Fowl Cholera (Avian Pasteurellosis)

Etiology, Pathogenesis, Clinical Signs, Diagnosis, Treatment, and Control

A Veterinary Reference Guide for Poultry Producers and Flock Owners

Fowl cholera is one of the oldest and most economically destructive bacterial diseases of poultry. Known also as avian pasteurellosis and avian hemorrhagic septicemia, it affects chickens, turkeys, waterfowl, and virtually all other avian species, as well as a wide range of mammals. The disease was first recognized and distinguished from other poultry diseases by Louis Pasteur in 1880, and the causative bacterium was subsequently named in his honor: Pasteurella multocida. Despite being among the best-characterized bacterial diseases of poultry, fowl cholera continues to cause significant mortality and production losses worldwide, and the emergence of antimicrobial resistance in P. multocida strains presents an increasing management challenge. This article provides a comprehensive, evidence-based review of fowl cholera for poultry producers and flock managers, drawing on peer-reviewed veterinary literature and authoritative clinical references.

Quick Reference Summary

Category	Detail	Notes
Causative Agent	Pasteurella multocida	Gram-negative, non-motile, non-spore-forming encapsulated rod
Subspecies	multocida, septica, gallicida	Subspecies multocida is the most common cause of fowl cholera
Capsular Serotypes	A, B, D, E, F	Serogroup A is most common in poultry; A:1, A:3, A:4 most prevalent
Primary Hosts	Turkeys, waterfowl, chickens	Turkeys and waterfowl more susceptible than chickens
Age Most Affected	Birds over 6 weeks; adults	Young chicks are less susceptible; older birds more so
Transmission	Nasal/oral secretions, fomites, wild birds, rodents	No significant vertical (egg) transmission
Mortality	Up to 100% in acute form	Lower in chronic infections; carrier state common
Key Acute Signs	Sudden death, cyanosis, mucous discharge, diarrhea	First sign may be finding dead birds
Key Chronic Signs	Swollen wattles/joints, torticollis, pneumonia	Species-specific; wattles in chickens, pneumonia in turkeys
Zoonotic Risk	Moderate	Primarily through cat/dog bites; direct avian transmission rare but documented
Diagnosis	Bacterial culture and isolation; bipolar staining	Somatic serotyping important for vaccine selection
Treatment	Antimicrobials (sulfonamides, tetracyclines, fluoroquinolones)	Reduces mortality but does not eliminate carrier state
Vaccination	Bacterins and live attenuated vaccines available	Bacterins are serotype-specific; live vaccines provide broader protection
Reportable?	Not federally reportable in U.S.	Check state regulations; some states require reporting

1. Historical Background

Fowl cholera has been recognized as a distinct disease of poultry for well over a century. It was previously known as “fowl plague” in the 1870s following outbreaks observed in poultry production in Italy and across Europe (PMC, Fowl Cholera in Chickens: Current Trends in Diagnosis and Phenotypic Drug Resistance, 2024). In 1880, the French chemist and microbiologist Louis Pasteur made the landmark observation that distinguished fowl cholera from other diseases affecting poultry at the time, isolating and characterizing the causative bacterium. The genus and species were subsequently named Pasteurella multocida in his honor — Pasteurella for Pasteur, and multocida from the Latin meaning “kill many” (Frontiers in Veterinary Science, 2023).

Beyond identifying the organism, Pasteur’s work on fowl cholera led to one of the most foundational discoveries in the history of vaccinology. While accidentally using an aged culture on chickens, Pasteur observed that the attenuated bacteria conferred protection against subsequent challenge with a virulent strain. This observation became the foundation of the principle of attenuation and the development of live, attenuated vaccines — a concept that transformed not only veterinary medicine but human medicine as well. The same International Microbiology review (2025) notes that vaccines targeting P. multocida have been developed since Pasteur’s 1880 work and many are now commercially available.

The Poultry Site’s disease guide notes that fowl cholera was one of the first infectious diseases ever formally recognized. Today it remains one of the most economically significant bacterial diseases of poultry worldwide.

