Introduction 
Few if any diseases have as many distinct 
epidemic and epizootic patterns as tularemia, 
which is a specific infectious disease of man 
and animals caused by the micro-organism 
I'axteurella tularensis of McCoy and Chapin (1). 
Early in the history of the disease several of 
t he epidemic types were referred to by such 
distinctive names as “deerfly fever,” “rabbit 
fever," “plaguelike disease of rodents,” and 
“glandular type of tick fever.” After a com- 
mon etiological agent was established for some 
of these the name “tularemia” was proposed by 
Francis (#). 
The main epidemic types 1 that can be recog- 
nized in North America at the present time are: 
1 . Tularemia from wild rabbits — rabbit fever. 
2. Tularemia from terrestrial rodents. 
3. Tularemia from the biting fly ( Chrysops 
(fiscal is Williston) — deerfly fever, or Pahvant 
Valley plague. 
4. Tularemia from tick bites — glandular type 
of tick fever. 
5. Tularemia from the semiaquatic mammals, 
such as muskrats and beavers, and from con- 
taminated water. 
6. Tularemia from game birds. 
1 Khatenever (S) outlines five main epidemic types 
of tularemia observed in the USSR, where rodents 
appear to be most important as sources of human 
infection, in contrast to North America, where rabbits 
are the chief source of infection. Khatenever’s 
epidemic types are: (a) Trade outbreaks resulting from 
hunting, skinning, and preparing hides and carcasses 
of rabbits, water rats, and other wild game or fur- 
bearing animals; ( b ) agricultural or mouse outbreaks 
from more or less direct contact with mice by farmers 
during epizootics; (c) contact outbreaks from food 
contaminated by infected mice; ( d ) water outbreaks 
from direct contact with or ingestion of contaminated 
water, but not including cases from contact with water 
animals; and (e) tularemia transmitted by arthropods 
(ticks and insects). 
Although these various types of tularemia 
are often interrelated, there are features of 
seasonal distribution, clinical manifestation, or 
occupational incidence that serve to distinguish 
them. It is becoming evident that virulence 
of the strains and, therefore, prognosis of the 
individual case may be related to the epidemic 
type involved. 
Range sheep in western United States are 
grazed on lands that are also favorable for 
cottontails and jackrabbits. Most of the area 
is infested with the Rocky Mountain wood tick, 
Dermacentor andersoni Stiles, a well-known 
vector of tularemia. Epizootics of tularemia 
among sheep occur in the spring of the year, 
when sheep often become heavily infested with 
ticks. Four such outbreaks have been studied 
by the Rocky Mountain Laboratory. Lambing 
often takes place at this season, and it is the 
usual season for shearing. Masses of infected 
ticks and infectious tick feces are encountered 
in shearing, and loose ticks readily transfer 
from sheep to man. Herders are continually 
exposed to ticks on the range. If sheep sicken 
and die, they are usually skinned, and the pelts 
are saved. A herder or a member of a shearing 
crew may occasionally take a wild rabbit for 
food. Thus, in the sheep industry, there is 
gross exposure of man to ticks and to infected 
animals, and many human tularemia cases 
result from this exposure. Most physicians 
in the area are aware of this association and 
anticipate a few sheep-contact cases each spring. 
We now have records of 189 such human cases 
of tularemia. These observations and records 
appear to warrant the consideration of tularemia 
in sheep and in sheep industry workers as a 
separate epizootic and epidemic entity, and it 
is the purpose of this paper to bring together 
our information on the subject. 
Public Health Monograph No» 28, 1955 
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