s disease usually during the first summer, and if
they are adult animals, particularly milch cows or fat cattle, nearly all
die. Calves are much more likely to survive. The disease is one from which
immunity is acquired, and therefore calves which recover are not again
attacked, as a rule, even after they become adult.

When the infection is disseminated beyond the permanently infected
district, the roads, pastures, pens, and other inclosures are dangerous for
susceptible animals until freezing weather. The infection then disappears,
and cattle may be driven over the grounds or kept in the inclosures the
succeeding summer and the disease will not reappear. There are some
exceptions to this rule in the section just north of the boundary line of
the infected district. In this locality the infection sometimes resists the
winters, especially if they are mild.

In regard to the manner in which the disease is communicated, experience
shows that this does not occur by animals coming near or in contact with
one another. It is an indirect infection. The cattle from the infected
district first infect the pastures, roads, pens, cars, etc., whence the
susceptible cattle obtain the virus secondhand. Usually animals do not
contract the disease when separated from infected pastures by a fence. If,
however, there is any drainage or washing by rains across the line of fence
this rule does not hold good.

The investigations made by the Bureau of Animal Industry demonstrate that
the ticks which adhere to cattle from the infected district are the only
known means of conveying the infection to susceptible cattle. The infection
is not spread by the saliva, the urine, or the manure of cattle from the
infected district. In studying the causation and prevention of this
disease, attention must therefore be largely given to the tick, and it now
seems apparent that if cattle could be freed from this parasite when
leaving the infected district they would not be able to spread the malady.
The discovery of the connection of the ticks with the production of the
disease has played a very important part in determining the methods that
should be adopted in preventing its spread. It established an essential
point and indicated many lines of investigation which have yielded and are
still likely to yield very important results.

_Nature of the disease._--Texas fever is caused by an organism which lives
within the red blood corpuscles and breaks them up. I