Intestinal nematode infections in India : a cross-sectional survey.
Intestinal nematodes encountered by clinicians and pathologists in India include roundworm (RW or Ascaris lumbricoides), hookworm (HW or Ancylostoma duodenale and Necator americanus), whipworm (WW or Trichuris trichiura) and threadworm (TW or Enterobius vermicularis). The first three are transmitted largely through the medium of soil whereas the last one is transmitted mostly through personal contact. Reviews, show that the prevalence of these worms has been studied in localised areas by several workers and excellent epidemiological studies in depth have also been carried out in well demarcated areas., However, the difficulties of organising a countrywide survey are obvious and the study of Chandler in 1927 still remains a classic.
The prevalence of intestinal nematodes reported by different workers shows wide variations,, probably due to the differences in time, place and method; used. As such, these reports are of limited help in formulating any conclusion about the country as a whole today. We, therefore, made an effort to approach the problem in a different way by conducting a survey of stool reports simultaneously across the country and by correlating the results with those of a survey of doctors' experience about the occurrence of these infections in their practice.
The study was carried out in 1983 with the assistance of our field staff. It was spread over 45 operational areas in the country with headquarters locations as shown in [Fig. 1]. Bombay and Calcutta represented 4 areas each while the remaining 37 locations represented one area each, thus making up a total of 45.
Survey of stool reports
Each of the 45 area managers was asked to list the pathology laboratories in his area under 3 categories: (a) public free hospital laboratories, (b) private paying hospital laboratories, and (c) private paying laboratories. One laboratory from each category was randomly selected for the study, making up a total of 135 laboratories. Each laboratory was requested to provide data from the last 100 consecutive stool reports in their records. The data were entered on special forms designed for computer analysis and included serial number, report number and the eggs or larvae of intestinal nematodes present. The occurrence rate of a worm infection was calculated as the percentage of stool reports positive for the eggs or larvae of that worm.
Survey of clinicians' experience
Each of the 300 officers covering 45 operational areas was asked to categorise the doctors on his list into family physicians, pediatricians and other consultants. By random process 2 family physicians, 2 pediatricians and one other consultant were selected for survey for each officer, making up a total of 1500. The survey was carried out through a questionnaire and covered many aspects of intestinal helminthiasis. The questionnaire forms were designed for computer analysis and they were distributed and collected personally by the detailmen. Those doctors who said they use stool examination to diagnose intestinal worm infection in at least some of their patients were asked to grade the occurrence of RW, HW, TW and WW in their practice as most common (4 +) frequent (3 +), occasional (2 +) and rare or never seen (1 +). It was permissible to assign the same grade to two or more worms if the respondent desired so. For analysis the percentage of doctors grading the worms as 4 +, 3 +, 2 + and 1 + was calculated.
The occurrence rates of worms were calculated for 13 zones corresponding to the states and metropolitan areas of the country. The percentage of doctors grading different worms from 4 + to 1 + was calculated for the whole country.
The percentage of doctors grading RW and TW as the most common worms were calculated separately for each of the 13 zones and the correlation between them was examined by computing Spearman's rank correlation coefficient.
[Table - 1] shows the occurrence rates of various intestinal nematodes in 13 zones of the country. RW was the commonest in all zones except Tamil Nadu where H:W was the commonest. The occurrence rate of RW varied from 31.3% in Assam and Siliguri to 5.5% in Punjab-Haryana and Rajasthan-Gujarat. HW occurrence rate was highest (17.4%) in Tamil Nadu and lowest (1.8%) in Rajasthan and Gujarat.
The occurrence Tate of WW was less than 5% in all zones except Kerala, Bombay City, Tamil Nadu and Calcutta City where it ranged from a high of 13.8% to a low of 6.0%. Strongyloides stercoralis was the rarest nematode found in 0.5-1.8% of stool samples mainly in Calcutta City, West Bengal and Orissa, and Andhra Pradesh.
TW occurrence rate was the lowest, which was expected since TW eggs are usually not found in faeces and therefore stool examination can detect not more than 5% of these infections. Despite this limitation, TW was relatively more common in Assam and Siliguri, Madhya Pradesh, Rajasthan and Gujarat, and Tamil Nadu.
Of the 1500 doctors selected for [lie survey of clinical experience, 1485 returned the questionnaire and only 770 of them said they use stool examination to diagnose intestinal worm infections. As seen from a majority of these 770 doctors graded RW as the most common, TW as the most common or frequent, HW as frequent or occasional, and WW as rare or not seen.
As shown in [Fig. 2] there was a significant and negative correlation between the percentage of doctors grading RW as tire commonest (4 +) and the percentage of doctors grading TW as the commonest (Spearman's r = - 0.69, p < 0.01).
The findings of this study need to be interpreted with an awareness of its limitations. Firstly, although the stool reports collected were a random sample of stool reports, the patients concerned were not a random sample of the country's population. They represented people who go for treatment to public hospitals, private hospitals and private practitioners in cities and district towns, and who, for some reason or the other, are asked to have their stools examined. Secondly, in most pathology laboratories routine stool examination is carried out by direct smear method which is less sensitive than the concentration method so that the true occurrence rates of intestinal nematode infections might be higher than the observed rates. However, the cross-sectional nature of the survey implying simultaneous collection of data from all over the country renders it of some value in assessing the pattern of intestinal nematode infections today.
By occurrence rate [Table - 1] RW was the most common, HW less common and WW the least common of these three. As the true occurrence rate of TW cannot be determined from stool examination, it may be extrapolated on the assumption that stool examination detects not more than 5% of TW infections. On this basis, the TW occurrence rate of 0.7% in stool reports would correspond to a true occurrence rate of 14% at the maximum, which would place TW next to RW in occurrence rate. Now if the worms are ranked by the grade given to them by the highest percentage of doctors [Table - 2] the order is again RW, TW, HW and WW. Thus the grading of worms by doctors for the frequency of occurrence agreed with the occurrence rates in stool reports.
The negative correlation between the percentages of doctors grading RW and 'TW as the most common infections [Fig. 2] shows that the two were inversely related, i.e. in areas where a high proportion of doctors perceived RW as the most common, a low proportion of doctors perceived TW as the most common, and vice versa. This observation is contrary to the belief that RW and TW, the two most common intestinal nematodes, often occur together. Further, it can be seen from [Table - 1] that, with the exception of Assam and Siliguri where the occurrence rates of both RW and TW were high, the zones with a high occurrence rate of RW had a relatively low occurrence rate of TW, and vice versa. Considering the difference in the modes of transmission of RW and TW, viz. through soil for RW and through direct contact for TW, an inverse relationship in their occurrence rates is not unlikely. Only a survey based on both stool examination for RW and perianal adhesive tape smear for TW will shed more light on this finding.
The authors are grateful to the pathologists and laboratories who contributed stool report data for this study. Their names are listed in the Appendix. Thanks are due to the clinicians who responded to the questionnaires and to the field officers who monitored the surveys.
Prof. A. B. Chowdhury, Emeritus Medical Scientist (I.C.M.R.), School of Tropical Medicine, Calcutta was kind enough to review the manuscript and suggest improvements.
Computer analysis of the data was carried out by Mr. A. R. Kannappan and Mr. Shripad Kulkarni. Mr. S. Nambi provided technical assistance.