ABSTRACTS
91
concentration of 0 .5 per cent. The reinforced toxoid was prepared by precipitating
toxoid with ammonium sulphate (600gm./litre) from anaculture (pH. adjusted to 7.4) and
dissolving in a further quantity of anaculture, thus concentrating the toxoid four to five
fold. The alum-precipitated toxoid was prepared by the addition of potassium alum to
anaculture (pH 7.4) to make a final concentration of 2 per cent. Zinc chloride was added
in a similar concentration.
The six vaccines were used on groups of six ewes of the Romney breed, two doses at
an interval of 15 days being given.
In the experiment in which lambs were actively immunized, the response of groups
of lambs (about 3½ months old) from vaccinated and unvaccinated dams to active immuni-
zation (2 doses at 7 weeks interval) with some of the toxoids was studied and the possibi-
lity of the presence of transmitted passive immunity in the lambs sera influencing the
subsequent response of these lambs to active immunization was investigated. The de-
gree of response to immunization was judged by two factors, viz.—antitoxic titre
and agglutination titre of the sera.
Judged by these standards, the highest degree of passive transmitted immunity in
lambs following the prenatal vaccination of their dams was acquired by the use of alum-
precipitated reinforced toxoid. Alum precipitated toxoid was superior an as antigen to
zinc chloride precipitated toxoid. Inoculation by the intraperitonial route did not pro-
duce a significantly higher antigenic response than the subcutaneous. Lambs from the
vaccinated dams (inspite of the fact that agglutinins and antitoxins had disappeared from
heir blood sera by the time they were taken up for immunization) did not respond as
atisfactorily to active immunization as lambs from the non-vaccinated dams. [V. R. R.]
Natural resistance to helminthic infections. J. E. ACKERT (1942). J. Parasit
28, 1-24
A CKERT gives a review and analysis of literature on such factors in natural resis-
tance as diet, genetic constitution, age, etc. and presents some new data on an in-
hibitory nematode growth factor in duodenal mucus of adult animals. The term natural
resistance is used to designate a host's resistance, at any age, to its helminths, as developed
in absence of helminthic antigens. Natural immunity is interpreted as representing a
natural incompatibility between the host and an invading helminth, while the term immu-
nity is used to represent the host condition, that is due to helminthic antigen.
Dietary factors in resistance to helminths
Vitamins
Experiments on Ascardia galli and Heterakis gallinae of chickens, Ascarids of pigs,
Trichinella spiralis of rats, the Ascarids of dogs and on Strongyloides ratti of rats, point to
the conclusion that a diet deficient in vitamin A lowers the resistance of the host to these
parasites and there is evidence that animals kept on vitamin B deficient diet harbour
more parasites than control animals.
Ackert and Spindler have shown that vitamin D protects chickens against the
effects of parasitism.
Other dietary supplements
A basal cereal ration with adequate vitamins and minerals when supplemented by
meat meal and skim milk produces the most resistant fowls, and rats fed on diet of plant
origin lack the resistance to parasites. Evidence indicates that deficiency of iron and
minerals (especially calcium and phosphorus) lowers the resistance of animals to parasites.
Absence of nutriment in the lumen of the intestine and in the somatic tissue is, however,
inimical to the propagation and growth of parasites.
Genetic constitution as a factor in resistance
Epidemiological evidence of group differences in helminthiasis
Epidemiological data in man indicate that negroes are more resistant to hookworms,
pinworms and dwarf tapeworms than whites. In the case of Ascaris infection, however,
it is found that whites are more resistant than negroes.