Salmonellosis: Pullorum Disease in Poultry

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

A Veterinary Reference Guide for Poultry Producers and Flock Owners

Pullorum disease is one of the most historically significant infectious diseases in the poultry industry. Once responsible for devastating losses in commercial and backyard flocks worldwide, it became the catalyst for the development of the National Poultry Improvement Plan (NPIP) in the United States — one of the most successful animal disease eradication programs ever conducted. While the disease has been eliminated from commercial poultry in the U.S. and many other developed nations, it remains a significant threat to backyard flocks, game birds, and wild bird populations globally, and continues to cause severe economic losses in countries with less-developed poultry industries. This article provides a comprehensive review of pullorum disease based on peer-reviewed veterinary literature and authoritative veterinary references.

Quick Reference Summary

Category	Detail	Notes
Causative Agent	Salmonella enterica serotype Gallinarum biovar Pullorum	Gram-negative, non-motile, non-spore-forming rod
Primary Hosts	Chickens, turkeys	Also affects quail, guinea fowl, pheasants, parrots, canaries
Transmission	Primarily vertical (egg)	Also horizontal via contact, feces, feed, water, litter
Age Most Affected	Chicks under 3 weeks	Adults often show no signs; become silent carriers
Mortality Rate	Up to 100% in young chicks	Lower in adults; highly variable by breed
Key Clinical Signs	White pasty diarrhea, huddling, weakness, vent pasting	May also include blindness, joint swelling, respiratory signs
Zoonotic Risk	Minimal	Host-adapted to avian species; rare human cases
Diagnosis	Culture + isolation; serology (whole blood plate agglutination)	Definitive diagnosis requires organism isolation
Treatment	Not recommended	Recovered birds become silent carriers
Reportable?	Yes — in the United States and most countries	Contact state veterinarian immediately
Prevention	NPIP-certified source flocks, biosecurity	Vaccination available in some countries; not federally licensed in U.S.

1. Historical Background

The history of pullorum disease is inseparable from the history of modern poultry production in the United States. The disease, historically known as bacillary white diarrhea (BWD), was rampant in poultry through the late 19th and early 20th centuries and could cause mortality approaching 80% or higher in recently hatched chicks (Trampel DW, Iowa State University, 2013).

According to a historical review published in Avian Diseases (Trampel, 2021), the bacterium responsible — Salmonella enterica serotype Pullorum — was first isolated and described by Leo F. Rettger in 1900. Rettger’s subsequent work fulfilled Koch’s postulates for the organism and, crucially, demonstrated for the first time in scientific history that a pathogen could be vertically transmitted from parent to offspring through the egg. The disease was renamed pullorum disease in 1929.

The development of a whole-blood serological test in 1913 was a turning point. This test allowed the identification and removal of carrier birds from breeding flocks, forming the basis of what would become the eradication strategy. The same historical review in Avian Diseases notes that by 1935, 34 states were participating in a testing program covering 4 million birds, representing 58.2% of all birds hatched. This program became formalized as the National Poultry Improvement Plan (NPIP). By 1948, 47 states were participating and more than 30 million birds were being tested annually.

The result of sustained testing and elimination was extraordinary: by 1967, all commercial chicken hatcheries participating in the NPIP were 100% free of pullorum and typhoid disease caused by Salmonella enterica serotype Gallinarum. The crowning achievement came in 1975, when no pullorum-typhoid reactors were detected anywhere in the United States (Trampel, Avian Diseases, 2021). This program stands as one of the most successful animal disease eradication efforts in agricultural history, and the Applied and Environmental Microbiology journal (Foley et al., 2011) credits it as having dramatically reduced the incidence of these infections in commercial U.S. flocks.

