81
Epidemiiiol. Infect. (1992), 109, 81-85 Pri-itte(l int G reat Br-ita iii
Helicobacter pylori, musings from the epidemiologic armchair T. KOSTER AND J. P. VANDENBROt'CKE Departnient of Clinical Epidemniology, Leiden UTnniiersity Hospital. Building 1 CO-P. P.O. Box 9600. 2300 RC Leiden. The Netherlands (Accepted 17 February 1992) SUITIA RRY
The literature on Helicobacter pylori has become enormous, but the epidemiology of the infection remains an enigma. Guided by epidemiologic principles we have tried to inter)ret the available data on the epidemiologic aspects of H. pylori. W'e conclude that conflicting results on familial clustering and seroprevalence curves might have logical explanations. However, the exact way this organism spreads among humans remains to be solved. INTROD)UCTION
The epidemiology and ecology of Helicobacter pylori remains an enigma. Much has been written about its association with chronic gastritis and peptic ulcer disease, but most reviews remain silent about the way this organism spreads among humans. 1tiitil now, the organism has only been found in places where an acid-secreting gastric mucosa is present (stomach, Barrett's oesophagus [1]. Meckel's diverticula [2]). An exception is one paper which reported H. pytori cultured from dental plaque [3]. Most investigators agree that the human stomach is the largest reservoir. Still, in western countries H. pylori infection has a prevalence of 500/, or more at age 50 or thereafter [4]. So the question remains: how does it spread from stomach to stomach? Most likely bv mouth. But how? WAith this question in mind, we set out to review the literature which describes the epidemiologic aspects of H. pylori, disregarding all other aspects. Wte started by looking at the data about its prevalence, and wondered what conclusions an epidemiologist would draw from such prevalence curves. Next, we wondered about the interpretation of the studies about its spread in families. SEROPRENTALENCE SURV'EYS
Serological tests for H. pylori IgG-antibodies have very high sensitivity and specificity. and become positive 2-3 months after colonization [5-71. As a rule of thumb, the age-specific seroprevalence of H. pylori in western society is the truncated age, i.e. at age 6 it is close to 0%, at age 17 it is 10%, up to age 59 when it becomes 50% and slightly higher thereafter [8]. This rule of thumb is probablv too crude an estimation. A somewhat finer assessment holds that the prevalence is very low in infancy, childhood and adolescence, and then
T.KOSTER AND J. P. VANDENBROUCKE quite suddenly rises somewhere between ages 20 and 40, after which the increase in prevalence per year slows down into some plateau [4]. Many seroprevalence studies have been performed and they reveal the same characteristic pattern. An exception to these findings is a recently published report on H.pylori infection in healthy children, which showed an overall infection rate of 31 %. However, this high prevalence might be a reflection of the over-represented lower socioeconomic 82
classes in addition to racial differences [9]. From volunteer and patient studies we also know that H. pylori is an extremely hard organism to eradicate, i.e. once a stomach is colonized it remains so [7]. Although all epidemiologic data always leave something to be desired, especially when looked at from a comfortable armchair position, two conclusions can be is drawn immediately: (1) in western society, most of the history of H. written in adulthood; (2) the yearly incidence of new colonizations must be low, except perhaps for a certain period in early adulthood. How might this type of accrual of colonization with age be explained? The small yearly incidence of colonization stands in contrast to the high prevalence among the middle-aged and the elderly: from birth onwards we are constantly surrounded by people with colonized stomachs. In general, infections with ubiquitous organisms are rare in adulthood: usually it is the children who suffer most of theso infectious disease burden when the organism is constantly around. Children do for two reasons: they are still immunologically naive, as they have not yet developed specific immunity, and they are physically close to each other, and to adults, with all the playing, caring and cuddling that goes on in the first years of life. Think about the age-prevalence curve of the tuberculin reaction: 80 per cent of its history is written before the age of 12 years. Again taking the armchair position, in principle two extreme types of explanation are possible: (1) H. is indeed ubiquitous, and contact with the organism is rampant, but we are born with very high natural resistance to developing the infection. Gradually, our natural resistance disappears (e.g. because the stomach's secretions alter). (2) Contact with H. pylori is rare; one needs to live long enough to encounter it at least once. If encountered, it stays. We favour explanation 1. Firstly, because the basic enigma which started our has yet been found, except the human inquiry is that no reservoir for H. stomach. It is not one of the habits of humans to have stomachic contact very often (in contrast to animals where the young might be fed regurgitated food). Secondly, children are by no means immune: some of the burgeoning literature on H. pylori gastritis comes from paediatrics. Explanation 2 entails the idea that we are at risk of H pylori infection every time we swallow some extraneous material If we concentrate (mostly food, or maybe another person's saliva when kissing). be small to explain must on food, the possibility of ingesting contaminated food the age-prevalence curve. Say that the prevalence of the disease increases at a rate of 1 % per year. In a given year, we will have three meals per day, each consisting of at least three different food items; in addition, there are snacks and sweets. If H. pylori is transmitted via contaminated food, only in half a million food items needs to be contaminated to explain its age-specific prevalence. What about the more intimate human routes? Again, these are the domain of infants and adolescents, or at most of young adults. If the quick rise in early
