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The brain was removed from ill mice and used for further passages in
the form of a 10% brain suspension.

Isolated agents were investigated in 3-4 passages. CF antigens to the
studied viruses were made by the sucrose-acetone method (Clarke and Casals
1958). Standard hyperimmune rat and rabbit sera and immune ascitic fluids
(LAF) to polygroup A, B, and TBE encephalitis viruses were used as specific
serum (Gaidamovich et al. 1969).

RESULTS

A total of 2,010 birds of 53 Species was serologically examined. We
trapped 275 birds in the spring, 1,630 in summer, and 105 in fall.

Ninety-three bird brain samples and parenchymal organs of 339 birds
of 22 species were virologically examined.

Of 2,010 bird blood samples, 56 contained CF antibodies to TBE virus
(titers 1:8, 1:64, and higher), i.e. 2.7 ±0.1%.

The maximum level of immunological responses was recorded in forest
birds, such as sparrow and woodcock.

Table 1 show results from virological investigations of bird organs.

Isolated strains were apathogenic for guinea pigs and rabbits and their
titers in tests on white mice were: 5.0 by intracerebral, 4.6 by intranasal,
3.8 by intravenous, and 1.8 lg LD50/0.02 ml by intraperitoneal inoculations
of strain 54, and 6.5 by intracerebral, 5.3 by intravenous, 3.2 by intranasal,
and 2.0 lg LD50/0.02 mi by intraperitoneal inoculation of strain 59.

The subcutaneous inoculation method was unsuccessful. The incubation
period of infected mice fluctuated between 4 and 5 days. Experimental
infection was characterized by disorders in coordination of movements and
death of animals.

Chicken embryos (9-10 days old) proved also to be sensible to the
viruses isolated. Strains 54 and 59 reproduced in the allantoic cavity and their
titers were 2.5-4.0 lg. The highest mortality rate of embryos inoculated
with a dose of 10 LD50 was observed on day 3-5 following infection.

For identification of strains. we used the CF test with standard hyperimmune
sera (Table 2).

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