Exposure to Helicobacter pylori-positive Siblings and Persistence of Helicobacter pylori Infection in Early Childhood

Abstract

Objectives

Cross-sectional studies suggest that Helicobacter pylori may be transmitted between siblings. The present study aimed to estimate the effect of a Helicobacter pylori infected sibling on the establishment of a persistent Helicobacter pylori infection.

Methods

The authors used data collected from a Texas-Mexico border population from 1998–2005 (the “Pasitos Cohort Study”). Starting at age 6-months, Helicobacter pylori and factors thought to be associated with Helicobacter pylori were ascertained every six month for participants and their younger siblings. Hazard ratios were estimated from proportional hazards regression models with household dependent modeling.

Results

Persistent Helicobacter pylori infection in older siblings always preceded persistent infection in younger siblings. After controlling for mother’s Helicobacter pylori status, breastfeeding, antibiotic use and socioeconomic factors, a strong effect was estimated for persistent Helicobacter pylori infection in an older sibling on persistent infection in a younger sibling [Hazard Ratio (HR): 7.6, 95% Confidence Interval (CI): 1.6, 37], especially when the difference in the age of the siblings was less than or equal to 3 years (HR: 16, 95% CI: 2.5, 112).

Conclusions

These results suggest that when siblings are close in age, the older sibling may be an important source of Helicobacter pylori transmission for younger siblings.

INTRODUCTION

Helicobacter pylori (H. pylori) is one of the most common chronic bacterial infections worldwide; it is estimated that more than half of the world’s population is infected with H. pylori (1). It is widely accepted that a persistent H. pylori infection may lead to serious disease outcomes such as gastric cancer, peptic ulcers and other gastrointestinal diseases (2). A persistent H. pylori infection is characterized by long-term colonization of the stomach, often for decades, up to a lifetime, regardless of developing an acquired immune response in the absence of antibiotic therapy (3). Newly acquired H. pylori infections often appear to be transient in very young children (4, 5), and it is not known what causes many H. pylori infections to persist into adulthood in the absence of directed treatment.

H. pylori’s mode of transmission is not well understood. Person-to-person is the most likely mode of transmission and evidence has demonstrated that oral-oral, fecal-oral and gastric-oral may be significant transmission routes (2). Previous cross-sectional studies have suggested that siblings may be a source of H. pylori transmission, although none of these studies have followed a large cohort of children throughout the first years of life (6–8). Few studies have longitudinally examined factors which influence H. pylori transmission. Past studies have been conducted on H. pylori transmission in children (6, 7) and H. pylori persistence (9, 10), but studies which prospectively examine sibling-to-sibling transmission and its role in acquiring a persistent infection are lacking. Therefore, our objective was to estimate the effect of exposure to a H. pylori infected (or persistently infected) sibling on the acquisition of a persistent H. pylori infection in early childhood.

MATERIALS AND METHOD

Data utilized in this analysis was collected from the Pasitos Cohort Study, a previously described prospective birth cohort study conducted from April 1998 through December 2005 at sites along the United States-Mexico border (11). Pasitos Cohort Study participants were drawn from Juarez, Chihuahua, Mexico and the communities of Socorro and San Elizario located in El Paso County, Texas. These sites were selected to investigate how diet, hygiene, socioeconomic and other household factors may play a role in the incidence and persistence of H. pylori infection in developed and developing countries (11, 12). Additional details pertaining to the study design have previously been reported elsewhere (11, 12).

The Pasitos Cohort Study began by recruiting pregnant women who were receiving Women, Infants and Children government assistance at San Elizario and Socorro clinics in El Paso County or who were seeking prenatal benefits at the Mexican Social Security Institute maternal-child clinics in Juarez (11).

