After a cow or heifer is bred, she should calve about nine months plus one week later (283 days, on average). But sometimes accidents of gestation terminate pregnancy early, or other factors (disease or toxins) kill the developing calf. Immediately after conception, when the tiny embryo is travelling down the fallopian tube into the uterus, it is safe from harmful influences. After it reaches the uterus a few days later, however, it becomes more vulnerable to problems. The conceptus is called an embryo during the first 45 days of pregnancy; after that, all the major organs and body systems have been formed and it becomes a fetus. If loss occurs before 45 days of gestation, it is termed early embryonic death.
Many pregnancy losses in early gestation take place without being noticed, since at that stage the embryo or early fetus may be reabsorbed by the cow's body, or isn't large enough to be seen if it is expelled -- simply coming out with fluid from the uterus. If the loss occurs quite early, the cow comes back into heat again and you may just think she didn't settle. In hot weather, for instance, embryos are sometimes lost during the first week after mating; the embryo fails to attach to the uterus. If the embryo dies after it has attached to the uterine lining and the pregnancy has been recognized by the cow's body, it may be longer before the cow comes back into heat.
The second phase in which pregnancy loss occurs is from about 45 days up to about 60 to 80 days. During this time heat stress can also cause pregnancy loss, especially in a hot, humid climate. A cow bred in May or June that goes through severe heat during July or August may end up open because of early loss -- due to high temperatures.
Another reason for pregnancy loss in this period is occasional miscommunication in hormonal signals between the growing embryo and the uterine tissue that will become the placenta; the CL (corpus luteum) on the ovary may not be maintained. The CL is an important key to continuation of pregnancy because it produces progesterone-- the hormone that is crucal to pregnancy. If the level of this hormone drops too low, the cow will come back into heat and lose the pregnancy. Once she's past 60 days, however, the pregnancy is considered safe from hormonal changes because the CL remains on the ovary and keeps progesterone levels high. But she is still vulnerable to other causes of pregnancy loss, the most common being some type of infection.
Non-Infectious Causes Of Loss -- Many things can terminate pregnancy including genetic abnormalities, malnutrition, stress, steroids, poisons. Some genetic defects kill the conceptus early on, while others create abnormalities that make it impossible for the fetus to survive outside the uterus and it dies shortly after birth.
If cows are severely underfed during early pregnancy they may reabsorb or abort. Malnutrition (inadequate or inbalanced diet) may also make a cow more susceptible to other problems -- including any infectious diseases she is exposed to -- and some of these may cause death of the embryo or fetus.
Some abortions in late pregnancy are due to injury or stress. Severe stress of any kind triggers production of cortisol, which is a natural steroid that can cause the cow to go into labor. Some people worry that injury (falling down, being jammed in the belly by another cow, etc.) might hurt the fetus and cause an abortion, but the fetus is very well protected from bumps and jolts since it is floating in the uterus in fluid. Usually if a cow aborts following an injury, it is the stress from pain, inflammation, blood loss, etc. that triggers abortion (due to the body being flooded with cortisol) rather than the injury itself.
For this reason, steroids such as dexamethasone should not be given to pregnant cows because this may cause them to go into labor and deliver the fetus prematurely. Dexamethasone is sometimes given to cattle to help reduce swelling, inflammation and pain due to injury or disease, but should not be given to pregnant cows.
High fever can make a cow abort. If she has a fever higher than 106 degrees for very long, this makes the uterine environment too hot for the developing embryo or fetus. If a cow is sick, she might abort anyway (due to stress) but it is always worth trying to treat pain and fever, in hopes of saving the pregnancy. Treating with Banamine (which will not have adverse affects on the fetus) can help reduce pain, fever and inflammation and thus reduce the stress she's experiencing, and reduce the risk for abortion--and is safer to give than dexamethasone.
Poisons like iodine can also be a cause of pregnancy loss, as can toxins in certain plants. Locoweed may cause abortion at any stage of pregnancy. Broomsnakeweed also contains toxins that can cause abortion. Ponderosa pine needles are a common cause of abortion during winter if cows eat them when feed is short or covered with snow. These "pine needle abortions" generally occur from six to nine months of gestation (last trimester). The calf may be born alive if it's close to full term, but is usually weak and small and may die soon after birth.
Abortion can also be caused by eating moldy hay or silage. Some molds are dangerous to the fetus during the third through seventh month, whereas Aspergillus usually causes abortion in the last trimester. Molds are thought to cause between three to 10 percent of abortions in cattle. There are often some gray-white thickened patches (that look like ringworm) on the aborted fetus and on the placenta. Much of the placenta is discolored (gray, yellow or red- brown) and the cotyledons ("buttons") are usually thick, wrinkled or leathery.
Infectious Causes -- Most abortions are caused by viral or bacterial infections. Premature separation of the placenta from the uterus is sometimes caused by inflammation or placentitis, which may be viral or bacterial. But sometimes the cause is never determined. The most common cause of pregnancy loss at any stage of gestation is infection, either from bacterial contamination of the uterus or from a viral or bacterial disease in the cow at certain stages of pregnancy when the embryo or fetus is vulnerable to that disease. The cow may not be very sick, but if the disease affects her unborn calf it may die. Sometimes a disease like lepto, for instance, is so mild you don't notice the cow being ill, but she may abort.
