According to LASI (2017-18), the States/UT with over 10% prevalence include Rajasthan, Chhattisgarh, Madhya Pradesh, Gujarat, Jharkhand, Dadra and Nagar Haveli, Haryana, Odisha, Uttar Pradesh and Arunachal Pradesh. Among these, Odisha, Chhattisgarh, Jharkhand and Madhya Pradesh are highly endemic for malaria. From July 2019, High Burden to High Impact (HBHI) strategy of WHO has been initiated in Jharkhand, Chhattisgarh, Madhya Pradesh and West Bengal [16, 17]. Considering the known endemicity of malaria in different States, the overall trends of malaria prevalence are on expected lines. However, there are surprising high and low prevalence data of malaria in specific States. Two striking observations stand-out in this survey: 1. Rajasthan at 23.3% ranks number 1 in the self reported cases and 2. The North-East State of Mizoram is among the lowest with 0.5%. Even though, malaria (esp. Pv) is prevalent in Rajasthan, it is not considered to be among the top 5 malaria endemic states in India . On the other hand, Mizoram is considered to be one of the highly malaria endemic states in India [20, 21]. One explanation for the unexpected numbers could be the study site where the survey was undertaken. For example, in Mizoram, the district of Aizawl reported 57 malaria cases in 2018, while the malaria-endemic districts of Lawngtlai, Lunglei and Mamit reported 2222, 1092 and 772 respectively. Therefore, if the survey was carried out at Aizawl, the capital of Mizoram, the self reported malaria cases will be lower. Another possibility could be the adults in Mizoram may be asymptomatic due to various types of adaptive or acquired immunity . In sub-Saharan Africa, many adults who harbour the parasites rarely show clinical symptoms . At Mamit, the average annual parasite index (API) from 2010 to 2018 was 34.4 (34 cases / 1000), one of the highest in the country . During 2014 to 2015, there was a big spike in malaria cases in Mizoram, and in Mamit district, nearly 50% of the total population (8766 cases out of 17731) were affected in 2015 .
Despite the significant strides India has made in decreasing malaria mortality and morbidity in the last two decades (from 2000 to 2019, malaria cases and deaths have declined by 71.8% and 73.9% respectively) , malaria remains a serious public health issue in several parts of the country. The NVBDCP has developed a comprehensive strategic plan to achieve malaria-free India by 2030 . For devising effective malaria control and elimination strategies, understanding the socio-economic and household variables that affect malaria transmission is imperative. Our analysis indicates rural residence, occupation (agricultural and allied), education levels (illiterates and primary), caste (ST), household size (≥ 6), sanitation (poor toilet facility), unclean cooking fuel, water source not in dwelling, damp wall/ceiling are the major socio-economic and household risk factors that affect malaria transmission.
Not surprisingly, ST population are at a higher malaria risk. Jharkhand, Chhattisgarh, Madhya Pradesh, Odisha and malaria-endemic NE States (Tripura, Meghalaya, Mizoram and Arunachal Pradesh) have a high ST population. The geographical terrain that includes many forested areas, poor accessibility, frequent natural hazards, perennial P. falciparum transmission, very efficient anthropophillic vectors, and socio-cultural practices greatly hinder malaria control efforts in many rural tribal areas of India [2, 25]. The orthodox health beliefs of tribal population have restrained them from accessing health services despite them being highly vulnerable to various health hazards, including malaria [25, 26]. In addition, the dense forest cover and high rainfall in the tribal belts are conducive for mosquito breeding, and malaria transmission [25, 26]. Many of the tribal pockets where malaria is endemic are characterized by poor housing conditions. Residents in kutcha houses have higher odds of malaria; kutcha houses may have holes and gaps that may allow easy entry of mosquitoes. This is in line with earlier Indian studies where kutcha houses/walls made of dung and earth have shown to be a risk factor for malaria [10, 13]. Positive association between mosquito bites / day and bamboo houses has been reported in Assam, India .
