A case–control study of epidemiological factors associated with leptospirosis in South Gujarat regionKT Desai, F Patel, PB Patel, S Nayak, NB Patel, RK Bansal
Department of Community Medicine, Surat Municipal Institute of Medical Education and Research, Surat, Gujarat, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.188551
Source of Support: None, Conflict of Interest: None
Background: The current study was planned to identify the epidemiological factors associated with leptospirosis in South Gujarat region using neighborhood controls. Methods: A total of 100 cases of leptospirosis occurred in South Gujarat region during the year 2012 were selected using simple random sampling. Three neighbors of the selected cases formed the controls (n = 300). A pretested structured questionnaire was used for data collection and data were analyzed using Epi Info 2007. Results: There was significant association of illiteracy (odds ratio [OR] =1.82, 95% confidence interval [CI] =1.14–2.89), working in waterlogged fields during the reference season (OR = 4.6, 95% CI = 1.6–17.9), swimming/bathing in canals, open air defecation practices, storage of cow dung in or surrounding house, residence in the house made up of cow dung walls, households with access of food to rodents, injuries over hands/foot during the endemic season (OR = 3, 95% CI = 1.8–4.8), and history of skin disease during the endemic season (OR = 4.2, 95% CI = 2–8.5), with leptospirosis. Only 10% of individuals had gumboots for protection. A total of 83 (83%) cases and 240 (80%) controls had taken oral doxycycline chemoprophylaxis (P > 0.05). Cases had taken chemoprophylaxis for a median 4 weeks (range: 1–8) while controls had taken the same for median 8 weeks (range = 1–8) (P < 0.002). Conclusions: Although the commonly established factors appear to be associated with leptospirosis, the role of host factors seems to play a more important role in determining susceptibility to leptospirosis in exposed individuals.
Keywords: Case–control study, epidemiology, leptospirosis, prevention, risk factors
Leptospirosis is a globally important zoonotic disease.,, In India, these outbreaks have increasingly been reported from Kerala, Gujarat, Tamil Nadu, and Karnataka. Leptospirosis is endemic in South Gujarat region which includes Tapi, Surat, Navsari, and Valsad districts. There were a total of 611, 919, and 156 suspected cases and 124, 178, and 26 suspected deaths from leptospirosis in 2010, 2011, and 2012, respectively, from the above districts. The presumptive diagnosis of leptospirosis is based on a positive result in IgM-based immune assays, slide agglutination test or latex agglutination test or immunochromatographic test or microscopic agglutination test (MAT) titer (>100) in single sample based on endemicity or demonstration of leptospires either directly or by staining methods. The confirmatory diagnosis is based on isolation of leptospires from clinical specimen or four-fold or greater rise in the MAT titer between acute- and convalescent-phase serum specimens run in parallel or positive test by any two different types of rapid test or seroconversion or polymerase chain reaction test.
The key reasons for endemicity of leptospirosis in the above districts are favorable environment condition for survival of Leptospira such as alkaline soil and wetness (due to heavy rainfall, repeated floods, and water logging); presence of reservoir hosts (e.g., rodents, cattle, goats, pigs, etc.); cultivation of high water requiring crops such as paddy, sugarcane, and banana; and presence of irrigation projects and water seepage from the canals originating from the Ukai dam project (the banks of the canals are made of mud which favors the survival of leptospira). The present study evaluates risk factors for leptospirosis in South Gujarat region using neighborhood controls.
Study design and period
A case–control study was conducted over a 6-month period from November 2012 to April 2013. During the year 2012 (July–October), there were a total of 156 confirmed clinical cases of leptospirosis and 26 deaths in South Gujarat region.
Sample size calculation
At 95% confidence interval (CI), assuming power of 80%, prevalence of exposure among controls to be 50%, odd's ratio (OR) of 2 as epidemiologically sufficient for evidence of risk, and a ratio of three controls per case, a sample size of 100 cases and 298 controls was calculated using the Epi Info (Division of Health Informatics and Surveillance (DHIS), Center for Surveillance, Epidemiology and Laboratory Services (CSELS), Centers for Disease Control and Prevention, Atlanta, USA) 2007 software.
Study procedure, inclusions, and process of randomization
A line listing of confirmed clinical cases of leptospirosis in South Gujarat region during the year 2012 was obtained from the Regional Deputy Director office at Surat. Out of this sampling frame, a total of 100 cases were selected with the help of simple random sampling using a random number table. Three neighbors of the selected cases who were not affected with clinical leptospirosis during the year 2012 were selected purposively (from the three nearest house) as the controls in this study. Thus, 300 controls were selected for the study.
