ABSTRACTS
103
Bacteriological and histological examination gave negative results, but microscopic examination
of the exudate showed numerous protozoa with elongated ends and of an average size of 16 × 6.l5μ.
In old and drying discharges these protozoa take oval and round forms. Microscopic examination
of vaginal material after 12 to 18 hours incubation gives better results.
Since the parasties were also found in the prepuce of bulls it is considered that bulls may act
as mechanical carriers of this infection.
When these protozoa were introduced into the pregnant uterus, the authors were able to pro¬
duce abortion in guinea-pigs, rabbits and in one heifer. [B. S. ]
The commercial possibilities and limitations of breeding for disease
resistance. COLE, L. J. 1933. (UNIV. WISCONSIN.) Proc. Amer. Soc. Anim. Prod.
25th Ann. Meeting, 1932 : 271-272.
Natural resistance to many diseases is apparently more prevalent in wild species' than in the
domesticated races. To incorporate general resistance in a breed it must be found in some members
of the breed or be brought in by crossing. The rules of most breed associations effectively prevent
introduction of resistance by cross-breeding. There is commonly difficulty in distinguishing natural
from acquired immunity, which interferes with the selective process. While genetic resistance may
in some cases be conditioned by a single gene pair, it is usually more complex. Resistance to infec
tions is commonly specific, requiring selection for each disease. This greatly limits what can be
accomplished. Selection for resistance adds to the characters already being selected for,
complicating the breeding programme. Variability (mutation) of the pathogen may upset
the results of long selection. Constant selection will be required to retain resistance after
it has been attained. The following general principles are offered : (1) Where methods of
prophylaxis or treatment are known and readily applicable, they will in general be preferable eco
nomically to attempting to breed for resistance. It must be borne in mind, however, that unless
the pathogenic agent can be eliminated completely the necessity for control, such, for instance, as by
vaccination, will be a continuous one. Under such treatment the stock is likely to come to have
less natural resistance rather than more. (2) The genetic method can be much more readily applied
to animals which produce a large progeny and in which the value of the individual is relatively
small. Progress depends on selection of large numbers and elimination on a large scale. Such
elimination is scarcely practicable in the larger animals, for the commercial value of the individual
is apt to be too high, and reproduction is so slow that the female breeding stock cannot be main
tained if selection is too rigil. (3) From the nature of the case, the number of diseases which might
be selected against in any stock is strictly limited. This is another reason for restricting the genetic
method to those diseases which cannot readily be brought under control by other means. (4) While
in general, as pointed out, the other methods tend away from genetic improvement, it is entirely
possible that, at least in certain cases, they may be made to work in harmony and towards the same
end. [ From Animal Breeding Abstracts, Vol. I, No. 2, July 1933. ]
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