Under normal conditions about 1 out of every 200 cows will abort for some reason or another and these miscellaneous losses are no cause for alarm, regarding herd health. A single abortion in the herd is usually an isolated incident; something went wrong with that particular pregnancy. But if abortion rate exceeds one or two percent of your cows, there's a chance that disease is involved. Several of the common diseases that cause abortion can be prevented by vaccination. But if a herd has not been vaccinated against a certain disease before (and the cows have no immunity), vaccination may not be effective in the short term -- in the face of an outbreak.
Brucellosis is the most common cause of abortion worldwide, but has been nearly eradicated in the U.S. by herd testing/culling and vaccination of heifers. There are only two states still involved in control programs (Idaho and Texas). Tuberculosis (TB) was also eradicated in the U.S. but in recent years was reintroduced to cattle in several states (Michigan, Texas, California) by infected wildlife or Mexican cattle. California and Texas have resolved their problem but Michigan is still fighting TB because it became established in their deer population in several areas.
Leptospirosis is the most common cause of infectious abortion in the U.S. There are many types of lepto bacteria, but only five that generally cause abortion in cattle and there are vaccines that include all of these. The bacteria are spread via urine of sick and carrier animals (rodents, pigs, cats, canines, wild animals such as deer, elk, antelope, etc.) and may contaminate feed and water. Bacteria don't have to be ingested; they can enter the cow through breaks in the skin on her feet and legs when she's walking in contaminated water, or enter through nose, mouth or eyes if she has contact with contaminated feed, water or urine. Lepto can also be transmitted by semen from an infected bull.
Incidence of lepto abortions in an unprotected herd may vary from 5 to 40 percent or more, depending on number of susceptible cows when the disease goes through them. Cows in the second half of pregnancy will usually abort one to three weeks after having an acute case of lepto. Not all affected cows abort; some give birth to weak calves that die within a few days. Vaccination gives good protection for about six months. Since lepto can cause problems at any stage of pregnancy (though abortions are most common in the second half), many vets recommend vaccinating cows twice a year.
Vibrio (now called campylobacteriosis) is a veneral disease which can be transmitted to cows at time of service from infected bulls. The cow conceives, but the bacterial infection causes an inflammation of the uterus and early embryonic death. Often the embryo dies so early that the cow returns to heat very soon, but it sometimes lives a few months and is then aborted. Infection can be prevented by vaccinating cows each year before the breeding season.
Trichomoniasis is another venereal disease that causes loss of pregnancy, but is caused by a protozoan -- spread to cows by infected bulls. Some states have a mandatory testing program for bulls. Trich usually results in early embryonic death and the cow returns to heat; this is a common cause of repeat breeders and infertility. Most cows clear themselves of infection after several heat cycles and losses, and finally become pregnant, but bulls usually remain infected; the protozoa live in his reproductive tract. Prevention is best accomplished by buying only virgin bulls, testing bulls every year, and making sure no cows are bred by infected bulls.
Several other diseases are caused by protozoa and result in abortion, including neosporosis (spread to cattle by canine fecal material), and sarcosystosis (also spread by predatory animals that shed the oocysts in feces). There's no vaccines for these diseases and best prevention is to keep canines, cats, etc. from defecating in feed (such as hay).
Some abortions are caused by viral diseases, the most common being IBR and BVD, which can both be controlled by vaccination. Best prevention is use of a modified live virus vaccine once a year, given to non-pregnant cows at least 3 weeks before breeding (to ensure strong immunity by the time the cows are bred). Some stockmen also give a killed vaccine in addition (as a booster) in mid to late pregnancy. The latter is safe to use in pregnant cows. There have been abortions caused by giving live virus vaccine to pregnant susceptible cows (that have low immunity) or to their calves ahead of weaning. Working with a vet to develop a good herd health program and proper vaccination can eliminate abortion problems due to IBR and BVD.
Other causes of abortion include salmonella (these bacteria can cause abortion as well as diarrhea in cattle), listeriosis (which most often affects the nervous system but can also cause abortion), and haemophilus somnus. Many healthy cattle have the latter bacteria in their urinary/genital tract without any signs of illness, but infection with this organism can cause pneumonia, septicemia, encephalitis or inflammation of the uterine lining and vagina. It's not a common cause of abortion, but may be a factor in infertility if cows have a uterine infection.
Chlamydia (an organism that is not a virus nor bacteria but somewhat similar to a virus) can inhabit the reproductive tract of bulls and cows and may cause sporadic pregnancy losses at seven to nine months' gestation. Foothill abortion is another problem (occurring in the central-eastern foothills of California, western Nevada and southern Oregon) but the cause has not yet been identified—though some researchers think it is the spirochete that also causes Lyme disease. Abortions occur about three to four months after exposure to a certain kind of tick that is always present in these regions.
If your cow herd experiences an abortion rate higher than one or two percent, work with your vet to determine the cause. In most cases you can develop a vaccination/management program to prevent these losses. Often the cause can be determined if your vet can send a freshly aborted fetus or some of the placental membranes to a diagnostic lab, or take blood samples from the aborting cow or the fresh placenta.