Page 924 - Veterinary Immunology, 10th Edition

because they are masked by host serum proteins and are not
VetBooks.ir recognized as foreign.

Many protozoa successfully employ repeated antigenic variation.
If cattle are infected with the pathogenic trypanosomes T. vivax, T.

congolense, or T. brucei and their parasitemia measured, it is found
that periods of high parasitemia alternate regularly with periods of
low or undetectable parasitemia (Fig. 28.4). Serum from infected
animals contains antibodies against trypanosomes isolated prior to

each bleeding but not against those that develop subsequently.
Each period of high parasitemia corresponds to the expansion of a
population of trypanosomes with a new surface glycoprotein
antigen. The elimination of this population by antibodies leads to a

rapid fall in parasitemia. Among the survivors, however, are
parasites that express new surface glycoproteins and grow without
hindrance. As a result, a fresh population arises to produce yet
another period of high parasitemia (Fig. 28.5). This cyclical

parasitemia, with each peak reflecting the appearance of a new
population with new surface glycoproteins, can continue for many
months.

FIG. 28.4 The time course of Trypanosoma congolense
parasitemia in an infected calf. Each parasitemic peak represents
the development of a new, antigenically original population of
organisms.

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