Development of culture-based serological assays to diagnose Babesia divergens infections

Babesioses are hematic tick-borne diseases that induce malaria-like disorders in domestic, wild animals, and humans. Although indirect fluorescent antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA) commercial kits are available to test the presence of antibodies against most Babesia species, no kit exists to serologically diagnose the infections due to Babesia divergens, one of the most important zoonotic species. To fill this gap and to develop assays to detect animal and human infections, in vitro cultures (microaerophilous stationary phase system) of B. divergens were organized. Infected erythrocytes were adsorbed as corpuscular antigen (CA) on IFAT slides and ELISA microwells. The supernatant medium of the cultures (metabolic antigen, MA) was collected and employed in ELISA and western blot (WB) assays. B. divergens was also used to produce positive sera in Meriones unguiculatus and to infect a calf. Serological tests were set up with sera from experimentally/naturally infected animals, and possible cross-reactions were evaluated using heterologous sera from cattle positive to other piroplasms. Sera from clinically healthy people at risk of infection were also tested. As expected, assays based on the purified MAs from in vitro cultures proved more sensitive and specific than CA-IFAT and CA-ELISA. In fact, MA-ELISA provided satisfactory performances (even if 8.4%-15.7% cross-reactions were evidenced), and the WB developed proved totally sensitive and specific. WB indicated as immunodominant antigens two major protein bands at 33 and 37 kDa, which were also evidenced in 2.2% of the human sera tested, proving the parasite transmission to humans also in Italy.