Evidence Against Rapid Emergence of Praziquantel Resistance in Schistosoma haematobium, Kenya

Charles H. King, Case Western Reserve University School of Medicine and University Hospitals of Cleveland, Cleveland, Ohio, USA; Eric M. Muchiri, John H. Ouma, Ministry of Health, Nairobi, Kenya

Disclosures

Emerging Infectious Diseases. 2000;6(6) 

In This Article

Discussion

Without clear evidence of emerging resistance in our treated population, we considered two related questions: Why was resistance not observed, and when might it be expected to emerge under the field conditions tested? To answer these questions, we turned to mathematical models of inheritance of drug resistance.

A number of factors may account for the lack of evidence of praziquantel resistance in the Msambweni project. Although S. haematobium resistance to praziquantel is likely to emerge at some future time, the interval required for detection of resistance may be much longer than our 8-year observation period. An effective praziquantel-resistance mutation may be so rare that the number of generations required for it to become the dominant phenotype have not yet occurred. In Egypt, for example, apparently resistant strains of S. mansoni are emerging >10 years after widespread availability of praziquantel treatment (32). For a single resistance gene mutation beginning at frequency of 10-6, a Hardy-Weinberg equilibrium analysis (28) predicts it would take eight or more generations for the resistant phenotype to become clinically detectable (i.e., 25% to 50% of worms) at the community treatment rates used in this study, if the resistance gene heterozygotes are fully resistant to treatment (i.e., a dominant trait [Figure 4a]). In our study, we estimate that 25% to 50% of infections were treated, but because the targeted school-age groups have the highest numbers of worms, 50% to 75% of worms may have been exposed to praziquantel. Emergence times for the resistance phenotype are longer if heterozygotes remain fully or partially susceptible to the drug (11 generations for 25% heterozygote loss after treatment and 14 to 18 generations for 50% heterozygote losses) and only the homozygotes are fully resistant (Figure 4b). Given the seasonal nature of rainfall and water exposure, the requirement of two sexes for schistosome reproduction, and the uneven aggregation of worms in the human population, the effective generation time for S. haematobium is likely to be at least 6 to 12 months, so we observed no more than 8 to 16 generations; our observation period may have been too short to identify praziquantel resistance. Our annual follow-up was an insensitive means to detect drug failure due to resistance. However, under the pressure of continued praziquantel treatment in the community, if resistant worms are fully fit, rapid predominance of resistant strains would be expected (Figure 4).

Figure 4. a. Hardy-Weinberg equilibrium analysis of the increase in resistance gene frequency in a parasite population where the initial R gene frequency is 10-6, heterozygotes and R gene homozygotes are fully resistant, and 75% of susceptible worms are lost to treatment each generation. b. As in a, but 40% of heterozygotes are lost to treatment each generation. c. As in a, but 99% of resistant homozygotes do not survive to reproduce.

Praziquantel failure on initial treatment in S. mansoni- endemic areas of Senegal suggests primary resistance to praziquantel[16,18] and a high prevalence of resistance genes in the local S. mansoni strain (21,33,34). More recent reports indicate, however, that in this area of Senegal, retreatment after 40 days adequately reduces infection levels and achieves better cure rates[19]. The latter results suggest that immature schistosomes, which are known not to be susceptible to praziquantel, are typically present in Senegalese patients at the time of treatment. If the treated patient has been recently exposed to infection (<=6 weeks ago) in an area with continuous--rather than seasonal--transmission, apparent treatment failure may be observed when unaffected juvenile worms reach maturity and pass eggs several weeks after praziquantel treatment[14]. Annual reinfection rates for S. haematobium may be high in some areas, such as Niger[35], and post-treatment infection detected after a single round of therapy should not be immediately interpreted as evidence of praziquantel resistance.

Nevertheless, animal studies of S. mansoni strains from Senegal indicate they are less sensitive than strains from other parts of the world[21,33,34]. There is another explanation for an apparent low-level prevalence of drug resistance that fails to predominate in the parasite population: if a praziquantel-resistance mutation compromises reproductive fitness, the homozygous, fully resistant worm will never predominate. Instead, under the pressure of continued treatment, the resistance gene will achieve a stable-equilibrium share of the worm population, at a level dependent on its lower survival efficiency relative to that of the praziquantel-sensitive genotype (Figure 4c). This situation is analogous to the effect of the sickle cell-hemoglobin gene in human populations exposed to malaria.

Figure 5. Three-dimensional graph of estimated mean community level of infection over time (20 years) as a function of the proportion of infections treated each year (20% to 100%). Results are obtained from the model described in the appendix. Resurgence of infection (due to emergence of resistance) was predicted to occur soonest (approximately 7 years) when yearly treatment coverage was greatest (100%). Under the conditions of our study (25% to 75% coverage), detectable increases in community level of infection (due to resistance) were estimated to take a minimum of 10 to 15 years.