2. Etiology and Classification

The Organism

Fowl cholera is caused by Pasteurella multocida, a small, gram-negative, non-motile, non-spore-forming rod with a polysaccharide capsule. The Merck Veterinary Manual describes P. multocida as a single species with three subspecies: multocida, septica, and gallicida, noting that subspecies multocida is the most common cause of fowl cholera, though the other subspecies may also cause cholera-like disease. Fresh isolates may show bipolar staining — a distinctive dark-staining at both poles of the rod when stained with Wright’s or Giemsa stain — which is a key feature used in presumptive diagnosis.

The organism is classified into five capsular serogroups (A, B, D, E, and F) based on the chemistry of the polysaccharide capsule, and into 16 somatic serotypes (1–16) based on lipopolysaccharide (LPS) composition. The PubMed review by Blackall (2000) notes that strains causing fowl cholera are most frequently designated A:1, A:3, or A:4, with serogroup A being the dominant serogroup in poultry worldwide. This classification is clinically significant because bacterin vaccines provide protection primarily against the homologous LPS serotype, making serotyping of isolates essential for vaccine selection.

Virulence Factors

The virulence of P. multocida is multifactorial. The PubMed review (Blackall, 2000, Fowl Cholera) identifies the key virulence factors as the polysaccharide capsule, endotoxin (lipopolysaccharide), outer membrane proteins, iron acquisition systems, heat shock proteins, neuraminidase production, and antibody-cleaving enzymes. A study published in PLOS Pathogens (Steen et al., 2010, DOI: 10.1371/journal.ppat.1000750) identified the Fis regulatory protein as essential for capsule production in P. multocida and a regulator of other key virulence factors, confirming that the capsule is central to the organism’s pathogenicity.

The Saudi Journal of Life Sciences review on Pasteurella multocida in veterinary medicine (2024) describes pathogenesis as driven by these virulence factors collectively, which promote colonization, immune evasion, and systemic infection. The polysaccharide capsule protects the organism from host phagocytosis, while LPS endotoxin triggers the systemic inflammatory cascade responsible for the acute septicemic form of the disease. Iron acquisition systems enable the organism to scavenge iron from host tissues, supporting growth during systemic infection.

Environmental Survival

multocida is susceptible to ordinary disinfectants, sunlight, drying, and heat (Merck Veterinary Manual). The organism does not survive long in the open environment under typical conditions. However, The Poultry Site disease guide notes that P. multocida may persist in soil for prolonged periods under favorable conditions (cool, moist, shaded), which has implications for disease recurrence on contaminated premises. A Journal of Wildlife Diseases study (Blanchong et al., 2006) documented persistence of P. multocida in wetlands following avian cholera outbreaks, underscoring the importance of environmental contamination in sustaining disease in wild bird populations.

3. Epidemiology and Transmission

Host Range and Susceptibility

Fowl cholera affects domestic and wild birds worldwide. All poultry species are susceptible, but susceptibility varies. The Merck Veterinary Manual and multiple clinical resources consistently note that turkeys and waterfowl are more susceptible than chickens. Within chickens, older birds are significantly more susceptible than young chicks — the disease typically strikes birds older than six weeks of age, with mature laying hens and breeder-aged birds representing the highest-risk population. In parental flocks, the University of Kentucky Extension resource (Dr. Jacquie Jacob) notes that cocks are far more susceptible than hens.

A peer-reviewed study published in Avian Pathology (Alemu et al., 2008, DOI: 10.1080/03079450701784891) evaluated susceptibility across age groups in chickens and ducks experimentally infected with P. multocida. The study found that 12-week-old chickens expressed significantly more clinical signs than 4-week-old, 8-week-old, and 16-week-old chickens, and that severe signs were more frequent in older birds. In ducks, 8-week-old birds showed the most signs. P. multocida was isolated from 25.9% of healthy-looking ducks and 6.2% of healthy-looking chickens at slaughter, demonstrating the prevalence of the carrier state in apparently healthy birds.

