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Transmission Dynamics of Visceral Leishmaniasis in the Indian Subcontinent – A Systematic Literature Review

Background
As Bangladesh, India and Nepal progress towards visceral leishmaniasis (VL) elimination, it
is important to understand the role of asymptomatic Leishmania infection (ALI), VL treatment
relapse and post kala-azar dermal leishmaniasis (PKDL) in transmission.


Methodology/ Principal Finding
We reviewed evidence systematically on ALI, relapse and PKDL. We searched multiple
databases to include studies on burden, risk factors, biomarkers, natural history, and infectiveness
of ALI, PKDL and relapse. After screening 292 papers, 98 were included covering
the years 1942 through 2016. ALI, PKDL and relapse studies lacked a reference standard
and appropriate biomarker. The prevalence of ALI was 4–17-fold that of VL. The risk of ALI
was higher in VL case contacts. Most infections remained asymptomatic or resolved spontaneously.
The proportion of ALI that progressed to VL disease within a year was 1.5–23%,
and was higher amongst those with high antibody titres. The natural history of PKDL
showed variability; 3.8–28.6% had no past history of VL treatment. The infectiveness of
PKDL was 32–53%. The risk of VL relapse was higher with HIV co-infection. Modelling
studies predicted a range of scenarios. One model predicted VL elimination was unlikely in
the long term with early diagnosis. Another model estimated that ALI contributed to 82% of
the overall transmission, VL to 10% and PKDL to 8%. Another model predicted that VL
cases were the main driver for transmission. Different models predicted VL elimination if the
sandfly density was reduced by 67% by killing the sandfly or by 79% by reducing their
breeding sites, or with 4–6y of optimal IRS or 10y of sub-optimal IRS and only in low
endemic setting.

 

Conclusion/ Significance
There is a need for xenodiagnostic and longitudinal studies to understand the potential of
ALI and PKDL as reservoirs of infection.

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