pylori
pylori
pylori
1
Epidemiology of Helicobacter pylori
83
adulthood is a fact, and not due to deficiencies in the collection of the data, the only diseases with similar patterns are cytomegalovirus infection and infectious mononucleosis. While most Epstein-Barr virus infection occurs in childhood, and is generally subelinical, mononucleosis is also nicknamed 'kissing disease' given its mode of spread in young adults who might develop clear-cut glandular fever. Other traditional patterns of infectious disease transmission such as person-toperson faeco-oral or droplet transmission are mainly infection routes of children. The purely sexual route, i.e. like a venereal disease, will never lead to such high prevalences: a prevalence of 50 % at age 50 and over is simply too high to consider H. pylori as a candidate for sexual transmission. THE STORY OF FAMILIAL CLUSTERING
Much attention has been focused on familial clustering of H. pylori. When children develop acute symptomatie gastritis (of a severity which leads to hospitalization and in-depth investigation) and are H. pylori positive, the great majority of their parents will also prove to be positive [1, 10]. The evidence is much less certain when looking at the spouses of adults with a chronic condition ascribed by H. pylori; the prevalence in the spouse might hardly exceed that in the general population [11, 12]. A seeming contradictory result is reported by Malaty and co-workers [13], who investigated the transmission of H. pylori in healthy families consisting of a husband and wife and at least one biologic child. None of them suffered from upper gastrointestinal symptoms. One of the parents was selected at random as the index subject. It turned out that 68 % of the spouses of H. pylori-infected index subjects was also seropositive for H. pylori. However, when races were analysed separately, the reported seroprevalences were 83 % for black people and 42 % for white people. Given the age of the subjects in the study, these figures are comparable to the prevalence in the general population, reported earlier by the same authors [14]. These observations about children and their parents and the discrepancy with the spouse situation might have logical explanations. Whenever the symptoms with the children are acute and alarming, it is likely that the infection has been massive. No wonder that those living in the same premises have shared the infection more often than not. In contrast, in the colonized adult the organism might already have been present for a long time, and the person might no longer be shedding the organism; in consequence, he or she will not infect the adult partner. Thus, there are two diametrically opposite explanations. First, the paediatric 'epidemics' might be a confusing factor. They have led to the notion that H. pylori is transmitted via simple person to person transmission. The latter is difficult to conceive, given that most of the infection is acquired in adulthood, when we are physically so much less close to each other. Also, children are constantly surrounded by many people who carry the organisms, e.g. grand-parents, which should give them ample opportunity to be infected. Second, the paediatric epidemics reveal some truth, but it is a more complex one. It is possible that only certain persons are infective during a brief time period. Imagine that only those people who have massive symptoms shed H. pylori via
84
T. KOSTER
AND
J. P. VANDENBROUCKE
droplets and faeces. But imagine also that only one person in so manv becomes massively symptomatic (or massively infectious). That situation would be analogous to tuberculosis in which only about 1 in 20 persons who develops open cavitarv tuberculosis transmits the disease. Still. it remains enigmatic why children are not more affected. ROUTE OF TRANSMISSION
Faecal-oral spread is postulated by Graham and co-workers [15], mostly because the seroprevalence curves look very similar to other well known faecaloral transmitted diseases like poliomyelitis and hepatitis A. Also the difference in sero-epidemiology between WNestern and non- W'estern societies reveals the same pattern for the three diseases mentioned, and relates to personal hygiene and sanitation [16]. However, this similarity does not close the argument since the same reasoning serves for oral-oral transmitted disease like Epstein-Barr V'irus infections, tuberculosis and other respiratory diseases. Lee and colleagues recently suggested that the inter-oral route is the more likely one [17]. They found that infected coprophagous mice did not spread the disease to their uninfected congeners. whereas infected dogs did. It was reasoned that dogs are more likely to have oral-oral contacts (saliva, vomit) than mice. who do not vomit and are coprophagous. I)espite this very interesting study one must keep in mind that H. pylori can travel along the alimentary tract, since some reports show that H. pylori can be cultured from gastric metaplasia areas in the rectum anid from Meckel's diverticula. However. until now it has never been cultured from the stools.
