Foothill Abortion

The Organism

Foothill abortion, or epizootic bovine abortion, is caused by Pajaroellobacter abortibovis, a bacterium identified after decades of research at UC Davis [1, 2]. It cannot be grown in laboratory culture. Researchers propagate it only in immunodeficient mice, or mice bred without a working immune system, which is also how they make the vaccine [3].

The disease affects the fetus, not the cow. After a tick bite, the bacterium reaches the fetus through the dam’s blood, and the fetus’s immature immune system overreacts, causing widespread inflammation that eventually leads to fetal death [1, 4].

Where It Lives

The organism lives in the pajaroello tick, Ornithodoros coriaceus. It is a soft tick and behaves nothing like the hard ticks most producers know. It does not embed: an adult feeds for 10 to 20 minutes, drops back into the soil, and feeds again only every 60 to 90 days. It can survive years without a meal. It is rarely seen on cattle, and bites are almost never observed [4, 5].

The tick lives in dry soil and leaf litter at deer and cattle bedding sites, under oak, juniper, pinyon and ponderosa pine, mountain mahogany, manzanita, sage and bitterbrush. Mule deer are its main native host. Wet ground — irrigated pasture, wet meadow and high-rainfall country — does not support it [5, 6]. Recorded elevations range from a few hundred feet to about 8,000-9,000 feet [4].

Peak tick activity runs from June to September in the intermountain region, and warm years extend the active season by 30 to 60 days [4]. Fewer than 10% of ticks are estimated to carry the organism, and fewer than 1% can transmit it [1].

How It Spreads

Foothill abortion spreads only by tick bite. It does not pass from cow to cow, through the bull, or through aborted material [1, 4].

Naive pregnant heifers and cows are at the highest risk. The risk window runs from about 45 days before breeding through five to six months of gestation, with fetal infection typically between 60 and 145 days. Before about 60 days, the fetus cannot mount an immune response; after six to seven months, it typically mounts a competent one and survives. A cow first bitten at six to seven months of gestation should produce a healthy calf [1, 5].

Animals with no prior exposure are at the highest risk: replacement heifers and cattle brought into tick country from elsewhere. Cattle raised on infested range are repeatedly bitten and remain immune [1].

Infection Pattern

The cow shows no signs at any point. About four months after the bite, she aborts, usually in the last trimester, or delivers a weak calf that dies soon after birth [1]. Because the loss comes months after the exposure, and the fetus is often never found, the cause is frequently missed. The ranch pattern is a bad calving season concentrated in first-calf heifers or newly purchased cattle [4, 5].

A cow that has aborted develops immunity that protects later pregnancies. UC Davis states that immunity may last up to three years but is unlikely to be lifelong. Cattle kept on infested range stay protected because repeated bites keep boosting immunity; cattle moved off tick ground for several years can lose it and abort again when they return [1, 5].

What to Do If You Suspect Foothill Abortion

Call your veterinarian and keep the aborted fetus. Diagnosis requires the fetus; you can’t reliably diagnose it from the cow’s blood. Chill the aborted fetus, do not freeze it, and get it to the laboratory as soon as possible, because fresh tissue gives the best PCR result [4, 7].

Write down which animals aborted or calved weak, their ages and whether they were purchased, and where each group grazed during the second to fifth month of pregnancy. Brushy, dry bedding ground under oak, juniper, or pine during that window points toward the tick [4, 5].

Cows are not contagious and do not need isolation [1]. Because foothill abortion has not been confirmed in Arizona, New Mexico, or Texas, a confirmed diagnosis in any of the three states should be passed to the State Veterinarian. Reporting emerging geographical diseases supports the health of the entire cattle industry.

Diagnostic Differential

Other infectious causes of late-term abortion in cattle include brucellosis, leptospirosis, neosporosis, IBR, BVD, campylobacteriosis, listeriosis, salmonellosis, Q fever, and mycotic, or fungal, abortion. Most diagnoses can only be determined by laboratory submission of the aborted fetus. In California diagnostic casework from 2007 to 2012, a cause was found in 57% of abortions, and foothill abortion was the leading identified infectious cause, at 16.2% of 709 fetuses [8].

In northern Arizona and New Mexico, ponderosa pine needles are a common non-infectious cause. Cows that eat the needles in late pregnancy, usually in cold or snowy weather or when short of feed, can abort 48 hours to two weeks later, and nearly all retain the placenta [9]. Pine needle abortion follows needle exposure within days; foothill abortion follows tick exposure by about four months.

