The incidence of Pasteurella tularensis in wild animals and their ectoparasites from selected areas of the Great Salt Lake Desert region was determined by using bacteriological and serological techniques. Approximately 31,300 mammals, 1,700 birds, and 141,000 ectoparasites were examined from 1951 through 1964. The sera from 5,047 head of livestock of the region also were tested.
Fifty-two isolates of P. tularensis were obtained, 26 from ectoparasites and 26 from animal tissues. The positive ectoparasites included three species of ticks (Dermacentor parumapertus, Dermacentor andersoni, and Ixodes kingi), a flea (Thrassis bacchi), and an anopluran louse (probably Hoplopleura hesperomydis). The tissue isolates were from black-tailed jack rabbits (Lepus californicus), an Audubon (desert) cottontail (Sylvilagus audubonii), a Great Basin pocket mouse (Perognathus parvus), Ord kangaroo rats (Dipodomys ordii), white-tailed antelope squirrels (Citellus leucurus), and beaver (Castor canadensis). Serum agglutinins at titers of 1:20 to 1:320 were found in 92 specimens of 25 species of wild mammals and birds; 31% of the cattle and 24% of the sheep had serum agglutinin titers of 1:40 or greater.
Eight mammalian and seven avian species were implicated within the natural ecology of tularemia for the first time: the Western spotted skunk (Spilogale gracilis), the Western harvest mouse (Reithrodontomys megalotis), the pinyon mouse (Peromyscus truei), the Great Basin pocket mouse, the chisel-toothed kangaroo rat (Dipodomys microps), the white-tailed antelope squirrel, the cliff chipmunk (Eutamias dorsalis), the bobcat (Lynx rufus), the shrike (Lanius ludovicianus), the horned lark (Eremophila alpestris), the common mallard (Anas platyrhynchos), the California gull (Larus californicus), the prairie falcon (Falco mexicanus), the red-tailed hawk (Buteo jamaicensis), and the Great Basin screech owl (Otus asio).
Four of the isolates of P. tularensis from rodents were found to be of maximal virulence (i.e., one organism was fatal for standard laboratory animals excluding the white rat), while one isolate from a desert cottontail and one from a Great Basin pocket mouse were found to be of low virulence. The latter findings are exceptions to the commonly accepted generalization that strains from North American ticks, lagomorphs, and sheep are of high virulence and those from beaver, rodents, and water are of low virulence. While the generally accepted concept is usually justified, this distinction does not exist invariably.
E and E Branch, U. S. Army Chemical Corps, Dugway Proving Grounds, Dugway, Utah.