Listeriosis

The Organism

Listeriosis is caused by Listeria monocytogenes, a small, rod-shaped bacterium found in soil and decaying plant matter. It is facultatively intracellular, meaning it can survive and multiply inside the host’s own cells [1].

Several biological properties help explain why this organism is important in agriculture and food production.

It can grow at temperatures ranging from approximately −1.5 °C to 45 °C, meaning it can multiply even under refrigeration. It tolerates relatively high salt concentrations, approximately 12% to 14%. Although acidic conditions inhibit its growth, the exact limits depend on factors such as moisture, the strain of bacteria, and other environmental stresses. In one study involving 138 strains, none grew at pH 4.4 or below under the tested low-moisture conditions [2, 3].

Refrigeration slows the growth of many foodborne bacteria, but it does not reliably stop Listeria. Contaminated refrigerated foods can become increasingly hazardous during extended storage.

Fortunately, Listeria is not particularly heat resistant. Proper pasteurization and cooking kill the organism [3].

Where Listeria Lives

Listeria monocytogenes is widespread in soil, plant debris, water, and animal feces. Many animals exposed to the organism carry it in their intestinal tract without becoming ill [1].

Poorly fermented silage has historically been an important source of listeriosis in ruminants. Properly fermented silage generally becomes acidic enough to inhibit bacterial growth. Poorly fermented, spoiled, or improperly stored silage may have a higher pH and provide conditions that allow the organism to survive or multiply. Outbreaks of livestock listeriosis have frequently been associated with feeding poor-quality silage [1, 2].

A 2025 systematic review and meta-analysis of ruminant listeriosis concluded that silage should be considered an important risk factor, but not the only source of infection [1, 4].

Other documented sources include moist, manure-contaminated feed; rotting round bales; and decaying vegetation in feeders [1].

Listeriosis is more common in temperate and colder regions. In cooler climates, cases tend to peak in winter and spring. In warmer climates, infection may occur where animals congregate near contaminated, wet environments [1].

The organism’s environmental persistence is important to livestock producers because contamination can occur through multiple pathways. Good feed storage and management reduce risk but cannot guarantee that animals will never encounter the bacterium.

How Listeria Spreads

Ruminants generally become infected through their mouths by consuming contaminated feed, vegetation, or soil [1].

In the encephalitic, or brain-infection, form of the disease, the organism is thought to enter through damaged oral tissues and travel along cranial nerves toward the brainstem. The trigeminal nerve, which carries sensation from the face and helps control chewing, is particularly important in this proposed pathway [1].

After an abortion, infected animals may shed the organism in vaginal discharge and milk for a month or longer [1]. Healthy intestinal carriers can also shed the organism in their feces, contributing to environmental contamination. Neurologic cases may recur on the same premises in successive years [1].

People are infected almost entirely through contaminated food. Direct infection from animals is uncommon and is most often associated with handling infected reproductive tissues, particularly aborted fetuses and placentas [1, 5].

For people working with livestock, this distinction matters: consuming contaminated animal products is the primary public-health concern, but direct exposure to infected reproductive materials is another preventable route of infection.

Infection Pattern

Listeriosis in ruminants occurs in several forms.

Encephalitis (brain infection)

Encephalitis is the most common form of listeriosis in ruminants. In a 2025 meta-analysis of 2,837 affected ruminants, 64.8% had the encephalitic form and 21.3% had the abortive form [4].

The organism produces small pockets of infection called microabscesses in the brainstem, often with inflammation concentrated on one side. This helps explain why the clinical signs are frequently more pronounced on one side of the animal’s body [1].

The brainstem controls important functions including facial movement, chewing, swallowing, and balance. Because Listeria commonly causes localized inflammation in this region, affected animals develop a distinctive combination of neurologic and facial abnormalities.

The signature clinical feature is dysfunction of the trigeminal nerve. Animals may have difficulty chewing, food may accumulate inside one cheek, and saliva may continuously drip from the mouth.

Facial nerve and vestibular nerve dysfunction can occur on the same side. Signs include a drooping ear, a deviated muzzle, a slack lip, and a lowered eyelid. Animals may become depressed or disoriented, circle toward the affected side, lean against objects, or press their heads into corners. As the disease progresses, they may become recumbent, often lying on the affected side [1].

Loss of the normal blinking reflex can damage the surface of the eye.

In sheep and goats, the disease can progress rapidly, and severely affected animals may die within 24 to 48 hours. Cattle generally have a less acute course. The prognosis becomes worse once an animal is recumbent [1].

In the published cases included in the 2025 meta-analysis, the estimated case fatality was 50.6% [4]. This is a pooled estimate from published reports, not a prediction of the outcome for every affected herd or individual animal.

Abortion

Listeriosis can cause abortion, usually during the last trimester of pregnancy. The pregnant animal may show no obvious signs of illness before aborting.

