Salmonella

Salmonella spp. are gram-negative straight rods, usually flagellated, facultative anaerobes.

Overview

• Important member of the enterobacteria
• Cause disease in humans and animals worldwide
• Reservior of infection in poulty, pigs, rodents, cattle, dogs
• Bacteria may be present in water, soil, animal feed, raw meat
• Cause enteritis and systemic infection (septicaemia and abortion)
• Salmonella may be carried sub-clinically
• Some human strains cause enteric fever (S. Typhi causes typhoid), also gastroenteritis, septicaemia or bacteraemia

Characteristics

• Gram negative bacilli
• Facultative intracellular pathogens
• Non-lactose fermentors, oxidase negative
• Do not produce urease or indole from tryptophan
• Utilise citrate as a carbon source
• Reduce nitrates to nitrites
• Grow on MacConkey
• Red colonies on brilliant green agar indicating alkalinity
• Ferment glucose to produce acid and gas
• Usually produce hydrogen sulphide - red colinies with black centre on XLD agar
• Most motile with flagellae (H antigen)
• H antigen can be in phase 1 or phase 2, depending on a genetic switch allowing for one of the H antigen genes to be transcribed at any one time

Classification

• Single species, Salmonella enterica
• Over 2400 pathogenic serotypes or serovars identified
• Grouped into 9 groups according to Somatic, O antigen (lipopolysaccharide) by the Kauffmann-White scheme - determined by slide agglutination of the bacteria with specific antisera
• Categorised into serovars depending on and H (Flagellar) antigen, e.g. Salmonella enterica subspecies enterica serovar Tymphimurium; must also determine phase of H antigen (isolates must be in phase 1 to be typed)
• Most animal and human isolates in Groups B to E

Pathogenicity

• Faecal-oral transmission
• Infection frequently transmitted from faeces of rodents and birds
• Young, immunocompromised animals particularly susceptible
• Comparitively large dose required for infection due to gastric acid, normal intestinal flora and local immunity
• Enterocolitis:
  • Acute enteritis
  • Bacteria adhere to intestinal epithelial cells in the ileum and colon, probably via fimbrae, O antigen and flagellar H antigen
  • Multiply in and destroy epithelial cells
  • Cytotoxin may cause epithelial cell damage by inhibiting protein synthesis and causing calcium escape from cells
  • Enterotoxin may induce fluid secretion into intestinal lumen
  • Degeneration of microvilli
• Systemic disease:
  • Bacteria invade and replicate in host cells and resist phagocytosis and destruction by complement
  • Bacteria internalised by intestinal epithelial cells by inducing ruffling of cell membranes and uptake into vesicles
  • The organisms replicate within the vesicles and are released from the cells
  • Stimulate immune response on reaching the lamina propria
  • Acute inflammation, possibly with ulceration; prostaglandin and cytokine production by epithelial cells; enterotoxin production damaging mucosa
  • Phagocytosis of bacteria by neutrophils and macrophages
  • Bacteria either destroyed by the phagocytic cells or survive and multiply in the cells to cause systemic disease
  • Resistance to phagocytosis and destruction by complement allows spread within the body - bacteraemia and septicaemia
  • LPS O antigens prevent damage to bacterial cell wall by complement
  • LPS also causes endotoxaemia, and may contribute to local inflammatory response damaging intestinal cells to cause diarrhoea
  • Endotoxic shock during septicaemic salmonellosis due to LPS
  • Septicaemia may cause cyanosis of extremities
  • Intracellular carriage if bacteria no completely removed
  • Invasive potential of certain strains e.g. Salmonella Dublin associated with carriage of a large plasmid, encoding genes to allow intracellular survival in macrophages and also to allow iron acquisition
  • Salmonellae are facultative intracellular organisms, allowing them to move from the gut in macrophages and cause a bacteraemia and lesions throughout the body
  • Possession of Pathogenicity Islands associated with virulence
• Carriage:
  • Salmonellae can persist in the gut or gall bladder
  • Excreted in faeces after clinical signs disappeared - active carriage
  • Bacteria can survive intracellularly, avoiding the immune system and antimicrobials
  • May have latent carriage and intermittent excretion in faeces
  • Stresses e.g. transportation, illness, parturition, overcrowding promote excretion in carrier animals and may cause clinical signs to be shown
  • Tortoises, terrapins, snakes and other reptiles ofter carry Salmonellae
  • Asymptomatic carriage allows faecal spread of infection

