Middle East DVEP - Armed Forces Pest Management Board

from bites of infected fleas in southern areas, such as the Asir region in Saudi Arabia, andin Yemen. Risk in the rest of Saudi Arabia is restricted to the wettest months, generallyfrom March to May, when flea populations are most abundant.Transmission Cycle(s). Plague is a disease of rodents. It is maintained in nature amongwild rodents and their fleas (Figure 7). This zoonotic cycle is termed sylvatic,campestral, rural, or wild plague and can be very complex, involving manyrodent and flea species. Worldwide, over 220 species of rodents have been shown toharbor Y. pestis. Gerbils are important rodent reservoirs in the Middle East.Some rodents are highly susceptible to infection, resulting in high mortality. Althoughlarge numbers of dead and dying rodents are a good indication of an epizootic of plague,rodent species that are resistant to the effects of infection are more important inmaintaining the zoonotic cycle. Most cases in military personnel would probably occuras a result of intrusion into the zoonotic cycle during or following an epizootic of plaguein wild rodents. Domestic cats and dogs may carry infected rodent fleas into buildings ortents. Cats may occasionally transmit infection by their bites or scratches, or by aerosolwhen they have pneumonic plague. Troops should not be allowed to adopt cats or dogsas pets during military operations.The entry of wild rodents or their infected fleas into human habitations can initiate anepizootic among commensal rodents, primarily Rattus spp., which are highly susceptibleto infection. Close association of humans and large populations of infected commensalrodents can result in an urban cycle of plague. A similar cycle can occur in militarycantonments harboring large infestations of commensal rodents. The most importantvector of urban plague worldwide is the Oriental rat flea, Xenopsylla cheopis.Plague is transmitted to humans primarily by the bite of infected fleas. Fleas oftenexhibit a host preference, but most species of medical importance readily pass from onehost to another. A lack of absolute host specificity increases the potential for infectionand transmission of pathogens. Plague may also be acquired by handling tissues ofinfected animals and infected humans, and by person-to-person transmission ofpneumonic plague. Crushed infected fleas and flea feces inoculated into skin abrasionsor mucous membranes can also cause infection. Not all flea species are competentvectors. The vector competence of the Oriental rat flea is attributed to enzymes producedby the plague bacilli that cause blood to coagulate in the flea’s digestive tract. The fleaattempts to clear the blockage in its digestive tract by repeated efforts to feed. In theprocess, plague bacilli are inoculated into the host. Fleas may remain infective formonths when temperature and humidity are favorable. Xenopsylla cheopis may require 2to 3 weeks after an infective blood meal before it can transmit plague bacilli.Vector Ecology Profiles. Xenopsylla cheopis occurs mostly in urban areas, inassociation with its rodent hosts. However, it may occur sporadically in villages, whenrats are present, or in highlands, associated with gerbils. The distribution of the Orientalrat flea is determined by the distribution of its hosts, primarily R. rattus, R. norvegicus,Mus musculus, Meriones spp. and Psammomys spp. (gerbils). In the Middle East,Xenopsylla astia is found only in southern and eastern Iran and replaces X. cheopis as the98