The average abortion rate across the published reports included in the 2025 review was approximately 13% [1, 4]. This figure should be interpreted in the context of the populations and studies included in the review.

Encephalitis and abortion generally occur as different disease patterns rather than simultaneously in the same herd or flock [1].

Septicemia (bloodstream infection)

Septicemia occurs primarily in young ruminants before the rumen is fully functional. In this form, the organism spreads through the bloodstream and causes a more generalized infection [1].

What to Do If You Suspect Listeriosis

Contact your veterinarian promptly.

Rabies can cause similar neurologic signs and is a serious zoonotic disease. Avoid unprotected contact with an affected animal’s saliva or nervous tissue. Do not handle the animal’s mouth or perform a necropsy without veterinary guidance and appropriate precautions [1].

If listeriosis is suspected:

• Stop feeding silage or other feed associated with the onset of cases.
• Remove spoiled or decomposing feed from feeding areas.
• Separate affected animals when practical.

• Wear gloves when handling aborted fetuses, placentas, and reproductive discharges.
• Pregnant women should avoid contact with abortion materials and animals suspected of having listeriosis.
• Do not consume raw milk from sick animals or animals suspected of having listeriosis.

Supportive care is important. Affected animals may become dehydrated because they cannot chew or swallow normally. A veterinarian should assess swallowing ability and determine the safest way to provide fluids and nutrition.

Do not force-feed or drench an animal with suspected swallowing dysfunction. Fluids, feed, or medications administered orally can enter the lungs if the animal cannot swallow properly.

Diagnostic Differential

Rabies is an especially important differential diagnosis because it must be considered before unprotected handling of an animal with neurologic signs.

Other causes of neurologic disease in ruminants include:

• Otitis media and interna (middle and inner ear infection).
• Brain abscess.
• Parasite migration through the nervous system.
• Thromboembolic meningoencephalitis.
• Polioencephalomalacia.
• Pregnancy toxemia or ketosis, depending on the species and clinical presentation [1].

Vestibular disease is relatively common in growing ruminants and can cause head tilt, loss of balance, and circling. However, animals with uncomplicated vestibular disease generally remain alert and retain the ability to chew normally.

The combination of facial weakness, difficulty chewing, and drooling is particularly suggestive of trigeminal and associated cranial nerve dysfunction, making listeriosis an important consideration [1].

For late-term abortion, the differential diagnosis includes brucellosis, leptospirosis, neosporosis, bovine viral diarrhea (BVD), infectious bovine rhinotracheitis (IBR), campylobacteriosis, trichomoniasis, Q fever, and fungal abortion.

Diagnosis

In a live animal, the diagnosis is initially based on the clinical signs and veterinary examination.

Cerebrospinal fluid (CSF), the fluid surrounding the brain and spinal cord, typically shows increased protein and a mild increase in white blood cells in affected animals [1]. These findings support a diagnosis of inflammation in the central nervous system but are not specific to listeriosis.

Serology is generally not useful for diagnosing active disease because many healthy animals have antibodies from previous exposure.

Similarly, finding Listeria in feces or feed demonstrates that the organism is present but does not prove that it is causing the animal’s clinical signs [1].

After death, laboratory examination of the brain is important. Brainstem tissue should be submitted for testing even when the brain appears grossly normal, because visible lesions may be minimal.

Microscopic examination commonly reveals microabscesses and inflammation around blood vessels in the brainstem. Bacterial culture, PCR, or immunofluorescence can help confirm the diagnosis [1].

When abortion is suspected, submit the fetus and placenta for laboratory examination. The placenta may have dead, crumbling cotyledons, while the fetus may have small areas of tissue death in the liver and lungs [1].

Risk to Humans

Listeriosis is an important foodborne disease in the United States. The country experiences approximately 1,250 cases and 172 deaths annually, making it one of the leading causes of death from foodborne illness. Nearly everyone diagnosed with invasive listeriosis is hospitalized, and approximately 20% die [5].

Healthy young adults rarely develop invasive disease. Risk is concentrated among older adults, pregnant people, and people with weakened immune systems. Adults 65 and older and people with certain underlying medical conditions account for a disproportionate share of invasive infections and deaths [5, 6].

Invasive listeriosis can be particularly dangerous during pregnancy. A pregnant woman may experience only mild, flu-like symptoms, yet the infection can cause miscarriage, stillbirth, premature delivery, or serious illness in the newborn. Approximately one in four recognized pregnancies affected by listeriosis results in pregnancy loss or death of the infant shortly after birth [5, 6].

Nearly all human cases are foodborne. Common sources include refrigerated, ready-to-eat foods with long shelf lives, soft cheeses, and raw milk.