The study and the informed consent was approved by the Committee for the Protection of Human Subjects at the University of Texas Health Science Center at Houston, and the institutional review boards at the Mexican Social Security Institute, the University of Texas at El Paso, and the Texas Department of Health. Upon informed consent, baseline information was collected by questionnaire; blood samples and breath samples were also collected as previously described (4, 11). Briefly, a questionnaire was administered to the pregnant women to ascertain basic demographic information as well as factors relating to socioeconomic status and hygiene. Questionnaires were available both in Spanish and English to allow for the language preference of the mother. While the index child was in utero, H. pylori status of the mother was assessed using the commercially available enzyme immunoassay kit HM-CAP (High-molecular-mass cell-associated protein enzyme immunoassay) previously described (12), which tests for IgG (Immunoglobulin G) antibodies against H. pylori.

The first follow-up visit was targeted at six months after the birth of the index child, the main subject of the study; follow-up visits were conducted at target intervals of six months thereafter. All younger siblings born during the study were invited to enroll. At all follow-up visits for index children and younger siblings, a questionnaire was administered to ascertain study factors, including illness history, antibiotic use and dietary intake during the preceding six month interval (12). H. pylori status was determined during each follow-up visit using the ¹³C urea breath test with Klein’s correction as previously described (4).

For younger siblings, information regarding baseline study factors was obtained via a baseline questionnaire adapted for younger siblings. Younger siblings’ follow-up visits occurred concurrently with the older siblings, rather than at targeted six month age multiples.

Survival analysis was utilized to estimate the effect of having a H. pylori infection (or a persistent H. pylori infection) in the older (index) sibling on the rate of acquiring an incident (or persistent) infection in a younger sibling. The direction of effect from younger to older siblings was not examined for reasons explained in further detail below. The proportional hazards regression model was used to obtain hazard ratios and 95% confidence intervals (CI). Since the observations within a household are not independent, we used the method proposed by Lee EW and colleagues (13) to adjust for the dependence within the household. This model uses a working independent model with a general estimation equation approach.

The first incident H. pylori infection and persistent infection in the younger sibling were used as the outcome variables in our models. An incident case was defined as a positive H. pylori breath test result after a previous negative test (or birth, if the first test was positive). Persistence was defined as three consecutive positive H. pylori breath test results with no subsequent negative test results. All breath test results including the first and last results were counted and utilized in the analysis. In relation to each outcome, the main exposures of interest were any H. pylori positivity and persistent positivity in the older (index) siblings. In households with three or more siblings in the study, these exposure variables (any H. pylori positivity and persistent positivity) came from the index subjects, and not from the second or subsequent siblings.

The time at risk (or time to event) variable was defined for the initial acquisition of H. pylori infection and persistent infection in the younger siblings. Given interval-censored infection onsets, time of initial acquisition of infection was estimated as the midpoint between the immediately preceding negative breath test and the first positive breath test. For younger siblings who tested positive at their first follow-up visit, the time of risk for the initial acquisition was defined as the midpoint between birth and the first visit. Time of acquisition of a persistent infection was defined as the midpoint between the first and third consecutively positive breath tests. For subjects who did not become infected (or persistently infected), the sum of all time intervals in which data was collected was defined as the time at risk of infection (or persistence). The proportionality assumption was checked by plotting the hazard of each exposure by person-time in months.

Several variables were included as possible confounders in the regression models. Socioeconomic status was controlled for based on a priori knowledge of which socioeconomic variables are associated with H. pylori frequency, and therefore included the variables of mother’s years of education and household size (14). The mother’s baseline H. pylori status (seropositivity) (15), the number of months the child was breastfed (14) and number of antibiotic courses taken (14) were also adjusted for based on a priori information. The difference in age between the siblings and the country of residence (United States or Mexico) were treated as effect modifiers by stratifying the models on these factors.

RESULTS

In the Pasitos Cohort Study, 472 index subjects and 143 younger siblings returned for follow-up and had one or more breath tests. A total of 3,114 follow-up visits were conducted, with index children attending an average of 5.5 visits and younger siblings attending an average of 3.7 visits. The mean follow-up time, starting at 3–9 months of age, was 3.8 years for index children and 2.5 years for younger siblings. Among index subjects with younger siblings, the average age at last follow-up was 5.6 years. One or more valid ¹³C urea breath test results were obtained from 468 index subjects (99%). Of these, 212 (45%) acquired H. pylori at least once during follow-up and 34 (7%) became persistently infected, while of 136 siblings with complete data for the current analysis, 49 (36%) acquired H. pylori and 10 (7%) became persistently infected.

