Rocky Mountain Area


lowing the injection of blood from a sick patient. After having fed on the
infected guinea pig for two days, the tick was removed, placed in a
ventilated pillbox for two days more and then attached to the base of the
ear of a guinea pig. After an incubation period of three and one-half
days, the temperature of the animal gradually rose to io6.40 F., near
which point it remained for seven days, when it gradually returned to
normal. The monkey injected with blood from the same patient came
down with an infection more like the human disease. When a fresh
supply of guinea pigs arrived, Ricketts found it possible to alternate
injections, monkey to guinea pig and again to a monkey, thus providing
a source of material for continued study of the disease beyond its brief
season of natural prevalence. Subsequently guinea pig to guinea pig
inoculations proved possible by obtaining blood earlier in the course of
the infection. All the infected animals ran similar courses of fever with
frequent development of a hemorrhagic eruption characteristic of the
disease in man, and the essential anatomical changes also agreed with
those in man. However, all cultures from blood or organs proved nega-
tive. That this experimental disease was actually Rocky Mountain
spotted fever and not some other similar infection was further demon-
strated by an active immunity developed both in guinea pigs and in
monkeys. Recovered animals remained well when injected with infec-
tious material that produced the typical disease in normal animals.
   Ricketts continued with a beautifully built-up series of experiments,
 some thirty papers in all, giving us much of what we know today about
 Rocky Mountain spotted fever. By 19o9 the following important re-
 sults had been repeatedly demonstrated:
   i. Although the number was small, infected wood ticks were found
 in the so-called infected districts.
   2. Adult ticks, both male and female, may acquire the disease by
 feeding on an infected animal and may transmit it to a normal suscep-
 tible animal for several weeks thereafter. Larval and nymphal ticks
 may also acquire the infection and prove infective when they become
 adults.
    3. The infected female tick may transfer the infection to her young
 through the egg, a transovarian passage as in Texas fever.
   4. The virus is widely distributed in the infected mammal and in the
 infected tick. The disease, however, is chronic in the tick, not highly
 destructive.
    5. Blood cells from infected animals retain the virus in spite of re-


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