MODELING TRYPANOSOMA-CONGOLENSE PARASITEMIA PATTERNS DURING THE CHRONIC PHASE OF INFECTION IN NDAMA CATTLE

We reanalysed parasitaemia profiles of the trypanotolerant N'Dama catt le (Bos taurus), consecutively infected with the same four clones of T rypanosoma congolense. Our analysis shows that each individual parasit aemia is characterized by progressively longer intervals between paras ites waves. This pattern is most visible during the chronic phase of i nfection. In addition, the last of the four infections had a significa ntly larger overall duration of inter-wave intervals. We retrieved the se patterns by numerical simulations of a mathematical model, which in corporates assumptions about the molecular basis of antigenic variatio n and about the anti-parasitic major immune processes. Six potential f actors that may determine parasitaemia pattern were studied: carrying capacity of the host environment, intrinsic growth rate of the parasit e, affinity maturation of the immune response, immune cell birth and d eath rate, levels of antibodies to variant surface glycoprotein and le vels of antibodies to invariant antigens. Our simulations suggest that the first five factors are not likely to determine the chronic phase parasitaemia pattern whereas the sixth one, namely, antibody response to invariant antigens, yielded profiles consistent with the experiment al data. Being cumulative, the immune response to anti-invariant antig ens may be increasingly effective as infection proceeds and in success ive infections. Comparisons between N'Dama and Zebu and between chroni c and acute phases will be needed to make a statement on the role of t his phenomenon in trypanotolerance.