76
THYROIDAL ACTIVITY AND CAECAL COCCIDIOSIS
[Vol. XXV, Part II,
the oocyst output. Consequently, count of oocyst output by an infective dose
would form a more foolproof method for judging the effect of changed thyroid
status upon growth of the parasite. Todd et al., [1949] counted oocysts 23 days
after infection, when majority of oocysts has been eliminated. Only the oocysts
which were produced by the progeny of slackers (lag-phase) could have been taken
into consideration by them. Even this factor is not regular and constant as it is
a matter of chance as to how many sporozoites lag behind in development and
reach their destination (i.e. oocystic stage) later than others.
Pathogenicity of coccidia depends upon the size of infective dose. Only ingestion
of a large number of viable and sporulated oocysts will cause mortality which, in
case of caecal coccidiosis, takes place on the 5th to the 7th day of infection. Survivors
will start voiding oocysts in faeces late on the 7th day, discharge being heaviest for
next 2—4 days when it subsides as most of macrogametes have been fertilised and
oocysts voided. Oocysts in faeces become progressively scantier with the age of
infection. Late maturing coccidia and oocysts enmeshed deep in the areolar
tissue probably keep up the later discharge of oocysts for another few days, so that
if reinfection is prevented the infection would eliminate itself in a fortnight or even
earlier. Large amount of blood in caecal pouches is organised into caecal cores
which, a week after infection, contains a variable number of oocysts imprisoned in
them. These caecal cores are voided after a varying interval of time. Their
presence in caeca prevents regular discharge of oocysts in caecal evacuations.
Therefore, if total oocysts production has to be studied, the infective dose should
be sublethal and sufficiently small as to cause less haemorrhage in caeca with no
formation of caecal cores or of only small caecal cores. Consequently, an unhindered
free discharge of oocysts in faeces is ensured.
In the light of above facts, it is apparent that it was not possible to compare
both mortality rate and total oocyst production in one and the same experiment.
Hence these two observations were recorded separately by designing two different
experiments.
EXPERIMENTAL
Oocyst production
Sixty-day-old White Leghorn battery-hatched chicks were kept coccidia-free
for three days in electrically heated batteries. At three days of age, they were divi-
ded into four lots of 13 chicks each*. Only healthy-looking chicks of almost equal
weights were selected. Each lot was weighed and kept separately in electrically
heated wire-bottomed cages underneath which were fitted sliding tin trays to receive
faeces. Two lots were kept on standard chick mash, the third on basal ration
containing 0.15† per cent methyl-thiouracil and. the fourth lot was fed basal ration
to which 0.04 per cent thyroprotein (' protamone ') was added.
* First it was thought to employ only 10 birds in each group. But with a view to allowing some
margin for mortality due to causes other than coccidiosis, a large number was taken so that the number
of experimentals may not fall below 10 during the period of observation.
† It was experimentally determined that ' protamone ' when fed for a fortnight in this concentra-
tion (0.15 per cent) in standard mash to day-old White Leghorn chicks (maintained at 80-90°F.) inhibits
thyroxine secretion of the thyroids and produces maximum enlargement of these glands. Their acini-
on histological examination were seen to be absolutely devoid of the colloid,