Salmonella Lecture Notes

Salmonella

Introduction

This lecture discusses Salmonella, a leading human foodborne pathogen. The lecture will cover the importance of studying the disease, organism characteristics, epidemiology, disease in animals (symptoms, prevalence, and transmission), and disease in humans (transmission, clinical signs/symptoms, laboratory diagnosis, treatment, prevention, and control).

Why Study Salmonella?

  • Salmonella is a leading human foodborne pathogen, causing more deaths than any other known human foodborne pathogen.
  • It is one of the four key global causes of diarrheal diseases.
  • Salmonellosis was the second most commonly reported foodborne gastrointestinal infection in humans in Europe.
  • Most cases are mild and don't require treatment, but it can be life-threatening.
  • The emergence and spread of multi-drug resistant (MDR) Salmonella is a concern, as these strains often cause more severe illness and increased risk of hospitalization.

Types of Salmonella Infection

Typhoidal

  • Causes typhoid fever, a severe systemic infection with non-specific gastrointestinal symptoms.
  • Caused by S. Typhi and S. Paratyphi.

Non-typhoidal

  • Often a mild, self-limiting infection causing gastroenteritis (enteric infections).
  • Caused by all other Salmonella serotypes.

Historical Incidence

The incidence of typhoid fever decreased by the mid-20th century due to modern sanitation separating human feces from food and water. However, non-typhoidal salmonellosis became a major problem with the industrialization of agriculture.

Focus on Non-Typhoidal Salmonellosis

The lecture will primarily focus on non-typhoidal salmonellosis.

Organism Characteristics

  • Belongs to the Enterobacteriaceae family.
  • Gram-negative rod.
  • Motile due to flagella on its cell surface.
  • Facultative anaerobe.
  • Can survive over a range of temperatures (approximately 2°C - 54°C depending on the serovar).
  • Can grow between pH 3.7 - 9.4.
  • Can survive for extended periods in dry environments.

Species and Serotypes

  • Two species:
    • S. bongori
    • S. enterica (responsible for the majority of cases, with six subspecies)
  • More than 2,600 known serotypes/serovars.
    • Less than 100 account for the majority of human infections.
    • Many are zoonotic.
    • All serotypes can cause disease in humans.
    • Some serotypes are host-specific and generally cause severe disease in infected individuals. Examples:
      • Salmonella enterica serotype Dublin: specific host is cattle
      • Salmonella enterica serotype Choleraesuis: specific host is pigs
      • Salmonella enterica subsp. enterica serovars Chameleon, Java, Marina, Poona and Stanley: specific host is reptiles

Serotypes and Serovars

  • Serotypes are microorganisms from the same species that differ in antigenic determinants expressed on their cell surface.
  • Variations in the composition of these antigens provide the basis for serotyping.
  • Serovars are referred to as Salmonella followed by the capitalized and unitalicized serovar name (e.g., S. Typhimurium).
  • Salmonella enterica serotype Enteritidis and Salmonella enterica serotype Typhimurium are the two most important serotypes transmitted from animals to humans in most parts of the world.

Epidemiology

World-Wide Distribution

Salmonella has a worldwide distribution among a wide range of hosts:

  • Poultry (chicks, chickens, ducklings, ducks, geese, and turkeys)

  • Wild birds

  • Reptiles (turtles, lizards, and snakes)

  • Amphibians (frogs and toads)

  • Rodents (mice, rats, hamsters, and guinea pigs)

  • Other small mammals (hedgehogs)

  • Farm animals (goats, calves, cows, sheep, and pigs)

  • Domestic canines and felines

  • S. enterica serovars can infect a range of different animal species.

  • Serovar distribution varies, with some serovars being geographically limited.

  • Salmonella spp. outbreaks cause huge mortality rates in several wildlife species, especially passerines (small perching birds, many are songbirds e.g., warblers, thrushes, sparrows, finches, jays, larks, and wrens).