C(ONCLUiSION Despite many interesting investigations and progress made, the epidemiology of H. pylori infections remains blanketed in fog. More secure information about the increase in prevalence with age, i.e. is there really a sudden jump in early adulthood, or is there a monotonic increase from childhood to senescence, might help to clear away the fog. )L EANEENTS ACKNOWNY'LE
The authors thank RI. A. Veenendaal, gastroenterologist, and R. Bosboom. microbiologist, for their enthusiastic discussions. REFERENCEiES 1. Mitchell HM.1 Bohane TD. Berkowicz J. Hazell SL, Lee A. Antibody to (anpylobacter Pylori in families of index children with gastrointestinal illness due to C. pylori. Lancet 1987;
2: 681-2. 2. De Cothi GA, Newbold KMI, O'Conner HJ. Campylobacter-like organisms andt heterotopic gastric mucosa in Meckel's diverticula. J Clin Pathol 1989; 42: 132-4. 3. Krajden S. Fuska M. Anderson J. Examination of human stomach biopsies. saliva and dental plaque for Canmpylobacter pylori. J Clin Microbiol 1989: 27: 1397-8.
Epidemiology of Helicobacter pylori
85
4. Perez-Perez GI, Dworkin BM, Chodos JE, Blaser MJ. Campylobacter pylori antibodies in humans. Ann Intern Med 1988; 109: 11-17. 5. Evans DJ, Evans DG, Graham DY, Klein PD. A sensitive and specific test for detection of 'ampylobacter pylori infection. Gastroenterologv 1989; 96: 1004-8. 6. Sobala GM, Crabtree JE, Dixon MF, et al. Acute Helicobacter pylori infection: clinical features, local and systemic immune response, gastric mucosal histology, and gastric juice ascorbic acid concentrations. Gut 1991; 32: 1415-18. 7. Morris AJ, Rafiq Ali M, Nicholson GI, Perez-Perez GI, Blaser MJ. Long-term follow-up voluntary ingestion of Helicobacter pylori. Ann Intern Med 1991; 114: 662-3. 8. Doolev CP, Cohen H, Fitzgibbons PL. et al. Prevalence of helicobacter pylori infection and histologic gastritis in asymptomatic persons. N Engl J Med 1989; 321: 1562-6. 9. Fiedorek SC, Malaty HD, Evans DL, et al. Factors influencing the epidemiology of Helicobacter pylori infection in children. Pediatrics 1991; 88: 578-82. 10. Drumm B, Perez-Perez GI, Blaser MJ, Sherman PM. Intrafamilial clustering of Helicobacter pylori infection. N Engi J Med 1990; 322: 359-63. 11. Jones DM, Eldridge J, Whorwell PJ. Antibodies to Campylobacter pyloridis in household contacts of infected patients. Br Med J 1987; 294: 615. 12. Perez-Perez GI, Witkin SS. Decker MID. Blaser M1J. Seroprevalence of helicobacter infection in couples. J Clin Microbiol 1991; 29: 642-4. 13. Malaty HM, Graham DY. Klein PD, Evans I)G, Adam E, Evans D)J. Transmission of Helicobacter pylori infection; studies in families of healthy individuals. Scand J Gastroenterol 1991; 26: 927-32. 14. Graham DY, Malaty HM, Evans DG, Evans DJ, Klein PD, Adam E. Epidemiology of Helicobacter pylori in an asymptomatic population in the United States: effect of age, race and socioeconomic status. Gastroenterology 1991: 100: 1495-501. 15. Graham DY. Helicobacter pylori: its epidemiology and its role in duodenal ulcer disease. J Gastroenterol Hepatol 1991; 6: 105-13. 16. Graham DY, Adam E, Reddy GT, et al. Seroepidemiology of Helicobacter pylori infection in India: comparison of developing and developed countries. Dig Dis Sci 1991; 36: 1084-8. 17. Lee A. Fox JG, Otto G. D)ick EH, Krakowka S. Transmission of Helicobacter spp. A challenge to the dogma of faecal-oral spread. Epidemiol Infect 1991; 107: 99-109.