H. pylori seropositivity status at baseline was obtained for 454 (96%) mothers in the Pasitos Cohort Study. Of these mothers, antibodies against H. pylori were detected in 283 (63%). Maternal H. pylori seropositive status occurred in 135 (64%) households with an infected index child and 30 (88%) households with a persistently infected index child.

Of 212 index subjects who acquired H. pylori, there was no instance in which the younger sibling’s H. pylori acquisition preceded that of the index subject. In all instances except four, the first incident infection in the older index sibling was acquired prior to the first incident infection in a younger sibling; in the other four occasions, the first incident infection was acquired simultaneously. All 34 cases of persistent H. pylori infection in the index subject preceded the acquisition of a persistent infection in the younger sibling. Therefore, we only present the results of analyses examining the effect of incident and persistent infections of H. pylori in the direction of the older index sibling’s status affecting incident and persistent infections of H. pylori in the younger sibling.

Results of survival analysis controlling for mother’s H. pylori status, mother’s education, household size, number of antibiotic courses, and months breastfed revealed that younger siblings whose older sibling was infected with H. pylori were roughly just as likely to be infected with H. pylori as those who did not have an infected older sibling (HR: 1.1, 95% CI: 0.61, 1.8) (Table 1). However, a younger sibling was nearly three times more likely to acquire an H. pylori infection when their older index sibling had a persistent infection compared to younger siblings whose older index sibling did not have a persistent H. pylori infection (HR: 2.8, 95% CI: 1.6, 4.9) (Table 1). Stratified analysis showed that the effect of exposure to a persistently infected older sibling on the acquisition of H. pylori in younger siblings was greater in the United States (HR: 3.5, 95% CI: 1.4, 9.1) than in the Mexico (HR: 2.5, 95% CI: 1.0, 5.9), and among siblings close in age (≤3 years apart) (HR: 3.7, 95% CI: 1.5, 8.7) versus those farther apart in age (HR: 2.4, 95% CI: 0.92, 6.2) (Table 1).

Table 1.

Effect of exposure to an H. pylori-infected older sibling on a younger sibling acquiring an incident H. pylori infection, Pasitos Cohort, Juarez, Mexico/El Paso, Texas, 1998–2005.

Exposure variable	Incident Infections (n)a	Person Time (Months)b	Hazard Ratioc	95% CIc,d
Overall	49	3113
Infected older siblinge	31	1911	1.1	0.61, 1.8
Uninfected older sibling	18	1202	1.0
Persistently infected older siblingf	13	307	2.8	1.6, 4.9
Not persistently infected older sibling	36	2806	1.0
≤3 years of age between siblings	25	1885
Infected older siblinge	14	1064	1.4	0.54, 3.6
Uninfected older sibling	11	821	1.0
Persistently infected older siblingf	6	140	3.7	1.5, 8.7
Not persistently infected older sibling	19	1745	1.0
> 3 years of age between siblings	24	1228
Infected older siblinge	17	847	1.2	0.53, 2.6
Uninfected older sibling	7	381	1.0
Persistently infected older siblingf	7	167	2.4	0.92, 6.2
Not persistently infected older sibling	17	1061	1.0
Mexico	34	1972
Infected older siblinge	19	1301	0.80	0.41, 1.6
Uninfected older sibling	15	671	1.0
Persistently infected older siblingf	6	144	2.5	1.0, 5.9
Not persistently infected older sibling	28	1828	1.0
United States	15	1141
Infected older siblinge	12	610	2.0	0.62, 6.7
Uninfected older sibling	3	531	1.0
Persistently infected older siblingf	7	163	3.5	1.4, 9.1
Not persistently infected older sibling	8	978	1.0

^aNumber of incident infections in the numerator of the crude rate

^bPerson months used in the denominator of the crude rate

^cConfidence Interval

^dAdjusted for mother’s H. pylori status, mother’s years of education, household size, number of months of breastfeeding, and number of antibiotic courses.

