’ is the responsible carrier. 
Hundreds of thousands of pounds have been expended 
upon the inquiry into the nature and identity of this 4 dudu.’ 
At the Pretoria Veterinary Laboratory very extensive experi- 
ments have been conducted for several years past. Probably 
every known mosquito in South Africa has been collected, is 
being bred under natural conditions, and is fed upon sick and 
healthy horses, so far — I am given to believe — without any 
positive result. 
Fly disease, or trypanosomiasis, is the condition of animals 
which offers the best example of Di'ptera as carriers of disease. 
The actual germ, or trypanosome, belongs to one of the 
classes of the Protozoa, and it is characterised by certain 
anatomical features. All trypanosomes are not disease- 
producing. One, Tr. Lewisi, is very common in wild rats — even 
in Nairobi itself — and is carried from rat to rat by fleas in which 
a cyclical development is undergone. Many fish, reptiles, 
