Salmonellosis

• There are many different serotypes of Salmonella.
  • All can produce disease BUT only a few commonly produce illness in UK.
  • Salmonella poses a serious risk to man.
  • Some serotypes tend to be more species specific, whereas others can affect a wide range of species. For example:
    • Salmonella enteritidis
    • Salmonella typhimurium
      • Widespread in most species.
    • Salmonella dublin
      • Cattle
    • Salmonella cholerae suis
      • Pigs
      • Usually speticaemic.
      • Not very common now.
    • Salmonella montevideo
      • Produces outbreaks from contaminated imported meat and bone meal.
  • Strains are often named after where they were first isolated.
  • Typing is important for epidemiology.
  • All strains can occur epizootically, enzootically and sporadically.
  • All strains can also produce very similar changes.
• Salmonellosis is NOT very common in the dog and cat.
• However, the horse is often a carrier.
  • Stress may precipitate the disease, meaning Salmonellosis is seen often in veterinary hospitals.

Pathogenesis

• Disease is often seen associated with stress.
• organisms penetrate enterocytes before crossing the mucosa and entering macrophages.
  • After entering macrophages, organins may then either remain localised to the gut, or are carried round the body to cause disease.
• There are 2 main types of disease- septicaemic and enteric.
  • Each outbreak causes only one type of disease.
  • Type of disease is linked to serotype- some serotyopres produce septicaemia, whereas others remain localised in the gut.

Septicaemic Salmonellosis

• Septicaemic salmonellosis is very dramatic and produces death quite suddenly.
• diarrhoea is often not seen before death.
• This form of the disease is unusual in the very young.

Pathogenesis

• The organism colonises bowel epithelium, where it affects the Peyers patches.
  • It then invades macrophages, which enable it to invade across the epithelium to the submucosa. It may then either
    • Remain localised to submucosa, or
    • Spread to the lymph nodes and enter the circulation to become septicaemic.
• Animals may die at this stage (30%), but this depends on such factors as the infecting dose and strain.
• Is similar to septicaemic E. Coli.

Clinical

• Is usually post-weaning (is unusual in the very young animal).
  • 6 to 9 months in calves.
  • 6 to 10 weeks in piglets.
• Animals suffer from pyrexia, and occasionally a little bit of diarrhoea.
• Skin is reddened diffusely
  • Bruise-like dark purplish-red blotched may be seen.

Pathology

• Intestines
  • May show mild catarrhal enteritis, becoming fibrinous lower down.
  • The bowel is generally flaccid, reddened and filled with fluid.
  • Ecchymotic and petechial haemorrhages on serosa and mucosa.
  • Enlarged, haemorrhagic mesenteric lymph nodes.
• Excessive blood-tinged peritoneal fluid.
• Lungs are collapsed and frothy.
• Heart is often dilated with ecchymotic haemorrhages.
• Viscera have a "half-cooked appearance"
  • Pale in colour.
  • The liver and kidneys are also flabby and may have subcapsular haemorrhages.
  • The liver may contain small white foci of necrosis known as paratyphoid granulomas.

Diagnosis

• By culture of blood and from mesenteric lymph nodes (which are oedematous and red).

Enteric Salmonellosis

• Enteric Salmonellosis shows differences in clinical presentation between species.
  • Horse - acute fatal colitis.
  • Cows - lingering febrile diarrhoea with passage of pseudomembranes.
  • Calves - acute diarrhoea like colibacillosis.
  • Dogs - acute bouts of diarrhoea.
  • Cats - febrile enterocolitis.
  • Pigs - septicemia or enterocolitis.

Clinical

• Affected animals produce acute diarrhoea, which causes many deaths.
  • Watery and yellow.
  • May be tinged with a little blood.
• Animals may die from dehydration.
• In some outbreaks, particularly in pigs, chronic low-grade diarrhoea only is seen.
• Calves usually die in acute stage, but may also recover.

Pathology

• Enteritis is seen throughout the gut, but is worse further along the gut.
  • Inflammation is catarrhal in the duodenum.
  • By the ileocaecal junction enteritis is often fibrinous, sometimes with formation of diptheric membranes on the mucosal surface.
• The necrotic and fibrinous changes particularly affect the Peyers patches and the caecal and colonic lymphoid nodules.
  • May lead to "button ulcers" in the terminal ileum and colon.
    • These ulcers rupture very rarely.
• Focal necrosis may also be seen.
  • Particularly in the liver, but also in the spleen.
  • Histologically, foci show a central zone of necrosis, surrounded by macrophages and lymphocytes- paratyphoid granulomas.
    • Although this indicates the animal has had a systemic incfection, paratyphoid granulomas may be present without showing signs of septicaemia.
    • Enteric cases of salmonella infection nearly always show some evidence of systemic spread.
• Septicaemic form may relocalise in the gut, resulting in enteric disease.

Sequelae

• Animals can remain carriers for months/years following recovery from the acute diarrhoea phase.
  • Bacteria is shed from the bile duct and mesenteric lymph nodes.
    • A source of infection for other animals and people.
    • Shed particularly in times of stress.
• Stricture of the rectum.
  • A possible sequel to acute salmonellosis, especially in the piglet.
  • Just a few centimetres in from the anus.
  • Gives rise to:
    1. Difficulty in passing faeces.
    2. Megacolon
    3. thin watery diarrhoea in small 'pencils'.
    4. blown up abdomen
  • Animals eventually die from the stricture if they are not first destroyed.

Ulcerative Enteritis

• • • • Salmonella typically produces a broad spectrum of disease.
      • Classically may produce 'button' ulcers, especially in the terminal small intestine and upper large intestine (Ileocaecocolic junction).
        • Seen particularly in Salmonella cholerae suis in the pig.

Hepatitis

• Salmonella dublin
• calves

Clinical

• fever
• dehydration
• diarrhoea

Gross

• severe, often haemorrhagic, inflammation in the ileum
• paratyphoid nodules - pale foci of necrosis in the liver

Microscopically

• foci of hepatocytic necrosis
• mixed mononuclear inflammatory cell infiltrate

NB: small foci of hepatocytic necrosis are often found as incidental lesions at post mortem examination