zoonoses and communicable diseases common to ... - PAHO/WHO

PLAGUE 211at home by the peasants. One factor that helped to increase the number of cases wasthat rodenticides were used without simultaneous or prior use of flea pulicides(Report from Dr. Alfonso Ruiz to the Pan American Health Organization, February8, 1994).Occurrence in Animals: Natural infection by Y. pestis has been found in 230species and subspecies of wild rodents. In natural foci, sylvatic plague is perpetuatedthrough the continuous circulation of the etiologic agent, transmitted by fleasfrom one rodent to another. It is generally believed that the survival of the etiologicagent in a natural focus depends on the existence of rodent species, or individualswithin a species, with differing levels of susceptibility. The most resistant individualsare host to and infect the fleas, which in turn infect susceptible animals in thearea and can spread to domestic rodents. Susceptible animals generally die, but theyincrease the population of infected fleas by means of their bacteremia. When thenumber of susceptible individuals is large and climatic conditions favorable, an epizooticmay develop in which many rodents die. As the epizootic diminishes, theinfection continues in enzootic form in the surviving population until a new outbreakoccurs. Infection may remain latent in enzootic foci for a long time, and theabsence of human cases should not be interpreted as a sign that the natural focus iseliminated.During the period 1966–1982, 861 isolates were taken of Y. pestis in foci in northeasternBrazil. Of these, 471 were from rodents or other small mammals, 236 werefrom batches of fleas, 2 from batches of Ornithodorus, and 152 from patients. In therodents, the highest number of isolates were taken from Zygodontomys lasiarus pixuna,which also provided the highest number of fleas, primarily of the genusPolygenis; on only one occasion was the agent isolated from cat fleas(Ctenocephalides felis). The agent was isolated from human fleas (Pulex irritans)found on the floor of dwellings on 10 occasions. Human flea infection suggests thepossibility of human-to-human transmission through flea bites, usually after a fatalcase in the family (Almeida et al., 1985).House cats that come into contact with rodents and/or their fleas can becomeinfected and fall ill, and can transmit the infection to man. In the US and SouthAfrica, several cases of the disease in cats have been described (Kaufmann et al.,1981; Rollag et al., 1981). In New Mexico (USA), 119 cases of plague werereported in domestic cats from 1977 to 1988 (Eidson et al., 1991). There is also evidencethat camels and sheep in enzootic plague areas can contract the infection andthat, in turn, man can become infected when sacrificing these animals. Such casesoccurred in Libya (Christie et al., 1980).The Disease in Man: The incubation period lasts from two to six days, though itmay be shorter. Three clinical forms of plague are recognized: bubonic, septicemic,and pneumonic. The symptoms shared by all three are fever, chills, cephalalgia, nausea,generalized pain, diarrhea, or constipation; toxemia, shock, arterial hypotension,rapid pulse, anxiety, staggering gait, slurred speech, mental confusion, andprostration are also frequent.Bubonic plague—the most common form in interpandemic periods—is characterizedby acute inflammation and swelling of peripheral lymph nodes (buboes),which can become suppurative. There may be a small vesicle at the site of the fleabite. The buboes are painful and the surrounding area is usually edematous.