Listeriosis and the Dangers That May Lurk Among the Flock

Listeria monocytogenes is a Gram-positive non-spore-forming bacteria that can be present in poultry products and live birds, as well as many common foods like deli meats, hot dogs, soft cheeses, and leafy greens. It is zoonotic, which means the pathogen is transferable between animals and humans, and may be of concern for individuals who raise or regularly interact with live poultry or their products. It is important to note that L. monocytogenes is somewhat rare in poultry and, while chickens can transmit this disease to humans, little is known about its prevalence in backyard flocks.  

L. monocytogenes lives in the intestinal tract of many healthy animals and is shed in higher amounts due to illness or stress. Stressful events, such as season or feed changes, can trigger higher shedding of bacteria. L.  monocytogenes has the potential to cause severe disease in humans, especially those that are most at risk, including pregnant women, individuals over the age of 65, and those with weakened immune systems. The purpose of this article is to highlight the risks associated with L. monocytogenes, so those who interact with poultry can reduce their risk through sanitation, proper biosecurity, and management practices. 

Listeria monocytogenes in Poultry 

Chickens can be affected by L. monocytogenes in two forms. The encephalitic form of the disease leads to neurologic signs, such as head tilt, loss of appetite, and circling. The septicemic form manifests as diarrhea, anorexia, and depression. Heart lesions are often seen in chickens with the septicemic form. Both forms of listeriosis may also include nonspecific clinical signs such as weakness and decreased egg production.

L. monocytogenes is ubiquitous, as it naturally exists in the environment in soil, stagnant water, and fecal material from other animals. This bacterium is transmitted via inhalation, ingestion, or contamination through a wound. Flocks that are raised outdoors are more likely to be exposed than those raised indoors, particularly if they have access to stagnant water such as puddles. Ingestion of this bacterium can also occur if birds are being fed commonly contaminated food items such as packaged mixed greens, raw sprouts, deli meats, smoked fish, raw milk products, celery, and melons. This is one reason why it is typically not recommended to feed certain treats or human food scraps to backyard flocks. Additionally, younger birds are more susceptible to developing severe infection from this bacterium, so reducing risks in this age group is critical to maintaining a safe and healthy flock.

If you suspect illness in your flock, please contact your veterinarian or the Animal Diagnostic Lab. For more information, see Submitting Poultry Cases to the Animal Diagnostic Lab, or call 814-863-0837. 

Listeria monocytogenes in Humans 

Listeriosis is zoonotic, and backyard poultry owners are at a higher risk of exposure to this pathogen. For many, this bacterium only causes mild symptoms that resolve within 1–3 days. Like poultry, there are two forms of disease in people: the less severe gastrointestinal form and the more severe systemic form. The less severe form includes symptoms of muscle aches, fever, vomiting, and diarrhea, which may last for 1–3 days. The more severe systemic form occurs when the bacteria spreads beyond the gastrointestinal tract via the bloodstream to other organ systems, including the nervous system. Symptoms of the systemic form include headache, stiff neck, confusion, incoordination, and convulsions. This form is potentially life-threatening for those who are immunocompromised or over the age of 65.

This disease is especially dangerous to pregnant women and can lead to complications with their pregnancy. The bacteria can cross into the placenta and infect the fetus, increasing the risk for miscarriage and stillbirth or lifelong health complications for the baby. Pregnant women with backyard flocks should exercise as much caution as possible around their birds. For example, the CDC recommends not putting poultry near your face to kiss or snuggle, and to wash your hands before and after handling poultry.

If you have chickens and are pregnant, are above the age of 65, or are immunocompromised, it is important to understand the risks associated with the bacteria and take appropriate precautions. Contact a human health professional regarding any specific health concerns. 

Prevention

Backyard flock owners should be aware of the dangers of Listeria monocytogenes and other pathogens, and exercise caution when interacting with poultry and poultry products. The bacteria can be inhaled or ingested, so proper PPE (personal protective equipment) such as an N95 mask and/or gloves should be worn while cleaning anything contaminated with poultry fecal matter. Keep a pair of boots solely for working with poultry, as this can keep these bacteria and other pathogens out of your home. Consider wearing protective coveralls or changing and washing your clothes immediately after interacting with poultry or cleaning their coop. The bacteria can exist on the surface of eggshells and can survive refrigeration temperatures. Always wash your hands before and after handling eggs, and cook eggs to an appropriate internal temperature of 160 degrees Fahrenheit before consumption. Overall, listeriosis in poultry is uncommon, but the risks are slightly elevated in backyard poultry with outdoor access. It is important for backyard flock owners to understand the risks and implement appropriate practices to prevent infection and illness in themselves and their flock. 

For information on the food processing side, please see Control of Listeria monocytogenes in Meat and Poultry.

Sophia Kutsaya (Penn State undergraduate student) is the primary author of this article, with support from Drs. Emily Frye, Kayla Niel, and Megan Lighty. 

References  

Backyard poultry. (2024, April 15). Healthy Pets, Healthy People.

Listeria (Listeriosis). (2025, January 16). U.S. Food and Drug Administration.

El-Gazzar, M. (2024, September 18). Listeriosis in poultry. Merck Veterinary Manual.

Rothrock, M. J., Davis, M. L., Locatelli, A., Bodie, A., McIntosh, T. G., Donaldson, J. R., & Ricke, S. C. (2017). Listeria occurrence in poultry flocks: detection and potential implications. Frontiers in Veterinary Science, 4.