Fresh, high-moisture cheeses, particularly queso fresco and requesón, have been associated with multistate outbreaks [7, 9]. Contamination can occur during production and processing, including through contaminated milk or processing environments.

A 2025–2026 outbreak linked to prepared pasta meals caused seven deaths, including one each in Arizona and Texas [8].

In a 2026 outbreak linked to requesón, 92% of patients with ethnicity recorded were Hispanic [9]. This finding describes the demographic distribution of reported patients in that outbreak. It should not be interpreted as evidence of an inherent biological susceptibility associated with ethnicity.

On farms and ranches, the main human-health concerns are contaminated raw milk and dairy products, and direct exposure to infected abortion materials. People handling aborted fetuses and placentas should wear gloves and practice careful hygiene [1].

Healthy-appearing animals can carry and shed Listeria, so avoiding milk only from visibly sick animals is not sufficient protection against foodborne infection. CDC recommends consuming pasteurized milk and dairy products [7].

Refrigeration is not a kill step. Contaminated refrigerated foods can continue to support bacterial growth during storage, which is why proper food processing, sanitation, storage, and adherence to food recalls are important.

Treatment and Control

No antibiotic is specifically labeled in the United States for treating listeriosis in ruminants. Antibiotic treatment is therefore extralabel and must be directed by a veterinarian.

Penicillin has historically been the drug of choice. Other antibiotics, including oxytetracycline and florfenicol, have been used in some circumstances. However, published evidence regarding antibiotic effectiveness in ruminants is limited [1, 4].

Treatment is challenging because the organism can enter the central nervous system. Early treatment, before severe neurologic damage develops, offers the best chance of success.

Veterinarians may also use anti-inflammatory medications such as flunixin meglumine or dexamethasone. These drugs should not be combined in ruminants without specific veterinary direction [1].

Drug selection, dosage, duration, and meat and milk withdrawal times must be determined by the veterinarian.

Supportive care is often critical to the outcome. Depending on the animal’s condition, this may include:

• Fluids with attention to electrolyte and acid-base balance.
• Nutritional support with small amounts of appropriate feed.
• Appropriate access to feed and water for animals that can safely swallow.

• Thiamine supplementation when indicated.
• Eye protection when normal blinking is lost.
• Nursing care and management of recumbent animals.

Prevention depends primarily on feed management and environmental hygiene.

Avoid spoiled or wet feed, remove decomposing vegetation from feeding areas, prevent manure contamination of feed and water, and investigate feed sources when multiple animals develop compatible clinical signs. If silage is associated with cases, discontinue feeding it pending veterinary evaluation [1].

No vaccine is approved for preventing listeriosis in animals in the United States [1].

What Listeriosis Costs Producers

Unlike major federally regulated livestock diseases, listeriosis does not ordinarily trigger a federal quarantine or eradication program. However, the principal economic consequences can still be substantial.

The direct costs to producers include:

• Deaths of affected animals.
• Veterinary examinations and treatment.
• Labor-intensive nursing and supportive care.
• Abortions and associated reproductive losses.
• Removal and disposal of spoiled feed.
• Additional sanitation and feed-management expenses.

Across the published reports included in the 2025 meta-analysis, approximately 12.6% of animals in affected populations became ill, and the estimated case fatality among affected animals was approximately 50.6% [4]. The reported average abortion rate was approximately 13% [1, 4].

These pooled figures describe the studies included in the review. They should not be interpreted as predictions for a particular farm, herd, or outbreak.

For producers who sell raw milk or manufacture farmstead cheese, the potential economic exposure extends beyond livestock losses. Contamination of a dairy product can lead to product recalls, lost sales, regulatory investigations, and serious public-health consequences.

Listeriosis is therefore an animal-health issue and a food-safety issue, particularly for farms producing foods intended for direct human consumption.

Where to Report a Case in the Southwest

Animal listeriosis is not included on the routine mandatory animal disease reporting lists in Arizona, New Mexico, or Texas and is not a federal program disease [10, 11, 12].

Human listeriosis is reportable in all three states and is nationally notifiable.

Documented Incidence in the Southwest

Animal listeriosis is not routinely reportable in Arizona, New Mexico, or Texas. Consequently, comparable statewide animal case counts are not available through mandatory disease reporting. This does not mean the disease is absent or uncommon in livestock. Cases may be diagnosed by private veterinarians or diagnostic laboratories without appearing in a centralized state surveillance database.

Regional surveillance information is therefore largely based on human foodborne disease reporting.

Listeriosis is more common in temperate and colder regions. In cooler climates, cases tend to peak during winter and spring [1].

Based on the organism’s environmental preferences and the seasonal patterns reported in cooler regions, northern Arizona and New Mexico may have conditions more favorable to some recognized risk factors, particularly winter feeding of stored forage. However, available animal surveillance data are insufficient to establish regional differences in livestock incidence.

