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

TULAREMIA 279Figure 19. Tularemia. Mode of transmission in the Americas.Infected wildanimals: septicemicwild lagomorphs androdents (Sylvilagus spp.,Castor canadensis,muskrats,Ondatra zibethicus)Tick biteTicksBiteSusceptible wildanimals: septicemicwild lagomorphs androdents (Sylvilagus spp.,Castor canadensis,muskrats,Ondatra zibethicus)Handling game animals,ingestion of contaminatedwater and meat, aerosolsBiteBiteBiteManSheepManDogsing the infection in nature. It is a matter of debate whether very susceptible lagomorphsand rodents (group 1) are true reservoirs or only amplifiers and the mainsource of infection for man. Less susceptible animals (group 2), together with ticks,are thought to be important reservoirs.Domestic animals, such as sheep and cats, are accidental hosts, but they may alsoconstitute sources of infection for man.Humans contract the infection upon entering the natural foci of tularemia. Thesources of infection and modes of transmission of the causal agent are many. InNorth America, the animals that most frequently serve as the source of infection forman are wild rabbits (Sylvilagus spp.), hares (Lepus californicus), beavers (Castorcanadensis), muskrats (Ondatra zibethicus), meadow voles (Microtus spp.), andsheep. The biovar tularensis is generally transmitted by wild rabbits or by their ticks(Dermacentor variabilis, D. andersoni, Amblyomma americanum). The biovarpalaearctica is more common among rodents, particularly aquatic rodents, but alsoin some species of lagomorphs, such as Lepus europaeus and L. variabilis.Tularemia in Sweden is transmitted from L. variabilis by means of mosquitoes. TheEuropean hare plays no role in Sweden, but it does in other European countries.Rodents such as beavers and muskrats are important in aquatic cycles. In differentecological areas, other ticks (e.g., Ixodes spp., Haemophysalis spp.) and arthropodsare also involved. In many enzootic areas, the principal route of penetration isthrough the skin (by means of hematophagous arthropods, scratches, or knife cuts).Another portal of entry is the conjunctiva, which can be contaminated by materialssplashed into the eyes or, in the case of hunters or sheep shearers, by hands soiledfrom handling sick animals. Infection via the oral route occurs as a result of ingestingwater contaminated by dead animals or the urine and feces of infected animals,or by eating undercooked meat of lagomorphs or other infected animals. In addition,