The results from multivariate logistic regression analysis and one way ANOVA also indicated that the parasite density of malaria was significantly associated with the co-infection status of study participants, and, the mean parasite density of malaria was higher in malaria-multiple STHs–co-infected patients as compared with patients infected with malaria with single STHs and Plasmodium parasite alone. Previous studies conducted in Nigeria, Cameroon, southern and northern Ethiopia also found similar findings with the current study [11, 13–15]. The reason might be due to the fact that helminths modulate the host immune response both to themselves and to coexisting infections or to the down regulation of the immune system by helminths. Consequently, the Plasmodium parasite could enter into the host and multiply at a faster rate in patients co-infected with STHs [10, 11.The present findings also support the hypothesis that co-infection of malaria and STHs could aggravate clinical malaria [11].
On the other hand, the results from logistic regression analysis and the independent samples t-test demonstrated that the malaria parasite density had a variable association with different species of soil-transmitted helminths infection intensities in malaria-STHs–co-infected study participants in the present study. In the present study, malaria parasite density increased with increasing parasite intensities of hookworm and T. trichiura infections. Study participants with moderate infection intensities of hookworm and T. trichiura had significantly higher mean malaria parasite density than study participants with light co-infection intensities. In this regard, the results of the present study were in agreement with the findings of studies from Ghana, Tanzania, and southern Ethiopia [10, 12, and 16]. The reason could be due to the fact that a strong cytokine production in response to helminths high infection makes individuals more susceptible to more replication of plasmodium parasites and advanced stages of malaria [14, 17]. The results also appeared in support of the hypothesis that co-infection of malaria and helminth infections render co-infected individuals more vulnerable to increased incidence of malaria and severe diseases associated with malaria [17, 18].
On the other hand, the parasite density of malaria decreased with the increasing parasite intensities of Ascaris lumbricoides, and study participants with light infection intensity of Ascaris lumbricoides had higher mean malaria parasite density than study participants with moderate Ascaris lumbricoides infection intensity.
This finding was in agreement with the findings of studies conducted in Thailand, Madagascar, northern Ethiopia, and Gilgel Gibe Dam area in Ethiopia [19, 20, 11, and 8]. The reason might be due to the fact that there is a mixed Th1/Th2 immune response during moderate infection intensity of Ascaris lumbricoides which leads to induced low level of malaria parasite density [20]. The results also were in agreement with the hypothesis of the protective effect of Ascaris lumbricoides against malaria parasite density and severe forms [11, 19]. Similarly, the study also showed that study participants infected moderately with S. mansoni had lower Plasmodium densities compared to lightly infected patients but the association was not statistically significant. This finding was in agreement with studies carried out in Senegal, and Tanzania [10, 21]. The reason might be due to down regulation of Th1 response which could leads to an exacerbation of Th2 dependent anti-body response. This would accelerate the process of parasite clearance and favors plasmodium parasite elimination with a better control of malaria parasite density [21, 22].
In the present study, the overall anemia prevalence was found 38.3% with only 0.9% of severe anemia prevalence. In this study anemia prevalence among malaria patients with multiple STHs co-infection group was found significantly higher than malaria mono-infected and malaria with single STHs co-infection group. The mean hemoglobin level significantly decreased with increasing co-infecting parasites. This finding is in agreement with findings of studies carried out in Cameroon, Tanzania, northern Ethiopia, and Alaba Kulito (southern Ethiopia) from where higher anemia prevalence among malaria and multiple soil-transmitted helminths co-infected study participants was reported[2, 7, 23, 24 22]. The reason for increased prevalence of anemia in co-infected study participants may be attributed to chronic blood and iron loss due to worm infections in addition to the loss due to malaria [23, 25].
The present study also demonstrates that the prevalence of anemia and the mean hemoglobin concentration had a variable association with STHs infection intensities depending on parasite species. Study participants with high infection intensity of hookworm and T. trichiura were found significantly more anemic and had lower mean hemoglobin concentration than their counterpart participants having low infection intensities. This finding was in line with findings of studies carried out in Peru, Brazil, Malaysia, Uganda, Alaba Kulito and Gilgel Gibe dam area (Southwest Ethiopia) [2, 15, 26–29]. The reason might be because of T. trichiura has been shown to be associated with the anemia through erythrocyte loss from the gut.
Also high hook worm infections lead to decreased erythropoiesis through Nitric oxide (NO) release Since NO can reduce erythrocyte deformability; it could lead to increased red blood cell destruction. On the other hand, contrary findings were reported from studies carried out in Côte d’Ivoire, Tanzania, and Uganda [10, 30, and 31]. These differences could be due to high number of hook worm and T. Trichiura light infections reported in previous studies and given, the young age, the infection may have been recently acquired and did not progress to chronicity to be associated with anemia. In addition the hookworm species involved may have been the less virulent [27, 32].
Interestingly, study participants with light infection intensity of Ascaris lumbricoides were found to be significantly less anemic and had high mean hemoglobin concentration than their counterpart participants with moderate infection intensities. This finding is in agreement with findings of studies carried out in Peru, Brazil, and the Gilgel Gibe dam area (southern Ethiopia) [15, 26, 33]. The reason may be due to the fact that increased titers of IgE and high level of eosinophilia during light infection of Ascaris lumbricoides infection can cause Th1 hypersensitivity which can cause anemia. On the other hand, the findings of studies carried out in Malaysia, Nigeria, and Alaba Kulito (southern Ethiopia) showed contrary results [2, 25, and 27]. Another study carried out in Cameroon found no effect of Ascaris lumbricoides on hemoglobin level [32].
These variations may be due to differences in, study participants because some of the studies included pregnant women. In most of addition in the present most of light Ascaris lumbricoides light infections were co-infected with other STH infections and previous studies suggest that the effect of Ascaris lumbricoides was not significant, unless there were two or more infecting species of intestinal helminths. Although intestinal helminth infection is mutually correlated with other factors to some degree, this infection does contribute a portion of risk for the development of anemia conditioned on the presence of other risk factors. For this reason, when analyzed alone or when independent of other risk factors, Ascaris lumbricoides may not seem to be a major factor for anemia [26, 27, and 33].