USA Epidemiology

According to the Centers for Disease Control and Prevention (CDC) estimates in the USA:

  • Around 1.35 million illnesses annually
  • 26,500 hospitalizations
  • 420 deaths

Pathogen Surveillance Data

The CDC's Foodborne Diseases Active Surveillance Network (FoodNet) provides data on pathogen surveillance, including:

  • Infections by year
  • Incidence per 100,000 population
  • Percentage of infections
  • Number of infections
  • Distribution of Salmonella infections by serotype

European Centre for Disease Prevention and Control (ECDC)

  • In 2023, there were 77,486 cases of illness in the EU, with a notification rate of 18.0 per 100,000 population.
  • 46,548 infections were acquired in the EU, 4768 were acquired outside the EU, and 26,170 had unknown travel status or country of infection.
  • 14,801 hospitalizations (41.0%) and 88 deaths (0.20%).
  • Strong-evidence outbreaks implicated eggs and egg products, mixed food, broiler meat, bakery products, and unspecified meat products.
  • In 2023 there were 1115 foodborne outbreaks. 258 strong-evidence outbreaks and 857 weak-evidence outbreaks.

UK Data from Food Standards Agency

While Salmonella increased from last year, rates remain in line with pre-COVID-19 levels and longer term rates.

Recent Outbreaks

Recent outbreaks have been linked to Kinder products, parrot food, and cantaloupes, and charcuterie meat products.

Disease in Animals

Morbidity/Mortality

  • Most animals don't become unwell.
  • Asymptomatic infections are common.
    • Many animals are carriers.
    • High rates of carriage in reptiles and birds.
  • Clinical disease is more common in young, pregnant/lactating, or stressed animals.
    • Mortality can reach 100% (septicemia).

Prevalence in Animals

  • Cattle: 6% commonly associated with S. Typhimurium, S. Dublin, S. Newport.
  • Sheep and goats: 86% commonly associated with S. Typhimurium, S. Diarizonae, S. Abortusovis, S. Hindmarsh, S. Brandenburg.
  • Pigs: 50% commonly associated with S. Typhimurium, S. Choleraesuis, S. Typhisuis, S. Heidelberg, S. Derby.
  • Horses: 1-36% commonly associated with S. Typhimurium, S. Agona, S. Anatum.
  • Poultry: 2-20% commonly associated with S. Pullorum, S. Gallinarum, S. Typhimurium, S. Enteritidis.

Animal Transmission

  • Fecal-oral transmission:
    • Carried asymptomatically by many animals.
    • Often spread via poor animal husbandry.
    • Consumption of contaminated food and water.
    • Consuming other infected animals or food scraps, consuming material from landfills or sewage (especially wild birds like gulls).
  • Transmission via fomites or mechanical vectors.
  • Vertical transmission (transovarial):
    • Reported in reptiles and commercial poultry.
  • In utero transmission
  • Perinatal transmission (at time of birth)

Transmission to Humans

  • Fecal-oral (primary route of transmission): direct or indirect contact.
  • Often via consumption of infected food of animal origin (e.g., eggs, meat, poultry, and milk).
  • Foods, including green vegetables contaminated by manure, have also been implicated in transmission.
  • Person-to-person transmission can also occur, as can infection via contact with pets or livestock:
    • Reptiles
    • Chicks
    • Ducklings
    • Livestock, dogs, cats, adult poultry
  • Direct contact: handling infected pet.
  • Indirect contact: contact with contaminated objects (free roaming of pet), bites, scratches.

Reptile-Associated Salmonellosis

  • Serotypes associated with reptiles (amphibians and fish) include: S. enterica subsp. enterica serovars Chameleon, Java, Marina, Poona and Stanley.
  • In the 1970s, pet turtles were identified as a major source of Salmonella infection in the U.S., with 280,000 reported cases per year.
  • In 1975, the FDA banned interstate shipment of pet turtles (reported cases decreased by 100,000).
  • Currently, around 4.5 million people in the U.S. own a pet reptile, and these continue to be a major source of infection.
  • The sale and distribution of turtles with shells less than 4 inches long as pets is banned under federal law in the U.S.A. because they have caused many illnesses.
  • Many are still sold illegally at stores, flea markets, and roadside stands.

Backyard Poultry

Keeping backyard poultry (chickens, ducks, geese, guinea fowl, and turkeys) is becoming more popular in the USA and UK, and is a major cause of many outbreaks in USA.

