A. F. ALEXANDER, C. S. CARD, R. S. JAENKE, J. L. HICKS, AND D. H. WILL 
195 
Table II. — Three-Year Herd History Implicating a Spe- 
cific Male in the Incidence of High Mountain Disease 
Total number calves produced — . 359 
Number developing HMD 8 
Total number sired by suspect bull - - 39 
Number of offspring developing HMD B 
Suspect sire also developed HMD. 
taneous disease. Likewise, in certain herds, a 
high incidence factor has been traced to 
offspring of particular dam or sire combina- 
tions (Table II). In contrast, the disease has 
not been observed in the native Criollo cattle in 
the Peruvian Andes where a high incidence oc- 
curs in imported cattle. 
For these reasons, studies of heritable suscep- 
tibility were undertaken. The objective was to 
establish two distinct groups of cattle, one clas- 
sified as "susceptible" and the other as "resist- 
ant" to the hypoxic stress of high altitude. The 
establishment of the parent susceptible and re- 
sistant herds was accomplished by the selection 
of two groups of cattle based on the presence or 
absence of high mountain disease in their his- 
tory, and their level of high altitude induced 
pulmonary hypertension. Susceptible parent 
stock females were selected from heifers be- 
tween 6 and 18 months of age, not necessarily 
native to high altitude, but which developed un- 
complicated high mountain disease at altitudes 
of 8,000 feet or higher. Similar criteria were 
used in the selection of a suitable sire for this 
group. Females for the resistant parent herd 
were selected from heifers of at least one year 
of age who were born, raised and resided at al- 
titudes of 9,000 feet or above and at those alti- 
tudes had low pulmonary arterial pressures. To 
identify these animals catheterization studies 
were performed on 49 heifers meeting the age, 
residence and altitude requirements. The 15 
heifers with the lowest mean pulmonary ar- 
terial pressures were selected. 
Similar criteria were used in choosing a male 
to serve as sire for the resistant group. Only 
three males could be found which met the re- 
quirements of being born and raised at altitudes 
of greater than 9,000 feet. The animal with the 
lowest mean pulmonary arterial pressure was 
chosen as group sire. The parent susceptible 
herd, therefore, consisted of animals which had 
developed and recovered from clinical high 
mountain disease while the resistant herd con- 
sisted of animals native to and residing at alti- 
tudes greater than 9,000 feet but which had low 
pulmonary arterial pressures and no clinical 
history of high mountain disease (Figure 2). 
To produce the first year progeny, ten of the 
susceptible parent females were bred to a sus- 
ceptible male. Eight viable calves were pro- 
duced, four females and four males. Fifteen re- 
sistant parent famales were bred to the resist- 
ant male. Eleven viable calves were produced, 
four females and seven males (Figure 2). Car- 
diorespiratory and hematological studies were 
carried out on each calf at 10, 30, 60, and 90 
days of age at an altitude of 5,000 feet (Fort 
Collins, Colorado) and at 10 and 90 days after 
two hours exposure in the hypobaric chamber at 
a simulated altitude of 15,000 feet." When the 
minimum age within the two groups of calves 
was three months, they were moved to a moun- 
tainous altitude of 10,000 feet for chronic hy- 
poxic exposure. During the ensuing six month 
period, periodic cardiorespiratory and hemato- 
logic studies were made. 
In the susceptible group of calves, higher lev- 
els of mean pulmonary arterial pressure were 
observed at 5,000 feet at 10, 30, 60, and 90 days 
and at 15,000 feet at both 10 and 90 days of age 
(Figure 3). The increase in pulmonary arterial 
pressure under the stimulus of acute hypoxia 
was also greater in the susceptible animals. In- 
creased resistance to pulmonary blood flow was 
RESISTANT CATTLE 
SUSCEPTIBLE CATTLE 
1 
19 
MALE 
FEMALES 
(NO HMD) 
(NO HMD) 
PARENT 
STOCK 
MALE 
(HAD HMD) 
19 
FEMALES 
(HAD HMO) 
FIRST YEAR 
2 
MALES 
— ' THIRD YEAR 
C 
8 
PROGENY 
12 
PROQCNY 
Figure 2. — Parent stock, breeding plan and progeny of 
susceptible and resistant cattle. 