2. Etiology and Classification

Pullorum disease is caused by Salmonella enterica enterica serovar Gallinarum biovar Pullorum, commonly abbreviated as S. Pullorum. According to the Merck Veterinary Manual (reviewed by Dr. Mohamed El-Gazzar, DVM, PhD, Iowa State University, 2024), S. Pullorum organisms are gram-negative, non-spore-forming rods measuring 1.0–2.5 micrometers by 0.3–1.5 micrometers. They are non-motile under normal conditions and exhibit a high degree of host specificity for avian species.

A comprehensive genomic review published in Antibiotics (MDPI, 2024) describes the classification of S. Pullorum within the Salmonella enterica subsp. enterica serovar Gallinarum group, noting that two distinct biovars exist: Pullorum (causing pullorum disease) and Gallinarum (causing fowl typhoid). Although these two biovars are closely related genetically, they differ in key virulence characteristics. S. Pullorum is the more host-adapted of the two and is strongly associated with vertical egg transmission, whereas S. Gallinarum is more likely to cause disease across all age groups.

Pullorum’s non-motility — a result of lost flagella genes — is a significant virulence-associated adaptation. The World Organisation for Animal Health (WOAH) Terrestrial Animal Health Standards chapter on Fowl Typhoid and Pullorum Disease notes that the loss of flagella genes is thought to help the organism evade host immune responses, contributing to its ability to establish persistent, silent carrier states in adult birds.

3. Epidemiology and Transmission

Vertical Transmission

The primary and most epidemiologically significant route of S. Pullorum transmission is vertical — from an infected hen to her offspring through the egg. This was first established by Rettger and colleagues in the early 1900s and remains the defining epidemiological characteristic of the disease. Infected hens that survive pullorum disease become long-term asymptomatic carriers, shedding the organism into eggs and passing it to their offspring without showing clinical signs themselves (Iowa State University Center for Food Security and Public Health, CFSPH, Factsheet: Fowl Typhoid and Pullorum Disease, 2019).

A 2022 study published in BMC Veterinary Research (Shen et al., 2022, DOI: 10.1186/s12917-022-03335-z) evaluated S. Pullorum dissemination and shedding patterns in experimentally infected chickens and confirmed that the organism colonizes the ovaries and reproductive tract of carrier hens, enabling consistent trans-ovarian transmission to developing eggs and chicks.

Horizontal Transmission

In addition to vertical transmission, S. Pullorum spreads horizontally within and between flocks through several routes, as detailed by the University of Kentucky Extension (Dr. Jacquie Jacob, poultry.extension.org):

Bird-to-bird contact
Cannibalism of infected carcasses
Fecal contamination of feed, water, and litter
Wound contamination
Wild birds, mammals, and insects acting as mechanical vectors

Host Range

Although chickens and turkeys are the primary hosts, the Iowa State University CFSPH factsheet documents confirmed S. Pullorum infections in turkeys, quail, guinea fowl, pheasants, ducks, pigeons, sparrows, canaries, bullfinches, and parrots. Infection has also been reported in mammals including pigs, cattle, cats, dogs, foxes, mink, rabbits, and guinea pigs, though mammals are considered incidental hosts rather than significant reservoirs.

The WOAH disease manual notes that game birds and backyard flocks may act as reservoirs of infection, and that S. Pullorum is particularly difficult to control in game birds reared in semi-wild conditions. Even in countries where the disease has been eliminated from commercial poultry, it persists in these populations.

Carrier State

One of the most clinically important features of pullorum disease is the carrier state. Birds that survive infection — particularly those infected after 4 weeks of age — often show no clinical signs but harbor the organism in the ovaries, testes, and other tissues for months or years (CFSPH, 2019). The Merck Veterinary Manual notes that the usual source of S. Pullorum for any new flock is other poultry, either through vertical or horizontal transmission from these silent carriers.

4. Pathogenesis

The virulence mechanisms of S. Pullorum are multifactorial, involving numerous genes and pathways. The Antibiotics review (MDPI, 2024) describes the pathogenicity as involving Salmonella pathogenicity islands (SPIs), particularly SPI-1 and SPI-2, which encode type III secretion systems that allow the bacteria to invade host epithelial cells and survive intracellularly within macrophages. This intracellular survival is key to the organism’s ability to establish systemic infection and persistent carrier states.

