Cattle abortion stems from a wide range of causes, including infections, toxic plants, nutritional problems, and environmental stress. Infectious agents account for the largest share of diagnosed cases, but roughly half to 80% of all submitted abortion cases never receive a definitive diagnosis, often because tissue samples are too degraded or the placenta wasn’t submitted for testing. Understanding the most common causes can help you narrow down what’s happening in your herd and take preventive steps.
Infectious Agents Are the Leading Identified Cause
A large meta-analysis of more than 9,000 aborted bovine fetuses found that the parasite Neospora caninum was the single most frequently detected infectious agent, showing up in 22.2% of cases. Opportunistic bacteria collectively appeared in 21.4% of cases, followed by Chlamydia-family organisms at 10.9% and the bacterium that causes Q fever at 9.5%. Viral agents like infectious bovine rhinotracheitis (IBR) and bovine viral diarrhea virus (BVDV) each appeared in roughly 5% of cases.
Detection doesn’t always equal cause. When researchers looked for actual tissue damage confirming the agent killed the fetus, the numbers dropped significantly. Neospora was confirmed as the cause in about 17% of cases, opportunistic bacteria in 13%, and Chlamydia-family organisms in about 7%. The Q fever bacterium, despite being detected in nearly 10% of samples, was confirmed as the cause in only 1% of cases. This gap is one reason abortion diagnosis is so difficult.
Neospora: The Most Common Parasite Involved
Neospora caninum is a microscopic parasite that spreads through two main routes. Cattle pick it up horizontally by eating feed or water contaminated with oocysts shed in dog or coyote feces. The second route, vertical transmission from an infected cow to her calf in the womb, is the one that sustains the problem in a herd over years. In one study of aborting cattle, 87% of seropositive dams passed the infection to their fetuses. That means an infected cow can silently transmit Neospora to calf after calf across multiple pregnancies, with some of those pregnancies ending in abortion and others producing live but subclinically infected calves that carry the cycle forward.
Neospora-related abortions can occur at any stage of pregnancy but are most commonly seen between the fifth and seventh month of gestation. There is no approved vaccine or treatment for Neospora in cattle, so management focuses on testing, culling seropositive animals over time, and keeping dogs and wild canids away from feed storage and calving areas.
Bacterial Causes
Leptospirosis
Leptospirosis, particularly the strain adapted to cattle (Leptospira borgpetersenii serovar Hardjo), causes pregnancy losses ranging from early embryonic death to late-term abortions. While textbooks traditionally describe leptospiral abortions as occurring in the final third of pregnancy, fetal death has been documented at all stages. Embryonic loss in early pregnancy is actually the more frequent outcome, though it often goes unnoticed because the cow simply appears to have failed to conceive. Cattle pick up leptospirosis from contaminated water, urine-soaked pastures, or direct contact with infected animals, and the organism can persist in the kidneys and reproductive tract for months.
Brucellosis
Brucella abortus was historically one of the most devastating causes of cattle abortion worldwide. In the United States, decades of testing and vaccination have made all 50 states, Puerto Rico, and the U.S. Virgin Islands brucellosis-free, with the only exceptions being occasional spillover from wild bison and elk in the Greater Yellowstone Area. It remains a serious problem in parts of Africa, Asia, and South America. Brucellosis is a reportable disease, meaning any suspected case must be reported to state and federal animal health officials.
Opportunistic Bacteria
A long list of common environmental bacteria, including Salmonella, E. coli, Listeria, and various Streptococcus and Staphylococcus species, can cause sporadic abortions. These organisms typically enter the bloodstream during illness or stress and reach the placenta. Because they’re common in the environment, these abortions tend to appear as isolated cases rather than herd-wide outbreaks. Placental infection and inflammation is usually the mechanism, and submitting the placenta along with the fetus dramatically improves the chances of getting a diagnosis.
Viral Causes
Bovine viral diarrhea virus and infectious bovine rhinotracheitis (caused by bovine herpesvirus type 1) are the two viruses most commonly linked to reproductive failure in cattle, each showing up in about 5% of abortion cases.
BVDV’s effects depend heavily on when during pregnancy the cow is exposed. Infection during roughly the first 120 days of gestation, before the fetus develops immune competence, can result in a persistently infected (PI) calf. These PI calves are born alive but shed massive amounts of virus for life, becoming a constant source of infection for the rest of the herd. Exposure at around 70 days of gestation with certain BVDV strains has been shown to cause outright abortion. Infection later in pregnancy, after the fetal immune system is functional, typically causes only a mild, transient infection in the calf.
IBR can cause abortion at any stage of pregnancy, though it most often occurs in the second half. Cows that carry latent herpesvirus can reactivate the virus during periods of stress, leading to abortion even without obvious respiratory disease in the herd.
