Salmonellosis

Introduction

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 and is hence a zoonosis and a public health issue. Disease in man is mostly contracted from poultry or cattle. Reptiles are also healthy carriers of Salmonella and these may also infect humans that are in frequent contact with these animals.

Salmonella enteritidis and Salmonella typhimurium are widespread in most species. Salmonella dublin is a cause of disease in cattle and Salmonella cholerae suis is prevalent in pigs and is usually septicaemic, although is now quite rare. Salmonella montevideo causes disease in humans and animals from contaminated imported meat and bone meal.

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.

Salmonella strains are often named after where they were first isolated. All strains can occur epizootically, enzootically and sporadically. The strains can all produce similar clinical signs. Infection is via the faecal- oral route and 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. The type of disease is linked to serotype- some serotyopres produce septicaemia, whereas others remain localised in the gut.

Septicaemic disease

Septicaemic salmonellosis is very dramatic and produces death quite suddenly and diarrhoea is often not seen before death. This form of the disease is unusual in the very young and usually occurs post-weaning. This is around 6 to 9 months in calves and around 6 to 10 weeks in pigs.
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 Signs

Animals suffer from pyrexia, and occasionally a little bit of diarrhoea. The skin is reddened diffusely and bruise-like dark purplish-red blotched may be seen. The intestines maay show mild catarrhal enteritis, becoming fibrinous lower down. The bowel is generally flaccid, reddened and filled with fluid. Ecchymotic and petechial haemorrhages are often see on the serosa and mucosa. There may also be enlarged, haemorrhagic mesenteric lymph nodes and excessive blood-tinged peritoneal fluid. The lungs are collapsed and frothy and the heart is often dilated with ecchymotic haemorrhages. The liver and kidneys are also flabby and may have subcapsular haemorrhages and 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