Veterinary Epidemiology and Zoonosis

Diagnosis of Salmonellosis

EXERCISE-11: DIAGNOSIS OF SALMONELLOSIS

• It is an economically important disease worldwide.
• Infect both warm and cold blooded animals.
• A common bacterial cause of food-poisoning worldwide. Over 1800 food-poisoning serotypes of salmonella (bacterium) exist.
• The most common disease is diarrhoea and some serotypes produce abortion and septicaemia. Infected animals shed their agents in feaces or milk.
• It is difficult to identify the agent due to intermittent shedding. Stress may result in shedding of the agent.
• Clinical salmonellosis causes peracute septicemia, acute enteritis and chronic enteritis.
• There are about 2200 serotypes. Prevalence of the serotypes constantly changes.
• The most common serotypes in various species are:
  • Cattle : S.typhimurium, S.dublin, S.newport
  • Sheep and goat : S.typhimurium, S.dublin, S.anatum
  • Pigs : S.typhimurium, S.choleraesuis
  • Horses : S.typhimurium, S.anatum, S.newport, S.enteritidis, S.arizona,
  • S.angona
  • Humans : S.typhi, S.paratyphi A and B (S.typhi, the cause of Typhoid fever)

Isolation and identification of Salmonella

• Materials to be collected
  • Animals : Faeces, urine, eggs, meat and necropsy materials like liver, lymphnodes etc.
  • Humans : Faeces, rectal swabs and blood.
• Culturing and isolation
  • Non-selective enrichment : Add macerated sample to lactose broth / buffered peptone water (1:10), pH 7.2. Incubate for 24 hr at 37°C.
  • Selective enrichment : The selective enrichment inoculation is done in selenite / tetrathionate broth from non-selective culture (1:10). Incubate for 48 hrs at 37°C. Mannitol selenite and cystine selenite broths are preferred for food samples.
  • Plating: Xylose Lysine Deoxy­cholate citrate agar (XLD) is preferred for faeces and rectal swabs. XLD and Brilliant Green Agar (BGA) are best for food samples. Incubate the plates at 37°C for 18-24 hr. Typical colonies characteristic of Salmonella can be seen.

Figure: Growth of Salmonella on XLD agar

 

Figure: Indole test positive for Salmonella

 

Figure: Salmonella in selenite broth

 

• Identification of Salmonellae sp by serological method
  • Take a loopful colony from selective media on to a clean glass slide and emulsify by adding one drop of normal saline followed by polyvalent sera. Mix well with a toothpick and observe for agglutination. The Salmonellae can be identified if agglutination occurs within 30 seconds to 1 minute.

Flowchart for isolation and identification of Salmonella