Histomonas meleagridis (Parabasala, Trichomonadea, Monocercomonadidae): presence of natural agglutinins in horse serum

Cultured Histomonas meleagridis cells were readily agglutinated in vitro by horse serum at concentrations as low as 5%, although clumping was more rapid and prominent at 15% or higher. For observation of clumping, the cultured organisms were washed twice in Hanks balanced solution (HBSS) by centrifugation (1,000×g for 15 min) and filtered through glass wool. The test sera were added and the mixture incubated in a Petri plate or 24-well culture plates at r.t. for 15–30 min. Formation of clumps was time- and concentration-dependent. Gentle agitation hindered agglutination at low serum concentration and accelerated agglutination at higher concentrations. The agglutinating factor (AF) was detected in several batches of serum from different sources, regardless of whether sera were heat-treated to inactivate complement. Histomonads were not clumped by either fetal horse or bovine serum (5–30%). Neither chicken nor turkey serum agglutinated histomonads to the extent seen with horse serum. Immune turkey serum lysed histomonads, hindering observation of clumping. Complement inactivation of immune serum slightly reduced lysis. AF in horse serum was precipitated with 25–40% ammonium sulfate and was active when cleaned by dialysis and reconstituted in HBSS. Clumping by serum facilitated the cleaning of histomonads for other studies where pure suspensions were needed.