Epidemiiiol. Infect. (1992), 109, 81-85 Pri-itte(l int G reat Br-ita iii
Helicobacter pylori, musings from the epidemiologic armchair T. KOSTER AND J. P. VANDENBROt'CKE Departnient of Clinical Epidemniology, Leiden UTnniiersity Hospital. Building 1 CO-P. P.O. Box 9600. 2300 RC Leiden. The Netherlands (Accepted 17 February 1992) SUITIA RRY
The literature on Helicobacter pylori has become enormous, but the epidemiology of the infection remains an enigma. Guided by epidemiologic principles we have tried to inter)ret the available data on the epidemiologic aspects of H. pylori. W'e conclude that conflicting results on familial clustering and seroprevalence curves might have logical explanations. However, the exact way this organism spreads among humans remains to be solved. INTROD)UCTION
The epidemiology and ecology of Helicobacter pylori remains an enigma. Much has been written about its association with chronic gastritis and peptic ulcer disease, but most reviews remain silent about the way this organism spreads among humans. 1tiitil now, the organism has only been found in places where an acid-secreting gastric mucosa is present (stomach, Barrett's oesophagus [1]. Meckel's diverticula [2]). An exception is one paper which reported H. pytori cultured from dental plaque [3]. Most investigators agree that the human stomach is the largest reservoir. Still, in western countries H. pylori infection has a prevalence of 500/, or more at age 50 or thereafter [4]. So the question remains: how does it spread from stomach to stomach? Most likely bv mouth. But how? WAith this question in mind, we set out to review the literature which describes the epidemiologic aspects of H. pylori, disregarding all other aspects. Wte started by looking at the data about its prevalence, and wondered what conclusions an epidemiologist would draw from such prevalence curves. Next, we wondered about the interpretation of the studies about its spread in families. SEROPRENTALENCE SURV'EYS
Serological tests for H. pylori IgG-antibodies have very high sensitivity and specificity. and become positive 2-3 months after colonization [5-71. As a rule of thumb, the age-specific seroprevalence of H. pylori in western society is the truncated age, i.e. at age 6 it is close to 0%, at age 17 it is 10%, up to age 59 when it becomes 50% and slightly higher thereafter [8]. This rule of thumb is probablv too crude an estimation. A somewhat finer assessment holds that the prevalence is very low in infancy, childhood and adolescence, and then
T.KOSTER AND J. P. VANDENBROUCKE quite suddenly rises somewhere between ages 20 and 40, after which the increase in prevalence per year slows down into some plateau [4]. Many seroprevalence studies have been performed and they reveal the same characteristic pattern. An exception to these findings is a recently published report on H.pylori infection in healthy children, which showed an overall infection rate of 31 %. However, this high prevalence might be a reflection of the over-represented lower socioeconomic 82
classes in addition to racial differences [9]. From volunteer and patient studies we also know that H. pylori is an extremely hard organism to eradicate, i.e. once a stomach is colonized it remains so [7]. Although all epidemiologic data always leave something to be desired, especially when looked at from a comfortable armchair position, two conclusions can be is drawn immediately: (1) in western society, most of the history of H. written in adulthood; (2) the yearly incidence of new colonizations must be low, except perhaps for a certain period in early adulthood. How might this type of accrual of colonization with age be explained? The small yearly incidence of colonization stands in contrast to the high prevalence among the middle-aged and the elderly: from birth onwards we are constantly surrounded by people with colonized stomachs. In general, infections with ubiquitous organisms are rare in adulthood: usually it is the children who suffer most of theso infectious disease burden when the organism is constantly around. Children do for two reasons: they are still immunologically naive, as they have not yet developed specific immunity, and they are physically close to each other, and to adults, with all the playing, caring and cuddling that goes on in the first years of life. Think about the age-prevalence curve of the tuberculin reaction: 80 per cent of its history is written before the age of 12 years. Again taking the armchair position, in principle two extreme types of explanation are possible: (1) H. is indeed ubiquitous, and contact with the organism is rampant, but we are born with very high natural resistance to developing the infection. Gradually, our natural resistance disappears (e.g. because the stomach's secretions alter). (2) Contact with H. pylori is rare; one needs to live long enough to encounter it at least once. If encountered, it stays. We favour explanation 1. Firstly, because the basic enigma which started our has yet been found, except the human inquiry is that no reservoir for H. stomach. It is not one of the habits of humans to have stomachic contact very often (in contrast to animals where the young might be fed regurgitated food). Secondly, children are by no means immune: some of the burgeoning literature on H. pylori gastritis comes from paediatrics. Explanation 2 entails the idea that we are at risk of H pylori infection every time we swallow some extraneous material If we concentrate (mostly food, or maybe another person's saliva when kissing). be small to explain must on food, the possibility of ingesting contaminated food the age-prevalence curve. Say that the prevalence of the disease increases at a rate of 1 % per year. In a given year, we will have three meals per day, each consisting of at least three different food items; in addition, there are snacks and sweets. If H. pylori is transmitted via contaminated food, only in half a million food items needs to be contaminated to explain its age-specific prevalence. What about the more intimate human routes? Again, these are the domain of infants and adolescents, or at most of young adults. If the quick rise in early