Beyond domestic poultry, P. multocida infects a wide range of mammals including pigs, cattle, dogs, cats, rabbits, rodents, and humans. The Saudi Journal review (2024) describes P. multocida as causing pneumonia in pigs, cattle and goats, progressive atrophic rhinitis in swine, and hemorrhagic septicemia in buffalo and cattle — illustrating the organism’s broad host range and economic significance across multiple species.

Routes of Transmission

Unlike pullorum disease, fowl cholera does not have a significant vertical transmission component. The Hendrix Genetics Laying Hens reference states clearly that vertical egg transmission does not appear to occur, and that fowl cholera is not a hatchery problem. Transmission is primarily horizontal, occurring through:

Nasal and oral secretions from infected birds contaminating feed and water — the primary route, as P. multocida is shed in large quantities from the respiratory tract of infected birds
Fomites: contaminated equipment, clothing, footwear, crates, and vehicles
Wild birds entering the flock or premises
Rodents — a particularly important reservoir; rats and mice can carry P. multocida asymptomatically
Cats and dogs, which can harbor and shed the organism
Chronic carrier birds within the flock that show no clinical signs

The Merck Veterinary Manual emphasizes that the most important sources of infection are chronically infected birds and asymptomatic carriers, whether avian or mammalian. The organism is not typically shed in fecal material, which differentiates its transmission dynamics from many other poultry pathogens.

Seasonal and Environmental Patterns

The Hendrix Genetics review notes that fowl cholera is more prevalent in late summer, fall, and winter, likely due to increased exposure to wild bird vectors during migratory seasons and suboptimal climate conditions in poultry houses. Stress factors that reduce immune competence — including changes in weather, transport, feed alteration, debeaking, overcrowding, and exhaustion — are recognized predisposing factors that increase susceptibility to clinical disease even in previously exposed or vaccinated flocks.

4. Pathogenesis

Following exposure, P. multocida colonizes the mucous membranes of the upper respiratory tract. The polysaccharide capsule is the organism’s primary defense against host phagocytosis, enabling it to survive in the bloodstream and establish bacteremia. From the bloodstream, the organism disseminates rapidly to the liver, spleen, lungs, and other organs, triggering endotoxin-mediated systemic inflammation that produces the acute septicemic form of the disease.

The PLOS Pathogens study (Steen et al., 2010) found that Fis — a global regulatory protein — controls the expression of the capsule biosynthesis genes and other key virulence determinants, and that Fis-mutant P. multocida strains were dramatically attenuated in virulence. This regulatory control of capsule expression is part of why certain strains are far more virulent than others.

In the acute form, endotoxin release from the large bacterial burden produces rapid vascular compromise, hemorrhage, and organ necrosis. Death can occur in as little as 6–12 hours after the onset of signs — or before clinical signs are observed at all. In the chronic form, the organism establishes localized infection in specific tissues such as the wattles, joints, sinuses, middle ear, or air sacs, producing the fibrinous and caseous lesions characteristic of this form. The Merck Veterinary Manual notes that lesions are typically found in the conjunctiva, other structures of the head, lungs, and liver in acute disease, while chronic disease causes caseous to fibrinous inflammation and necrosis in localized tissues.

5. Clinical Signs

Per-Acute Form

In highly susceptible flocks or when a virulent strain is introduced, per-acute disease produces sudden death with no premonitory signs. The Merck Veterinary Manual notes that the disease usually occurs as a septicemia of sudden onset with high morbidity and mortality rates. Flock managers may arrive to find multiple dead birds without any prior indication of illness. Mortality can approach 100% in highly susceptible flocks.

Acute Form

The PoultryDVM.com clinical resource describes the following signs in the acute form:

Fever, loss of appetite, and depression
Ruffled feathers and lethargy
Mucous discharge from the beak and nostrils
Increased respiratory rate, labored breathing, and coughing
Cyanosis — darkening and purplish discoloration of the comb, wattles, and face
Diarrhea progressing from yellow-gray to watery green with mucus
Retraction of the head (opisthotonus) in severe neurological involvement
Swelling of the face, comb, or wattles

Chronic Form

Birds that survive the acute stage, or those infected with less virulent strains, often develop the chronic form. The Merck Veterinary Manual describes species-specific chronic manifestations:

