Alejandro Gómez

A Community-Based Seroepidemiologic Study of Helicobacter pylori Infection in Mexico

A nationwide community-based survey for Helicobacter pylori infection had not been done. This study sought to determine the seroprevalence of infection in Mexico, and the socioeconomic and demographic variables that are risk factors for infection. The survey assessed 11,605 sera from a sample population representing persons ages 1-90 years from all socioeconomic and demographic levels and from all regions of Mexico. Antibodies against H. pylori were studied by ELISA using whole cell antigen. Among the findings were that 66% of the population was infected and that age was the strongest risk factor for infection. By age 1 year, 20% were infected and by age 10 years, 50% were infected. Crowding (odds ratio [OR], 1.4), low educational level (OR, 2.42), and low socioeconomic level (OR, 1.43) were risk factors for infection. Prevalence was similar in urban and in rural communities (OR, 0.95). This study is the largest community-based seroepidemiologic study of H. pylori to date.

Reassessment of the epidemiology of amebiasis: State of the art

The epidemiology of amebiasis has dramatically changed since the separation of Entamoeba histolytica and Entamoeba dispar species, and the worldwide prevalence of these species has not been estimated until recently. The most cited data regarding prevalence, morbidity, or mortality due to amebiasis is the 1986 Walsh report, in which 100,000 deaths are reported to occur worldwide each year due to medical complications of invasive amebiasis. However, the prevalence values of Entamoeba histolytica infection could be completely erroneous since the estimations were performed prior to the molecular characterization of E. histolytica and E. dispar species. Moreover, Entamoeba moshkovskii, another morphologically indistinguishable human parasitic Entamoeba, was not mentioned or considered as a contributor to the prevalence figures in endemic areas. However, recent available prevalence and morbidity data obtained through molecular techniques allow the construction of a more reliable map of endemic regions of amebiasis all over the world [the Asian subcontinent (India, Bangladesh), Africa, Asian Pacific Countries (Thailand, Japan), South and Central America (Mexico, Colombia)]. The epidemiology of infectious diseases focuses on identification of factors that determine disease distribution in time and space, transmission factors responsible for the disease, clinical manifestations, and progression in the host, with the goal being the design of realistic intervention and prevention strategies in a reasonable period of time. In the present review, we will describe how molecular tools have made actual knowledge regarding the epidemiology of amebiasis possible. We will also analyze the most relevant available data on prevalence, morbidity, geographic distribution, patterns of transmission, exposure, and risk factors for infection in the human host. Our intention is to emphasize the recent molecular typing methods applied in genotyping Entamoeba species and strains, and to assess their value and limitations. Finally, we will discuss those areas of the host-parasite relationship that are still not fully understood, and the scientific challenges to approach this important public health problem in the future.

Validation of a Serologic Test for the Diagnosis of Helicobacter pylori Infection and the Immune Response to Urease and CagA in Children

Objective:Little is known about Helicobacter pylori infections and the immune response to urease and CagA in pediatric populations. Our aims were: 1) to validate serological assays for antibodies against whole cell extract, CagA, and urease of H. pylori; 2) to examine their role in diagnosis of infection in children with recurrent abdominal pain (RAP); and 3) to examine the antibody responses to CagA and urease in children.Methods:An enzyme-linked immunosorbent assay (ELISA) for diagnosis of H. pylori infection using whole cell extracts was validated in 50 children with biopsy-confirmed infection. The IgG and IgA antibody responses against recombinant CagA and urease were compared by ELISA in 82 children with RAP and in 246 age- and sex-matched healthy children.Results:The whole-cell extract ELISA had a sensitivity of 85% and specificity of 87%. Children with RAP were more infected with H. pylori than were healthy control subjects; however, IgG and IgA CagA seropositivity was lower among those with RAP than among asymptomatic children (34% and 23%vs 76% and 55%, respectively; p < 0.0001). In both groups of children, the immune response to urease was low.Conclusion:A serodiagnosis of H. pylori infection using native strains was developed. The difference in the immune response between children with RAP and control subjects suggests that RAP occurs during the acute phase of H. pylori infection. Our results also suggest that urease is a poor immunogen.