pylori
pylori
pylori
1
Epidemiology of Helicobacter pylori
83
adulthood is a fact, and not due to deficiencies in the collection of the data, the only diseases with similar patterns are cytomegalovirus infection and infectious mononucleosis. While most Epstein-Barr virus infection occurs in childhood, and is generally subelinical, mononucleosis is also nicknamed 'kissing disease' given its mode of spread in young adults who might develop clear-cut glandular fever. Other traditional patterns of infectious disease transmission such as person-toperson faeco-oral or droplet transmission are mainly infection routes of children. The purely sexual route, i.e. like a venereal disease, will never lead to such high prevalences: a prevalence of 50 % at age 50 and over is simply too high to consider H. pylori as a candidate for sexual transmission. THE STORY OF FAMILIAL CLUSTERING
Much attention has been focused on familial clustering of H. pylori. When children develop acute symptomatie gastritis (of a severity which leads to hospitalization and in-depth investigation) and are H. pylori positive, the great majority of their parents will also prove to be positive [1, 10]. The evidence is much less certain when looking at the spouses of adults with a chronic condition ascribed by H. pylori; the prevalence in the spouse might hardly exceed that in the general population [11, 12]. A seeming contradictory result is reported by Malaty and co-workers [13], who investigated the transmission of H. pylori in healthy families consisting of a husband and wife and at least one biologic child. None of them suffered from upper gastrointestinal symptoms. One of the parents was selected at random as the index subject. It turned out that 68 % of the spouses of H. pylori-infected index subjects was also seropositive for H. pylori. However, when races were analysed separately, the reported seroprevalences were 83 % for black people and 42 % for white people. Given the age of the subjects in the study, these figures are comparable to the prevalence in the general population, reported earlier by the same authors [14]. These observations about children and their parents and the discrepancy with the spouse situation might have logical explanations. Whenever the symptoms with the children are acute and alarming, it is likely that the infection has been massive. No wonder that those living in the same premises have shared the infection more often than not. In contrast, in the colonized adult the organism might already have been present for a long time, and the person might no longer be shedding the organism; in consequence, he or she will not infect the adult partner. Thus, there are two diametrically opposite explanations. First, the paediatric 'epidemics' might be a confusing factor. They have led to the notion that H. pylori is transmitted via simple person to person transmission. The latter is difficult to conceive, given that most of the infection is acquired in adulthood, when we are physically so much less close to each other. Also, children are constantly surrounded by many people who carry the organisms, e.g. grand-parents, which should give them ample opportunity to be infected. Second, the paediatric epidemics reveal some truth, but it is a more complex one. It is possible that only certain persons are infective during a brief time period. Imagine that only those people who have massive symptoms shed H. pylori via
84
T. KOSTER
AND
J. P. VANDENBROUCKE
droplets and faeces. But imagine also that only one person in so manv becomes massively symptomatic (or massively infectious). That situation would be analogous to tuberculosis in which only about 1 in 20 persons who develops open cavitarv tuberculosis transmits the disease. Still. it remains enigmatic why children are not more affected. ROUTE OF TRANSMISSION
Faecal-oral spread is postulated by Graham and co-workers [15], mostly because the seroprevalence curves look very similar to other well known faecaloral transmitted diseases like poliomyelitis and hepatitis A. Also the difference in sero-epidemiology between WNestern and non- W'estern societies reveals the same pattern for the three diseases mentioned, and relates to personal hygiene and sanitation [16]. However, this similarity does not close the argument since the same reasoning serves for oral-oral transmitted disease like Epstein-Barr V'irus infections, tuberculosis and other respiratory diseases. Lee and colleagues recently suggested that the inter-oral route is the more likely one [17]. They found that infected coprophagous mice did not spread the disease to their uninfected congeners. whereas infected dogs did. It was reasoned that dogs are more likely to have oral-oral contacts (saliva, vomit) than mice. who do not vomit and are coprophagous. I)espite this very interesting study one must keep in mind that H. pylori can travel along the alimentary tract, since some reports show that H. pylori can be cultured from gastric metaplasia areas in the rectum anid from Meckel's diverticula. However. until now it has never been cultured from the stools.
