STUDIES ON BOVINE MASTITIS
357
" Healthy " and " Infected " refer to cows free from or infected with Str. agalactiæ.
" Healthy became Infected " refers to animals which became infected during the
year stated, but which were in the healthy section on December 31st of the previous
year ; the percentage is calculated on the number in the healthy section at that date.
" Healthy Added " and " Infected Added " refer to animals which were added to the
herd during the year in question and which were free from or infected with Str.
agalactiæ, respectively at their first test. "Healthy Removed" and "Infected
Removed " refer to healthy or infected animals (whether infected at first test or
subsequently) which died or were sold during the year in question.
The figures in brackets express percentages.
Of the 29 cows classed as infected in 1935, 9 had been free from infection for at
least two years. The number of cows actually infected at the end of 1935 is, therefore,
20, and the real incidence 13.1 per cent. (See comment.)
Since 1929 milk samples have been examined at three-monthly intervals from
all animals in milk, including those in the infected section. Additional samples have
been taken seven to ten days after calving, and from any animal which showed ab-
normality of the udder or milk. It is seen from the table relating to Herd A that the
number of cows at December, 1929, was 55 and that 290 ("Healthy added" plus
" Infected added ") have been added since. Of these 345 cows, 138 were under obser-
vation for one lactation or less, 76 for two, 66 for three, 32 for four, 24 for five and 9
for six lactations. The number of tests made on the milk of each cow was from 2 to
22, a total of 2,482 tests, or an average of a little over 7 for each cow.
Comment
This herd is notable in the first place because, although the original incidence
of Str. agalactiæ was below 10 per cent, milk samples were regularly examined
and infected cows milked last or partially segregated, the number of new infections
with Str. agalactiæ showed a marked increase in 1932, nearly half of the cows in
the herd showing infection in that year. The number of new infections has since
gradually decreased, 16.7 per cent being found in 1933, 14.3 per cent in 1934,
and 5.3 per cent in 1935. At the time of this report the total number of new
infections which have appeared among the 337 cows passed into the healthy sec-
tion of the herd is 88. More than half of the infected cows were retained for a
long time, chiefly on account of the high incidence of other forms of mastitis in
this herd, with the result that at December, 1934, the section of the herd described
as infected still formed 28.9 per cent of the whole, whilst at November, 1935, it
formed 19.9 per cent. A striking feature of these new infections with Str. agalactiæ,
however, is that in 29 cases the organisms have apparently disappeared from the
udder. In nearly all of these cases the streptococci were detected once only and
usually only by means of the glucose broth enrichment medium, in spite of nume-
rous examinations during the following one to three years. Such cows have,
however, always been kept in the infected section and the number shown as in-
fected is, therefore, unduly high for all years since 1932. The number of cows