Malaria is a focal disease in India, influenced by several local ecological and social factors [
21]. The study team collected samples and clinical data from 945 individuals of tribal origin living in closely placed hamlets in the forested hills of Jharkhand in the Eastern parts of India. This study involved four cross-sectional surveys two each during the high (October–December) and the low (June–August) transmission seasons and fortnightly surveillance during the periods intervening the cross-sectional surveys. In summary: (1) malaria transmission was hyper-endemic, and (2) there was evidence suggesting that the population develops naturally acquired immunity (NAI) as a function of age and exposure. The observations that children had (i) a high incidence and prevalence rate of
P. falciparum infections, (ii) a high malarial attack rate, and (iii) higher parasite densities compared to adults are indications of hyper-endemicity in this study area. It is generally observed that individuals living in hyper-endemic areas gradually develop an anti-parasite immunity which protects them from febrile malaria. The age-dependent variation of malaria attack rates, parasite prevalence and density levels in the present study was characterized by a gradual decrease with increasing age. This relationship is typical of NAI against malaria and is similar to findings in highly endemic areas of Africa [
22‐
24] and Asia [
25]. The development of NAI is further underscored by the very high difference in parasite densities between acute malaria cases and asymptomatic carriers. The authors observed that parasite densities in acute malaria cases were much higher than those in asymptomatic carriers. This difference was particularly high in young children and decreased with age i.e. the parasite density causing fever decreases with age and exposure. This finding is related to the phenomenon of the pyrogenic threshold which has been described in Africa [
26]. Similar age-dependent decrease in the prevalence of parasitaemia and febrile malaria has also been observed in Odisha, which is located in a neighboring state of India [
27]. Another study in the Sundargarh district in Odisha, also demonstrated high levels of agglutinating antibodies against
P. falciparum infected erythrocytes in healthy adults but not in children below 3 years of age [
28]. Collectively these findings support the notion that individuals who live in highly malaria endemic areas of India develop NAI.
Another important observation of the present study was the apparent decrease in the malaria parasite burden between 2014 and 2016. In the first cross-sectional survey during high transmission season of 2014 the rate of P. falciparum-positive samples was around 44% with a clinical malaria prevalence of 11.4%. This was reduced to 19.3% for P. falciparum-positive samples and clinical malaria prevalence of 8.6% during the next high transmission season. This reduction in the parasite burden in the study population is likely due to the combination of active case detection and ACT of all identified positive cases.
The entomological studies revealed that the study area is under the influence of three vector species
An. fluviatilis and
An. culicifacies as the primary vectors and
An. annularis as the secondary vector for malaria transmission. The climatic conditions in the forest ecotype are more conducive for higher survival of vectors that are associated with a predominance of
P. falciparum [
3].
An. fluviatilis is widely distributed in India although its role in malaria transmission varies from place to place depending on the local prevalence of different sibling species [
29].
An. fluviatilis (species S) is among the most efficient vectors of malaria particularly in hills and foothills of India and it has previously been incriminated as a vector in the tribal districts of Koraput, Orissa [
28] and Bastar, Madhya Pradesh [
30]. In contrast, other studies have found that
An. fluviatilis (species T), though prevalent in high densities in the foothills of Shiwalik range was not playing role in malaria transmission [
31]. However,
An. fluviatilis (species T) has been found to be susceptible to
P. vivax [
32] and
P. falciparum infections (MK Das, unpublished) and has been incriminated as an efficient vector of malaria for transmission in the mountainous areas of the Hormozgan province, south Iran [
33]. The role of
An. fluviatilis (species T) as the primary vector in malaria transmission in this study has been established by way of its high prevalence period coinciding with the peak transmission period, more preference for human biting, high human blood index and susceptibility to plasmodial infections (MK Das, unpublished). The other primary vector,
An. culicifacies (species C), is responsible for 60–70% of all malaria cases in India despite being predominantly a zoophagic species and being prone to environmental factors with epidemiological implications in different ecotypes [
12,
34].
An. culicifacies may be regarded as playing a complementary role in maintaining perennial transmission during pre-monsoon and monsoon months where transmission is low. The secondary vector
An. annularis though primarily zoophagic, exophilic and exophagic, is associated with low human biting rate and sporozoite rate, and has a marginal role in malaria transmission in this study area. While
An. annularis plays a limited role for malaria transmission in the present study area, it is responsible for year-round transmission as observed in inlands of Odisha [
32] and forests of Assam [
35]. The presence of the three vector species and their sibling species complexes reported here together with a high prevalence of parasite positive individuals in the study population explains the year-round malaria transmission.
Besides technical inadequacies, the problem posed by malaria in tribal areas of India is multifaceted. Operational constraints such as inaccessible remote areas, poor surveillance, inadequate vector control and general lack of motivation and health awareness all facilitate persistent transmission. Therefore, focus should be on strengthening of health infrastructure at the periphery to ensure early case detection and prompt treatment. This should be supported by quality vector control measures, creating health awareness in the community and ensuring economic growth of the tribal areas. These efforts along with the development of an efficacious malaria vaccine will lead to a sustained reduction in malaria morbidity and mortality.