Diagnosis

Diagnosis rests on the fetus. On necropsy, the most consistent finding is enlarged lymph nodes, especially the prescapular nodes in front of the shoulder. Other findings include petechiae (pinpoint hemorrhages) on the gums and in the thymus; clear, straw-colored fluid in the abdomen that gives a pot-bellied appearance; and an enlarged, mottled liver and spleen [4].

Under the microscope, the characteristic combination includes meningoencephalitis (inflammation of the brain and its lining), vasculitis (inflamed blood vessels), and inflammation of the lymph nodes and thymus. The thymus is the most useful single organ to submit [4].

PCR on fetal tissue is a routine commercial test. Oregon State University’s laboratory accepts fetal lung, kidney, liver, thymus, spleen, or placenta, and reports in two to four days; fresh tissue gives the most accurate result [7]. Antibody testing of fetal blood or body fluid can support the diagnosis alongside PCR.

The organism cannot be cultured, so isolation-based confirmation is not possible. Antibody in the cow’s blood shows she has been exposed, not that foothill abortion caused this loss. However, a cow with no antibody helps rule out EBA if she has been grazing in an endemic area [4].

Risk to Humans

No human infection with P. abortibovis has been reported [1].

The tick does bite people, usually campers and hunters who rest at deer or cattle bedding sites. The bites are notoriously painful and cause a local inflammatory reaction, attributed to a substance the tick injects while feeding [10].

The vaccine is live and is not weakened. Veterinarians are the only ones who can administer it, in part because its risk to a human fetus is unknown [11].

Vaccination and Control

There is no treatment for an infected fetus. Prevention relies on vaccination or on managing when pregnant cattle encounter the tick.

A live vaccine developed at UC Davis and the University of Nevada, Reno, and made by Hygieia Biological Laboratories, received a conditional USDA license in September 2020. Veterinarians supply it in 30-dose vials to open females at least six months old, at least 60 days before breeding [12]. It is shipped and stored in liquid nitrogen [11]. In controlled challenge trials, it prevented every diagnosed case of foothill abortion; counting fetal losses of undetermined cause, protection ranged from 64% to 95% [3]. Field use in more than 60,000 head has been reported as more than 95% effective [11].

The 60-day rule matters. Because the vaccine is live and not weakened, it can itself cause fetal loss: in trials, loss occurred when vaccination preceded breeding by 21 days but not by 56 days. Never vaccinate a pregnant animal [3].

Without the vaccine, management focuses on timing [5]:

• Keep pregnant cattle on tick-free ground — irrigated pasture, wet meadow — for the first six months of pregnancy, then move them to tick pasture.
• Expose heifers to tick pasture before breeding so they are immune when bred. This does not guarantee that every heifer is bitten.
• Graze stocker steers through tick pasture first. Because ticks feed only every 60 to 90 days, that leaves about a two-month window of lower exposure for the pregnant cattle that follow.
• Shift breeding dates. One Lassen County, California ranch cut its abortion rate by 50% to near zero by shifting calving dates and rotating pastures.

What Foothill Abortion Costs Producers

Foothill abortion carries no quarantine, movement restriction, or trade consequence. The cost is the calf crop.

A long-standing estimate suggests that 5% to 10% of California’s beef calf crop, or 45,000 to 90,000 calves, may be lost each year. That figure dates from 1991 [6]. A more recent conservative estimate puts losses at more than $10 million a year across California, Oregon and Nevada [12]. Losses concentrate in replacement heifers and cattle new to tick country, which are also the animals a producer has invested in most [6].

Where to Report a Case in the Southwest

Foothill abortion is not a federal program disease and is not listed by name on any of the three states’ reportable disease lists. It is not a human disease, so it is not subject to public health reporting.

Documented Incidence in the Southwest

Foothill abortion is documented in California, northern Nevada and southern Oregon, and the tick and disease have long been recognized in Mexico [1, 16]. The UC Davis researchers who identified the organism state that they do not know whether it is spreading. They attribute its recognition in new places largely to management changes — breeding heifers at one year old, bringing in cattle from outside tick country — and to better diagnostics and tick collection [16].

