Fowl Cholera in Chickens: Clinical Signs, Postmortem Lesions, Treatment, Vaccination, and Prevention

Etiology and Bacteriology

Fowl cholera is a contagious bacterial disease of domestic poultry caused by Pasteurella multocida, a Gram-negative, non‑motile, facultative anaerobic coccobacillus belonging to the family Pasteurellaceae. The organism possesses a polysaccharide capsule that serves as a major virulence factor, together with lipopolysaccharide (LPS) and several outer membrane proteins. P. multocida is classified into five capsular serogroups (A, B, D, E, F) and 16 somatic LPS serotypes (1 through 16) by the Heddleston scheme. In chickens, the most commonly isolated strains belong to capsular serogroups A and D, with somatic serotypes 1, 3, and 4 predominating [1].

The bacterium is relatively fragile in the environment, being inactivated by drying, sunlight, and common disinfectants (phenolic compounds, quaternary ammonium compounds, and hypochlorites). However, it can survive for weeks in moist organic material, such as soil or carcass tissues, especially at cool temperatures.

Pathogenesis

Infection occurs primarily via the respiratory or oral route after inhalation or ingestion of P. multocida shed from carrier birds or contaminated fomites. The bacteria colonize the upper respiratory tract, particularly the nasal cavity and pharynx, and then invade the bloodstream via the tonsils of the cecal tonsils or the mucosal epithelium. Bacteremia leads to dissemination to multiple organs, including the liver, spleen, lungs, and heart.

The capsular polysaccharide of P. multocida inhibits phagocytosis and complement-mediated killing, allowing the bacterium to survive and multiply within macrophages and extracellularly. Endotoxins released from the LPS trigger a severe systemic inflammatory response, resulting in increased vascular permeability, disseminated intravascular coagulation (DIC), and multi-organ failure. The rapid progression of septicemia often causes death within 24–48 hours in acute cases [2].

Clinical Signs

Clinical presentation of fowl cholera ranges from peracute death to chronic localized infections. The most common forms in chicken flocks are:

The pathognomonic clinical sign most commonly searched for is swollen wattles, which results from bacterial invasion of the wattle interstitial tissue, leading to serofibrinous or purulent exudate accumulation. This lesion may be warm and painful on palpation.

Differential diagnoses include Infectious Coryza in Poultry and Ducks (Avibacterium paragallinarum), Highly Pathogenic Avian Influenza (H5N1), Newcastle disease, salmonellosis, and colibacillosis.

Postmortem (PM) Lesions

Gross pathological findings vary with the form of the disease. The hallmark of acute fowl cholera is septicemic hemorrhages.

Acute Form

Chronic Form

Microscopic examination shows multifocal coagulative necrosis in the liver, bacterial emboli in capillaries, and fibrin thrombi consistent with DIC.

Diagnostic Confirmation

Definitive diagnosis relies on isolation and identification of P. multocida. Samples for culture include swabs from wattles, liver, spleen, bone marrow, or heart blood. The organism grows on blood agar or MacConkey agar (some strains) as small, grey, mucoid colonies. It is oxidase‑positive, catalase‑positive, and indole‑positive. Biochemical profiling and serotyping can be performed.

Molecular detection via conventional or real‑time PCR targeting the kmt1 gene (species‑specific) or capsular typing genes (hyaD-hyaC for serogroup A, dcbF for serogroup D) is increasingly used for rapid confirmation [3]. Serological tests (e.g., Enzyme-Linked Immunosorbent Assay (ELISA)) are available but generally used for flock serosurveillance rather than individual diagnosis.

Differential diagnosis must rule out other causes of sudden death with hemorrhagic lesions, such as Avian Influenza, Newcastle disease, and other septicemic bacterial infections (Escherichia coli in Chickens, Gallibacterium spp., Riemerella anatipestifer).

Treatment

Treatment should be initiated immediately based on clinical suspicion, ideally after culture and susceptibility testing. P. multocida is susceptible to many antibiotics, but resistance has been reported, particularly to tetracyclines and sulfonamides.

Treatment regimes typically last 3–5 days (or 2 days beyond clinical resolution). Medicated water is preferred for mass medication, but individual birds with chronic lesions (e.g., wattle abscesses) may benefit from surgical drainage and local antibiotic therapy.