Chickens: Swollen wattles (caseous exudate-filled), lameness from arthritis in joints of the legs and wings, torticollis (twisted neck from middle ear infection)
Turkeys: Pneumonia, air sacculitis, sinusitis, and swollen snoods
Waterfowl: More likely to harbor the organism without severe clinical signs, acting as subclinical reservoirs

Torticollis — a persistent twisting of the neck caused by inner ear infection (otitis interna) — is a particularly distressing chronic sign in chickens that typically does not resolve and results in the affected bird being unable to compete for food and water. PoultryDVM.com notes that birds exhibiting torticollis from fowl cholera generally require humane euthanasia.

Effects on Production

Even in flocks where mortality is not severe, fowl cholera causes significant production losses. The PMC review on current trends in diagnosis (2024) reports morbidity rates of up to 52% and mortality rates of up to 56% in affected flocks, with the disease leading to fatalities, weight loss, carcass rejection at processing, and substantial medication expenses. Reduced egg production and poor egg quality are commonly reported in affected laying flocks.

6. Gross Pathology and Post-Mortem Lesions

Acute Lesions

Post-mortem examination of birds dying from acute fowl cholera typically reveals:

Petechial (pinpoint) hemorrhages on the epicardial fat, heart muscle, intestinal serosa, and mesentery
Congestion and hemorrhage throughout the abdominal organs
Swollen, friable, and congested liver — often with small, pale necrotic foci
Swollen, congested spleen
Fibrinous pericarditis (fibrin deposits around the heart)
Consolidated, dark-red lungs

Chronic Lesions

Caseous (cheese-like) exudate in swollen wattles of chickens
Fibrinous or caseous material in joints, tendon sheaths, or footpads
Fibrinopurulent pneumonia and air sacculitis, particularly in turkeys
Caseous exudate in the sinuses, middle ear, or infraorbital sinuses
Fibrinous peritonitis in laying hens

The Merck Veterinary Manual and the AAAP Gross Pathology reference (Abdul-Aziz & Barnes, 2018) note that while the gross lesions of fowl cholera are characteristic, definitive diagnosis cannot be made on pathology alone and must be confirmed by bacteriological culture.

7. Diagnosis

Presumptive Diagnosis

A presumptive diagnosis of fowl cholera can be made based on clinical signs, flock history, age of birds affected (older birds), and the characteristic gross pathological findings, particularly the hemorrhagic pattern and liver necrosis in acute cases. The bipolar staining pattern of P. multocida on Wright’s or Giemsa stain of fresh tissue impression smears provides a rapid, practical presumptive indicator. The Merck Veterinary Manual notes that bipolar staining is a hallmark of fresh P. multocida isolates.

Bacterial Culture and Identification

Definitive diagnosis requires bacterial culture and isolation. The Merck Veterinary Manual and The Poultry Site disease guide recommend submitting fresh liver, spleen, bone marrow, and lung tissue to a state veterinary diagnostic laboratory. P. multocida grows well on blood agar and chocolate agar, producing small, grayish, non-hemolytic colonies with a characteristic musty odor. Biochemical testing confirms species identification.

The PMC review on current trends in diagnosis (2024) notes that culturally, P. multocida can be isolated using standard bacteriological and biochemical tests from birds infected with fowl cholera, and that the isolation rate and accuracy are improved by submitting samples from birds in the acute stage of illness before antimicrobial treatment is initiated.

Somatic Serotyping

Somatic (LPS) serotyping is a critical step in diagnosis because bacterin vaccines are only protective against the same somatic serotypes included in the vaccine. The Merck Veterinary Manual emphasizes that because bacterins are only effective in preventing disease caused by the same serotypes included in the vaccine, somatic serotyping is important and it is critical to know the most prevalent serotypes within an area. Autogenous bacterins — custom vaccines produced from the specific isolate recovered from the affected flock — are recommended when polyvalent bacterins are found to be ineffective.