Arizona

A 2024 article by the UC Davis research group states that “northern Arizona has been added to the list of states suffering from foothill abortion,” but gives no county, year, case count, or diagnostic basis [16]. UC Davis’s own disease page, updated July 2026, still lists only California, northern Nevada, and southern Oregon [1]. Northern Arizona’s ponderosa pine and pinyon–juniper country, with its mule deer, matches the tick’s described habitat and elevation range [4, 5]. We found no published Arizona case report or tick collection.

New Mexico

No documented case or tick record was located.

Texas

No documented case or tick record was located.

References

1. Foothill abortion (epizootic bovine abortion). UC Davis School of Veterinary Medicine, Health Topics, updated 15 July 2026. https://healthtopics.vetmed.ucdavis.edu/health-topics/livestock/epizootic-bovine-abortion↑ a b c d e f g h i j k l
2. Welly BT, Miller MR, Stott JL, Blanchard MT, et al. Genome report: identification and validation of antigenic proteins from Pajaroellobacter abortibovis. G3: Genes, Genomes, Genetics 2017;7(2):321–331. https://escholarship.org/uc/item/43r0n4hv↑
3. Blanchard MT, Teglas MB, Anderson ML, Moore PF, McNabb BR, Collins KM, Yeargan BV, Stott JL. Protection of cattle against epizootic bovine abortion using a live Pajaroellobacter abortibovis vaccine. Vaccines 2022;10(2):335. https://doi.org/10.3390/vaccines10020335↑ a b c
4. Scott MA. Epizootic bovine abortion (foothill abortion): the disease, diagnosis and control strategies. Clinical Theriogenology 2014;6(3). https://cdn.ymaws.com/www.therio.org/resource/collection/DB0508BA-DE64-46F0-BE6B-2CE2A338D52C/2014_v3_017.pdf↑ a b c d e f g h i j k l
5. Oliver M, Nader GA, Maas J, Blanchard M, Stott J, Teglas M, Becchetti T, Bushnell R. Cattle management strategies to minimize foothill abortion. University of California ANR Publication 8566, October 2016. https://naes.agnt.unr.edu/PMS/Pubs/430_2016_12.pdf↑ a b c d e f g h
6. Nader GA, Oliver MN, Finzel JA, Blanchard MT, Stott JL. Solving the puzzle of foothill abortion in beef cattle. California Agriculture 2014;68(4):114–115. https://calag.ucanr.edu/Archive/?article=ca.v068n04p114↑ a b c
7. Epizootic bovine abortion PCR (test 8173). Oregon State University Veterinary Diagnostic Laboratory. https://vetmed.oregonstate.edu/vdl-test/epizootic-bovine-abortion-pcr-eba-foothill↑ a b
8. Clothier K, Anderson M. Evaluation of bovine abortion cases and tissue suitability for identification of infectious agents in California diagnostic laboratory cases from 2007 to 2012. Theriogenology 2016;85:933–938. https://ccfruitandnuts.ucanr.edu/sites/default/files/2023-05/384244.pdf↑
9. Ponderosa pine (Pinus ponderosa). USDA Agricultural Research Service, Poisonous Plant Research Laboratory. https://www.ars.usda.gov/pacific-west-area/logan-ut/poisonous-plant-research/docs/ponderosa-pine-pinus-ponderosa↑
10. California ticks. UC Davis, Human Skin Parasites, 1 March 2021. https://skinparasites.ucdavis.edu/california-ticks↑
11. Epizootic bovine abortion (foothill abortion) producer brochure. UC Davis School of Veterinary Medicine, 2020. https://healthtopics.vetmed.ucdavis.edu/sites/g/files/dgvnsk6721/files/inline-files/20201027%20tri-folder%20brochure%20EBA%20FINAL_2.pdf↑ a b c
12. Vaccine now available for cattle producers to fight foothill abortion. UC Davis School of Veterinary Medicine, 3 September 2020. https://www.vetmed.ucdavis.edu/news/vaccine-now-available-cattle-producers-fight-foothill-abortion↑ a b
13. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
14. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board, effective 16 July 2024. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑
15. Reportable diseases. Texas Animal Health Commission. https://www.tahc.texas.gov/regs/reportable.html↑
16. Chandler J, Blanchard MT, Stott JL. Foothill abortions: all from a little tick. Progressive Cattle, 4 June 2024. https://www.agproud.com/articles/59550-foothill-abortions-all-from-a-little-tick↑ a b c