High Rates of Recurrence and of Transient Reinfections of Helicobacter pylori in a Population With High Prevalence of Infection

OBJECTIVES:Little is known concerning the magnitude of reinfection versus recrudescence of Helicobacter pylori (H. pylori) infection after eradication treatment. The aims of this study were to determine the magnitude of H. pylori reinfection versus recrudescence, and to identify possible risk factors for reinfection.METHODS:Children and adults with upper GI symptoms treated at the Centro Médico Nacional Siglo XXI (Instituto Mexicano del Seguro Social, in Mexico City, Mexico) were studied. H. pylori infection was diagnosed with urea breath test (UBT), histology, and culture. Infected patients received triple therapy, and those who became UBT negative 4–6 wk after treatment were considered as eradicated and were included in the study. A cohort of 141 patients in whom the disease was eradicated was monitored for recurrence with UBT at 3, 6, 9, 12, 18, and 24 months. H. pylori was isolated from gastric biopsy samples before treatment and at recurrence and isolates compared by genotyping.RESULTS:During this period, 32 (22.7%) cases of recurrence were documented the majority occurring during yr 1. In nine of the 32 (28.1%) cases, recurrence was eradicated spontaneously, suggesting these were transient reinfections. Recurrence rates were significantly higher in the subjects 41–60 yr of age than in younger or older subjects. H. pylori isolates from 12 recurrence cases were genotyped; nine (75%) were classified as true reinfection and three as recrudescence.CONCLUSIONS:In our population, recurrence rate is high in adults and transient reinfection is common. In several cases, reinfection occurred by multiple strains, which suggests that soon after eradication, patients are exposed to multiple sources of reinfection.

Diarrhea morbidity and mortality in Mexican children: impact of rotavirus disease.

Aim: To analyze changes in prevalence and seasonality of diarrhea morbidity and mortality and to evaluate the impact of rotavirus disease among Mexican children younger than 5 years old. Methods: Diarrhea surveillance was performed from 1990 to 2002. Rotavirus testing was performed on stool specimens from 1996 to 2002. Data were obtained from different surveillance systems considering a nationwide representation in Mexico. Diarrhea morbidity and mortality rates were analyzed against time to determine trends or seasonal patterns. Results: Improvement of surveillance for all diarrhea episodes denoted an initial morbidity increase from 1995 to 1999, followed by a decrease by 2002, without any seasonal pattern. However, from 1990 to 1995, morbidity for severe diarrhea decreased 63%. From 1996 to 2002, 62–68% of severe diarrhea episodes occurring during the fall-winter season (FWS) were rotavirus-positive compared with 6–12% in the spring-summer season (SSS). From 1990 to 2002, diarrhea mortality decreased 84%. Higher mortality rates for children younger than 1 year old coincided precisely during the FWS, annually. Both severe diarrhea episodes and diarrhea deaths denoted a changing seasonal pattern. In 1990–1991, 2 waves of increased diarrhea activity occurred. The increase in SSS was much more pronounced than that in FWS. From 1992 to 1995 for severe diarrhea and from 1993 to 2002 for diarrhea deaths, the SSS frequencies subsequently reduced, whereas the FWS peaks remained annually. Conclusions: A significant reduction in morbidity and mortality of severe diarrhea has occurred from 1990 and 2002 in Mexican children younger than 5 years old. This is a consequence of preventive programs initiated for cholera control since 1991, which had greater impact on SSS diarrhea and limited response for FWS diarrhea, when rotavirus is mainly present. Currently rotavirus diarrhea requires new prevention strategies and specific control measures, such as a specific national vaccine program.