Another possible factor likely to be slowing the emergence of praziquantel resistance in Schistosoma is the parasite's obligate dioecious sexual reproduction. Unlike drug-resistant bacteria, praziquantel-resistant schistosomes must find a mate of the opposite sex to reproduce, which requires a sufficient density of human infection. In a disease-endemic area where most of the population has been treated, the initial heavy loss of susceptible worms (>80%) may actually reduce mean number of worms sufficiently (i.e., to fewer than one male and one female per host) to prevent most resistant worms from finding suitable mates[29]. Worm distribution is highly aggregated in the human population, with most (75%) patients having light infections and a small proportion (approximately 5%) having heavy infections. Heavy infection can result in reduced worm fecundity, slowing the production of eggs and thus the transmission of genes from a resistant worm, even though the chances of mating are enhanced. After treatment, the reduced number of worms in a heavily infected human could increase fecundity, so the net effect of treatment on praziquantel-resistance gene transmission would be difficult to predict. In an age-targeted program such as ours, praziquantel-sensitive parasites would also persist in the untreated adult and infant human subpopulations, slowing the dominance of drug resistance gene(s). This would occur by allowing interbreeding of resistant and susceptible worms in host locations not having the environmental pressure (i.e., praziquantel treatment) that favors the resistance gene.

Using a dynamic model of parasite transmission that accounts for these reproductive features of macroparasite transmission (Appendix), we examined the impact of treatment coverage and duration of control programs on the emergence of resistant infection, to estimate the time required for drug failure. We also examined the impact of varying reproductive fitness of the resistant worms on the time to recrudescence of infection. Figures 5 and 6 show the predicted recrudescence of infection over a 20-year period, as influenced by treatment coverage and by reproductive fitness of resistant worms. Both simulations indicate at least an 8- to 10-year delay in the return of infection to pretreatment levels. This delay is especially noted if treatment coverage is incomplete (Figure 5) and resistant parasites are less reproductively fit than susceptible ones (Figure 6). The model further indicates that under the conditions of our program (targeted mass treatment, with 25% to 75% of infections praziquantel-treated), resistance becomes clinically apparent (>50% resistant worms) only after a period years longer than that we were able to observe.

Figure 6. Three-dimensional graph of mean community level of infection, over time, as a function of the reproductive fitness of resistant parasites during an ongoing control program in which 50% of infected patients are treated each year. Limited fitness of resistant parasites (<0.6) delays or severely limits the reemergence of parasite infection intensity.

In our model, we assumed an effective spontaneous mutation rate for praziquantel resistance of 10-6. Recent work by Moxon and colleagues[36,37,38] on the evolution of bacterial pathogens indicates that genetic mutation becomes accelerated (i.e., nonrandom) in a subset of organisms under environmental stress, such as drug pressure. The "hypermutable" phenotype prevails under these rapidly changing conditions because of its ability to adapt quickly to the new environment. Rapid genetic diversification is enhanced by simultaneous modular changes in DNA, which are facilitated by the presence of multiple copies of critical genes, transposons, and repeat regions or Z-DNA boundaries flanking key intron/exon boundaries[39]. Higher organisms, such as schistosomes, are likely to use similar mechanisms in adaptation. Drake[40] has estimated that the effective spontaneous mutation rate per genome per sexual generation may be as high as 10-2 for the nematode C. elegans and 10-1 for the insect Drosophila. In sensitivity analysis of our differential model, reducing the mutation rate from 10-6 to 10-1 in our base case (50% coverage with 85% efficacy) reduced the expected time to emergence of resistance from 10 to 7 years. At higher levels of drug use (90%-100% coverage), the time to resistance was reduced from 7 to 5 years at the highest mutation rate (10-1). This effect is not as dramatic as would be predicted by the simpler Hardy-Weinberg model (i.e., a reduction from 14 to 3 generations for dominance of resistance), apparently because of the retarding effects of aggregation and mating on emergence of new genotypes.

In summary, our results indicate that significant resistance to praziquantel treatment had not emerged in S. haematobium in the Msambweni area as of 1991. As noted since its first use, praziquantel treatment was not 100% effective in eliminating infection[41]. It was, however, reliably capable of suppressing intensity of infection in each of six rounds of treatment spaced over 8 years. These findings, together with our modeling analysis, support targeted use of chemotherapy for disease control in S. haematobium-endemic populations[1,26,42,43,44]. In our program, praziquantel treatment was focused only on schoolchildren, the subpopulation having the highest numbers of worms[26,27]. Higher treatment coverage increases evolutionary pressure on the schistosome population and hastens emergence of resistance. The limited coverage of a targeted (vs. mass-treatment) program may have reduced the tendency for resistance to emerge, compared with the experience reported for S. mansoni in Egypt[32].

For the treated Msambweni population, the benefits of treatment appeared to be maximal after 2 years of repeated yearly treatment[26,27]. After this, a 2- to 3-year interval may be sufficient to maintain effective suppression of infection and disease[27]. Because reduced exposure to the drug slows the emergence of drug resistance, the efficacy of longer intertreatment intervals in suppressing S. haematobium-related disease should be studied. Although efficacy should be maintained for a number of years, regular monitoring of drug efficacy is appropriate for all long-term praziquantel-based control programs[14]. Emergence of praziquantel resistance should be anticipated within 10 to 20 years, and continued antischistosomal drug development should be pursued.

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