Yersinia
The genus yersinia includes Yersinia pestis, the cause of plague; Yersinia pseudotuberculosis and Yersinia enterocolitica, important causes of human diarrheal diseases; and others .Yersinia Pestis
Plague is an infection of wild rodents, transmitted from one rodent to another and occasionally from rodents to humans by the bites of fleas. Serious infection often results, which in previous centuries produced pandemics of "black death" with millions of fatalities.Morphology & Identification
Y pestis is a gram-negative rod that exhibits striking bipolar staining with special stains . It is non motile , facultative anaerobe , Growth is more rapid in media containing blood or tissue fluids and fastest at 30 C. In cultures on blood agar at 37 C, colonies may be very small at 24 hours.
Antigenic Structure
All yersiniae possess lipopolysaccharides that have endotoxic activity when released. . The envelope contains a protein (fraction I) that is produced mainly at 37 C and confers antiphagocytic properties. Y pestis produces a coagulase at 28 C but not at 35 C .Y pestis also produces a bacteriocin (pesticin); the enzyme isocitrate lyase, which is said to be distinctive; and other products.
Pathogenesis & Pathology
When a flea feeds on a rodent infected with Y pestis, the ingested organisms multiply in the gut of the flea and transmit to human via bite of flea . The inoculated organisms may be phagocytosed by polymorphonuclear cells and monocytes. Y pestis are killed by the polymorphonuclear cells but multiply in the monocytes; The pathogens rapidly reach the lymphatics and an intense hemorrhagic inflammation develops in the enlarged lymph nodes leading to necrosis . Y pestis often reach the bloodstream and become widely disseminated. Hemorrhagic and necrotic lesions may develop in all organs; meningitis, pneumonia, and serosanguineous pleuropericarditis are prominent features.
Primary pneumonic plague results from inhalation of infective droplets usually from a coughing patient , with hemorrhagic consolidation, sepsis, and death.
Clinical Findings
After an incubation period of 27 days, there is high fever and painful lymphadenopathy, tender nodes ("buboes") in the groin or axillae. Vomiting and diarrhea may develop with early sepsis. Later, disseminated intravascular coagulation leads to hypotension, altered mental status, and renal and cardiac failure. Signs of pneumonia and meningitis can appear .
Diagnostic Laboratory Tests
Specimens
Blood , aspirates sputum and C.S.F for smear and culture. Acute and convalescent sera may be examined for antibody levels.
Smears
Material from needle aspiration is examined after staining with Giemsa's stain and with specific immunofluorescent stains. With Wayson's stain, Y pestis may show a striking bipolar appearance.
Culture
All materials are cultured on blood agar and MacConkey's agar plates and in infusion broth. Growth on solid media may be slow, but blood cultures are often positive in 24 hours. Cultures can be tentatively identified by biochemical reactions , immunofluorescence and PCR .
Serology
In patients who have not been previously vaccinated, a convalescent serum antibody titer of 1:16 or greater is presumptive evidence of Y pestis infection. A titer rise in two sequential specimens confirms the serologic diagnosis.
Treatment
Unless promptly treated, plague may have a mortality rate of nearly 50%; pneumonic plague, nearly 100%. The drug of choice is HYPERLINK "javascript:showDrugInfo(575);" streptomycin. HYPERLINK "javascript:showDrugInfo(598);" Tetracycline is an alternative drug and is sometimes given in combination with streptomycin. Drug resistance has not been noted in Y pestis.
Control
Plague is an infection of wild rodents (field mice, gerbils, moles, skunks, and other animals) that occurs in many parts of the world , the commonest vector of plague is the rat flea (Xenopsylla cheopis), but other fleas may also transmit the infection.
The control of plague requires surveys of infected animals, vectors, and human contactsin the United States this is done by county and state agencies with support from the Plague Branch of the Centers for Disease Control and Preventionand by destruction of plague-infected animals. If a human case is diagnosed, health authorities must be notified promptly. All patients with suspected plague should be isolated, particularly if pulmonary involvement has not been ruled out. All specimens must be treated with extreme caution. Contacts of patients with suspected plague pneumonia should receive HYPERLINK "javascript:showDrugInfo(598);" tetracycline, as chemoprophylaxis.
A formalin-killed vaccine is available for travelers to hyper endemic areas and for persons at special high risk.Yersinia Enterocolitica & Yersinia Pseudotuberculosis
These are non-lactose-fermenting gram-negative rods , urease-positive and oxidase-negative. They grow best at 25 C and are motile at 25 C but nonmotile at 37 C. They are found in the intestinal tract of a variety of animals, in which they may cause disease, and are transmissible to humans .
Y enterocolitica has been isolated from rodents and domestic animals (eg, sheep, cattle, swine, dogs, and cats) and waters contaminated by them. Transmission to humans probably occurs by contamination of food, drink, or fomites. Y pseudotuberculosis occurs in domestic and farm animals and birds, which excrete the organisms in feces. Human infection probably results from ingestion of materials contaminated with animal feces. Person-to-person transmission with either of these organisms is probably rare.
Pathogenesis & Clinical Findings
An inoculum of 108109 yersiniae must enter the alimentary tract to produce infection. During the incubation period of 510 days, yersiniae multiply in the gut mucosa, particularly the ileum. This leads to inflammation ulceration and leukocytes appear in feces. Early symptoms include fever, abdominal pain, and diarrhea which is ranges from watery to bloody and may be due to an enterotoxin or to invasion of the mucosa. At times, the abdominal pain is severe and located in the right lower quadrant, suggesting appendicitis.
Diagnostic Laboratory Tests
Specimens
Specimens may be stool, blood, or material obtained at surgical exploration.
Culture
The number of yersiniae in stool may be small and can be increased by "cold enrichment": a small amount of feces or a rectal swab is placed in buffered saline, pH 7.6, and kept at 4 C for 24 weeks; many fecal organisms do not survive, but Y enterocolitica will multiply. Subcultures made at intervals on MacConkey agar may yield yersiniae.
Serology
In paired serum specimens taken 2 or more weeks apart, a rise in agglutinating antibodies can be shown; however, cross reactions between yersiniae and other organisms (vibrios, salmonellae, brucellae) may confuse the results.
Treatment
Most yersinia infections with diarrhea are self-limited, and the possible benefits of antimicrobial therapy are unknown. Y enterocolitica is generally susceptible to HYPERLINK "javascript:showDrugClass('aminoglycosides');" aminoglycosides, HYPERLINK "javascript:showDrugInfo(115);" chloramphenicol, HYPERLINK "javascript:showDrugInfo(598);" tetracycline, HYPERLINK "javascript:showDrugInfo(631);" trimethoprim- HYPERLINK "javascript:showDrugInfo(581);" sulfamethoxazole, HYPERLINK "javascript:showDrugInfo(490);" piperacillin, HYPERLINK "javascript:showDrugClass('third-generation%20cephalosporins');" third-generation cephalosporins, and fluoroquinolones; it is typically resistant to HYPERLINK "javascript:showDrugInfo(34);" ampicillin and to first-generation HYPERLINK "javascript:showDrugClass('cephalosporins');" cephalosporins.
Prevention & Control
Contact with farm and domestic animals, their feces, or materials contaminated by them probably accounts for most human infections. Conventional sanitary precautions are probably helpful. There are no specific preventive measures.