In newly hatched chicks, the organism enters through the gastrointestinal tract or directly from the egg, rapidly producing an acute septicemia. In older birds, the immune response is generally sufficient to limit acute septicemia, but S. Pullorum localizes in the ovaries and other organs, establishing the long-term carrier state. A 2023 outbreak report published in Avian Pathology (Tandfonline, 2023) documented that in adult laying hens, gross pathological findings were characterized by hepatomegaly, splenomegaly, and oophoritis, with S. Pullorum consistently isolated from liver, spleen, ovary, and bone marrow.

The SR Publications review by Dr. Pokyem Ering and Dr. Himani Ravi of the Indian Veterinary Research Institute (2023) notes that upon necropsy of recently hatched birds, characteristic lesions include enteritis, peritonitis, hepatomegaly, splenomegaly, congested lungs, and unabsorbed yolk sacs. Birds that survive past the acute phase often have typhlitis with hard, caseous material in the cecum and small, grey-white necrotic foci in the liver, spleen, lungs, and heart.

5. Clinical Signs

In Young Chicks (Under 3 Weeks)

The Merck Veterinary Manual and the CFSPH factsheet both describe the classic presentation in young chicks as an acute septicemia with the following signs:

Depression, somnolence, and weakness
Drooping wings
Huddling near heat sources
Loss of appetite and failure to thrive
Labored breathing
White, viscous, chalk-like diarrhea — the hallmark sign that gave the disease its historical name “bacillary white diarrhea”
Fecal pasting around the vent
Ruffled feathers and dehydration
In some cases: blindness, swollen joints (arthritis), or respiratory signs

The disease course in young chicks can be per-acute — some birds die before notable clinical signs develop, particularly those hatched from heavily infected eggs. Mortality can approach 100% in highly susceptible flocks and breeds. The CFSPH notes that heavier breeds are generally more susceptible than lighter breeds.

In Older Birds and Adults

The Merck Veterinary Manual notes that in adult chickens, mortality may be high in severe outbreaks, but frequently there are no clinical signs whatsoever. The WOAH disease manual confirms that in breeding and laying flocks, susceptibility increases at the point of lay, but reduced egg production and poor hatchability may be the only observable signs. The Avian Pathology case report from the Netherlands (2023) documented an unusual adult flock outbreak in which neurological signs were observed alongside increased mortality and decreased egg production, illustrating that clinical presentations in adults can be atypical and easily confused with other diseases.

Effects on Production

In infected breeding flocks, S. Pullorum can produce significant reductions in egg production, hatchability, and fertility. The IVING/SR Publications review notes that economic losses in affected flocks arise from high chick mortality, reduced egg production, poor hatchability, the cost of testing, medication, and eradication programs.

6. Diagnosis

Clinical and Post-Mortem Diagnosis

Presumptive diagnosis based on clinical signs and post-mortem lesions is possible, but definitive diagnosis requires laboratory confirmation. The Merck Veterinary Manual emphasizes that because several other diseases can cause similar signs — particularly fowl typhoid, which is caused by the closely related S. Gallinarum biovar Gallinarum and produces essentially identical clinical and gross pathological findings — clinical diagnosis alone is insufficient and organism isolation with serotyping is required.

Bacterial Culture and Isolation

Definitive diagnosis is made by isolation and identification of S. Pullorum from tissues or feces using standard bacteriological culture methods. The CFSPH factsheet recommends submitting fresh liver, spleen, lung, cecal contents, and yolk sac from affected birds to a state veterinary diagnostic laboratory. Serotyping at a Salmonella Reference Typing Laboratory is needed to confirm the biovar as Pullorum versus Gallinarum.