Molecular Diagnostics

The PMC review on current trends (2024) describes advances in molecular diagnostics, including capsular and lipopolysaccharide genotyping by PCR assay, which allow rapid identification and serotyping of P. multocida directly from tissue samples. Multilocus sequence typing (MLST) and whole genome sequencing (WGS) are increasingly used for epidemiological investigation and tracking the spread of specific strains between flocks and geographic regions, as described in the Saudi Journal review on emerging challenges in P. multocida (2024).

8. Treatment

Unlike pullorum disease, fowl cholera can and should be treated when it occurs in a flock, as antimicrobial therapy can significantly reduce mortality and morbidity. However, treatment does not eliminate the carrier state from the flock, and this is a critical management consideration.

Antimicrobial Options

The Merck Veterinary Manual states that P. multocida is susceptible to some antimicrobials and that treatment reduces mortality. The University of Kentucky Extension (Dr. Jacquie Jacob) confirms that because fowl cholera is caused by a bacterium, it can be treated with antibiotics. Antimicrobials commonly used include:

Sulfonamides: Among the oldest and most commonly used antimicrobials for fowl cholera. Administered in feed or water. Sulfa drugs reduce mortality but do not eliminate the carrier state.
Tetracyclines: Widely used; can be administered in feed or water. The PMC Brazil study (Blackall et al., 2013) found tetracycline resistance in approximately 6% of U.S. isolates and higher resistance rates in some international studies.
Fluoroquinolones (enrofloxacin): Highly effective but subject to regulatory restrictions; enrofloxacin is not approved for use in poultry in the United States.
Penicillin and ampicillin: Generally effective; resistance to ampicillin and cephalotin was found in less than 5% of U.S. isolates in a 2001–2003 study (Huang et al., 2009, cited in PMC Brazil review).
Erythromycin and other macrolides: Used in some formulations; the Saudi Journal review (2024) notes antimicrobial resistance is an emerging concern particularly against tetracyclines and macrolides.

Limitations of Treatment

The PMC review on current trends (2024) notes that widespread and prolonged antibiotic use has led P. multocida to develop resistance to many antimicrobials. Antimicrobial sensitivity testing of the specific isolate from an affected flock is strongly recommended before initiating treatment. Treatment should always be combined with an aggressive biosecurity and vaccination program, as antimicrobials alone will not prevent recurrence in an infected premises.

Treatment regimens typically continue for 3–5 days after clinical signs resolve. Early withdrawal of treatment often allows the infection to resurge in carrier birds. After treatment, the flock should be considered to contain carrier birds and managed accordingly.

9. Prevention and Control

Biosecurity

Because the most important source of P. multocida in any flock is carrier birds — both avian and mammalian — biosecurity measures are the foundation of prevention. The Merck Veterinary Manual, the University of Kentucky Extension, and the Hendrix Genetics review all emphasize the following:

Exclude wild birds from poultry housing. Wild birds are a primary source of P. multocida introduction. All openings should be screened.
Implement rigorous rodent control. Rats and mice are important P. multocida reservoirs and vectors. Rodent control should be a continuous, not seasonal, program.
Exclude cats and dogs from poultry areas. Both species can harbor and shed P. multocida without showing disease.
Quarantine and test all incoming birds. New birds should be isolated for a minimum of 30 days and observed carefully for any clinical signs before introduction to the main flock.
Practice all-in, all-out flock management. Depopulate, thoroughly clean, and disinfect premises before restocking. P. multocida is susceptible to ordinary disinfectants; a thorough cleaning program is effective in reducing environmental load.
Minimize stress factors. Overcrowding, sudden feed changes, extreme temperature fluctuations, transport stress, and debeaking all predispose birds to clinical disease and should be managed carefully.

Vaccination

Both killed bacterins (inactivated vaccines) and live, attenuated vaccines are available for fowl cholera in the United States and internationally. The Merck Veterinary Manual, the PMC vaccine review (Mostaan et al., 2020, PMC7368114), and the MDPI vaccine review (Vaccines, 2025) all describe the available options:

Killed bacterins: Whole-cell formalin-killed preparations, usually oil-emulsion adjuvanted. Widely used and generally effective against the homologous serotype. The PubMed candidate vaccine review (Blackall, 1999, PMID 10486918) notes that whole-cell bacterins can provide some degree of protection, but only against the homologous LPS serotype. Bacterins provide approximately 4–6 months of protection depending on adjuvant formulation. The Merck Veterinary Manual states that autogenous bacterins are recommended when polyvalent commercial bacterins prove ineffective, underscoring the importance of serotyping.