Clinical Signs/Symptoms in Humans

  • Symptoms can vary from none (asymptomatic) to severe disease (septicemia: deadly).
  • All serovars can produce all forms of disease.
  • However, when animal host-specific serotypes cause disease in humans, the disease is often much more severe; often invasive disease occurs, which is ultimately life-threatening.
  • Reptile-associated infection causes a very severe disease:
    • May be fatal (usually severe symptoms occur after 10-14 days)
    • Septicemia
    • Meningitis
  • Most cases occur in children <10 and immunocompromised persons.

Gastroenteritis

  • Most other serotypes cause uncomplicated gastroenteritis that does not require treatment.
  • The young, elderly, and patients with weakened immunity may experience more severe disease.
  • Main symptoms are nausea, vomiting, abdominal cramps, diarrhea (may be bloody).
  • May also have headache, fever, chills, myalgia.
  • Severe dehydration: infants, elderly.
  • Symptoms usually last 4-7 days (2-7 days on average).

Clinical Sequela: Reiter's Syndrome

Reiter's syndrome is a triad of:

  • Asymmetrical arthritis
  • Urethritis/cervicitis or Diarrhea

May be associated with:

  • Mucocutaneous disease (balanitis, ulcers, keratoderma)
  • Conjunctivitis

Laboratory Diagnosis

  • Isolate organism from feces, body fluids, blood.
  • Grows on a wide variety of media.
    • Enrichment - Selenite Cystine Broth
    • Selective agar – Xylose-Lysine-Desoxycholate (XLD) agar
  • Key biochemical tests are fermentation of glucose, negative urease reaction, positive lysine decarboxylase, negative indole test, H_2S production, and fermentation of dulcitol.

Serological Tests

  • Antigen detection - polyvalent antisera for flagellar (H) and somatic (O) antigens.

Rapid Tests

  • Latex agglutination
  • EIA, ELISA

Molecular Tests

  • DNA hybridization, PCR

Public Health Monitoring

  • Serotyping – once the core of public health monitoring and epidemiological tracing.
  • Newer techniques – Next Generation Sequencing Technology provides information on species, serovar, and subtype of bacteria in just one test.
  • E.g., Whole Genome Sequencing (WGS)

Treatment

  • Antibiotics – choice should be based on results of antibiotic susceptibility tests.
  • Common choices are: ampicillin, amoxicillin, gentamicin, trimethoprim/sulfamethoxazole, fluoroquinolones.
  • MDR is a problem (in particular for ceftriaxone, azithromycin, and ciprofloxacin (a fluoroquinolone antibiotic)).
  • Treatment required:
    • Septicemia
    • Elderly, infants, immunosuppressed
    • Most people recover without needing antibiotics
    • Reluctance to prescribe due to resistance issues

Prevention and Control

At Production/Farm Level

UK has successful measures for controlling Salmonella in eggs and minimizing on-farm environmental Salmonella:

  • Vaccination to protect birds against Salmonella enteritidis & Salmonella typhimurium
  • High standards of hygiene on farms and throughout the food supply chain
  • Ensuring proper and effective rodent control
  • Eradication programs in some countries
    • Sweden: alternative agriculture system called “Kretslopp,” which means agriculture in harmony with the cycle of nature.

Food-Borne Diseases

  • Avoid raw or undercooked eggs, poultry, meat; unpasteurized milk/dairy.
  • Ensure proper hygiene standards in the home:
    • Wash fruit/veg foods before eating
    • Avoid cross-contamination of food:
      • Keep uncooked and cooked foods separate
      • Wash hands and kitchen utensils
  • Personal hygiene – wash hands after visiting the toilet and before preparing food.

Animal Contact

  • Wash hands after contact (hot water and soap).
  • If immunocompromised, avoid contact with reptiles, young chicks, ducklings.
  • Reptiles - children under 10 years of age are particularly susceptible:
    • Wash hands, cages, and surfaces
    • Change clothes
    • Supervision
    • Do not allow reptiles to roam freely
  • No human vaccines exist to prevent non-typhoidal salmonellosis (a vaccine is available to prevent typhoid fever and is around 75% effective).