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.
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.
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 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.
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.
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.
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.
- 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
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