Infectious Contagious Abortion in Horses: Bacterial Pregnancy Loss Causes

Infectious contagious abortion in horses means a mare loses her pregnancy because an infectious organism damages the placenta, fetus, or both. In bacterial cases, the problem is often placentitis, which is inflammation and infection of the placenta. The placenta is the foal's lifeline, so when it becomes inflamed, oxygen and nutrient delivery can fall quickly.

In mares, bacterial placentitis is one of the most commonly diagnosed infectious causes of late-term abortion. Most bacterial cases are ascending infections, meaning bacteria move upward through the cervix and infect the placenta near the cervical star. Less common patterns include blood-borne infection, such as Leptospira reaching the placenta from the bloodstream, and nocardioform placentitis, which tends to affect a different area of the placenta.

One challenging part is that some mares show few outward signs until the problem is advanced. A mare may seem normal, then develop udder enlargement too early, vaginal discharge, or abort late in gestation. In other cases, the foal is born weak or premature rather than being aborted.

This is not something to monitor at home without veterinary guidance. If a pregnant mare shows suspicious changes, your vet can help confirm whether the pregnancy is still viable, whether infection is likely, and which care options fit the mare, foal, farm setup, and budget.

The most common bacterial pathway is ascending placentitis. In these cases, bacteria from the mare's lower reproductive tract move through a relaxed or compromised cervix and infect the placenta. Merck notes that, aside from Leptospira spp. and nocardioform infections, most bacterial placentitis in mares is ascending. Organisms reported in ascending placentitis include Streptococcus equi subsp. zooepidemicus and other streptococci, along with bacteria such as E. coli, Klebsiella, Pseudomonas, Staphylococcus, and Actinobacillus.

Other bacterial pregnancy losses happen differently. Leptospira can spread through the bloodstream after exposure to urine-contaminated water or wildlife reservoirs, then infect the placenta and fetus. In endemic areas, leptospiral abortion can make up a meaningful share of infectious equine abortions. Nocardioform placentitis affects a different placental region and may lead to abortion, stillbirth, or a weak foal at term.

Some mares are at higher risk if they have poor vulvar conformation, cervical incompetence, twins reduced late, previous reproductive tract damage, or environmental conditions that increase exposure to contaminated water, urine, or infectious reproductive fluids. Farm-level spread risk depends on the organism involved. Some causes are more opportunistic than truly horse-to-horse contagious, while others raise stronger herd biosecurity concerns.

Because abortion in mares can also be caused by viruses, fungi, twins, umbilical cord problems, and noninfectious placental disease, it is important not to assume the cause from signs alone. Your vet may recommend testing the mare, fetus, and placenta before making breeding or herd-management decisions.

Diagnosis starts with history and timing. Your vet will want to know the mare's gestation stage, vaccination status, recent travel, herd reproductive history, discharge, premature udder development, and whether any other horses are ill or aborting. A physical exam is usually paired with reproductive ultrasound to assess fetal heartbeat, placental thickness, fetal fluids, and signs of placental separation.

If the mare is still pregnant, your vet may recommend transrectal and sometimes transabdominal ultrasound, bloodwork, and in selected cases vaginal examination or culture. Ultrasound measurement of the combined thickness of uterus and placenta can help support a diagnosis of placentitis, especially when paired with clinical signs.

If abortion has already happened, the most useful samples are often the entire placenta and fetus submitted for necropsy, histopathology, culture, and targeted PCR or special testing. Merck notes that leptospiral abortion is best confirmed with tests such as fluorescent antibody testing or immunohistochemistry on placenta, umbilical cord, fetal liver, or fetal kidney. Depending on the case, your vet may also request bacterial culture, antimicrobial susceptibility testing, and testing for viral causes that can mimic bacterial abortion.

Costs vary widely by region and how much testing is pursued. A farm call and exam may run about $75-$250, reproductive ultrasound often adds $150-$300, and laboratory abortion workups commonly range from about $250-$350+ for a basic panel, with additional pathology, culture, PCR, shipping, and emergency care increasing the total.

Prevention starts with breeding management and mare conformation. Mares with poor vulvar seal, cervical problems, or a history that suggests ascending infection may benefit from closer reproductive monitoring before and during pregnancy. Good perineal conformation, prompt attention to vaginal discharge, and regular pregnancy checks can help your vet catch problems earlier.

Farm hygiene also matters. Isolate mares that abort until your vet advises otherwise, and handle aborted fetuses, placentas, fluids, and contaminated bedding as potentially infectious. Use gloves, dedicated equipment, and careful cleaning and disinfection. Limit nose-to-nose contact and unnecessary movement of horses if an infectious abortion is suspected.

Reducing environmental exposure is especially important for Leptospira. Clean water sources, control standing water when possible, and reduce contact with wildlife or rodent urine contamination around feed and barns. In breeding operations, quarantine new arrivals when feasible and review vaccination and reproductive records before mixing horses.

No prevention plan removes all risk. The most practical approach is a layered one: good broodmare management, timely veterinary exams, fast response to early warning signs, and diagnostic testing after any abortion event. That combination helps protect the current mare, future pregnancies, and the rest of the herd.