C(ONCLUiSION Despite many interesting investigations and progress made, the epidemiology of H. pylori infections remains blanketed in fog. More secure information about the increase in prevalence with age, i.e. is there really a sudden jump in early adulthood, or is there a monotonic increase from childhood to senescence, might help to clear away the fog. )L EANEENTS ACKNOWNY'LE
The authors thank RI. A. Veenendaal, gastroenterologist, and R. Bosboom. microbiologist, for their enthusiastic discussions. REFERENCEiES 1. Mitchell HM.1 Bohane TD. Berkowicz J. Hazell SL, Lee A. Antibody to (anpylobacter Pylori in families of index children with gastrointestinal illness due to C. pylori. Lancet 1987;
2: 681-2. 2. De Cothi GA, Newbold KMI, O'Conner HJ. Campylobacter-like organisms andt heterotopic gastric mucosa in Meckel's diverticula. J Clin Pathol 1989; 42: 132-4. 3. Krajden S. Fuska M. Anderson J. Examination of human stomach biopsies. saliva and dental plaque for Canmpylobacter pylori. J Clin Microbiol 1989: 27: 1397-8.
Epidemiology of Helicobacter pylori
85
4. Perez-Perez GI, Dworkin BM, Chodos JE, Blaser MJ. Campylobacter pylori antibodies in humans. Ann Intern Med 1988; 109: 11-17. 5. Evans DJ, Evans DG, Graham DY, Klein PD. A sensitive and specific test for detection of 'ampylobacter pylori infection. Gastroenterologv 1989; 96: 1004-8. 6. Sobala GM, Crabtree JE, Dixon MF, et al. Acute Helicobacter pylori infection: clinical features, local and systemic immune response, gastric mucosal histology, and gastric juice ascorbic acid concentrations. Gut 1991; 32: 1415-18. 7. Morris AJ, Rafiq Ali M, Nicholson GI, Perez-Perez GI, Blaser MJ. Long-term follow-up voluntary ingestion of Helicobacter pylori. Ann Intern Med 1991; 114: 662-3. 8. Doolev CP, Cohen H, Fitzgibbons PL. et al. Prevalence of helicobacter pylori infection and histologic gastritis in asymptomatic persons. N Engl J Med 1989; 321: 1562-6. 9. Fiedorek SC, Malaty HD, Evans DL, et al. Factors influencing the epidemiology of Helicobacter pylori infection in children. Pediatrics 1991; 88: 578-82. 10. Drumm B, Perez-Perez GI, Blaser MJ, Sherman PM. Intrafamilial clustering of Helicobacter pylori infection. N Engi J Med 1990; 322: 359-63. 11. Jones DM, Eldridge J, Whorwell PJ. Antibodies to Campylobacter pyloridis in household contacts of infected patients. Br Med J 1987; 294: 615. 12. Perez-Perez GI, Witkin SS. Decker MID. Blaser M1J. Seroprevalence of helicobacter infection in couples. J Clin Microbiol 1991; 29: 642-4. 13. Malaty HM, Graham DY. Klein PD, Evans I)G, Adam E, Evans D)J. Transmission of Helicobacter pylori infection; studies in families of healthy individuals. Scand J Gastroenterol 1991; 26: 927-32. 14. Graham DY, Malaty HM, Evans DG, Evans DJ, Klein PD, Adam E. Epidemiology of Helicobacter pylori in an asymptomatic population in the United States: effect of age, race and socioeconomic status. Gastroenterology 1991: 100: 1495-501. 15. Graham DY. Helicobacter pylori: its epidemiology and its role in duodenal ulcer disease. J Gastroenterol Hepatol 1991; 6: 105-13. 16. Graham DY, Adam E, Reddy GT, et al. Seroepidemiology of Helicobacter pylori infection in India: comparison of developing and developed countries. Dig Dis Sci 1991; 36: 1084-8. 17. Lee A. Fox JG, Otto G. D)ick EH, Krakowka S. Transmission of Helicobacter spp. A challenge to the dogma of faecal-oral spread. Epidemiol Infect 1991; 107: 99-109.
