The tree rat, Thamnomys surdaster, is highly susceptible to Plasmodium berghei. Ninety percent of all laboratory-bred or wild Thamnomys inoculated with one million parasitized cells succumbed to their progressive plasmodial infection within 11 to 15 days. Innate immunity to small inocula of P. berghei is encountered among individual Thamnomys. This natural immunity is quantitative in its nature and an increase in the strength of the inoculum overcomes it.
The reaction of the golden hamster towards P. berghei greatly resembles that of Thamnomys in the course of the plasmodial infection, in the individual innate quantitative immunity encountered and in the physiological ripening of the gametocytes.