All confirmed deaths from leptospirosis and cases who had migrated from their original place of residence at the time of the disease were excluded from the study.
A pretested structured questionnaire was designed to collect exposure-related information. Data collection was carried out by the pretrained residents of the Community Medicine Department in a personal interview. Questionnaire on exposure to water-related activities was asked with reference to activities from July to September. Housing characteristic of the participants was observed by the trained data collectors. Data entry form designed in Epi Info 2007 software was used for data entry as well as analysis.
Statistical analysis plan
The Chi-square test and Fisher's exact test were used for categorical data whereas quantitative data were assessed for normality using the Kolmogorov–Smirnov test, followed by Mann–Whitney U-test for between-group comparisons for quantitative data.
The study was approved by the Institutional Ethical Committee of SMIMER medical college. Written informed consent was taken from all the participants of the study.
A total of 97% of cases and controls were from rural areas. Among the cases, there were 63 males and 37 females; while out of controls, there were 175 (58%) males and 125 (42%) females. The mean age of cases was 38.28 ± 13.45 years and that of the controls was 37.36 ± 12.74 years. There was no significant difference between cases and controls with regard to socioeconomic status (as per Modified Prasad's Classification), marital status, and type of family. However, cases were more likely to be illiterate (OR = 1.82; 95% CI = 1.14–2.89) as compared to controls (P = 0.011).
Nonoccupational and occupational exposure
In total, more than 90% of houses had open drainage system, 60% had peridomestic waterlogging during the reference season, and 50% had rodent burrows inside the house. On further analysis, most of the nonoccupational as well as occupational [Table 1] including animal associated [Table 2] risk behaviors and environments were not found to be associated with leptospirosis, except working in waterlogged fields in the reference season (OR = 4.6, CI = 1.6–17.9), swimming in canals (OR = 3.2, 95% CI = 1.3–10.6), open air defecation (OR = 1.7; 95% CI = 1.03–2.97), access of rodents to stored raw (OR = 2.95, CI = 1.2–3.3) as well as cooked (OR = 1.9, 95% CI = 1.2–3.0) food, and residing in brick house (OR = 0.6, 95% CI = 0.98–0.4). On assessment of practices related to personal protection, it was found that only 10% of the total subjects owned gumboots, among whom more than 70% had bought the same on their own, and in only 3% of subjects, it was provided by their employer.(P > 0.05). The most common reason for not using the gumboot was reported as it was uncomfortable to work with.
Totally, 6% of cases and 30% of controls had history of contact with confirmed case of leptospirosis (OR = 0.1, 95% CI = 0.10.3, P < 0.001).
For almost all cases and controls houses, health workers were coming for fever surveillance regularly, most commonly at weekly frequency. A total of 83 (83%) cases and 240 (80%) controls had taken oral doxycycline chemoprophylaxis (P > 0.05). Totally, 9 (9%) cases and 32 (10.7%) control had received doxycycline but not taken it, whereas 8 (8%) cases and 28 (9.3%) controls had not received chemoprophylaxis.
Cases had taken chemoprophylaxis for median 4 weeks (range = 1–8) while controls had taken the same for median 8 weeks (range = 1–8) (P < 0.002).
The median weight of cases was 46 kg (range = 30–75 kg) and controls was 47 kg (range = 30–92 kg) (P > 0.05). The median height of individuals in both the groups was 1.5 m (P > 0.05). The median body mass index of cases was 19.03 kg/m 2 while of controls was 19.47 kg/m 2 (P = 0.01).
More cases than controls were current tobacco chewer (OR = 2.1, 95% CI = 1.3–3.5, P = 0.002) as well as current alcoholics (OR = 2.6, 95% CI = 1.5–4.3, P < 0.001) although no association was found for tobacco smoking or duration and amount of alcohol consumption. Around 48 (48%) cases and 71 (23.7%) controls had history of injury over hand/foot during the reference months (OR = 3, 95% CI = 1.8–4.8) (P < 0.001) [Table 3]. Further, 19 (19%) cases and 16 (5.3%) controls had history of skin disease present during the reference months (OR = 4.2, 95% CI = 2–8.5) (P < 0.001).
