Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

IV. Clinical Aspects of Reproductive Endocrinology
653
patterns in nonpregnant mares, an approach that was difficult
to do with the determination of free estradiol-17 β
( Daels et al. , 1991 ; Makawiti et al. , 1983 ).
4 . Other Pregnancy-Associated Substances
A test (a lateral flow assay using antibodies with colloid
gold as the indicator) for equine early conception factor
(a pregnancy-associated immunosuppressive protein) is
suggested to be a quick, easy, and noninvasive method for
detecting nonpregnant mares. However, a controlled study
shows that a large proportion (60%) of animals give a positive
test result even before breeding ( Horteloup et al. , 2005 ).
5 . Testosterone
Testosterone values vary in the mare according to the reproductive
state. Values, usually less than 15 pg/ml ( 52 pmol/
liter) during anestrus, range between 20 and 40 pg/ml
(69 to 139 pmol/liter) during cyclic ovarian activity with the
higher values being observed during the follicular phase of
the cycle immediately before ovulation. Testosterone determinations
have been used to aid the diagnosis of granulosatheca
cell tumors in the mare ( Stabenfeldt et al. , 1979 ) and
to differentiate granulosa-theca cell tumors from ovarian
teratoma ( Panciera et al. , 1991 ). Leydig-like cells in the
theca appear to be the source of testosterone. In cases of
granulosa-theca cell tumors, testosterone values vary with
values ranging from 40 pg (139 pmol/liter). Values over
100 pg/ml (347 pmol/liter) are considered diagnostic of
granulosa-theca cell tumors in mares. These tumors are generally
slow in development, and it is not known whether this
slow development also reflects a slowly developing capacity
for testosterone production or whether there is variability as
to the number of testosterone secreting cells among tumors.
Aggressive stallion-like behavior is associated with high
values of testosterone ( Stabenfeldt et al. , 1979 ). Inhibin is
usually elevated in granulosa-theca cell tumors in mares and
thus suppresses pituitary FSH release ( Bailey et al. , 2002 ).
Testosterone determinations in the male horse have been
used as an aid in the diagnosis of cryptorchidism. Cox (1975)
reported that horses with 40 pg/ml (139 pmol/liter)
plasma should be considered castrated, whereas animals
with concentrations 100 pg/ml (347 pmol/liter) should
be considered as having testicular tissue present. Although
some cryptorchid animals have testosterone concentrations
100 pg/ml (347 pmol/liter), most have values ranging
from 200 to 1000 pg/ml (693 to 3467 pmol/liter) (Cox,
1975 ). Testosterone concentrations in intact males usually
range from 1000 to 2000 pg/ml (3467 to 6934 pmol/liter)
(Berndtson et al. , 1974 ; Cox et al. , 1973 ). HCG administration
for the purpose of stimulating testosterone production
by the testes has been suggested as a means of resolving
cases in which values are between 40 and 100 pg/ml (139 to
347 pmol/liter). Dosages of injected hCG have been 6000 to
12,000 IU. ( Cox et al. , 1986 ; Silberzahn et al. , 1989 ) and
the second sample for analysis of testosterone obtained after
30 to 120 min (Cox et al. , 1986 ) or after 3 days ( Silberzahn
et al. , 1989 ). The responses to hCG may be lower in horses
less than 18 months of age and during the nonbreeding season
(winter).
It has been suggested that the analysis for estrone sulfate
conjugates for the diagnosis of cryptorchidism is preferred
over testosterone on the basis that it requires only
one analysis and that the accuracy is slightly improved
( Cox et al. , 1986 ). It should be noted, however, that
estrone sulfate analysis for cryptorchidism cannot be used
in horses less than 3 years of age or in donkeys because
little estrone sulfate is produced in either of these situations.
Androstenedione is the precursor hormone of androgens
and estrogens and can also be used for the diagnosis
of cryptorchidism ( Illera et al. , 2003 ).
Castration and thus withdrawal of androgens causes
increased levels of gonadotropins, which thus can be measured
as a complement to testosterone.
The presence of sexual behavior in suspected cases of
cryptorchidism is not necessarily dependent on elevated
testosterone concentrations in that many patients (about
one-third) referred because of behavioral problems are, in
fact, castrated. In essence, some animals can maintain normal
libido with low circulating concentrations of testosterone.
In this situation, testosterone analysis is helpful from
the point of view of either eliminating unnecessary surgery
or indicating to the surgeon that testicular tissue is present
and should be found on surgical entry.
E . Dog
1 . Progesterone
In the bitch, there is little difference between the progesterone
pattern of the pregnant and nonpregnant luteal phase. Plasma
concentrations of progesterone are elevated throughout the
luteal phase. However, although mean concentrations of progesterone
are higher during the latter part of the pregnant
luteal phase compared with the nonpregnant luteal phase,
these differences are not significant to allow progesterone
determinations to be used as a pregnancy diagnosis test.
Bitches usually ovulate at the onset of sexual receptivity,
remain sexually receptive for about the first week of luteal
activity, and, in fact, are fertile during this period of time
( Holst and Phemister, 1974 ). Progesterone analysis can be
used to confirm the occurrence of ovulation with concentrations
usually increasing slightly above baseline in the periovulatory
period reaching about 15 nmol/liter at ovulation
followed by a sustained increase beginning 24 h following
ovulation (Fig. 21-9 ) (Concannon et al. , 1975 ). This approach
may be used in a retrospective analysis of a breeding cycle in
an attempt to correlate the time of ovulation with other criteria
such as vaginal cytological changes. The determination of