For almost 100 years, cattle producers in the West experienced an awful phenomenon: large numbers of strange, late-term abortions in otherwise healthy cattle. In the mid-1900s, it became clear that the disease was not caused by any recognized pathogens and was first referred to by researchers as epizootic bovine abortion or, more simply, EBA. The disease was reported most commonly in beef cattle pastured in foothill regions of California and therefore was given the nickname the “foothill abortion.”
Fetal losses can from this disease can be devastating. In cattle raised from birth in endemic areas, losses may range from 5% to 10%, to as high as 60% to 70% in replacement heifers. Naïve cattle brought into endemic areas have been reported to suffer up to 90% losses in their calf crop. With such staggering numbers, producers with a keen eye on the bottom line quickly asked veterinary researchers for help in figuring out how to prevent it. Unfortunately, this peculiar disease hasn’t made that easy.
What causes it, where is it and how is it transmitted?
An extremely unusual bacteria (Pajaroellobacter abortibovis) causes the disease and is spread by an equally unusual and little-known soft-shell tick, commonly called the pajaroello (or pajahuello) tick (Ornithodoros coriaceus). For as much trouble as it causes, most never see this tick. It has been recognized for centuries in semiarid and more mountainous regions of Mexico and California. The tick and disease were later formally recognized in northern Nevada and southern Oregon. Recently, northern Arizona has been added to the list of states suffering from foothill abortion.
The pajaroello tick lives in decaying leaf litter, under a wide variety of trees and brush, including oak, juniper, ponderosa and Jeffrey pines, pinion pine, mahogany, large sage and bitterbrush. This unique tick does not embed in the host like the more widely recognized hard-shell ticks. This unique tick lies in wait for animals to bed down, attracted by the CO2 exhaled from the animal, then attaches and feeds in less than an hour, detaches and returns to the soil. While feeding, the P. abortibovis bacteria is transmitted through the tick’s saliva to the host. Not all pajaroello ticks are infected with the bacteria, but studies have found that there are always some P. abortibovis-infected ticks within a population. The tick, the bacterial pathogen and disease all go hand in hand; find one and you’ll find the other.
Has the disease been spreading?
Researchers currently don’t know if foothill abortion is spreading. Most likely the tick, and therefore the disease, have been in the now-recognized endemic areas for centuries. If it is spreading, it is likely doing so very slowly, since only the larval stage of the tick stays attached to an animal for any length of time. When the tick detaches, it must find a safe haven and wait for an animal to come along and bed down before it dies of starvation or dehydration. That said, cattle herds and wildlife provide a reliable supply of food, and increased animal density likely increase tick populations.
The question is often asked: If foothill abortion is not spreading, why is it being found in more places over time? Recognition of EBA in a herd can often be tied to management changes. One critical change can be breeding replacement heifers as 1- instead of 2-year-olds. These younger animals have had less opportunity to have been bitten by an infected tick, and therefore are more likely to be naïve when they become pregnant with their first calf. The disease can also appear when producers bring in heifers or cows from non-foothill endemic areas instead of raising replacement heifers on-site. Alteration of ranges or the time of year certain pastures are utilized can also affect the incidence of EBA, as tick activity changes with location, weather and time of year. Better diagnostics and field collection of the pajaroello ticks have also played a role in disease recognition.
How do you recognize EBA?
To make an already strange disease even more complicated, EBA has an unusually long incubation period of three-and-a-half to five months between exposure from an infected tick and subsequent abortion. Pregnant cows are most susceptible in the late first and early second trimesters of pregnancy, but a cow infected one to two months before breeding can also transfer the bacteria to its fetus. Because ticks are most active in warm summer months, losses in spring calvers are typically greater than those in fall calvers.
The outward appearance of the fetus may range from relatively normal to a more classic “foothill abortion” presentation, which includes small hemorrhages under the eyelids, on the tongue and gums, enlarged lymph nodes, a distended belly and umbilicus due to fluid in the abdomen and being smaller in size than expected. EBA infection extremes range from losses at five-and-a-half to six months gestation of a hairless, small fetus all the way to fully haired, near-term calf or even live birth. Those born alive, however, often die shortly thereafter. Early-term fetal infections have recently been recognized to occur at or soon after conception, resulting in the loss of the fetus about 90 to 100 days old. Such events are unlikely to be observed by producers, and animals suffering these early losses typically breed back if bulls are present, resulting in a younger subset of calves at weaning.
Is it EBA?
The first thing to ask when trying to diagnose EBA in your herd is whether your cattle are grazing in an area consistent with the tick vector. If your herd is east of the Rocky Mountains or lives in an area with high rainfall, there’s little chance the cause is EBA. If your cattle are pastured in an area that could harbor the tick, the best sample to collect is an aborted fetus.
Submitting the entire aborted carcass to a diagnostic laboratory is ideal, such that all possible causes of death can be investigated. Alternatively, tissue samples can be sent to California Animal Health and Food Service Laboratories (CAHFS) or the Oregon Veterinary Diagnostic Laboratory for detection of the bacteria by polymerase chain reaction (PCR). Fetal tissues most likely to contain the bacteria are the thymus, lymph nodes and spleen. Fetal serum/fluid is also an important diagnostic tool and should be sent to CAHFS for P. abortibovis antibody testing.
Adult cows within the herd can also be tested for the presence of antibody for P. abortibovis, indicating exposure to the bacteria. Once exposed, cows develop antibodies one to two months later and are then immune. As with fetal fluid, cow serum from a blood sample can be sent to CAHFS.
What can you do if your herd suffers from the disease?
Historically, pasture management was the only way for ranchers to mitigate EBA losses. This included managing breeding schedules and strategic movement of animals to minimize exposure to infested pastures during pregnancy. Producers should look back three to five months from the time of abortion to identify where cows were grazing when infected. Attempts to manage around the disease have variable results, and pasture management may not be practical in all cases.
The good news is that producers now have one more option. The most effective form of prevention is through use of a recently developed epizootic bovine abortion agent (EBAA) vaccine, now commercially available. This live vaccine takes advantage of the fact that only the developing fetus shows signs of clinical disease; adult animals never appear sick following infection. Because the vaccine is live, cows must be vaccinated at least 60 days before breeding to allow adequate time for the cow to clear the bacteria from her system and not infect the developing embryo. Pregnant cows should not be vaccinated. The vaccine must be administered by a veterinarian. Consult your local practitioner as to whether use of the vaccine is right for you.
More information regarding foothill abortion diagnosis, management and vaccination can be found on the University of California – Davis Veterinary Medicine “Health Topics” website.
