Selenium (Se) is an essential trace element for several organisms and is present 
in proteins as selenocysteine (Sec or U), an amino acid that is chemically 
distinct from serine and cysteine by a single atom (Se instead of O or S, 
respectively). Sec is incorporated into selenoproteins at an in-frame UGA codon 
specified by an mRNA stem-loop structure called the selenocysteine incorporating 
sequence (SECIS) presented in selenoprotein mRNA and specific selenocysteine 
synthesis and incorporation machinery. Selenoproteins are presented in all 
domains but are not found in all organisms. Although several functions have been 
attributed to this class, the majority of the proteins are involved in oxidative 
stress defense. Here, we discuss the kinetoplastid selenocysteine pathway and 
how selenium supplementation is able to alter the infection course of 
trypanosomatids in detail. These organisms possess the canonical elements 
required for selenoprotein production such as phosphoseryl tRNA kinase (PSTK), 
selenocysteine synthase (SepSecS), selenophosphase synthase (SelD or SPS), and 
elongation factor EFSec (SelB), whereas other important factors presented in 
mammal cells, such as SECIS binding protein 2 (SBP) and SecP 43, are absent. The 
selenoproteome of trypanosomatids is small, as is the selenoproteome of others 
parasites, which is in contrast to the large number of selenoproteins found in 
bacteria, aquatic organisms and higher eukaryotes. Trypanosoma and Leishmania 
are sensitive to auranofin, a potent selenoprotein inhibitor; however, the 
probable drug mechanism is not related to selenoproteins in kinetoplastids. 
Selenium supplementation decreases the parasitemia of various Trypanosome 
infections and reduces important parameters associated with diseases such as 
anemia and parasite-induced organ damage. New experiments are necessary to 
determine how selenium acts, but evidence suggests that immune response 
modulation and increased host defense against oxidative stress contribute to 
control of the parasite infection.