Awareness about leptospirosis
A total of 80 (80%) cases and 213 (71%) controls had ever heard of leptospirosis (P = 0.078). The most common source of information on leptospirosis was health worker for both cases and controls. Only 44% of subjects knew about the zoonotic nature of leptospirosis. Among those who were aware, rat as a source of leptospirosis was identified correctly by 83% subjects, whereas cattle was identified by 88% subjects (P > 0.05). Fifty percent of cases and controls had no information on mode of transmission of leptospirosis [Figure 1]. Only half of subjects knew that farmers are highly susceptible to leptospirosis (P > 0.05) [Figure 2]. There was no significant difference in awareness regarding preventive measures for leptospirosis between the cases and controls [Table 4].
Although the basic determinants of leptospiral transmission, for example, presence of carrier animals, environment suitable for the survival of leptospires, predisposing behavioral and occupational factors, are common worldwide; the magnitude and nature of these factors vary from community to community. Therefore, the specific risk factors for acquiring leptospiral infection could be unique to each community. The current study shows that both cases and controls are exposed to almost similar type of occupational environment, indicating that perhaps other factors such as host factors may be responsible for the development of disease in the same community. The role of host factors such as immunity is further indicated by the Northeast shift in the leptospiral cases concentration over the years in South Gujarat.
For an instance, in a study from Calicut, leptospirosis seroprevalence was evaluated among high-risk groups and the highest values were found in hospital sanitary workers (56.2%), followed by fishermen (52.7%), construction workers (40%), agricultural workers (30%), sewage workers (28.2%), veterinarians (13.3%), and laboratory staff (3.3%). However, in the present study, no one occupation was found to be associated with leptospirosis. This is because the cases and controls were belonging to the same neighborhood and were involved in similar occupational and nonoccupational exposure-related activities. In such cases, it will not be wrong to hypothesize the role of host factors (e.g., genetic, immunity, nutrition) in susceptibility to developing leptospirosis in exposed individuals.
Besides occupational expositions, situations related to travels and recreational activities have emerged as important routes of leptospirosis transmission. Swimming/bathing in open canal is very common tradition in rural and tribal area of South Gujarat. This may be because of poor availability of domestic water at doorstep. This is an issue of concern due to it being associated with the leptospirosis in the present study.
Prevention of leptospirosis may be achieved by avoidance of high-risk exposures, adoption of protective measures, immunization, and use of chemoprophylaxis throughout the season, in varying combinations. As per the guidelines, during the peak transmission season, doxycycline 200 mg, once a week, may be given to high-risk persons in endemic areas, maximally to a period of 6 months. The chemoprophylaxis is required to be given throughout the exposure period for effective prevention. At local level, doxycycline prophylaxis is given in peak season period for 8 weeks by outreach workers at home. One problem here is that the 8 weeks cycle of the chemoprophylaxis is usually provided in June–July. However, the delay in monsoon in recent years, with heavy rainfalls extending in August–September, the effectiveness of the chemoprophylaxis to protect these late exposed cases reduces. In the present study, although there was no significant difference in taking chemoprophylaxis, leptospirosis cases had taken chemoprophylaxis for significantly shorter duration than controls, suggesting that duration of chemoprophylaxis was important in prevention of leptospirosis rather than simply giving chemoprophylaxis. Minor cuts and abrasions are unavoidable for the laborer and immediate treatment can reduce exposure. However, for early care seeking, awareness needs to be improved along with easy availability and access to primary health care services. There is a need for future interventions to raise awareness of the importance of chemoprophylaxis adherence throughout the season and use of other preventive measures as primary prevention measures. In addition, poor usage of gumboots due to them being uncomfortable to use with also points toward the need of redesigning the shoes in an innovative way for personal protection in partnership with the National Institute of Design or similar other partners.
Based on the current study we can definitely say that there are few possible risk factors which are different in Leptospirosis cases compare to controls. However, when it comes to the individuals living in similar environmental habitation, and having similar occupational and exposures profile with regards to above variables; why some individuals develop Leptospirosis where as others do not; is something not very clear yet. In this context probably host factor plays more important role than environment. Hence further detail study in future on role of various host factors including genetic markers and immunity in Leptospirosis amongst individuals living in similar environment needs to be planned.
We would like to thank the faculty and residents of the department of community medicine, SMIMER for their support in successful completion of the study.
Financial support and sponsorship
The study was funded by the Department of Health and Family Welfare, Government of Gujarat.
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]