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

PLAGUE 213Natural infection in cats has come under close scrutiny, as they have been a sourceof infection for man in several instances. Feline plague is characterized by formationof abscesses, lymphadenitis, lethargy, and fever (Rollag et al., 1981). Secondary pneumoniamay also be present, as in the case at Lake Tahoe, California, where a kittentransmitted the infection to a man by aerosol. Fatality is over 50% in cats infectedexperimentally. In contrast, dogs inoculated with the plague agent react only withfever. Other carnivores are not very susceptible, with the exception of individuals withgreater than normal susceptibility, as might be expected in any animal population.Natural infection has been recorded in camels and sheep in the former SovietUnion and Libya (Christie et al., 1980) and, more recently, in camels from SaudiArabia (A. Barnes, personal communication).Source of Infection and Mode of Transmission (Figure 15): Wild rodents arethe natural reservoir. The maintenance hosts vary in each natural focus, but they arealmost always rodent species with low susceptibility, i.e., the animals becomeinfected but do not die from the disease. Very susceptible species, in which manyanimals die during an epizootic, are important in amplification and diffusion of theinfection as well as in its transmission to man, but they cannot be permanent hosts.The epizootics that afflict prairie dogs (Cynomys spp.) are devastating. In one epizootic,only some animals survived in two of seven colonies. Another explosive epizooticannihilated an entire colony of 1,000 to 1,500 animals in two months. A thirdepizootic reduced the population by 85% (Ubico et al., 1988). R. rattus is very susceptible,but the infection usually dies out rapidly in this species. Only in some circumstances,as occurred in India, can it serve as a temporary host, but not for manyyears. Consequently, the persistence of a focus depends on rodent species that havea wide spectrum of partial resistance.In a natural focus, the infection is transmitted from one individual to another byfleas. Different species of fleas vary greatly in their efficiency as vectors. BiologicalFigure 15. Plague. Domestic and peridomestic transmission cycle.InfectedrodentsTick biteXenopsylla cheopis(rat flea)BiteSusceptiblerodentsBiteMan