Approach Considerations
Y pseudotuberculosis infection is diagnosed primarily via isolation of the organism from stool, tissue, blood, or joint fluid in patients with clinical history and manifestations suggestive of enteritis or scarlet fever–like syndromes. The laboratory should be alerted as to the possibility of the organism in order to use special media, cold enrichment procedures, or longer monitoring of cultures.
Polymerase chain reaction (PCR) testing may be available from specialized reference or research laboratories.
Laboratory Studies
Bacteriology
Y pseudotuberculosis is both aerobic and facultatively anaerobic; it is a gram-negative coccobacillus that grows slowly on blood and chocolate agar plates, forming small gray and translucent colonies at 24-72 hours. It has a good growth pattern on MacConkey or eosin-methylene blue (EMB) agar plates but is enhanced noticeably at lower temperatures (for example, 4°C cold enrichment in buffered saline) and is motile at temperatures lower than 28°C. Biochemically, it is oxidase-negative, urease-positive, and catalase-producing, and it does not ferment lactose.
Stool
Isolation of Y pseudotuberculosis from stool is difficult given the slow growth pattern and overgrowth of normal fecal flora. However, stool culture yield may be increased with cold enrichment or cefsulodin-Irgasan-novobiocin [CIN] agar.
Blood, peritoneal fluid, pharyngeal exudate, and synovial fluid
Blood, peritoneal fluid, pharyngeal exudate, and synovial fluid may yield Y pseudotuberculosis.
Serology
Numerus serological assays have been used for Yersinia infection but are neither sensitive nor specific for clinical use and not readily available, except from reference or research laboratories.
Enzyme-linked immunosorbent assay (ELISA) and agglutination tests may be obtained; the antibodies (against the O antigen) may appear soon after the onset of illness and typically wane over 2-6 months. Paired serum specimens taken 2 weeks apart that indicate a 4-fold rise in agglutinating antibodies support the diagnosis. Hemagglutination reaction tests that detect the pili (fimbriae) of either Y pseudotuberculosis or Y pestis have also been developed. Hemagglutination titers of 1:160 or higher are considered generally significant and indicative of true infection.
Imaging Studies
In patients with mesenteric lymphadenitis, CT scanning and, in some cases, ultrasonography of the abdomen and pelvis may reveal enlarged mesenteric lymph nodes and/or peritoneal findings, including appendiceal inflammation, peri-appendiceal fluid, and/or terminal ileitis.
In patients with pneumonic or septic presentations, chest radiography may reveal infiltrates.
Procedures
Exploratory laparotomy is not infrequently performed in critically ill patients with prominent mesenteric lymphadenitis. Diagnostic tissue may confirm the otherwise occult diagnosis.
Histologic Findings
Although the affected appendix may appear normal, involved lymph nodes (mesenteric) typically show epithelioid granulomatous changes, lymphoid hyperplasia, coagulative necrosis, and histiocytic cell hyperplasia. Enteric lesions may be associated with crypt hyperplasia, microabscesses, and villus shortening.
