A major obstacle to malaria eradication outside of Africa is P. vivax, because unlike P. falciparum, this malaria species can form latent hypnozoites in a persons liver that can reactivate months to years later causing blood- stage disease. Unfortunately the number of people latently infected with P. vivax remains unknown in the absence of a diagnostic test for latency, but with over 2.5 billion people exposed to P. vivax annually, the number is certainly high. Relapse is not only detrimental to the patient, but also poses the risk of re-initiating the transmission cycle in previously malaria-free areas. The development of a non-invasive diagnostic for latent P. vivax infection is challenging, since the latent hypnozoite form of the parasite resides in the liver, exists in small numbers and is not detectable in the human host. To address challenges such as this, we have developed a liver-chimeric humanized mouse model, which for the first time has allowed observation of P. vivax hypnozoites in vivo in a small animal model. This model serves as an attractive system for the identification of circulating biomarkers to detect hypnozoite infection. We propose to isolate exosomes, secreted nanovesicles 30-100 nm in size that are formed by numerous cell types and are excreted into various biological fluids, from the plasma of infected mice. We hypothesize that some of the exosomes will be derived from the P. vivax infected hepatocyte and will thus contain host- and parasite-derived proteins and nucleic acids that are indicative of latent P. vivax infection. We have indeed shown that parasite proteins can be observed in plasma-derived exosomes from mouse plasma when acute P. vivax liver infection is also present, but therfore could not differentiate between markers that were predictive of latent v. active infection. We will develop antibodies and nucleic acid probes that can be used to determine the specificity of biomarkers identified in mouse plasma specifically from mice with P. vivax latent infection. We will test these latent biomarkers for specificity of liver vs. blood-stage infection and specificity for P. vivax compared to P. falciparum. Lastly we will determine the specificity of identified biomarkers for Plasmodium infection versus infection by another hepatotropic pathogen and will conduct a pilot study to determine if these biomarkers can be observed in patients that have relapsed with P. vivax infection. The identification of a biomarker for P. vivax latent infection would have significant implications for the global goal of malaria eradication.
Relapsing malaria occurs when latent liver forms of the parasite reactivate and cause symptomatic disease months to years after the initial infection. There is currently no diagnostic tool to detect latent infection, therefore this proposal seeks to identify biomarkers of latent P. vivax infection which could be used to determine persons that are at-risk for relapse and could be treated with antimalarials. A biomarker would significantly advance progress towards malaria eradication.
