Sago spleen is seen in -
Appearance of Spleen with deposits of Amyloid in the stroma of the red pulp is known as lardaceous spleen and within the stroma of the white pulp is known as sago spleen.
                                                           Spleen involvement in Amyloidosis
            ↓                                                                                                                              ↓
Sago Spleen                                                                                                    Lardaeeous Spleen
Splenomegaly is not marked                                                                Splenomegaly Marked (Moderate to large spleen)
Cut surface lookalike it is impregnated with cranes                             Cut surface shows a diffuse large map like the area of pale waxy translucency and hence called Lardaeeous (Lard like; Fatty)
of sand (nodules resembling tapioca-like granules)                                Deposits of Amyloid in Red pulp (Connective tissue and splenic sinus spare the follicles
i.e. sago grains and hence called Sago's spleen.
Deposition of amyloid in the while pulp walls of 
the arterioles replace the follilcle
Amyloidosis
Gaucher’s disease
Malaria
Felty's disease