Clinical infections

• Zoonotic
• Most human infections contracted from animals, especially poulty and cattle
• Some serotypes are host-specific, some infect a wide range of species
• Healthy adult carnivores are resistant to salmonellosis
• Clinical outcome depends on number of bacteria ingested, virulence of serotype, susceptibility of host
• Young and debilitated animals susceptible
• Salmonella serotypes:
  • S. Typhimurium infects many species; causes severe diarrhoea; non-invasive; causes of food poisoning in humans, e.g. from infected poultry
  • S. enteritidis: non species-specific; losses in young birds; causes food poisoning in humans
  • S. Dublin: invasive serovar; infects cattle
  • S. Cholerae-Suis: primarily infects pigs; also causes severe human disease
  • S. Pullorum: infects poultry; egg-transmitted; causes bacillary white diarrhoea, known as pullorum disease
  • S. Gallinarum: infectes older birds; known as fowl typhoid
  • S. Abortis-ovis: infects sheep
  • S. Abortus-equi: infects horses outside of the UK
  • S. Typhi, S. Paratyphi: infect humans
  • S. Montevideo produces outbreaks from contaminated imported meat and bone meal
• Enteric salmonellosis:
  • Enterocolitis occurs in most farm animal species affecting all ages
  • Ulcerative enteritis
  • Fever, depression, anorexia, foul-smelling diarrhoea containing blood, mucus and epithelial casts
  • Dehydration and weight loss
  • Abortion
  • Fatal within days in severely young animals
  • Milder syndrome where endemic on farms, possibly due to acquired immunity
  • Chronic enterocolitis can occur in surviving pigs, cattle, horses, causing intermittent fever, soft faeces and gradual weight loss
• Septicaemic salmonellosis:
  • Most common in calves, neonatal foals, pigs under one month
  • Sudden onset fever, depression, recumbency
  • Die within 48 hours if not treated
  • Persistent diarrhoea, meningitis, arthritis or pneumonia may occur in surviving animals
  • Found in arthritis of horses
  • Can cause haemorrhagic disease by secondary thrombocytopenic disease
  • S. Cholerae-Suis in pigs causes blue discoloration of ears and snout; co-infection with viruses causes severe clinical forms of disease
• Bovine salmonellosis:
  • Syndrome of fever and diarrhoea (with dysentery), often fatal, in calves and adult cattle
  • Abdominal pain in adult cattle due to necrotic bowel
  • Recumbency and depression, with death after 7-10 days during severe infection - mortality up to 75% in untreated adult animals
  • Antibiotic treatment reduces mortality to 10%
  • Diarrhoea lasts for up to 2 weeks, and complete recovery may take months
  • May cause abortion of pregnant cattle in absence of other signs
  • Septicaemia in neonates; accute enteritis in older calves
  • Calves are dull, lethargic, inappetent, pyrexic, with profuse, fowl-smelling diarrhoea
  • Death in calves can occur after 2-3 days
  • Diarrhoea in survivors may last 2 weeks
  • Caused by infection with various Salmonella serotypes, e.g. S. Dublin and S. Typhimurium
  • An important zoonosis and reportable
  • Carrier animals important for spread
  • Salmonella Dublin:
    • Causes enterocolitis with blood-stained, foul-smelling diarrhoea containing mucus and epithelial cells
    • Can cause fatal septicaemia - fever, depression, drop in milk yield; calves may develope arthritis, meningitis, pneumonia
    • Abortion with no other clinical signs
    • Chronic infections with S. Dublin in calves cause dry gangrene of extremities due to disseminated intravascular coagulation; tips of ears, tail and limbs may slough
    • Can cause Osteomyelitis in young animals
    • Most survivors become subclinical excretors
    • May become latent carriers with no excretion
• Salmonellosis in poultry:

  • S. Pullorum and S. Gallinarum now rare in UK due to eradication programs including the Pullorum test (whole blood slide agglutination to detect antibody to both S. Pullorum and S Gallinarum)
  • These Salmonellae can infect the ovaries of hens and be transmitted via eggs
  • Pullorum disease infects young chickens and turkeys (under 3 weeks); high mortality rates; anorexia, depression, white diarrhoea; white nodules throughout lungs; focal necrosis of liver and spleen
• Fowl typhoid causes similar lesions to pullorum disease in young birds; septicaemic condition in adult birds with sudden death (enlarged, friable, bole-stained liver and enlarged spleen). On post mortem inspection bronzing of the organs is notable.
• Paratyphoid caused by non host-specific Salmonella serotypes, e.g. S. Enteritidis and S. Typhimurium; often subclinical infections

Diagnosis

• History of previous outbreaks; clinical signs
• Post mortem: enterocolitis; blood-stained intestinal contents; enlarged mesenteric lymph nodes
• Laboratory confirmation by detection in faeces and blood from live animals; intestinal contents and tissue samples from dead animals
• Isolation from blood or tissues confirms septicaemic salmonellosis
• Heavy growth on plates innoculated with faeces or intestinal contents from infected animals suggests Salmonella as cause
• Light growth may suggest carrier state
• Culture specimens on BG and XLD agar; also add to enrichment broth such as selinite or tetrathionate broth; incubate plates and broth under aerobic conditions at 37 degrees centigrade for 48 hours; subculture from enrichment broth at 24 and 48 hours
• Suspicious colonies should be identified biochemically by reactions in TSI agar and lysine decarboxylase
• Slide agglutination using antisera for O and H antigens confirm the serotype
• The antigens in both phases of the H antigen must be identified
• Phage typing is used for epidemiological studies of isolates
• A rising antibody titre using paired serum samples in ELISA indicates active infection

Treatment

• Intravenous antibiotics used to treat septicaemic salmonellosis
• Effective antimicrobials include tetracyclines, chloramphenicol, trimethoprim-sulphonamides, ampicillin, amoxicillin, 3rd generation cephalosporins, fluoroquinolones, but depend on the susceptiblity of individual isolate
• Fluid and electrolyte replacent to prevent dehydration and shock

Control

• Reduce exposure of young animals from fomites, food, water, infected animals
• Avoid stresses e.g. overcrowding
• Purchase animals from reliable sources and isolate incoming animals
• Separate animals according to age
• Rodent control, good hygiene, pasture rotation
• Avoid grazing animals on pasture fertilised by slurry for at least 2 months after spreading
• Attenuated live S. Typhimurium and S. Dublin vaccines used in cattle
• Avoid oral prophylactic antimicrobials

In Reptiles

It is estimated that between 36 and 77% lizards harbour Salmonella - the most recognized reptilian zoonosis. Owners must be educated regarding the public health hazard (especially for the very young, the old and the immune compromised).