Literacy has a negative association with malaria; illiterates and those with just primary education have higher risk of malaria. This was expected as literacy gives a better understanding of infectious diseases and the protective measures required. However, earlier studies [10, 14] in India did not find an association between education and malaria risk. In Yadav et al, 2014 , the sample size was just 71 households, while in Sharma et al, 2015, only no schooling, primary and secondary grades were included, and college education was not included in the education characteristics. ST who are at higher risk of malaria have lower literacy rate (59%), when compared to the national average of 73% .
Malaria risk was higher in those who carry out agricultural and allied activities when compared to respondents who are self employed or get wage/salary. Agricultural activities require significant time to be spent outdoor, and consequently are highly exposed to malaria bites. A recent study from Mandla district in Madhya Pradesh shows households having own farmlands to have a significant association with malaria .
Size of the household (> 6 members) contributed a significant risk on the malaria incidence. Family size/number of people in the house/number of people per room have been shown to be an important risk factor for malaria [10, 13, 28–31], as crowding attracts more mosquitoes due to strong olfactory signals . As observed in studies carried out in India [10, 13, 33], Indonesia , and sub-Saharan Africa , access to outside water source is a major risk factor of malaria, as dependence on outside source for water, especially in dusk and dawn increases the chances of mosquito bite. Furthermore, households using tube-wells as outside water source have risk of malaria , as tube-wells are suggested to have more stagnant water around them due to improperly maintained drainage facilities . Poor toilet facility (sanitation) is another important household risk factor of malaria, and is in-line with earlier studies carried out in India  and elsewhere [33–35, 37]. Use of unclean cooking fuel is also a major risk factor (odds increase by 1.5 times) for malaria. In addition, damp wall/ceiling was also associated with increased malaria risk; damp walls favour indoor resting of mosquitoes . The three household determinants: outside-water source, improper toilet facility and unclean cooking fuel increases the likelihood of bitten by mosquitoes outside the house. Increased time required for outdoor cooking using unclean cooking fuel could be a reason for its higher odds. Free clean cooking fuel (liquid petroleum gas connection) has been given to > 80 million Indian households through the Pradhan Mantri Ujjwala Yojana (PMUY) scheme . Through this scheme, it is expected that 80% of the households will have clean cooking fuel by 2019 . In addition to improving the standard of living, the PMUY scheme may also help in malaria control and elimination efforts.
Malaria is considered to be a disease of the poor [41–43], and several studies have shown significant association between poverty and malaria [13, 44–46]. There are also studies that have shown no association between malaria and socio economic status of the household [13, 44, 47, 48]. Interestingly, even though prevalence of malaria is higher in poorest, after adjusting the other socio-economic variables, richest were found to have slightly higher risk of malaria than the other economic categories. Urban malaria is predominantly caused by Pv, and as this is a pan-India study, a higher proportion of respondents positive for Pv could have been from urban cities, and are likely to be socio-economically forward. For example, Uttar Pradesh, the most populous State in India has predominantly Pv . Furthermore, richest, especially in urban cities may get tested promptly, and report accurately. However, the socio-economic-housing risk factors like rural residence, caste (ST), education levels, housing conditions, sanitation, unclean cooking fuel, improper water source and damp wall/ceiling strongly suggest poverty to be a risk factor for P. falciparum malaria, especially in the tribal dominated States of Jharkhand, Chhattisgarh, Madhya Pradesh, Odisha, Mizoram, Tripura and Meghalaya.
The major limitation of the study is that it is limited to adults ≥ 45 years old. Malaria affects all age groups, and this study captures only a particular age group. Furthermore, as malaria prevalence is self-reported, the accuracy cannot be verified.
Overall, the study gives important insight on socio-economic and housing determinants of malaria. In parallel to parasite and vector control strategies, improving the socio-economic and living conditions, especially in malaria dominated tribal pockets would greatly accelerate the malaria elimination efforts.