^eOne or more urea breath tests positive for H. pylori during follow-up.

^fThree or more consecutive urea breath tests positive for H. pylori during follow-up.

Younger siblings were at an even greater risk of acquiring a persistent infection if their older index sibling had a persistent infection. Younger siblings whose older index sibling had a persistent infection were roughly eight times more likely to become persistently infected compared to those whose older index sibling was not persistently infected with H. pylori (HR: 7.6, 95% CI: 1.6, 37) (Table 2). The effect of exposure to an older persistently-infected sibling on persistent infection in a younger sibling was greater in Mexico (HR: 37, 95% CI: 6.0, 224) than in the United States (HR: 8.9, 95% CI: 0.74, 107), and greater among siblings close in age (≤3 years) (HR: 17, 95% CI: 2.5, 113) than those farther apart in age (HR: 1.8, 95% CI: 0.15, 21).

Table 2.

Effect of exposure to a persistently H. pylori-infected older sibling on a younger sibling acquiring a persistent H. pylori infection, Pasitos Cohort, Juarez, Mexico/El Paso, Texas, 1998–2005.

Exposure variable	Persistent Infections (n)a	Person Time (Months)b	Hazard Ratioc,d	95% CIc,d
Overall	10	3906
Persistently infected older siblinge	6	552	7.6	1.6, 37.0
Not persistently infected older sibling	4	3354	1.0
≤3 years of age between siblings	6	2265
Persistently infected older siblinge	4	237	17	2.5, 113
Not persistently infected older sibling	2	2028	1.0
> 3 years of age between siblings	4	1641
Persistently infected older siblinge	2	315	1.8	0.15, 21
Not persistently infected older sibling	2	1326	1.0
Mexico	5	2466
Persistently infected older siblinge	4	228	37	6.0, 224
Not persistently infected older sibling	1	2238	1.0
United States	5	1440
Persistently infected older siblinge	2	324	8.9	0.74, 107
Not persistently infected older sibling	3	1116	1.0

^aNumber of persistent infections in the numerator of the crude rate

^bPerson months used in the denominator of the crude rate

^cConfidence Interval

^dAdjusted for mother’s H. pylori status, mother’s years of education, household size, number of months of breastfeeding, and number of antibiotic courses.

^eThree or more consecutive urea breath tests positive for H. pylori during follow-up.

In considering the other covariates in our model for the effect of exposure to an older infected sibling on the incident infection of a younger sibling, none of the covariates appeared to have an independent effect on the incident infection in the younger sibling (Table 3). These results are similar when evaluating other covariates in estimating the effect of exposure to a persistently infected older sibling on the incident infection of a younger sibling (Table 3).

Table 3.

Hazard ratios for the effect of exposure to an H. pylori-infected older sibling on a younger sibling acquiring an incident or persistent H. pylori infection and model covariates.

	Younger sibling acquiring incident infection - Model A		Younger sibling acquiring incident infection Model - B		Younger sibling acquiring persistent infection
	Hazard Ratiob	95% CIa, b	Hazard Ratiob	95% CIa, b	Hazard Ratioc	95% CIa, c
Infected older index sibling	1.1	0.61, 1.8
Persistently infected older index sibling			2.8	1.6, 4.9	7.6	1.6, 37.1
Mother’s Education(per year increase)	0.95	0.83, 1.1	0.96	0.85, 1.1	0.74	0.50, 1.1
Household Size(per person increase)	1.1	0.94, 1.3	1.1	0.96, 1.3	0.86	0.58, 1.3
Mother’s H. pylori Seropositivity (baseline)	1.2	0.62, 2.2	0.98	0.52, 1.9	3.9	0.69, 22.7
Antibiotic Courses(per course increase)	0.92	0.69, 1.2	0.95	0.73, 1.2	1.1	0.53, 2.3
Months Breastfed(per month increase)	1.0	0.95, 1.0	1.0	0.96, 1.0	0.93	0.72, 1.2

