Pregnancy may be terminated prematurely, resulting in abortion due to death of the conceptus or failure of the uterine environment to support the fetus. Abnormal fetal development may result in abortion or in a calf that dies soon after birth. Many cases of bovine abortion are not diagnosed. (Also see Abortion in Large Animals.)
Viruses, bacteria (including rickettsia and chlamydiae), molds, protozoa, or other infectious agents may attack the placenta or the fetus, or both. Some of these microorganisms reach the uterus hematogenously; others (such as venereal infections) are contracted during mating.
Infectious abortion may be sporadic or a herd problem. Herd problems usually are associated with significant losses and may be caused by infectious bovine rhinotracheitis, bovine viral diarrhea, brucellosis, leptospirosis (various serotypes), campylobacteriosis, trichomoniasis, anaplasmosis, ureaplasmas, mycoplasmas, and others not yet identified.
Mycotic abortion usually is caused by Aspergillus or Mucor spp, which reach the uterus hematogenously and cause abortion in late gestation. In many of these fetuses, the skin is not affected; in others, ringworm-like lesions are seen. The placenta frequently is severely affected with necrosis of the cotyledons and thickening of the intercotyledonary areas. Diagnosis is based on identification of the fungus through culture of the fetal or placental tissues, histologic examination of these tissues, or direct examination of cotyledons after clearing with potassium hydroxide solution. These abortions are almost always sporadic, and the only means of control is to reduce exposure to the fungi.
Sporadic losses may result from Listeria sp (a bacterium occasionally present in silage when pH is >7); miscellaneous bacteria such as Haemophilus sp, Trueperella pyogenes, Staphylococcus aureus, Bacillus cereus, Pasteurella multocida, Pseudomonas aeruginosa, Streptococcus bovis, Chlamydia sp, and others; or viruses (eg, bluetongue).
Noninfectious causes of abortion are numerous; the most common include 1) recessive or lethal genes (or both) such as cervical vertebral malformation, hydrocephalus, osteopetrosis (“marble bone” disease), arthrogryposis (“crooked calf” syndrome), and several others, some not fully identified; 2) toxins (eg, excessive nitrates from feed or water), certain pine needles, poisonous plants (eg, lupine, locoweed), or mycotoxins (moldy feeds); 3) hormonal imbalances in the pregnant dam; 4) injuries affecting the pregnant cow; and 5) nutritional deficiencies, particularly of vitamin A, vitamin E or selenium (or both), iodine, and manganese.
Heat stress in cattle can cause early embryonic death and lower the herd pregnancy rate. The mechanism by which heat stress affects embryonic survival is complex. Heat stress can disrupt early embryonic development. Effects of heat stress on embryonic survival decrease as embryos advance in development. Heat stress at day 1 or days 1–3 after breeding reduces embryonic survival. In contrast, heat stress of superovulated cows at day 3, 5, or 7 after estrus did not affect embryonic development in one study.
Accurate diagnoses of reproductive loss contribute to the cumulative herd history and provide criteria to evaluate the impact on herd performance and the need for implementation of preventive measures. Laboratory assistance is needed in most cases. Carefully selected, properly preserved, quality specimens should be submitted to a diagnostic laboratory for analysis. Even with these, the exact cause of an abortion may not be detected, especially if it is noninfectious. Laboratory diagnosis of abortion may include serology and examination of the fetus and placenta. However, a definitive diagnosis of bovine abortion remains challenging because, in many cases, the causative agent may have challenged a cow months previously and may no longer be present when abortion occurs.
Defective newborn calves can be recognized only by a thorough examination and sometimes only after some time has passed.
Several factors are critical to prevent and control abortion and development of defective calves. Measurement and management of herd health status and planning to control infectious disease risk with appropriate boundary and purchased stock biosecurity alongside vaccination programs are essential. A balanced nutritional program helps control losses associated with mineral or vitamin deficiencies and poor-quality feeds, including moldy grains and forages. Genetic selection and accurate record-keeping help to detect and eliminate bloodlines that prove to be carriers of recessive or lethal genes. Appropriate housing and handling facilities decrease the incidence of accidents and provide an environment conducive to health. The cattle producer and veterinarian should work together to assess the herd’s reproductive performance, tailor a vaccination program to the herd’s specific needs, and diagnose and control potential herd problems.
For successful abatement of heat stress, the environment of the cow must be modified to maintain the cow within a normal temperature range of 101.3º–102.8ºF (38.5º–39.3ºC). Common approaches include providing shade (to intercept solar radiation) and fans or sprinklers to promote evaporative cooling. Tactically avoiding breeding at hottest times of the year may be advisable.
Infectious diseases in a herd can disrupt and reduce reproductive efficiency by causing embryonic or fetal death, abortion, or illness and death of neonates. A complete vaccination program will not eliminate reproductive problems but may prevent or reduce losses associated with specific infections (see Table: Vaccination/Immunization Program to Protect Against Prepartum Diseases of the Breeding Herd and Vaccination/Immunization Program to Protect Against Diseases of the Neonatal Calf).