Dogs are the domestic reservoirs of zoonotic visceral leishmaniasis caused by Leishmania
infantum. Early detection of canine infections evolving to clinically patent disease may be
important to leishmaniasis control. In this study we firstly investigated the peripheral blood
mononuclear cell (PBMC) response to leishmanial antigens and to polyclonal activators
concanavalin A, phytohemagglutinin and pokeweed mitogen, of mixed-breed dogs with natural
L. infantum infection, either in presymptomatic or in patent disease condition, compared to healthy
animals. Leishmania antigens did not induce a clear proliferative response in any of the animals
examined. Furthermore, mitogen-induced lymphocyte proliferation was found strongly reduced not
only in symptomatic, but also in presymptomatic dogs suggesting that the cell-mediated immunity
is suppressed in progressive canine leishmaniasis. To test this finding, naive Beagle dogs were
exposed to natural L. infantum infection in a highly endemic area of southern Italy. Two to 10
months after exposure all dogs were found to be infected by Leishmania, and on month 2 of
exposure they all showed a significant reduction in PBMC activation by mitogens. Our results
indicate that suppression of the lymphoproliferative response is a common occurrence in dogs
already at the beginning of an established leishmanial infection. # 1999 Elsevier Science B.V. All
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