Serological Testing

The WOAH Terrestrial Animal Health Standards chapter identifies the rapid whole blood plate agglutination test (WBPAT) as the standard field serological test and the basis for NPIP surveillance programs. In this test, a drop of whole blood is mixed with a stained antigen on a plate; agglutination indicates the presence of antibodies against S. Pullorum. The test is rapid, inexpensive, and practical for large-scale flock screening, but has important limitations:

False negatives can occur in recently infected birds that have not yet mounted an antibody response
False positives can occur in birds vaccinated against Salmonella Enteritidis or Typhimurium
The test is unreliable in turkeys and ducks, as many uninfected birds of these species may give positive reactions

For these reasons, the Merck Veterinary Manual states that serological testing is used as a surveillance tool, but definitive diagnosis requires isolation and identification of the organism.

Molecular Diagnostics

The Antibiotics review (MDPI, 2024) notes that molecular-based methods are becoming increasingly important due to their rapidity, high sensitivity, and specificity. PCR-based assays can identify and differentiate S. Gallinarum and S. Pullorum directly from tissues, and loop-mediated isothermal amplification (LAMP) assays have also been developed and published. Whole genome sequencing (WGS) is increasingly being used for outbreak investigation and phylogenetic analysis, as demonstrated in the Dutch outbreak report (Avian Pathology, 2023), where WGS confirmed clonality of isolates within the affected flock.

7. Treatment

Treatment of pullorum disease is not recommended as a flock management strategy. The Merck Veterinary Manual states clearly that the goal for control of pullorum disease is elimination of the pathogen, particularly from breeding flocks, and therefore treatment is not recommended. The reasoning is straightforward: antimicrobial therapy may reduce mortality and suppress clinical signs, but it does not eliminate the carrier state. Birds that recover from infection become long-term silent carriers and continue to shed the organism vertically through their eggs and horizontally through their environment.

The poultry.extension.org resource (Dr. Jacquie Jacob, University of Kentucky) echoes this position, stating that it is best to depopulate a flock that tests positive for S. Pullorum. Recovered carrier birds in a breeding flock represent an unacceptable ongoing risk to the flock’s offspring and to any flock with which they come into contact.

Antimicrobial Resistance

The Antibiotics review (MDPI, 2024) raises an important concern: S. Pullorum has developed resistance to multiple antimicrobial agents, with multidrug-resistant strains increasingly reported in countries where treatment has historically been practiced. This further underscores why the eradication-based approach — rather than treatment — is the preferred strategy in the veterinary community.

8. Control, Prevention, and Eradication

The National Poultry Improvement Plan (NPIP)

The NPIP, administered by the USDA Animal and Plant Health Inspection Service (APHIS), remains the cornerstone of pullorum disease control in the United States. The program requires routine serological testing of breeding stock to assure freedom from infection. The Merck Veterinary Manual states that the NPIP outlines the essential components for eradication of S. Pullorum and that birds should be purchased from NPIP-certified sources.

The Avian Diseases historical review (Trampel, 2021) credits the NPIP as one of the primary reasons pullorum disease was successfully eradicated from commercial U.S. poultry by the 1960s-1970s. Participation in the program rapidly expanded from 34 states in 1935 to 47 states by 1948, covering tens of millions of birds annually.

Biosecurity Measures

The CFSPH factsheet, the Merck Veterinary Manual, and the DVM360 proceedings on backyard poultry diseases all emphasize that biosecurity is essential for preventing the introduction of S. Pullorum into clean flocks. Key measures include:

Purchase birds exclusively from NPIP-certified sources. This is the single most important preventive measure for both commercial and backyard flocks.
Strict quarantine of new birds for a minimum of 30 days before introducing them to an existing flock, with fecal testing and serological testing performed during this period.
Do not mix NPIP-certified flocks with non-certified birds. The extension.org resource emphasizes that this practice can re-introduce the pathogen to clean flocks.
Wild bird exclusion. Wild birds are a significant reservoir of S. Pullorum and can introduce the pathogen to otherwise clean backyard flocks.
Hatchery sanitation. The Avian Diseases historical review notes that formaldehyde gas fumigation of incubators and hatcheries was a significant early breakthrough in controlling the spread of S. Pullorum through hatchery environments.
Depopulation of positive birds. Any bird confirmed as a reactor on serological testing should be removed from the flock and the flock re-tested to confirm freedom from infection.

