Salmonella

It is the causative organism of typhoid fever. Persons exposed to contaminated meat, eggs, poultry or in close contact with animals such as turtles, snakes, lizards, iguanas and dogs are at risk of Salmonellosis. Salmonellosis is a reportable disease.

Human pathology

Vi antigen is a heat labile surface antigen that acts as a virulence factor for Salmonella by protecting the bacteria from opsonization. Salmonella causes enteric or typhoid/ paratyphoid fever, enterocolitis, septicaemia and osteomyelitis.

Once Salmonella enter the gastrointestinal tract from food or water sources, they grow and multiply in the intestinal Peyer’s patches from where they enter the bloodstream to spread all over the body with a predilection for gallbladder and the reticuloendothelial system. Persistent infection of the gallbladder can lead to a chronic carrier state. Peyer’s patches may ulcerate causing intestinal perforation in fulminant infections.

Classically a “step ladder” pattern of fever is seen. Patients present with diffuse abdominal pain, constipation, delirium, splenomegaly, bradycardia in later stages etc. Symptoms become progressively worse without treatment. Rose spots are seen on the trunk. They are salmon-colored, blanching, maculopapular rashes typically fewer than 5 in number. They are caused due to bacterial embolisation to the dermis.

Patients with sickle cell anemia are predisposed to Salmonella osteomyelitis. Salmonella enterocolitis presents with abdominal pain and diarrhea usually with blood in stools.

Laboratory diagnosis of Salmonella infections

Samples are stool, blood for culture, bone marrow and bile. Culture shows non lactose fermenting colonies with characteristic jet black color and metallic sheen on Wilson and Blair medium. Blood culture should be examined by subculture for 10 days before giving a negative report. Biochemically all strains ferment glucose with acid and gas production except for S.typhi which forms acid only. H2S is produced by all except S.paratyphi A and S.cholerae-suis. Slide agglutination tests can be done for epidemiological purposes.

Antibody detection by Widal test (not used that much these days), Indirect haemagglutination, Counter Immunoelectrophoresis (CIEP), ELISA or Radioimmunoassay can be done. Demonstration of a rise in titre of antibodies is diagnostic. The antibody tests may be negative during the first 10 days of infection.