^aConfidence Interval

^bOverall person-time was 3,113 person months

^cOverall person-time was 3,283 person months

When taking into account the effect of exposure to an persistently infected older sibling on the persistent H. pylori infection of a younger sibling, other covariates in the model indicate that increasing household size (HR: 0.86, 95% CI: 0.58, 1.3) and years of education obtained by the mother (HR: 0.74, 95% CI: 0.50, 1.1) appeared somewhat protective (Table 3). We observed that a younger sibling was almost four times more likely to become persistently infected with H. pylori if the mother was seropositive for H. pylori at the beginning of the study compared to a younger sibling whose mother was seronegative for H. pylori at the beginning of the study, however the confidence interval was wide and the direction of the association less certain (HR: 3.9, 95% CI: 0.69, 22.7) (Table 3).

DISCUSSION

Due to the role children are thought to play in H. pylori transmission, our aim was to estimate the effect of exposure to H. pylori infection (or a persistent infection) in siblings on the establishment of a persistent H. pylori infection. Our findings are consistent with person-to-person transmission occurring from older to younger siblings living in the same household. We found that the establishment of an H. pylori infection occurred first in the older sibling and later in the younger sibling, but never vice versa. Having an H. pylori-infected older sibling was associated with an increased rate of acquiring a persistent H. pylori infection in a younger sibling, and having a persistently infected older sibling increased the rate of acquiring a persistent infection in a younger sibling by approximately eight-fold. Further, persistent infections were most likely to be transmitted between siblings closer in age. A 17-fold increased rate of persistent infection was observed when the difference in the age of the siblings was three years or less. All of these effects were observed after controlling for the mother’s H. pylori infection and other potential confounders.

Previous cross-sectional studies have also suggested that family composition, specifically siblings, may influence H. pylori transmission (6, 8). In the study by Goodman and Correa, the odds of a prevalent H. pylori infection in Aldana, Colombia increased as the number of two to nine year-old siblings in the household increased, and H. pylori infection was especially likely when siblings were less than four years apart in age (6). In the study by Kivi et al, shared strains of H. pylori were most commonly observed among siblings, followed by mother-child relationships (8). These studies were limited by their cross-sectional design, and therefore were not able to evaluate the time sequence for H. pylori transmission or persistence.

The present prospective cohort study provides evidence to suggest that transmission occurs from the older to the younger siblings. It also estimates rate ratios for the effect of an older sibling’s infection on the incidence and persistence of a younger sibling’s infection. H. pylori transmission between siblings was the focus of the present analysis as mother’s H. pylori status was not associated with the incident infection of the older index sibling in previous Pasitos Cohort analyses (16). Although in the current study the seropositivity status of the mother was associated with the persistent H. pylori status of the younger sibling, the effect estimate was not as large as observed for the persistently infected older (index) sibling and the direction of effect was less certain. While there have been reports examining populations in which strong associations between mother’s and children’s infection status was observed, this is not consistent across populations and study designs (6).

We should mention that the wide confidence intervals for the hazard ratios from the stratified analyses indicate a lack of precision for stratum-specific estimates (Tables 1 and 2). Thus, though the results strongly support the claim of association, which we believe is causal, estimates of the strength of associations are imprecise. We were not able to simultaneously stratify on country of origin and the siblings’ age difference due to sparse numbers and the resulting instability of the models. Additionally, we did not have bacterial genotype information to examine the specific strain of H. pylori in the siblings to provide further evidence to support the hypothesis that the older sibling was the source of the younger siblings’ infection. As such, we cannot rule out the influence of other household members besides the mother and siblings enrolled in our study.

This study contributes to the overall knowledge regarding transmission patterns of H. pylori and provides evidence to suggest possible unilateral transmission of H. pylori in early childhood from older sibling to younger siblings, particularly when they are close in age. Although further cohort studies that examine the specific strain of H. pylori in household members may provide more direct evidence of transmission routes within families, our study provides a strong basis and direction for additional research possibly leading to more effective treatment and prevention strategies.