219 



by mere immersion in the clipping- fluid, can have little effect hi practice, 

 inasmuch as real success is only attained by killing the ticks before 

 thev can lay eggs. Experiments to determine the effect of an emulsion 

 on "the dipjping-fluid showed that a solution of sodium arsenite con- 

 taining only 0-153 percent, of As.,0,„ to which sufhcient emulsion 

 had been added, was as efficient as a pure solution contahiing 0-225 

 per cent, of As^O^. The increased killing power is mainly due to 

 the increased wetting power resulting from the addition of an emulsion. 

 The tick does not obtain a lethal dose from arsenic present in the 

 circulating blood, but from that present on the skin of the host. To 

 prevent reinfestation, dipping must be repeated at short hitervals ; 

 injury to the stock is avoided by reduction in the concentration of the 

 dipping bath. It is then found" that at the reduced strength, although 

 a single dipping fails to kill all the ticks, repeated dippings render the 

 host poisonous'to all ticks, both those attached at the time of dipping 

 and those which may be picked up later. The conclusion is reached 

 that the arsenic is ciimulative in its action and the quantity absorbed 

 by the skin is augmented until a certain maximum is reached. Arsenical 

 solutions applied to the skin penetrate the cellular tissues by osmosis 

 and it is presumed that the arsenic enters into combination with some 

 organic constituent of the cells. About 5-4 grams of arsenic are 

 applied to the skin at each dipping. The lethal dose for cattle varies 

 from 15 to 30 grms. and it follows that quantities of arsenic which 

 would prove fatal if applied internally, may be safely applied to the 

 skin. The epidermal cells possess an affinity for arsenic and as soon 

 as this affinity is satisfied, the excess is available for absorption by the 

 blood. If dipping is repeated at frequent intervals, the amount of 

 arsenic present in the blood of the peripheral vessels of the skin must 

 be considerable.' In the case of diseases transmitted by external 

 parasites, the infective organisms can only be injected into the most 

 peripheral cutaneous capillaries, where the concentration of arsenic 

 is great enough to kill them. Such being the case, regularly dipped 

 animals on infected pastures would remain free from disease. This 

 hypothesis was shown to be correct at Elliotdale, where 500 head of 

 cattle remained free from disease for a period of 18 months. It remains 

 to be ascertained for how long, after ceasing to dip, cattle will remain 

 immune to East Coast fever when on infected pasture. It is possible 

 that they will not act as carriers for several days after dipping. If 

 this point could be proved, it would be possible to relax the present 

 system of quarantine so as to allow some movement of stock. If by 

 dipping, the amount of arsenic which can be introduced into the peri- 

 pheral vessels is sufficient to kill ticks, it is possible that it might kill 

 other blood-sucking insects, such as tsetse-flies. The natural infection 

 of stock by tsetse-flies should be also preventible by the same means, 

 and as regards trypanosomiasis, it is highly probable that dipping 

 would exert a pronounced effect. Preliminary investigations have 

 been made in this direction. The results of the various experiments 

 performed are recorded in tabiflar form. 



Williams (C). The Chemical Control of Cattle Dipping Tanks.— 



South African Journal of Science, Cape Town, xi, no. 8, May 1915, 



pp. 287-296. [Received 21st October 1915.] 

 The bulk of this paper consists of a recapitulation of previous work 

 (C223) »2 



