by: Heather Smith Thomas
Abortion is expulsion of a premature fetus before it reaches a viable stage of life, or expulsion of a dead fetus at any stage of gestation. Many early abortions take place without being noticed; at early stages of gestation the embryo or fetus may not be large enough to be easily seen. Abortions before 5th month of pregnancy often have few external signs, and are seldom followed by retention of the placenta. But abortions after the 5th month are often characterized by retained placenta; the cow fails to shed the fetal membranes for a few days (up to two weeks) after losing the fetus.
Some abortions in late pregnancy are due to injury or extreme stress. Stress can trigger release of hormones in the body that can start the cow into labor, calving prematurely. Usually when a cow aborts following injury, it is the stress (pain, inflammation, etc.) that triggers the actual abortion, rather than the injury itself, for the uterus and its fluids cushion the fetus very well and protect it from trauma even if the cow is seriously injured.
An injection of dexamethasone during the last three months of pregnancy will often cause a cow to go into labor, losing the calf because it is born too early. Dexamethasone is sometimes given to cattle to reduce swelling, inflammation and pain from injury or disease. High fever may also cause a cow to abort. Poisons such as iodine can be another cause.
Poison Plants -- Abortions can be caused by toxins in certain plants. One of the most common causes is pondorosa pine needles. These abortions occur most frequently in late Fall to early Spring. Cattle may eat needles when hungry or cold, or when available, such as down trees or branches easy to reach, logging slash, windfalls, dried fallen needles, or discarded Christmas trees. Cattle seeking shelter from storms, or being herded through the timber may eat the needles, or they may be eaten inadvertantly when cows graze grass around the trees, or are fed hay on top of fallen needles.
With pine needle abortion, retained placenta occurs regardless of stage of gestation, and cows should be closely watched for signs of illness. Some of these abortions may be complicated by weak contractions, excessive bleeding, and incomplete dilation of the cervix. Some cows may develop toxemia and die before or shortly after the abortion, and will need prompt treatment to save them.
A few abortions are caused by eating certain types of mold, such as moldy hay or silage. Some molds are dangerous to the fetus during the 3rd through 7th month of gestations, while Aspergillus usually causes abortion in the last trimester. Molds are thought to cause between 3 and l0 percent of abortions in cattle.
Infectious Agents -- Most late-term abortions are caused by infections. Under normal conditions, about 1 out of 200 cows will abort for some reason or another, and this is usually no cause for alarm. But if abortion rate exceeds one or two percent of the herd, an infection of some kind is likely involved.
The most common cause of abortion in cattle worldwide is Brucellosis (Bang's Disease), but in the U.S. and Canada it's been well controlled by widespread use of vaccination and a more common cause of infectious abortion is Leptospirosis. There are many types of lepto bacteria. Abortion from any serotype can occur at any stage of pregnancy, but it is usually thought of as a third-trimester abortion, just because these are more readily noticed. The lepto bacteria are spread in urine of sick and carrier animals and can contaminate feed and water. The bacteria can enter the cow through breaks in the skin on feet and legs when walking in contaminated water, or enter through the nose, mouth and eyes -- by contact with contaminated feed, water or urine.
Lepto affects unborn fetuses. Cows in the last half of gestation will usually abort one to three weeks after recovery from the acute stage of the disease. Even if a cow did not appear sick, she may abort. The incidence of lepto abortions in a herd may vary from 5 to 40 percent of a herd, depending on number of susceptible cows in the last half of gestation. But not all infected cows will abort. Sometimes an infected cow will give birth to a live, weak calf that dies within a few days of birth.
Vaccine gives immunity for only about six months. Since lepto can be a problem at any time of pregnancy, most vets recommend that cattle be vaccinated twice a year for lepto. Abortion outbreaks can occur any time of year but are common during summer and early fall. Lepto can be introduced into a herd by purchasing an infected cow, from cows infected at sales, fairs or shows, from pigs mingling with cattle, or contact with infected wildlife or rodents. Deer, elk or antelope urinating on a feed ground, or infected rodents contaminating feed or water, can spread the disease. Outbreaks sometimes follow concentration of cattle for feeding, vaccinating, preg-checking, etc. Abortions from lepto often cause retention of placenta and uterine infection, but cows usually recover and breed.
Another common cause of third-trimester abortions is IBR (infectious bovine rhinotracheitis, often called "red nose"). The virus can cause upper respiratory disease -- common in feedlots or wherever cattle are confined together. Abortions often occur in the last third of pregnancy, and can be mistaken for lepto.
If a cow is pregnant, the virus may infect the fetus, causing abortion within a few days. Abortions from IBR may occur at any time during gestation but are most common during the second half. In a herd outbreak, more than half the cows may abort, depending on the severity of the outbreak and the number of susceptible cows in advanced pregnancy. When ranchers routinely vaccinate for IBR with modified-live virus before breeding season, number of IBR abortions usually drops dramatically. Intranasal IBR vaccine can be used in the face of an outbreak; it can halt the infection going through a herd, but may not be effective immediately in stopping abortions, since some fetuses may have already been infected and can be aborted for some time after vaccination.
Intranasal vaccination is usually safe for pregnant cows or calves nursing pregnant cows, but modified live virus injection for controlling respiratory disease may cause pregnant cows to abort (unless they already have immunity due to earlier vaccinations). Abortions due to vaccination usually occur 2 to l0 weeks after injection. To avoid this problem, vaccinate cows each year when they are not pregnant, and vaccinate young stock at weaning to start building immunity. If calves are vaccinated while still on their mothers, intranasal or killed vaccine should be used.
Abortion can also be caused by vibrio, transmitted to a cow at time of service from an infected bull. The infection causes early embryonic death; the embryo dies early and the cow returns to heat--and the rancher doesn't think she settled. In some cases a fetus is carried a few months and then aborted. This bacterial disease can be prevented by vaccination and by not using infected bulls. Another disease spread by infected bulls is trichomoniasis, causing cows to abort sometime during the first four months of pregnancy. The cow returns to heat but does not settle again until she recovers from the infection several heat cycles later.
Another common cause of abortion is BVD (Bovine Virus Diarrhea). It can cause abortion or mummification of the fetus, or calves carried full term but born with abnormalities (eye lesions, partial hairlessness and other problems) from the infection. A BVD abortion is rarely noticed, because the aborted fetus is so small.
BVD can also cause abortion indirectly. The virus can inhibit a cow's immune system and make her more suceptible to other types of infection such as leptospirosis or IBR, even though she may have been vaccinated against these diseases. Failure to include BVD as part of herd health program can negate the other vaccinations. Heifer calves should be vaccinated for BVD at weaning and cows vaccinated after calving and before rebreeding.
If a herd experiences abortion rate higher than one percent, work with your veterinarian to determine the cause, and develop a vaccination or management program to prevent losses. Often the cause of abortion can be determined by sending a freshly aborted fetus to a diagnostic laboratory, or in some cases blood samples from the aborting cow, or the placenta.
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