Applying Routh-Hurwitz conditions [5] the unique equilibrium of the above system is locally asymptotically stable if the following conditions are satisfied: For the special case r 1 = r 2 the above results simplifies significantly. The unique equilibrium solution become The existence and stability conditions are 5 a 1 > 0 , a 3 > 0 , a 1 a 2 > a 3 (7) a 1 = r 1 + r 2 + r 3 a 2 = r 1 r 2 + r 1 r 3 + r 2 r 3 − r 4 − r 5 a 3 = r 1 r 2 r 3 − r 1 r 4 − r 2 r 5 q 11 = q 22 = (1 − q 21 ) /r 1 (8) q 21 = ( r 1 − r 4 − r 5 ) / ( r 1 r 3 − r 4 − r 5 )