Economic losses are reduced by early identification of nonpregnant farm animals, which can either be rebred or culled. In cattle, for example, commercial producers cannot afford to carry open cows until the next breeding season - nearly one year away. Several less than ideal methods to detect pregnancy in livestock are available. The impetus is to develop reliable and practical on-farm/ranch tests for early pregnancy based on the presence of hormones or pregnancy-associated proteins.
A variety of qualitative physician's office and home pregnancy test kits are now available. Highly sensitive quantitative assays, such as RIA, must be carried out in licensed laboratories.
Palpation. Manual examination of the uterus per rectum is routinely used to diagnose pregnancy in cattle and horses. Landmarks indicating stage of bovine pregnancy are listed in Table 5-14.
A rectal-abdominal technique can be used for diagnosis of pregnancy in sheep. A lubricated plastic rod is inserted into the rectum of a ewe in the supine position in a restraining crate. The fetus is palpated through the abdominal wall by moving the stick side-to-side while applying slight upward pressure; the method is reliable after 50 days of gestation.
Radiography and ultrasound. The fetal skeleton can be identified by standard X-ray; this method is only useful for diagnosis during late pregnancy, is costly, and poses a radiation hazard.
Ultrasonography, developed originally for observation of pelvic organs in humans, is now being used for early diagnosis of pregnancy in sheep, pigs, and horses (with confidence, around Day 30 of gestation). A real-time scanning probe is passed over the abdominal wall that transmits two-dimensional images (representing about 10 mm slices) to an oscilloscope (Figure 5-32). Ultrasonic waves are reflected at different levels of energy by tissues of varied density. Dense tissue, like bone, readily reflect waves (bats have used echo-location to navigate for ages). Human pregnancy can be detected by ultrasound at approximately five weeks postconception. As pregnancy progresses, virtually every organ can be monitored. Ultrasound equipment is expensive.
Biochemical tests. An elevated milk or plasma progesterone as monitored by RIA or ELISA (eg., on Day 22 postestrus in the cow) is indicative of failure to return to estrus, and thus pregnancy. False negatives are negligible. False positives can result from long estrous cycles or early embryonic mortalities. Commercial application, especially with RIA, has been limited by laboratory costs and turnaround time. An assay which measures a pregnancy-specific (aspartic proteinase) protein (PSPB) has been developed for use in cattle.
A hemagglutination-inhibition assay has been used for detection of PMSG in horses; hormone is coupled to erythrocytes. Choriogonadotropin in the test sample inhibits binding of antisera against PMSG to red cells, thereby preventing agglutination. A variation is to coat erythrocytes with antibody. Presence of hormone acts as a cross-linking agent between antibodies, causing hemagglutination; this procedure has been used to detect hCG in urine.
A rosette-inhibition test has been developed for detection of EPF. Early pregnancy factor is an immunosuppressant that was first reported in pregnant women, and subsequently in livestock. Formation of rosettes between lymphocytes and sensitized erythrocytes (coated with antilymphocyte antibodies) is inhibited by EPF (Figure 5-33). The EPF test is time-consuming and unreliable.
Most urinary hCG test kits are ELISAs; they have replaced less sensitive and cumbersome sensitized red cell and latex particle agglutination systems. Choriogonadotropin ELISAs are performed in approximately five minutes on a first-voided morning urine sample; pregnancy can be accurately detected by the time of the first missed period (hCG can be measured by RIA within seven days after ovulation).