Ratio gametocytes/trophozoites

In an infected person, countless parasites can often be seen in the peripheral blood. Since gametocytes are the only forms which are responsible for transmission in nature, it is remarkable how few gametocytes are generally found. Often the number of gametocytes is only a small percentage of the number of trophozoites. Yet higher gametocyte densities must lead to higher transmission. Why gametocytes occur in such low proportions compared to the number of trophozoites is still not clear. Generally there are more female than male gametocytes, certainly early on in an infection. This is explained by the fact that one male gametocyte can form 8 viable male gametes, unlike the female gametocyte. This is in fact the case when infections are monoclonal, yet in mixed infections the ratio would have to be 1/1 according to population genetics. Later in the infection agglutinating antibodies are produced which immobilise male gametes and thus inhibit their function. Agglutination of female gametes has no effect on their function. To compensate for this the parasite produces more male gametocytes later in infection, possibly under the influence of increasing concentrations of erythropoietin. The latter hormone increases as anaemia increases. A single haploid clone of P. falciparum can produce both male and female gametocytes. Precisely how this works is not clear. Plasmodium falciparum gametocytes need 7-10 days for maturation. The sex ratio is determined by the erythropoietin content 7-10 days before they mature. It is interesting to note that P. falciparum exhibits increased infectibility in humans with sickle cell anaemia, regardless of the gametocyte density. This is explained by the chronically increased erythropoietin in this disorder, which leads to a higher percentage of male gametocytes. Nevertheless much study is still needed before the details will be fully understood.

Category: Medicine Notes