Pregnancy loss is not uncommon in dairy cows, with estimates as follows:
In the early embryonic period (from conception to maternal recognition of pregnancy at 14–19 days after fertilization), 35%–40%
In the late embryonic period (from maternal recognition of pregnancy to 28 days), and from 28 to 42 days, 5%–10%
In the fetal period (from 42 days to normal calving), 11%
Etiology of Reproductive Loss in Cattle
The varied causes of pregnancy loss in cattle include infectious disease, poor nutrition, twinning, genetics, and inflammation.
Viruses, bacteria (including rickettsias 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 are usually associated with substantial losses and may be due to infectious bovine rhinotracheitis, bovine viral diarrhea, brucellosis, leptospirosis (various serotypes), campylobacteriosis, trichomoniasis, anaplasmosis, ureaplasmas, and mycoplasmas, among others.
Mycotic abortion is usually caused by Aspergillus spp or Mucor spp, which reach the uterus hematogenously and cause abortion in late gestation. In many of these fetuses, the skin is not affected; others have ringworm-like lesions. The placenta is frequently 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. Mycotic abortions are almost always sporadic, and the only means of control is to decrease exposure to the fungi.
Sporadic losses may result from Listeria spp (a bacterium occasionally present in silage when the pH is > 7); miscellaneous bacteria, such as Histophilus spp, Trueperella pyogenes, Staphylococcus aureus, Bacillus cereus, Pasteurella multocida, Pseudomonas aeruginosa, Streptococcus bovis, Chlamydia spp; or viruses (eg, bluetongue virus or Schmallenberg virus).
Noninfectious causes of abortion are numerous. The most common include the following:
Recessive or lethal genes (or both) that result in disorders such as cervical vertebral malformation, hydrocephalus, osteopetrosis (marble bone disease), arthrogryposis (crooked calf syndrome), and several others, some not fully identified
Toxins (eg, excessive nitrates from feed or water), certain pine needles, poisonous plants (eg, lupine, locoweed), or mycotoxins (moldy feeds)
Hormonal imbalances in the pregnant dam
Injuries affecting the pregnant cow
Nutritional deficiencies, particularly of vitamin A, vitamin E or selenium (or both), iodine, and manganese
Heat stress in cattle can result in 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 decreases embryonic survival. In contrast, superovulated cows appear to be more resistant to heat stress in terms of early pregnancy.
Diagnosis of Reproductive Loss in Cattle
Accurate diagnoses of reproductive loss contribute to the cumulative herd history and provide criteria for evaluating the impact on herd performance, as well as the need for implementation of preventive measures. Laboratory assistance is needed in most cases. Carefully selected, properly preserved, high-quality specimens should be submitted to a diagnostic laboratory for analysis. Even with these criteria met, the exact cause of an abortion may not be detected, especially if it is noninfectious.
Techniques required for laboratory diagnosis of abortion may include serologic testing and examination of the fetus and placenta. However, a definitive diagnosis of bovine abortion remains challenging because often the causative agent challenged a cow months previously and is no longer present or detectable when abortion occurs.
Prevention and Control of Reproductive Loss in Cattle
Several factors are critical to prevent and control both abortion and development of defective calves:
Measurement and management of herd health status, appropriate boundary and purchased stock biosecurity measures, and vaccination programs are essential.
A balanced nutritional program helps control losses associated with mineral or vitamin deficiencies and low-quality feeds, including moldy grains and forages.
Genetic selection and accurate record keeping help to detect and eliminate bloodlines that carry 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 genomics and vaccination programs to the herd’s specific needs, and diagnose and control potential herd problems overall.
For successful abatement of heat stress, the cow's environment must be modified to maintain the cow's body temperature within a normal range: 38.5°C–39.3ºC (101.3°F–102.8ºF). Common approaches include providing shade, as well as fans or sprinklers to promote evaporative cooling. Tactically avoiding breeding at the hottest times of the year may be advisable.
Infectious diseases in a herd can disrupt and decrease reproductive efficiency by causing embryonic or fetal death, abortion, or illness and death of neonates. A complete vaccination program will not eliminate reproductive problems; however, it may prevent or decrease losses associated with specific infections.