Live, attenuated vaccines: Available for turkeys (administration in drinking water) and chickens (wing-web inoculation). The Merck Veterinary Manual states that live vaccines can effectively induce immunity against different serotypes of P. multocida and are recommended for use in healthy flocks only. Commercially available products include CholVax, Cholermune M, Multimunem, and M-Ninevax-C. The PubMed review notes that because the basis for attenuation in empirically derived live vaccines is undefined, reversion to virulence is not uncommon — an important safety consideration.

Autogenous bacterins: Custom-produced vaccines using the specific strain isolated from the affected flock. Recommended when commercial polyvalent bacterins fail, as they provide serotype-matched protection. Generally produced by regional veterinary diagnostic laboratories or commercial vaccine manufacturers on request.

The University of Kentucky Extension notes that vaccination is not recommended unless fowl cholera is already present and problematic on a farm — a practical guideline that reflects both the cost and the limitation of current vaccines in providing cross-serotype protection in flocks with no known exposure history.

Depopulation and Premises Management

In flocks with recurrent, untreatable, or severe fowl cholera, the Merck Veterinary Manual and clinical extension resources recommend complete depopulation, followed by thorough cleaning, disinfection, and a resting period before restocking with healthy birds from NPIP-certified sources. Because P. multocida may persist in soil, a minimum rest period of several weeks to months is advisable for outdoor or free-range premises with a history of fowl cholera outbreaks.

10. Zoonotic Considerations

multocida is considered a zoonotic pathogen, though the primary zoonotic risk to humans comes from companion animal bites rather than direct poultry contact. The Frontiers in Veterinary Science review (2023) states that P. multocida can spread to humans through cat or dog bites, causing severe zoonotic infections including cellulitis, bacteremia, meningitis, and in immunocompromised individuals, septicemia.

Direct transmission from poultry to humans is documented but uncommon. The Saudi Journal review (2024) notes that antimicrobial resistance in P. multocida isolates represents an increasing zoonotic risk, as resistant strains that are difficult to treat in animals may potentially transfer resistance genes to human pathogens. For flock owners and farm workers, standard hygiene precautions — handwashing after handling birds, avoiding contact between mucous membranes and poultry secretions, and wound care after any scratches from birds — are the primary protective measures.

11. What This Means for Backyard and Small Flock Owners

Fowl cholera can and does occur in backyard and small farm flocks, particularly in flocks with access to wild birds, in mixed-species flocks, or in areas with high wild bird or waterfowl activity. The disease’s ability to kill birds rapidly and without warning makes early recognition and a prepared response plan critical.

Key Points for Flock Owners

Finding dead birds without prior signs of illness in adult birds — particularly in late summer, fall, or winter — should always prompt consideration of fowl cholera. Contact a veterinarian and submit fresh birds to a state veterinary diagnostic laboratory immediately.
Rodent control is not optional. Rats and mice are among the most common sources of P. multocida introduction into backyard flocks. A comprehensive, year-round rodent control program is one of the most effective preventive measures available.
Wild bird exclusion matters. Migrating waterfowl in particular are significant P. multocida reservoirs. If your flock has access to ponds, wetlands, or areas frequented by wild birds, risk is significantly elevated.
Treatment should begin promptly under veterinary guidance when fowl cholera is suspected. Early treatment significantly reduces mortality. Do not wait for laboratory confirmation before initiating treatment in the face of rapidly progressing mortality.
Treated birds may become carriers. Birds that recover from fowl cholera following treatment should be considered potential carriers. They may shed P. multocida and remain a source of infection for flock mates and future birds introduced to the premises.
Vaccination should be discussed with your veterinarian if fowl cholera has been identified on your premises or is prevalent in your area. Serotyping of the local isolate is important for selecting an effective vaccine product.