Bio 119 PLAGUE 7/21/ Distinguish between sylvatic, bubonic, septicemic, and pneumonic plague.

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1 PLAGUE Reading in BOM-12: 35.7 Plague p Discussion Questions 1. Distinguish between sylvatic, bubonic, septicemic, and pneumonic plague. 2. Describe the Hemin Storage Phenotype (Hms) phenotype and its significance for plague. 3. Yersinia Murine Toxin (YMT) is an important gene product (i.e. protein) of Y. pestis. What is the function of YMT in the Y. pestis infective cycle? 4. Y. pestis requires several specific virulence factors for survival and growth in mammalian hosts. List as many as you can. HISTORY Plague is the quintessential epidemic infectious disease. According to some medical historians, responsible for more deaths than any other infectious disease in history. Historical Epidemics Biblical Philistines stole the Ark of the Covenant from the conquered Israelites; they were subsequently punished by the ravages of plague. This and numerous other accounts of plagues in classical antiquity are difficult to ascribe with certainty to any specific modern disease. 540 AD First epidemic outbreak in Europe to be generally accepted as plague in the modern sense. 14 th Century European Pandemic; est. 50 million deaths (1/4 population); triggered Renaissance according to some historians. Catapulting corpses of plague victims into besieged towns implies recognition of contagion. Continuous small-scale outbreaks and limited epidemic outbreaks in Asia 17 th C European pandemic; Defoe Journal of the Plague Year 1885 Chinese epidemic; 12 million deaths 1894 Hong Kong epidemic spread to French Indochina; etiology determined by Yersin and Kitasato related to French and Japanese Imperialism. 1 of 6

2 Yersinia is a genus of Gram-negative, facultatively anaerobic bacteria in the Family Enterobacteriaceae i.e. reasonably close relatives of E. coli. Pathogens include: Y. enterocolitica Free-living in soil and water. Zoonotic. Birds and mammals are reservoirs. Occasionally causes water or food borne enteric infections in humans following ingestion of contaminated water or food. Y. pseudotuberculosis Free-living in soil and water. Zoonotic. Birds and mammals are reservoirs. Occasionally causes water or food borne enteric infections in humans following ingestion of contaminated water or food. A very close genetic relative to Y pestis by DNA sequence homology. Some propose considering Y. pseudotuberculosis and Y. pestis as strains of the same species. Y. pestis Obligate flea-rodent-flea cycle, not free-living. (formerly Pasturella pestis) Plague is a classic example of a zoonosis, a disease of other animals that is sporadically transmitted to humans by incidental contact with infected wild animals or by vectors. Y. pestis is a natural pathogen of many wild and domestic rodents, particularly rats (i.e. sylvatic plague), marmots, prairie dogs, rabbits and ground squirrels. Hunters sometimes infected directly while dressing game. Chronic asymptomatic infection of rodents constitutes a permanent reservoir of infection. Many flea species, some specific for certain rodent hosts, transmit plague. The Rat flea (Xenopsylla cheopsis) has been the primary vector in large-scale epidemics. Typically fewer than 10 indigenous cases per year in US, mostly in SW. Death is in 3-5 days from onset of infection. Successful treatment by chloramphenicol or tetracyclines (natural resistance to penicillins) depends on early diagnosis. 2 of 6

3 Infection of the Flea Flea ingests blood meal from septicemic mammal. Establishment of Y. pestis in the flea requires ingestion of a large number of cells in the small meal volume (i.e. cell densities as high as 10 9 ml). This would select for high virulence in mammalian host (i.e. cell densities as high as 10 9 ml). Y. pestis is capable of avoiding clearance from the normally sterile midgut of the flea (due to YMT). Yersinia Murine toxin (Ymt) is a phosphlipase D (PLD) enzyme localized to the bacterial cytoplasm and coded by the gene ymt on the 100-kb plasmid pmt1 (=pcd1, = pfra), which is found in pathogenic strains of Y. pestis, but not in the closely related enteric pathogen Y pseudotubelculosis. Purified Ymt is highly toxic for mice and rats, and this suggested that Ymt contributes to the virulence of the pathogen for the mammalian hosts. However, ymt- mutant strains retain infectivity for mammals, but not for fleas. Moreover, expression of ymt is higher at room temperature than at 37 C. Therefore Ymt is now thought to be a virulence factor for infection of the flea host rather than the mammal. i.e. Mouse toxicity of injected purified Ymt is a red herring. Intracellular PLD activity potects Y. pestis from a cytotoxic digestion product of mammalian blood plasma produced in the flea gut, enabling bacterial colonization of the otherwise sterile flea midgut. The molecular mechanism by which PLD protects the flea is not known. It seems reasonable that acquisition of PLD (via aquisition pfra) may have initiated the evolutionary transition of Y. pestis to obligate arthropod-borne transmission. This transition probably occured within the past 20,000 years. Y. pestis multiplies to high density in flea gut without dissemination to other tissues. Bacterial cells aggregate in clumps, eventually blocking the proventriculus (between esophagus and midgut). Blockage of proventriculus causes flea death by starvation. Increases feeding behavior. This can be viewed as a MANIPULATION of the host to facilitate transmission. 3 of 6

