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ABSTRACTS
Pure Zebu cattle in Trinidad resist the ravages of strongyles better than the
other breeds of cattle ; it is also found that sheep succumb to the attacks of helminths
more easily than cattle.
Experimental evidence of breed and strain differences in resistance
It has been observed that such heavy breeds and varieties as Rhode Island Reds,
White Plymouth Rocks and Barred Plymouth Rocks had significantly fewer and smaller
Ascaridia than the lighter White Leghorn and White Minorca breeds. Certain breeds
of sheep are more resistant than others to the infection of stomach worms such as Hae-
monchus, Ostertagia and Nematodirus and the resistant lines of sheep to stomach worms
could be established without sacrificing type, conformation, quality of carcase or desirable
wool characteristies.
Age, resistance to helminthic infections
Experimental evidence
Looss in 1911 showed that some of the hookworm larvæ fed to young dogs were
able to reach maturity, whereas previous investigators had failed to obtain such results in
adult dogs. Similar observations were also made in the case of Ascaris infections in pigs
and Syngamus trachea infection of chickens. A gradual increase in the resistance of
chickens, as they became older, to the infection of Ascaridia and Rallietina has been notice¬
ed. All the experimental data obtained so far indicate that the phenomenon
of increased resistance with age may occur in mammalian, avian, amphibian and mullus¬
can hosts to trematode, cestode or nematode helminth.
Epidemiological evidence of age resistance
Available data show that after the age of 15 to 19 years, people develop resistance to
hookworms. In the case of human Ascaris, it was found that the peak of infection occurs
in the age group of 7 to 14 years. Cram and Reardon found that pinworm infection in
youths from 6-18 years was 51 per cent as compared with 22 per cent for adults. In
Belgian Congo, Fisher found that the clinical manifestations of schistosomiasis is es-
pecially common in children under 10 years of age but never in persons over 30.
Attempts to explain age resistance
Attempts to explain the age resistance on the basis of an abnormal host hypothesis,
the action of body defence cells and a growth-inhibiting substance have been made by
various workers. Foster found a parallel between resistance and the haemoglobin level
of the host.
Duodenal mucus and age resistance
While attempting to ascertain the possible structural differences in the habitat of
Ascaridia in younger than in older chickens Ackert and Edgar made histological studies
of portions of duodena of such chickens. There were some differences in the thickness of
muscular layers, but the most striking difference was in the increasing number of goblet
cells in the chickens as they grow older. It was* felt that the secretion of the goblet cells
might be a factor in the elimination of the worms from the host, since it had been ob-
served repeatedly on opening the duodenum that the mucus was much more abundant in
the older chickens than in the younger ones. These observations were confirmed even in
the case of rats. As a result of a series of well conducted experiments, they came to the
probable conclusion that the duodenal mucus secreted by the goblet cells contains a.
ther mostable substance that inhibits the development of intestinal helminths. [G. D. B.]