Most cattle 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 leptospirosis 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. If the 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.
Related Article: Non-Infectious Pregnancy Losses in Cattle >>
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. 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 and 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 venereal 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 are 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), Listeria (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 neither 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. 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.