Evaluation of the effectiveness of vaccination as an intervention tool in the control of visceral leishmaniasis in Sudan

Abstract

Background: Visceral leishmaniasis (VL) is a parasitic vector-borne neglected tropical disease (NTD), transmitted by sand flies. VL is endemic in Sudan, where it is mainly caused by Leishmania donovani. The WHO targets in Sudan are 100% detection and treatment of cases by 2020. However, with current control measures an estimated 80% of the cases remain undetected and untreated. Therefore additional intervention measures seem necessary, such as active case finding or potentially a vaccine. In this study we analysed a longitudinal dataset from Mushrau Koka, a village in East Sudan, to obtain insight in 1) the immune response of people against L. donovani over time, 2) the risk factors for development of subclinical or clinical infection and 3) the possible protective effect of previous infections. The aim of this study was to investigate the potential effectiveness of a vaccine as an intervention tool for VL control in Sudan.

Methods: A longitudinal epidemiological study was conducted in Mushrau Koka, Sudan between 1994 and 1996, using 5 different surveys. Various data regarding VL were collected from 1142 persons, among others about age, sex, place of birth, weight, length, medical history, clinical signs and whether VL was diagnosed. All individuals were also tested for antibodies against Leishmania parasites by using the direct agglutination test (DAT, humoral immunity), and the leishmanin skin test (LST, cellular immunity).

Results: The current leishmaniasis control programme in Sudan is not sufficient to achieve the WHO targets of 100% detection and treatment. Currently, there are VL vaccines in development, but it can take a long time before effective vaccines are available for the field. If an effective human vaccine comes available, it will likely be implementable in the current vaccine program in Sudan. Clinical VL incidence was found to be higher in persons with a high DAT titer, (between 1:25600 and 1:102400), than in persons with a low (1:200 to 1:800) or medium (1:1600 to 1:12800) DAT titer. LST positivity was found to increase with higher age. Contrary to our expectations, in Mushrau Koka, LST positivity did not show a protective effect against developing VL (diagnosed in retrospect). VL incidence was found to be higher in men than in women. In literature we found that VL incidence is higher in people with a household member with VL. These risk factors could assist in formulating additional intervention tools for VL control in Sudan.

Conclusions: This study did not show a protective effect of previous exposure to L. donovani. Since it might also take a long time before an effective human vaccine comes available, we suggest that additional interventions should focus on screening people in high risk groups, such as house hold members of a clinical VL case, with a diagnostic test (DAT), and follow up individuals with a high DAT titer, to increase the detection and treatment rates in Sudan.