Respiratory and diarrheal diseases are two major causes of morbidity and mortality among children residing in the urban slums of India [8
] and other developing countries [16
]. In our study, respiratory and gastrointestinal illnesses together accounted for 87% of all childhood morbidities. The overall morbidity burden was high with children suffering from an average of 12.5 episodes of illnesses/year during the first two years of their lives. The findings of our study are comparable to the morbidity experience of children participating in a previous birth cohort study on rotavirus infections between the years 2002 and 2006 in the same slum population (Table ), where they had an average of 11 episodes of different illnesses/year during the first three years of their lives [8
]. However, another study conducted in 1965 among residents of 3 localities in Vellore town had reported a much higher morbidity burden in infants and children, with infants experiencing an average of 17.4 morbidities/child-year of observation (Table ) [18
]. Together, these findings suggest a decreasing trend in the burden of childhood diseases over the years in this region, possibly due to better access to health care, although the overall disease burden still remains high.
Selected longitudinal studies on morbidities of children residing in urban slums of India and other developing countries, sorted chronologically
Respiratory illnesses were the commonest cause of morbidity among children in our study, which is in concordance with findings of the previous birth cohort study, in which respiratory illnesses contributed to more than 50% of the total disease burden [8
]. A higher burden of respiratory illnesses has also been reported from longitudinal studies conducted among underprivileged children in Peru [19
] and Brazil [16
]. The incidence of acute respiratory infections (ARI) among children in developing countries is estimated to range between 12.7 and 16.8 episodes/100 child-weeks [28
], which translates to an annual incidence rate of 6.6-8.8 episodes per child. It is also possible that the high incidence of respiratory illnesses in our study is partly due to the active follow up.
The majority of respiratory illnesses reported in our study were upper respiratory infections, which is in agreement with other prospective studies from developing countries [20
]. The incidence of respiratory morbidities was highest during infancy, decreasing thereafter, which differed from our previous birth cohort study where the incidence of respiratory infections was comparable across the three years of follow-up [8
]. A decrease in the incidence of acute respiratory infections (ARI) with increasing age has also been noticed among children in rural Kenya [30
]. In general, the rates of ARI seem to peak among infants aged 6–11 months [18
], although higher rates in older children have also been observed [31
]. An interesting finding was the significantly higher respiratory morbidity in the municipal drinking water cohort, which is possibly due to the greater use of firewood as the primary cooking fuel among these families (Table ). The relationship between indoor air pollution from use of solid fuels such as firewood and respiratory illnesses in children has previously been documented [32
GI morbidities were the next most common cause of morbidities among children in our study, almost all of which were episodes of either diarrhea or vomiting (without diarrhea). Taken alone, diarrhea accounted for approximately one-fifth of the overall disease burden. The diarrheal incidence in our study closely resembles the findings from a community-based longitudinal study among children under the age of five years residing in two urban slums of Brazil, where an overall incidence of 2.8 episodes of diarrhea/child/year was reported [23
]. However, in another longitudinal study among rural Bangladeshi children, an incidence rate of 1.8 diarrheal episodes/child-year was reported [26
], possibly due to the relatively older age (2–5 years) of the children. The morbidity estimates from the global burden of diarrheal disease study among under-five children in developing countries are, however, much higher at 3.2 episodes/child-year [35
]. The temporal and regional difference in the diarrheal incidence (Table ) thus underscores the need for contemporaneous, region-specific data to accurately estimate the diarrheal disease burden.
Although it was expected that the incidence of GI morbidities will be lower among children drinking bottled water, none of the illness categories other than respiratory illnesses were found to have lower rates. The apparent lack of negative association between bottled water and GI illnesses in our study could be due to contamination of drinking water at the point-of-use. In a study among peri-urban households in Lima, Peru, boiling of water failed to ensure its safety at the end-user level because of contaminated containers and poor domestic hygiene [36
]. In the same study, boiled water was found to be more contaminated when served in a drinking cup than when taken directly from a container [36
]. Most households in our study poured their drinking water into small, wide-mouthed containers prior to consumption, which were cleaned with locally available water sources that are likely to be contaminated. It has been hypothesized that the contamination of household drinking water may be influenced by several factors such as water storage and handling practices, domestic hygiene and surrounding environment, and social and cultural beliefs and practices [37
]; and that improvement of drinking water quality alone might not be sufficient enough to reduce the incidence of diarrhea in places with poor environmental sanitation [38
In this study the proportions of clinic visits and hospitalizations due to diarrhea and other illnesses were much higher than has previously been reported [27
]. Although the high rate of clinic visits were expected because the field staff were instructed to encourage caregivers to bring their children to the study clinic whenever they were sick, the high hospitalization rate is possibly a more accurate estimation of the need for disease-related admissions among children residing in Indian slums than current hospitalization rates outside the setting of a study. A similar high rate of hospitalization and low mortality was also observed in the previous birth cohort study [9
], which was attributed to good access to health care.
Per the National Family Health Survey 3 (NFHS-3) data, almost half of Indian children under the age of five years are stunted, about 20% are wasted and about 43% are underweight [40
]. However, community-based studies have found a much higher prevalence of malnutrition among slum children in urban India [41
]. In our study too, a large proportion of children were found to have one or more nutritional deficiencies during the follow-up period. It has been proposed that the high prevalence of childhood malnutrition in Indian slums is mainly due to factors such as inadequate food intake, recurrent illnesses and poor child care practices, as well as other issues such as lack of reach and coordination of public sector services, improper training and supervision of service providers, compromised efficiency of the nutritional programs and inadequate targeting of the urban poor [41
As with other community-based studies, this study has several limitations. Most of the data on morbidities were collected through reporting by the primary caregiver. This could have resulted in over-reporting of certain morbidities that were perceived to be important by the caregivers and under-reporting of others considered not so important. Additionally, the fact that the children who were never breastfed were excluded from participation might have introduced an inherent selection bias, which in turn, could have resulted in underestimation of the true morbidity burden, since bottle-fed infants tend to have a higher risk of enteric and other infections [42
]. However, we know from other observational studies in the same area that the proportion of children never breastfed is less than 3% (unpublished data).
Even though we provided sufficient quantities of bottled drinking water to cover the needs of the entire household by ensuring that water was available on demand, it was not possible to monitor or expect complete compliance. It is possible that children drank water from other sources or swallowed untreated water during bathing and other activities. In a recent study from an urban slum in Kolkata, India, it was found that water used for domestic purposes had a higher probability of contamination than that used for drinking purposes [45
]. Also, no information was available on the quantity of water utilized for different household activities in our study. Studies have suggested that quantity of water available for domestic use, rather than the quality of source water might be a better predictor of diarrheal disease [46
]. The lack of difference in GI illnesses between the bottled and municipal water cohorts could be explained partially by these limitations or by a high general level of environmental contamination which permits transmission of enteric infections through routes other than drinking water.