Database Reference
In-Depth Information
Sickle Cell (SS) Malaria Infection
S allele TR
Anemia DR
Infected Vector
INF LAG
SS S D
Trans SS
INF DR
SS births
SS I D
SS birth
SS INFECT
SS SUSC
SS INF
Anemia DR
SS IMMUNE
~
IMMUNITY LOSS
SS RECOV
SS IM LOSS
RECOV R
SS IM D
Anemia DR
Fig. 4.11
The source for each parameter value is given in the document section of the pa-
rameter menus. After entering parameter values from the literature, the normal TR
and S allele TR were determined empirically. First, we adjusted both equally to a
value that caused epidemics of reasonable size before the disease becomes endemic
as the model runs. When the two TRs were equal, the S allele frequency decreased
over time and eventually approaches zero. Then we reduced the S allele TR to ac-
count for the fact that carriers of this allele are more resistant to malaria. This TR
was adjusted to a value that resulted in 16% frequency of the S allele. With these
empirical values, any initial stock values can be used, and the model will come to
an equilibrium in the frequency of the S allele and the level of malaria infection.
Antimalaria drugs would decrease the death rate due to malaria and increase the
recovery rate, so in the model, we changed the INF DR from 0.1127 to 0.05 and
increased the RECOV R from 0.0125 to 0.1. This resulted in the S allelic frequency
reducing over time rather than leveling off at 16%. After 1,000 cycles, the frequency
is at 2.6%. This would be expected, since a reduced malaria infection level would
no longer make the individuals with the AS genotype have an advantage over the
individuals with the AA genotype (Figure 4.13). The corresponding numbers of
susceptible, infected, and immune is shown in Figure 4.14.
If sickle cell anemia treatment becomes available, affected individuals will live
longer and reproduce. If the Anemia DR is decreased, the equilibrium S allelic
frequency increases. Figure 4.15 shows a sensitivity run with the Anemia DR