Arizona

Eight human cases were reported in 2022 [16]. One of the seven deaths in the 2025–2026 prepared pasta outbreak occurred in Arizona [8]. Comparable statewide animal case counts are not available through mandatory reporting.

New Mexico

Two human cases were reported in 2022, one confirmed and one probable [16]. Comparable statewide animal case counts are not available through mandatory reporting.

Texas

Forty-seven human cases were reported in 2022 [16]. One of the seven deaths in the prepared pasta outbreak occurred in Texas [8]. Comparable statewide animal case counts are not available through mandatory reporting.

These numbers represent documented human disease, not the total burden of Listeria exposure in the region. The absence of mandatory animal reporting makes it difficult to compare livestock disease occurrence across the three states.

The Bigger Picture

Listeriosis illustrates how animal health, agricultural practices, food production, and human health are connected.

On farms, the disease is associated with environmental contamination, feed quality, and the handling of infected animals and reproductive tissues. In people, the greatest risk comes from contaminated foods, particularly products that are consumed without further cooking.

For livestock producers, good feed management, prompt veterinary attention, and careful handling of abortion materials help reduce risk. For consumers, pasteurized dairy products, proper food handling, and attention to food recalls are important safeguards.

The absence of routinely reported animal cases does not mean the organism is absent from livestock or the farm environment. Listeria is widespread, and controlling it requires attention to the entire food-production system—from the farm to the consumer.

References

1. Depenbrock S. Listeriosis in animals. Merck Veterinary Manual, full review May 2026, updated August 2026. https://www.merckvetmanual.com/generalized-conditions/listeriosis/listeriosis-in-animals↑ a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~ 
2. van der Veen S, Moezelaar R, Abee T, Wells-Bennik MHJ. The growth limits of many Listeria monocytogenes strains under combinations of stresses show serotype- and niche-specific traits. Journal of Applied Microbiology. 2008;105(5):1246–1258. https://research.wur.nl/en/publications/the-growth-limits-of-a-large-number-of-listeria-monocytogenes-str↑ a b
3. Listeria monocytogenes (Agents of Foodborne Illness). Food Standards Australia New Zealand. 2013, updated 2018. https://mta-sts.foodstandards.gov.au/sites/default/files/publications/Documents/listeria-monocytogenes.pdf↑ a b
4. López-Almela I, Sheth CC, Gomis J, Gómez-Martín Á, Lecuit M, Quereda JJ. Epidemiology, clinical and pathological features and outcomes of listeriosis in ruminants: a systematic review and meta-analysis. Veterinary Quarterly. 2025;45(1). https://doi.org/10.1080/01652176.2025.2598257↑ a b c d e f g
5. Clinical overview of Listeria infection. US Centers for Disease Control and Prevention. Reviewed 24 September 2025. https://www.cdc.gov/listeria/hcp/clinical-overview/index.html↑ a b c d
6. Listeria risk factors. US Centers for Disease Control and Prevention. Reviewed 24 September 2025. https://www.cdc.gov/listeria/risk-factors/index.html↑ a b
7. How Listeria spreads: soft cheeses and raw milk. US Centers for Disease Control and Prevention. Reviewed 16 January 2025. https://www.cdc.gov/listeria/causes/dairy.html↑ a b
8. Listeria outbreak linked to prepared pasta meals. US Centers for Disease Control and Prevention. Updated 12 February 2026. https://www.cdc.gov/listeria/outbreaks/chicken-fettuccine-alfredo-06-25/investigation.html↑ a b c
9. Listeria outbreak linked to soft cheese (requesón). US Centers for Disease Control and Prevention. Updated 26 August 2026. https://www.cdc.gov/listeria/outbreaks/soft-cheese-06-26/investigation.html↑ a b
10. 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↑
11. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑
12. 4 TAC § 45.3: Reportable and actionable disease list. Texas Animal Health Commission. https://www.law.cornell.edu/regulations/texas/4-Tex-Admin-Code-SS-45-3↑
13. Reportable diseases list for health care providers, effective 2 June 2025. Arizona Department of Health Services. https://www.azdhs.gov/documents/preparedness/epidemiology-disease-control/communicable-disease-reporting/reportable-diseases-list.pdf↑
14. 7.4.3.13 NMAC: Notifiable conditions and reporting timeframes. New Mexico Department of Health. https://www.srca.nm.gov/parts/title07/07.004.0003.pdf↑
15. Texas notifiable conditions, 2025. Texas Department of State Health Services. https://www.dshs.texas.gov/sites/default/files/IDCU/investigation/Reporting-forms/notifiable-conditions-2025.pdf↑
16. Nationally Notifiable Diseases Surveillance System, annual table 2j, 2022. US Centers for Disease Control and Prevention. https://wonder.cdc.gov/nndss/static/2022/annual/2022-table2j.html↑ a b c