292 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ I, IV

fortnight during the period of risk should be capable of warding off the disease ".
More recently, Allen [ 1930 ], in India, has also made a recommendation which is
essentially to the same effect, namely, that for prophylaxis against equine surra,
the drug should be used at the rate of 10 c. c. of a 10 per cent. solution per 1,000 lb.
body weight.

In the rationale of successful prophylaxis, it would seem necessary that the drug
should be maintained in a sufficiently trypanocidal concentration in the circulation
for the desired period. This brings us to the much disputed question of the
mechanism of the trypanocidal action of " Bayer 205 ". In regard to this, Kligler
and Weitzman [ 1924-25 ] would appear to have adduced sufficiently convincing
evidence (based on the results of experiments mentioned earlier in this paper) in
support of the contention that the drug confers protection so long as it remains in a
concentration sufficient to affect the parasites, so that it would seem unnecessary,
as some workers have done, to invoke the aid of certain hypothetical substances
produced, as they believe, as a result of the interaction between the drug on the one
hand and the tissues of the host on the other. It has been urged in support of the
latter contention that the drug behaves very differently when tested against the
parasites in vitro and in vivo. As to this, Kligler and Weitzman [ 1925 ] have
demonstrated that such differences are more apparent than real: if the amount of
dilution undergone by the drug when introduced into the circulating blood is taken
into consideration, then " the effective dose in vivo " will be found to " correspond
fairly well with those in vitro " so that, according to these authors, the rational
conclusion is that " the therapeutic property of ' Bayer 205 ' is due to a direct
injury to the trypanosomes which renders them avirulent for the host and thus
readily destroyed and eliminated ".

The findings of Kligler and Weitzman are of some practical importance in that
they point to the possibility that, within certain limits, the dosage of " Bayer 205 "
might be increased to lengthen the intervals between two successive injections, and
this would, obviously, be a distinct advantage for field workers having to treat
prophylactically a large number of animals within a brief space of time. As a
matter of fact, the recent findings of Launoy and his co-workers [ 1930] in regard
to the prophylactic action of " Bayer 205 " against T. brucei infection would seem
to bring such a possibility within the bounds of practical realization, for these
workers found that when "Bayer 205" was used protectively on cats in doses
ranging from 0.0025 to 0.04 gramme, the period of resistance to infection increased
with the dose of the drug used, but not in arithmetical progression. In the case
of mice, the drug was used prophylactically in doses varying from 0.0001 gramme
to 0.0006 gramme, and within this range the protective action increased in a regular