fever (Kozyelka, 1929; Riek, 1962), heartwater 
(Bonsma, 1949), African trypanosomiasis, 
anaplasmosis, piraplasmosis, leptospirosis, 
and East Coast fever in cattle. Most of these 
observations are based on a comparison of 
zebu and European breeds and could be due to 
differences in the degree of infection rather 
than genetic immunity. The greater resistance 
of the zebu may be because the vector has 
difficulty penetrating the skin. 
Wilkinson (1962) suggests some evidence 
in Illawarra Shorthorn heifers in tick resist- 
ance, which was attributed to differences in 
hair follicle depth. Observations by McDowell 
et al. (1964) for an 8-year period, however, 
showed no significant differences among Red 
Sindhi (a zebu breed from India), Red Sindhi- 
European crosses, Brahma, Angus, Brangus, 
Holsteins, Jerseys, and European crosses in 
resistance to anaplasmosis where horse flies 
and mosquitoes were the major vectors. 
Mahadevan (1964) reported a heritability for 
East Coast fever of near zero. 
Based on present evidence, it appears that 
the animal breeder should continue to rely 
on induced immunity and vector control pro- 
grams until a better understanding is obtained 
of the feasibility of selecting for modification 
of skin and hair coat traits as natural repel- 
lents, 
Internal Parasites 
What little is known about disease resistance 
in sheep is largely concerned with internal 
parasitism; however, at present, there is no 
clear evidence that selection can be effective 
in developing resistant strains. Stewart et al. 
(1937) showed that breeds differ in resistance 
to continuous exposure to Ostertagia circum- 
cinta, with the Romney being most resistant, 
followed by the Rambouillet, Southdown, Shorp- 
shire, and Hampshire, with some variation 
between individuals. Gregory et al. (1940) also 
noted variation between individuals of the 
Romney breed for this same parasite. Emik 
(1949) suggests evidence of genetic differences 
between lambs in resistance to trichostrongylid 
infection and calculated a heritability of 
0.22 £ 0,41. Warwick et al. (1949), showed that 
selection of survivors after an intensive ex- 
posure to Haemonchus contortus caused a 
significant increase in survival rate over a 
9-year period. There are similar bits of evi- 
dence for these and additional parasites in 
other classes of livestock. 
Thus, meager evidence suggests selection 
could be effective, but, as with mastitis, the 
measures employed so far are to subjective 
to warrant consideration by animal breeders. 
Furthermore, assuming the same rate of 
genetic mutation in a host and parasite, the 
fecundity of the parasite is much greater; 
therefore, the genetic changes in the host will 
be relatively unimportant. Since young animals 
are most susceptible, systems of feeding and 
management and methods of induced immunity 
appear as the most fruitful areas of research. 
Pests 
Observations made in this field, such as those 
by Bonsma (1949), Veiga (1964), Kozyelka 
(1929), and others, point to the lesser attack 
of zebu cattle by ticks. Among the reasons 
advanced are the shorter hair coat, greater 
number of hair follicles per unit of area, 
thicker skin, denser epithelial layer, greater 
depth of hair follicle, and perhaps peculiar 
secretions from the sebaceous gland exudated 
through the skin, which acts as repellents. 
These observations have not as yet been suf- 
ficiently proven as contributors against sus- 
ceptibility for the animal breeder to utilize 
them in the identification of resistant indivi- 
duals. Nevertheless, more specific experi- 
mentation should prove fruitful, particularly 
in view of the increasing concern over pesticide 
residues in human and animal health and the 
fact that insects rapidly develop resistance to 
pesticides (Johnston et al., 1954), 
OTHER TRAITS 
Heat Resistance 
Over the past two decades, much has been 
written on the resistance of some groups of 
animals versus others for heat resistance, 
such as Leghorn chickens, Merino sheep, and 
zebu cattle. Based on this evidence, those 
writing on breeding for disease resistance 
(Hutt, 1958; Fredeen, 1963; Shrode and Lush, 
1947; and others) have advocated exploitation 