Vaccination

There are no federally licensed vaccines for pullorum disease in the United States. The Merck Veterinary Manual notes that live vaccines (such as the SG 9R vaccine used in some countries) exist but are not approved for use in U.S. flocks. Vaccination is available in some countries and can reduce clinical disease, but it does not prevent infection or the carrier state, and vaccinated flocks can yield false-positive results on serology testing.

Reportable Disease Status

Pullorum disease is a reportable disease in the United States and in many countries worldwide. The poultry.extension.org resource notes that any flock owner who suspects pullorum disease must contact their state veterinarian. Affected flocks are generally depopulated under the supervision of state regulatory agencies.

9. Current Status: Where the Disease Stands Today

In the United States and most highly developed poultry-producing nations, pullorum disease has been successfully eradicated from commercial poultry. The Applied and Environmental Microbiology journal (Foley et al., 2011) notes that the NPIP effectively eliminated S. Pullorum from commercial flocks by the mid-1960s. As the Merck Veterinary Manual states, pullorum disease may now occur in other avian species and in small backyard or hobby flocks, but is no longer a commercial-sector concern in the U.S.

However, the disease remains a serious ongoing problem in many parts of the world. The Antibiotics review (MDPI, 2024) notes that S. Pullorum continues to cause significant economic losses in countries with less-developed poultry industries, particularly in parts of Asia, Africa, and South America where NPIP-equivalent surveillance programs are not in place or are inconsistently enforced.

The emergence of multidrug-resistant strains of S. Pullorum in some regions adds an additional dimension of concern. As the Antibiotics review notes, rapid and accurate diagnosis is crucial for effective control and prevention, and continued research into pathogenesis, drug resistance, and novel diagnostic approaches is needed.

Zoonotic Considerations

The WOAH disease manual states that S. Gallinarum and S. Pullorum are host-adapted to avian species and are considered to pose a minimal zoonotic risk to humans. This is in contrast to non-typhoidal Salmonella serovars such as Salmonella Enteritidis and Typhimurium, which are significant human pathogens. Nonetheless, the CDC Salmonella outbreaks page on backyard poultry notes that any backyard poultry can carry Salmonella germs and that standard hygiene practices — handwashing, not bringing birds indoors, and avoiding contact between poultry and food-preparation areas — should always be observed.

10. What This Means for Backyard and Small Flock Owners

The near-eradication of pullorum disease from commercial U.S. poultry has created a false sense of security among some backyard and small flock owners. It is important to understand that S. Pullorum can and does persist in backyard flocks, game birds, and wild birds, and that the disease can be reintroduced to any flock through untested birds, contaminated equipment, or wild bird contact.

Practical Steps for Flock Owners

Always purchase birds from NPIP-certified hatcheries. This is the most reliable protection against introducing pullorum disease into your flock. Reputable farms selling 2-month-old guaranteed pullets source from NPIP-certified hatcheries.
If chicks are dying in large numbers in the first 2–3 weeks with white pasty diarrhea, pullorum disease must be on the differential list and laboratory testing is essential. Do not wait — contact a veterinarian and your state veterinary diagnostic laboratory immediately.
Do not purchase birds from sources that cannot confirm NPIP certification or that lack documentation of testing. Swap meets, auctions, and roadside sales represent high-risk acquisition points.
Pullorum is a reportable disease. If your veterinarian confirms a positive result, you are legally required to report it to your state veterinarian.
Treatment is not a solution. Do not attempt to treat and retain birds that test positive for S. Pullorum. Carrier birds will perpetuate the disease in your flock indefinitely through egg transmission.

