highly suggestive of past infection. Again, 
not a single definite positive, 4+ at 1:20 or 
higher, was encountered in any of the 70 samples 
from sheep industry workmen other than shear- 
ers. Only 6 of the 31 positive individuals gave 
a definite history of illness with a diagnosis of 
tularemia; one thought he “probably had 
tularemia in 1920” and another “suspected 
tularemia in 1938” (table 9). Three had ex- 
perienced severe ulceroglandular infections, 
but if a specific diagnosis had been established 
they had not been informed of it. During the 
surveys we encountered several active cases of 
influenzalike illness that gave positive ag- 
glutinations at a moderate titer, but in no 
instance did we find a frank case of typical 
glandular or ulceroglandular tularemia infection 
among the shearers or other employees. 
Perhaps the most striking result of the sur- 
veys is that not a single one of the 31 individuals 
with a positive agglutination titer suggestive of 
past infection with tularemia had come to our 
attention through the reports of physicians or - 
State health departments, although 64 cases 
in shearers had been reported from such 
sources, and some of them had been treated by 
physicians who have reported other tularemia 
cases to us. 
Very little information was obtained in this 
survey as to locality where the infection was 
acquired. Places mentioned by various in- 
dividuals as the locality where they contracted 
some illness were Rawlins, Dixon, Lander, 
Evanston, and Ivemmerer, Wyo.; Huntley, 
Mont.; Antonito and Craig, Colo.: and 
Shoshone, Idaho. 
Discussion and Summary 
Tularemia in epizootic proportions occasion- 
ally occurs in range sheep in the western 
United States and in the adjacent areas of 
Canada. Every outbreak studied so far has 
been associated with heavy infestations of 
wood ticks, Dermacentor andersoni. Infection 
has been repeatedly demonstrated in ticks 
from affected sheep and we conclude that ticks 
are the source of infection. It is likely that a 
high population of ticks builds up in some area 
where there is an abundance of rodents or 
rabbits on which the immature ticks feed and 
from which they acquire then* infection. Then, 
at a favorable time during the tick season, a 
band of sheep moves through the tick-infested 
area and picks up a few infected ticks and 
many more noninfected ones. The burden of 
ticks, along with the infection, produces the 
observed symptoms and often, death. The 
feeding habits of this tick preclude the pos- 
sibility of sheep-to-sheep transfer of infection 
except perhaps in rare instances when partially 
engorged ticks leaving a sheep that had died 
of tularemia might reattach themselves to 
another sheep. The prompt recovery of most 
sheep after removal of the ticks suggests that 
infection is not the sole cause of death. Losses 
are greater among lambs and yearlings than 
among mature sheep. Infection has been found 
to persist in lambs several months after apparent 
recovery. 
Losses can be avoided by keeping sheep out 
of tick-infested sagebursh areas in the spring 
months. Dipping or spraying with a per- 
sistent repellent or acaricide before the tick 
season begins may be effective. Should the 
disease appear, prompt destruction of the ticks 
by spraying or dipping the sheep is recom- 
mended. In one recorded instance, treatment 
of sick animals with streptomycin appeared 
to be effective. 
Since range sheep are grazed on areas that 
are favorable for various kinds of wild animals 
which serve as reservoirs of tularemia and that 
are also favorable for ticks and deerflies, it 
is obvious that persons caring for sheep have 
considerable exposure to infection. Risk of 
infection is increased at times of epizootics, 
when sick animals are treated and dead animals 
are skinned. 
Sheepshearers experience gross exposure to 
16 
Tularemia in Sheep and in Sheep Industry Workers in Western United States 
