Of Cattle, Sand Flies and Men: A Systematic Review of Risk Factor Analyses for South Asian Visceral Leishmaniasis and Implications for Elimination

Abstract

BackgroundStudies performed over the past decade have identified fairly consistent epidemiological patterns of risk factors for visceral leishmaniasis (VL) in the Indian subcontinent.Methods and Principal FindingsTo inform the current regional VL elimination effort and identify key gaps in knowledge, we performed a systematic review of the literature, with a special emphasis on data regarding the role of cattle because primary risk factor studies have yielded apparently contradictory results. Because humans form the sole infection reservoir, clustering of kala-azar cases is a prominent epidemiological feature, both at the household level and on a larger scale. Subclinical infection also tends to show clustering around kala-azar cases. Within villages, areas become saturated over a period of several years; kala-azar incidence then decreases while neighboring areas see increases. More recently, post kala-azar dermal leishmaniasis (PKDL) cases have followed kala-azar peaks. Mud walls, palpable dampness in houses, and peri-domestic vegetation may increase infection risk through enhanced density and prolonged survival of the sand fly vector. Bed net use, sleeping on a cot and indoor residual spraying are generally associated with decreased risk. Poor micronutrient status increases the risk of progression to kala-azar. The presence of cattle is associated with increased risk in some studies and decreased risk in others, reflecting the complexity of the effect of bovines on sand fly abundance, aggregation, feeding behavior and leishmanial infection rates. Poverty is an overarching theme, interacting with individual risk factors on multiple levels.ConclusionsCarefully designed demonstration projects, taking into account the complex web of interconnected risk factors, are needed to provide direct proof of principle for elimination and to identify the most effective maintenance activities to prevent a rapid resurgence when interventions are scaled back. More effective, short-course treatment regimens for PKDL are urgently needed to enable the elimination initiative to succeed.

How effective is dog culling in controlling zoonotic visceral leishmaniasis? a critical evaluation of the science, politics and ethics behind this public health policy

“…Currently, India has been implementing the use of DDT in the State of Bihar since 1971, but control has not been achieved 32 , even though the country plans to eradicate the disease by 2015 33 . Some current studies have compared the effectiveness of residual spraying of insecticides with mosquito nets impregnated with long-term insecticides and with environmental modification, and the best results have been with indoor residual spraying 34,35 .…”

Leishmaniasis: A Review on Parasite, Vector and Reservoir Host

“…The human may inter this cycle in search of agricultural land, human settlement, timber production, road construction and other economic benefits in forests and other enzootic areas [49]. With regard to human settlement, the disease transmission tends to be high in agricultural villages where houses are frequently constructed with mud walls and earthen floors, and cattle and other livestock are kept close to human dwellings [50][51][52]. The human behavior of keeping domestic animals in the house and the surrounding attracts sand flies into human settlement, and therefore favor transmission of the disease to human [51].…”

“…With regard to human settlement, the disease transmission tends to be high in agricultural villages where houses are frequently constructed with mud walls and earthen floors, and cattle and other livestock are kept close to human dwellings [50][51][52]. The human behavior of keeping domestic animals in the house and the surrounding attracts sand flies into human settlement, and therefore favor transmission of the disease to human [51]. In endemic areas, building houses near the natural habitats of the vector and the reservoir hosts would increase human-sand fly contact, and hence increases the risk for human leishmaniasis [49].…”

Optimizing Insecticide Allocation Strategies Based on Houses and Livestock Shelters for Visceral Leishmaniasis Control in Bihar, India

“…5 The foregoing discussion verifies the importance of considering cattle sites in insecticide residual spraying (IRS) efforts. Previous studies 6 showed that spraying cattle sheds in Brazil caused increased sandfly density in unprotected human dwellings. The average number of annual VL cases in India was reported to be 28,382 cases in 2010 7 and the provisional number of kala-azar cases in 2011 was 31,000.…”