STUDIES ON BOVINE MASTITIS                              363

between samplings and because newly added cows—many of which were infected—
were not examined until the next herd test ; also because cows found to be infected
were sometimes not removed for weeks or months. With one exception the
herd has been free from mastitis due to causes other than Str. agalactiæ, whilst
obvious mastitis due to this organism has disappeared, such infections as have
occurred being detectable by laboratory methods only.

                                                     HERD K

This herd now consists of about 50 animals of the Friesian, Ayrshire and
Guernsey breeds. It is maintained solely by homebred stock, and is free from
tuberculosis. The eradication of contagious abortion was begun in 1932 and the
herd is now free from reactors. Milking is carried out by hand.

Mastitis control was begun in November, 1931, and by the end of 1932, 93
animals of the original herd had been examined for the first time. Of these 93
animals, 50 (53.8 per cent) were infected.

                                        HERD K

Year

State of
Herd at end of
each Year

Additions, Removals and Cows which
became infected during previous 12
Months

Healthy
became
Infected

Healthy
Added

Healthy
Removed

1931

36

—

—

—

1932

27

16 (44.4)

7

0

1933

40

9(33.3)

31

9

1934

56

4(10.0)

30

11

1935

49

4(7.8)

21

23

                        For explanation see table for Herd A.

All animals in milk were examined at intervals of three months during 1932
and subsequently at six-monthly intervals, those found to be infected at the
first or subsequent tests being removed as soon as convenient to the owner. Post-
calving samples were not examined and animals found to be infected at any given
test had, therefore, been in the herd for some time. As already stated, 50 animals
were found to be infected at their first test up to the end of 1932 ; in 1933 and 1934
the corresponding numbers were 7 and 3. The total number of animals classed
as healthy at their first test in 1931 or subsequently was 125. Of these 125, 46
were under observation for one lactation or less, 39 for two lactations, 30 for three
and 10 for four lactations. The number of examinations of each cow was from 2
to 10, a total of 480 or an average of nearly 4 for each animal. The table shows
the number of animals in the herd at the end of each year and the events during
the previous twelve months.