269 
December, 1902, were still alive, apparently healthy, and were constantly worked 
in November, 1906.* 
The first part of the history of Horse Yl is another instance of tolerance of 
trypanosome infection. 
\s will be gathered from the notes on the animals present at various posts 
in the Congo, individual cattle, horses, &c., seem to be resistant, since they live 
for some years in places where the disease is of a very severe type. C attle are 
susceptible to inoculation with Trypanosoma dimorphon, yet at many places in the 
Gambia, at Cape St. Mary for example, large herds of cattle in splendid condition 
graze over the same ground as the horses. (Captain Todd found trypanosomes 
in Gambia cattle, and he states that it is a not infrequent cause of death.) 
From all these facts it is not unreasonable to suppose that, like the game, 
domesticated animals may thrive in spite of actual trypanosome infection and 
exposure to constant re-infection. 
It was a “ tolerance ” of this sort which Schilling had in mind when he spoke 
of a “symbiosis ” of trypanosomes with their mammalian hosts (18). Ho describes 
cattle which have apparently recovered from infection with 'trypanosoma brucu. 
His later publications (19), we believe, simply confirm his former opinion 
expressed in (18). 
Malaria in African natives forms a striking parallel to this conception. 
Children suffer severely from the disease.t Those who survive have parasites 
constantly in their blood until they become adult, and even then they are not 
infrequently infected, and may have short attacks of fever. 
An animal long infected with trypanosomes may have shoit 
attacks of fever, with the appearance of numerous parasites in the 
Wood, and afterwards completely recover its former tolerant status 
(Horse VI). From the fact that adult antelopes are sometimes heavil) 
infected with trypanosomes, it seems likely that there may he reciude- 
scences of the disease in the same way in them. From the conditions 
obtaining in the Gambia, where “ sleeping sickness ’ has been endemic, 
for many years, it seems probable that man may acquire a similar 
tolerance of Trypanosoma gambiense (b page 23). Almost nothing 
!S known of the causes which may make the latent form 
ot infection in animal or man virulent, and so produce the 
beath of the individual host or an epidemic of trypanosomiasis in 
P !il ces where it was formerly sporadic ( 10 , page 319)' ( ’’ P a £ e 2 7 )‘ 
Mc '.i an effect might conceivably be produced by (tf) a diminution m 
“Resistance of the host, or by (< b ) an increase in the virulence of the 
^ {a) Epidemics of sleeping sickness have seemed to follow 
ard Hmes produced by wars and famines, 5 ' 1,1 hi the individual 
lhese animals has been treated at intervals with arse JV?V n the f ° rm 
5 solution. In March, 1907, he was still in excellent condition. 
*e®s Pani^ c . onnecl ' on it is noteworthy that the mortality from trypanosom 
Particularly great among calves ■ see Eala. Nouvelle Anvers, Tshota, 