4 Proventricular Blockage requires expression of Hemin Storage Phenotype (Hms) Required for proventricular blockage and transmission to mammal, but not for sustained infection of flea midgut, or for mammalian virulence. Adsorbtion of hemin and Congo Red to Yersinia outer membrane in artificial culture media when incubated < 30 degrees. (HMS expression is temperature-regulated.) 5 proteins are required for Hms+; 4 are located within a 100 kb Pathogenicity Island. Several of the Hms proteins are similar in sequence to proteins implicated in biofilm formation in other bacteria. i.e. extracellular polysaccharide biosynthesis Hms gene expression response to temperature is not regulated at transcription. Regulation mechanism unknown but probably related to protein stability. Hms is lost following large (100 kb) deletion mediated by recombination between two flanking IS100 elements. This region contains some genes necessary for Hms Phenotype as well as 4 of 6

5 pathogenicity-island containing genes for yersiniabactin (Ybt) iron transport system. Therefore, this deletion blocks transmission cycle in 2 ways. Attempts to feed dislodges bacteria from proventriculus, infecting site of wound. Fleas leave dead animal soon after death (catapulting corpses is probably not effective). Not all infected rats die, some develop chronic asymptomatic infections and act as carriers (reservoirs of infection). Rat abatement measures, and high rodent mortality due to plague, may increase rates of human infection as fleas seek alternate hosts. BUBONIC PLAGUE SEPTICEMIC PLAGUE PNEUMONIC PLAGUE Y. pestis colonizes human lymph nodes, particularly those near site of infection, often the armpit and groin. Bacterial growth causes lymph node swelling to produce lumps or buboes. Infection spreads to other nodes and progresses to general septicemia. General disseminated infection of circulatory system. Typically develops after bubonic phase. Multiple subcutaneous hemorrhages cause dark blotches (Black death). Pneumonic plague is a form of the disease involving respiratory tract infection either by direct respiratory infection or by spread from circulation. Bloody sputum in last 2 days before death is highly contagious, requiring quarantine. Virulence Factors in Mammals Cytotoxic Proteins for phagocyte killing (70kb Plasmid) Although Y. Pestis is able to survive intracellular killing in phagocytes, most growth is extracellular. Pla protease; active > 35 de.; cleaves plasminogen; required for dissemination from wound (10kb Plasmid) Extracellular capsule required to avoid phagocytosis. Non-encapsulated mutants are avirulent. (100kb Plasmid) V and W antigens, protein/lipoprotein complexes in outer membrane Pathogenicity-islands (chromosomal) containing genes for yersiniabactin (Ybt) iron transport system and for HMS. 5 of 6

6 Summary of Patogenicity Factors in Y. pestis GENETIC ELEMENT CHROMOSOME (Ybt Patogenicirty Island) 100 kb pmt1 (=pmt1, = pfra) 100kb pyv 70 kb DISTRIBUTION Y. pestis only Y. pseudotuberculosis Y. pestis FLEA-SPECIFIC HMS (Hemin Storage Phenotype) MAMMAL-SPECIFIC YBT (Yersiniabactin Iron Transport) Ymt (Yersinia Murine Toxin) Caf1 (anti-phagocytic capsule) Cytotoxic Proteins (phagocyte killing) ppla 10 kb Y. pestis only Pla protease Plague is one of the diseases physicians are required to report to public health officials and the CDC. Plague is considered a potential candidate for biowarfare/bioterrorism. 6 of 6