References and Resources

Merck Veterinary Manual. (Reviewed/Revised 2024). Pullorum Disease in Poultry. Reviewed by Dr. Mohamed El-Gazzar, DVM, PhD, Iowa State University. com/poultry/salmonelloses-in-poultry/pullorum-disease-in-poultry
Merck Veterinary Manual. (2024). Overview of Salmonelloses in Poultry. com/poultry/salmonelloses-in-poultry/overview-of-salmonelloses-in-poultry
Merck Veterinary Manual. (2024). Fowl Typhoid. com/poultry/salmonelloses-in-poultry/fowl-typhoid
Trampel DW. (2021). Pullorum Disease: Evolution of the Eradication Strategy. Avian Diseases, 65(2). DOI: 10.1637/aviandiseases-D-21-00024. PMID: 34412452.
Gast RK, Porter RE. (2020). Salmonella Infections. In: Swayne DE, Boulianne M, et al., eds. Diseases of Poultry, 14th Edition. Wiley-Blackwell. pp. 719–753.
Shivaprasad HL, Barrow PA. (2013). Pullorum Disease and Fowl Typhoid. In: Swayne DE et al., eds. Diseases of Poultry, 13th Edition. Wiley-Blackwell. pp. 678–693.
Fulton RM. (2019). Bacterial Diseases. In: Boulianne M, ed. Avian Disease Manual, 8th Edition. American Association of Avian Pathologists. pp. 101–108.
Shen X, Zhang A, Gu J, et al. (2022). Evaluating Salmonella pullorum dissemination and shedding patterns and antibody production in infected chickens. BMC Veterinary Research, 18, 237. DOI: 10.1186/s12917-022-03335-z
van den Boogaard LJM, et al. (2023). A Salmonella Pullorum outbreak with neurological signs in adult layers and outbreak investigation using whole genome sequencing. Avian Pathology. DOI: 10.1080/03079457.2023.2268027
Foley SL, et al. (2011). Population Dynamics of Salmonella enterica Serotypes in Commercial Egg and Poultry Production. Applied and Environmental Microbiology, 77(13), 4273–4279.
Ering PK, Ravi H. (2023). Pullorum Disease in Poultry. SR Publications Veterinary Review. Indian Veterinary Research Institute, Bareilly.
Jacob J. Pullorum Disease in Poultry. University of Kentucky, College of Agriculture, Food and Environment. extension.org
Center for Food Security and Public Health (CFSPH). (2019). Fowl Typhoid and Pullorum Disease. Iowa State University College of Veterinary Medicine. iastate.edu/Factsheets/pdfs/fowl_typhoid.pdf
World Organisation for Animal Health (WOAH). Pullorum Disease. Disease Card and Terrestrial Animal Health Standards Manual. org/en/disease/pullorum-disease
Trampel DW, DVM, PhD. (2013). Pullorum Disease, Poultry, and the NPIP. Iowa State University. Published in: Murray McMurray Hatchery Blog.
Antunes P, Mourao J, Campos J, Peixe L. (2024). Salmonella enterica Serovar Gallinarum Biovars Pullorum and Gallinarum in Poultry: Review of Pathogenesis, Antibiotic Resistance, Diagnosis and Control in the Genomic Era. Antibiotics (MDPI), 13(1), 23.
Davison S. (2018). Pullorum Disease in Poultry. In: Kahn CM, Line S, Aiello SE, eds. The Merck Veterinary Manual [online]. Merck and Co.
USDA Animal and Plant Health Inspection Service (APHIS). National Poultry Improvement Plan (NPIP). usda.gov/npip
Centers for Disease Control and Prevention (CDC). Salmonella Outbreaks Linked to Backyard Poultry. gov/salmonella/outbreaks

DVM360. (2026). Diseases of Backyard Poultry (Proceedings). dv