Etiologic agent = Leishmania chagasi
(new world) L.infantum (old world)
In humans, leishmaniasis occurs in 3 clinical forms: cutaneous,
mucosal/mucocutaneous and visceral. Different species of the Leishmania
protozoan parasite predominate in different parts of the world
and tend to be associated with specific forms of leishmaniasis.
Visceral leishmaniasis ("Kala-azar") has become
an important emerging disease in dogs and people throughout the
developing world, but particularly throughout the Mediterranean
countries, India and Latin America. Members of the L. donovani
complex of organisms cause visceral leishmaniasis - in the Mediterranean
region, L. infantum; in Latin America, L. chagasi.
- In the U.S., cases have historically been reported only sporadically
in a few states.
- In some instances, cases in the U.S. may reflect travel by
dogs to endemic areas, but in others it appears that the organism
exists locally.
- Since 1999, however, visceral leishmaniasis has been diagnosed
among dogs (predominantly hunting foxhounds) in 21 states and
2 provinces in Canada.
- The largest outbreak during this time involved dogs at a
hunt club in Millbrook, NY in 1999 in which 39/93 (42%) dogs
were seropositive, the organism was identified in biopsies or
aspirates of 15/15 dogs tested, and a number of dogs died.
In Latin America, L. chagasi is maintained in wild canid
reservoirs in nature, with spill-over into domestic dogs via the
sandfly vector. (Similarly, dogs are also suspected to be an important
reservoir of Leishmania braziliensis and peruviana,
the causative agents of primary cutaneous leishmaniasis in the
Americas.)
- Transmission between domestic dogs and from dogs-to-humans
occurs primarily via sandflies (Phlebotomus spp. in the
Old World; Lutzimyia spp. in the Americas). The Leishmania
organism replicates as a promastigote in the sandfly, then transforms
to the non-flagellated amastigote in the mammalian host and replicates
in macrophages (without being killed).
- There are at least 14 species of Lutzomyia phlebotomine
bugs in the United States. Even as far north as Wisconsin, they
may be able to overwinter in rodent burrows, and thus avoid freezing.
Three species have been implicated as possible vectors for Leishmania,
but none have been proven to be competent vectors.
- Rarely, infections in humans have been associated with direct
contact with an infected dog (i.e., without a sandfly vector),
but there are no proven cases of such transmission in the literature.
Direct dog-to-dog transmission via exchange of blood during fighting
or exudates from skin lesions has been suggested, and a recent
study (Owens et al. 2001) has proven direct Leishmania
transmission from dog-to-dog via transfusion with packed RBCs.
In addition, congential transmission has also been reported in
dogs.
- Interestingly, the widespread infections among foxhounds
in the U.S. have been due to infection with L. infantum,
typical of the Mediterranean region, rather than the Latin American
L. chagasi. It is also of interest that a survey of seropositivity
among dogs other than foxhounds in the U.S. in 2000-2001 revealed
an exceedingly low rate of seropositivity.
(Cats, coyotes, foxes, horses and rodents can also be infected,
but are rarely clinically affected.)
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