Toxic Plants and Feed Contaminants
Ponderosa Pine Needles
In North America, ponderosa pine needles are a well-documented cause of late-term abortion in beef cattle. The culprit is a compound called isocupressic acid, found in the needles and bark of the tree. This compound blocks progesterone production by the ovary’s corpus luteum, the structure responsible for maintaining pregnancy. Without adequate progesterone, the pregnancy cannot be sustained. Cattle typically encounter pine needles when grazing in wooded pastures during winter, especially when other forage is scarce. Juniper, lodgepole pine, and some cypress species contain similar compounds and pose the same risk.
Nitrate Toxicity
Forages high in nitrates, particularly drought-stressed crops like sorghum, Sudan grass, oats, and certain weeds, can trigger abortions 5 to 14 days after excessive exposure. Nitrates are converted to nitrites in the rumen, which enter the bloodstream and bind to hemoglobin, reducing the blood’s ability to carry oxygen. Forage nitrate concentrations above 5,000 ppm (dry-weight basis) are considered unsafe for pregnant beef cows. Concentrations above 10,000 ppm can cause acute poisoning in animals not gradually acclimated to the feed. Even levels as low as 1,000 ppm have proven lethal to hungry cows that gorge on the feed in a single feeding, so the total amount consumed at one time matters as much as the concentration.
Heat Stress and Early Pregnancy Loss
Heat stress is a major but often underappreciated cause of pregnancy failure, particularly in the first few weeks after conception. High temperatures suppress the hormonal signals needed to maintain early pregnancy. Specifically, heat reduces pulses of luteinizing hormone from the pituitary gland, which leads to poor corpus luteum function and declining progesterone levels. Progesterone is essential for preparing the uterus and supporting the embryo, so even a modest drop can result in pregnancy loss.
The damage goes beyond hormones. Heat stress increases the production of reactive oxygen species, damaging developing embryos at the cellular level. It also disrupts the cells surrounding the egg (granulosa cells), reducing estrogen production and sometimes silencing the estrous cycle entirely, so cows fail to show heat and aren’t bred. In chronic or extreme heat, uterine blood flow decreases, restricting fetal growth and increasing the risk of late pregnancy loss. The stress response itself compounds the problem: the adrenal glands release glucocorticoids that interfere with egg maturation. Together, these effects explain why conception rates and pregnancy survival can drop sharply during hot summer months, particularly in regions without adequate shade, ventilation, or cooling systems.
Why So Many Cases Go Undiagnosed
Diagnostic success rates for bovine abortion hover between 20% and 50% of submitted cases. That means for every two or three fetuses sent to the lab, at least one comes back with no definitive answer. The most common reasons include poor-quality or insufficient tissue samples, failure to submit the placenta (which is critical for diagnosing many infections), and tissue decomposition that has already destroyed the evidence by the time the fetus is found. Some causes, particularly non-infectious ones like nutritional deficiencies, hormonal problems, or chromosomal abnormalities, simply don’t leave identifiable lesions on post-mortem exam.
An inconclusive result isn’t necessarily a failure. It rules out the major infectious causes, which can be valuable information in itself. To maximize the chance of a diagnosis, submit both the fetus and the placenta as fresh as possible, ideally refrigerated rather than frozen. Paired blood samples from the dam can also help detect exposure to common agents like Neospora, Leptospira, or BVDV.
Vaccination and Prevention Strategies
Vaccination is the most practical tool for preventing abortions caused by IBR, BVDV, and leptospirosis. The standard recommendation is to administer a combined viral respiratory vaccine (covering IBR, BVD, PI3, and BRSV) along with a 5-way leptospirosis vaccine four to six weeks before breeding. For open heifers at least six weeks from breeding, modified-live vaccines are generally preferred because they produce a stronger immune response.
Timing and vaccine type matter enormously for pregnant animals. If a cow is already pregnant or within 30 days of breeding, only killed vaccines should be used, because modified-live vaccines carry a risk of causing the very abortion you’re trying to prevent. If a cow was vaccinated with a modified-live product within the past 12 months (check the specific product label), she can typically receive it again safely even while pregnant, but this requirement must be confirmed before proceeding.
Beyond vaccination, prevention involves managing the environmental and nutritional risk factors: keeping dogs and wildlife away from feed to reduce Neospora exposure, testing forages for nitrate levels before feeding to pregnant cows, fencing cattle out of areas with ponderosa pine during late gestation, and providing shade and cooling during hot months to reduce heat-related embryonic loss. For herds with recurring abortion problems, working with a veterinarian on a systematic diagnostic and testing protocol is the most efficient path to identifying the cause and stopping it.