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201.15.9 Plague One of the most horrific bacterial diseases transmitted by insects is the plague. It has several names including, bubonic plague, Black Death, black plague or just plain plague. It has been the scourge of Humans for several thousand years. Plague vectors Several species of fleas can transmit the plague. Hosts Rodents are the primary host but under the appropriate conditions, humans can also serve. Pathogen The pathogen is a gram negative bacteria with the appropriate name of, Yersinia pestis. Life cycle of Yersinia pestis Rodents are the primary host for pathogen and are too killed by the disease. The infected flea, needing more blood, moves off the cooling body to find another host. While the flea is searching, the pathogens are reproducing in the gut. Life cycle of Yersinia pestis To release nutrients from the red blood cells, they must be broken. This is accomplished by the proventriculus – the grindomatic of the insect gut. Hemoglobin from the RBCs, bacteria and other debris begin to accumulate on the proventriculus. Eventually, the proventriculus becomes blocked as does the rest of the gut. The flea continues to feed and the blood accumulates in the esophagus. The esophagus expands like a balloon but it can expand only so far. The elasticity of the crop expels the bacteria back into the host. The blockage drives the flea wild with hunger since it cannot get food into the midgut. It moves around biting furiously and at each bite, more pathogens are transmitted. To cause the disease, about 25 pathogenic organisms are needed, but when the flea expels the gut contents, there are about 100,000 bacteria introduced into the host. There is cosmic justice as the flea also dies, not from the disease, but from hunger. Flea proventriculus The image on the left side shows the proventriculus. It is a series of sharp spines encased in a muscular capsule. It connects the mouth to the midgut and is responsible for breaking red blood cells to release their nutrients. The image on the right is a colorful close-up of the spines. The detritus on the spines are bacteria and broken red blood cells.Symptoms The flea usually bites areas in the armpit or groin and the injected pathogens move via the circulatory system to the nearest lymph gland. There is an incubation period from two to five days and then painful lumps develop. This swollen lymph gland is called a bubo. The strange word comes from the Greek for groin. Bubo The lymph glands swell further due to the increase in bacteria. The swollen lymph gland then allows the bacteria to re-enter the blood stream and transit to the liver. Symptoms When the bacteria enter the blood stream, there is a tremendous amount of hemorrhaging under the skin which gives the victim a darker color. This is due to the blood leaking from capillaries. If you have ever pinched yourself and formed a “blood blister” you have an idea of the color. Symptoms Note this victims fingers. The bacteria produce several toxins that are the actual killing agents. These toxins work on mitochondria to inhibit ion pumps which are responsible for molecular respiration. DEV: this audio appears missing. Pneumonic Plague The plague comes in several forms. First, there is the transmission from the flea to the human. Once the contagious stage is reached in humans, it can then be spread by coughing. This is the most fatal form of the disease. There is not even time for the bubos to form as the individual is dead within several days. Distribution of the plague Geographically, the disease is world-wide but has areas that are endemic including Africa, Latin America, Southeast Asia and Russia. The western U.S. also harbors the pathogen. Plague in the U.S. Plague distribution in the U.S. centers mainly in the southwest and scattered pockets along the west coast. Contracting plague in the U.S. The likelihood of catching the disease is very small unless you come into frequent contact with rodents. Americans who have recently contracted the disease are usually hunters who skin animals and may have open wounds on their hands through with the pathogen can enter. The current world cases stand at about 3000/yr depending upon the level of global violence. As with a number of the other diseases, civil strife leads to increased outbreaks. For example, in1966, there were 353 cases of plague in Viet Nam. The next year, it jumped to 4,700 cases as the fighting

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UW-Madison ENTOM 201 - Plague

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