It is critical to note that antibiotic treatment does not eliminate the carrier state; recovered birds may continue to shed P. multocida intermittently. Therefore, treatment must be combined with rigorous biosecurity and, where feasible, depopulation of affected flocks.

Vaccination

Vaccination is a cornerstone of prevention in endemic areas or in flocks with a history of fowl cholera. Two main types of vaccines are available:

Vaccination programs typically include two doses of bacterin at 8 and 12 weeks of age, followed by a booster every 6 months for layers or breeders. Live vaccines are often given once between 8 and 12 weeks. Autogenous bacterins prepared from the specific field isolates can be used when commercial vaccines fail due to serotype mismatch.

The response to vaccination can be monitored by ELISA antibody titers. However, protection is not solely antibody‑mediated; cellular responses play a significant role.

Prevention and Biosecurity

Prevention of fowl cholera hinges on breaking the transmission cycle of P. multocida.

Biosecurity Measures

1. Rodent and vector control: Rodents, wild birds, and insects can mechanically carry P. multocida into flocks. Implement integrated pest management.
2. Quarantine: New birds should be isolated for a minimum of 2 weeks and tested before introduction to the main flock.
3. All‑in/all‑out production: Complete depopulation and thorough cleaning between flocks reduce environmental contamination.
4. Sanitation: Effective cleaning and disinfection of houses (including feeders, drinkers, and manure pits) using approved disinfectants.
5. Flock monitoring: Daily observation for early signs of depression, swollen wattles, or mortality spikes.
6. Carrier culling: Recovered birds that continue to shed bacteria should be culled, especially in breeder or layer flocks.

Farm Management

• Avoid overcrowding and ensure adequate ventilation.
• Reduce stress factors such as sudden temperature changes, feed changes, or vaccination reactions.
• Remove dead birds promptly; proper disposal by incineration or composting.

Vaccination Integration

Vaccination should not be used as a replacement for biosecurity; it is an adjunct. In the face of an outbreak, emergency vaccination with an appropriate bacterin (or live vaccine outside the withdrawal period) can help limit spread.

Decision Tree for Fowl Cholera Outbreak Management

flowchart TD
    A[Sudden high mortality and/or swollen wattles in flock], > B{Clinical Assessment & Necropsy}
    B, > C[Suspicion of Fowl Cholera]
    C, > D[Collect samples: liver, spleen, wattle exudate]
    D, > E{Laboratory Confirmation}
    E, Gram stain & culture positive, > F[Isolate *P. multocida*; AST]
    E, PCR positive, > G[Confirm *P. multocida*]
    F, > H{Antibiotic Susceptibility}
    H, Susceptible, > I[Initiate mass medication (water/feed)]
    H, Resistant, > J[Switch based on AST results]
    I, > K[Remove dead birds daily; monitor mortality]
    K, > L{Mortality declines within 48 hr?}
    L, Yes, > M[Continue treatment; implement biosecurity; consider vaccination]
    L, No, > N[Re‑evaluate diagnosis; check for co‑infections; adjust antibiotic]
    M, > O[Stable flock: long‑term vaccination program + biosecurity]
    N, > P[Consider depopulation of affected houses]
    P, > Q[Thorough cleaning & disinfection; restock with vaccinated birds]

Public Health and Zoonotic Considerations

Pasteurella multocida is a zoonotic pathogen capable of causing localized wound infections, cellulitis, and respiratory infections in humans, typically after bites, scratches, or contact with infected birds. Poultry workers and veterinarians should use personal protective equipment (gloves, masks) when handling suspected cases. However, this article focuses on avian disease; detailed discussion of human clinical aspects is outside the scope.

References

[1] Christensen H, Bisgaard M. The genus Pasteurella. In: Garrity GM, et al., editors. Bergey’s Manual of Systematic Bacteriology. 2nd ed. Springer; 2005. vol. 2, part B, p. 890–904.

[2] Glisson JR, Hofacre CL, Christensen JP. Fowl cholera. In: Swayne DE, et al., editors. Diseases of Poultry. 14th ed. Wiley‑Blackwell; 2020. p. 807–824.

[3] Townsend KM, Boyce JD, Chung JY, et al. Genetic organization of Pasteurella multocida cap loci and development of a multiplex capsular PCR typing system. J Clin Microbiol. 2001;39(3):924–929. doi:10.1128/JCM.39.3.924-929.2001.