The direct agglutination test: a non-specific test specific for the diagnosis of visceral leishmaniasis?

Experimental Evidence on the Nature of the Antigen in the Direct Agglutination Test for Visceral Leishmaniasis

The direct agglutination test (DAT) for visceral leishmaniasis (VL) is the serodiagnostic test for VL that has the most robust sensitivity and specificity in the field across all endemic regions. It is based on trypsin-treated and formaldehyde-fixed whole promastigote cells from Leishmania donovani. The exact identity and nature of the epitopes on the DAT antigen that cause agglutination with VL patients' sera are currently unknown. In this study, we performed antigen-inhibition studies which revealed that lipophosphoglycan (LPG) and the DAT antigen share epitopes. Antibody inhibition with a monoclonal antibody directed against the phosphoglycan repeat epitope of LPG showed that this is not the epitope that reacts with human sera. Oxidation of carbohydrates by sodium metaperiodate did not alter the reactivity of human sera with the DAT antigen and LPG. This indicates that carbohydrates do not play a role in the reaction of the DAT antigen with antibodies in serum from VL patients, and that they also are not involved in the reaction of LPG with the same serum. We conclude that the noncarbohydrate moiety of LPG, that is, the core-anchor fragment, and potentially other noncarbohydrate epitopes on the surface of the DAT antigen are responsible for its agglutination with antibodies from VL patients. As LPG plays a role in the DAT reaction, this could facilitate the following: 1) incorporation of LPG, preferably the synthetic version of the core-anchor fragment, into an immunochromatographic test format that is more adapted as a point-of-care test (short incubation, little training, and equipment needed) than DAT and 2) enhancing the quality control for the production of the DAT antigen.

The Unknown Nature of the Antigen in the Direct Agglutination Test for Visceral Leishmaniasis Hampers Development of Serodiagnostic Tests

Current diagnostic tests for visceral leishmaniasis (VL) are either not adapted for use in resource-poor settings or are insufficiently accurate in Eastern Africa. Only the direct agglutination test (DAT), based on whole Leishmania promastigotes, is highly reliable in all endemic regions, but its implementation is hampered by the need for a cold chain, minimal laboratory conditions, and long incubation times. Integrating the DAT antigen(s) in an immunochromatographic rapid diagnostic test (RDT) would overcome these disadvantages. Unfortunately, the identity of the DAT antigen(s) involved in the agglutination reaction is unknown. For this study, we reviewed all publications that might shed some light on this issue. We conclude that the DAT antigen is a mixture of Leishmania-specific epitopes of protein, carbohydrate, and lipid nature. To develop an accurate RDT for VL diagnosis in Eastern Africa, we suggest to complement the classical protein antigen discovery with approaches to identify carbohydrate and lipid epitopes.

A new PCR-ELISA for diagnosis of visceral leishmaniasis in blood of HIV-negative subjects

The PCR-ELISA represents a promising advance for diagnosis of visceral leishmaniasis (VL) in blood samples. However, the method has been validated mostly with HIV-positive patients who are known to have high levels of parasitaemia. We developed a new PCR-ELISA assay for specific detection of Leishmania in patients' blood and validated it in Nepalese subjects with clinically suspected VL, almost all of whom were HIV-negative. For blood samples, PCR-ELISA was more sensitive (83.9%) than conventional PCR (73.2%), and demonstrated 100% and 87.2% specificity when using healthy controls who had never travelled to a VL-endemic area and controls from a VL-endemic area as references, respectively. We have demonstrated the ability of PCR-ELISA to detect parasites in blood of HIV-negative patients. The method could be used for epidemiological as well as clinical purposes, as it reduces the need for traumatic bone marrow sampling and risky spleen aspiration.

Pediatric visceral leishmaniasis in Albania

OBJECTIVE

Visceral leishmaniasis (VL) in children is endemic in southern Europe but has not been previously reported from Albania. This prospective study reports the clinical and laboratory findings in 50 children with visceral leishmaniasis, the value of a direct agglutination test (DAT), and the result of treatment with meglumine antimonate.

MATERIALS AND METHODS

Sera obtained from 50 children with VL confirmed by bone marrow examination, 40 household contacts, and 30 hospitalized children with other infections were examined using DAT.

RESULTS

Clinical features included fever (100%), hepatosplenomegaly (100%), pallor (100%), weight loss (98%), vomiting (68%), diarrhea (32%), and bleeding disorders (8%). Laboratory findings were anemia (94%), neutropenia (85%), hypergammaglobulinemia (70%), and thrombocytopenia (22%). Thirty children who developed secondary bacterial infections had significantly lower hemoglobin and neutrophil counts (P<0.0001). Direct agglutination test had a sensitivity of 98%, a specificity of 100%, and a positive predictive value of 100%. One child with severe generalized bleeding died within 48 hours of admission before receiving treatment.

CONCLUSION

The direct agglutination test was highly valuable in diagnosis of VL in this series. Meglumine antimonate was an effective therapeutic agent. Post-treatment bone marrow examination confirmed recovery in all patients. There were no relapses of VL during one-year follow up.