Seroepidemiology of Hepatitis E Virus Infection in Mexican Subjects 1 to 29 Years of Age

BACKGROUND Hepatitis E virus (HEV) infection causes an acute, self-limited hepatitis associated with high mortality in pregnant women. Community-based surveys are scarce and information on HEV infection in populations is needed. The aim of this work was to study seroprevalence to HEV in young adults and children in Mexico, using a community-based survey. METHODS Serum samples from 3,549 individuals were studied; the population included subjects from 1 to 29 years old from all regions of the country representing all socioeconomic levels. IgG anti-HEV was determined by ELISA. RESULTS Anti-HEV antibodies were found in 374 (10.5%) individuals. Seroprevalence increased with age from 1.1% in children younger than 5 years to 14.2% in persons 26 to 29 years of age (p = 0.006). Risk factors for infection included living in rural communities and a low educational level. Seroprevalence was not associated with the level of regional development. CONCLUSIONS HEV infection is endemic in Mexico. Age, type of community, and educational level were identified as risk factors for infection.

Novelties on Amoebiasis: A Neglected Tropical Disease

In accordance with the 1997 documents of the World Health Organization (WHO), amoebiasis is defined as the infection by the protozoan parasite Entamoeba histolytica with or without clinical manifestations. The only known natural host of E. histolytica is the human with the large intestine as major target organ. This parasite has a very simple life cycle in which the infective form is the cyst, considered a resistant form of parasite: The asymptomatic cyst passers and the intestinal amoebiasis patients are the transmitters; they excrete cysts in their feces, which can contaminate food and water sources. E. histolytica sensu stricto is the potentially pathogenic species and E. dispar is a commensal non-pathogenic Entamoeba. Both species are biochemical, immunological and genetically distinct. The knowledge of both species with different pathogenic phenotypes comes from a large scientific debate during the second half of the 20th century, which gave place to the rapid development of diagnostics technology based on molecular and immunological strategies. During the last ten years, knowledge of the new epidemiology of amoebiasis in different geographic endemic and non-endemic areas has been obtained by applying mostly molecular techniques. In the present work we highlight novelties on human infection and the disease that can help the general physician from both endemic and non-endemic countries in their medical practice, particularly, now that emigration is undoubtedly a global phenomenon that is modifying the previous geography of infectious diseases worldwide.

The effect of formalin fixation on the polymerase chain reaction characterization of Entamoeba histolytica

Formalin fixation is the most common storage, transportation and preservation method for stool samples. However, fixation dramatically reduces our ability to extract from stool samples DNA that is a suitable template for polymerase chain reaction (PCR)-based diagnostic tests. In this study we evaluated the effects of formalin concentration and of the time stored in fixative on the success of PCR amplification. We found that the deleterious effects of formalin are both time and concentration dependent and may result from fragmentation of fixed DNA during its purification.

Genotypic testing for HIV-1 drug resistance using dried blood samples

In third-world countries, dried blood samples (DBS) are a convenient alternative to plasma for monitoring viral load during HIV-1 therapy. In this study, we evaluated the feasibility of using DBS to perform HIV-1 drug resistance genotyping in a ViroSeq assay in which the protease and reverse transcriptase regions of the pol gene are analyzed. Fifty-seven antiretroviral genotypes from plasma samples were tested, and drug resistance genotypes were determined. Only 38.6% paired DBS samples were sequenced. Failure to amplify DNA from DBS samples generally correlated with plasma viral loads below log10 5.1. The majority of the mutations identified in plasma pol sequences were also found in their DBS counterpart, with a concordance in genotype interpretation of 96.4%. Several factors were identified that could potentially improve both the sensitivity and the quality of genotype data, such as sample storage conditions and sequence analysis. Therefore, DBS sampling is useful to determine viral load and drug resistance genotypes in HIV patients.

Susceptibility to varicella-zoster infection in individuals 1 to 29 years of age in Mexico.

BACKGROUND The prevalence of varicella zoster virus (VZV) infection has been poorly studied in Latin America. The aim of this work was to study the seroprevalence of antibody to VZV infection in Mexico. Infection was determined in 3,737 individuals. METHODS Samples were collected during a national serologic survey performed during 1987-1988 and represented individuals 1- to 29-years-old from all socioeconomic levels and from rural and urban communities throughout the country. Antibodies anti-VZV were measured with a commercially available enzyme-linked immunosorbent assay (VARELISA Merck, Germany). RESULTS In the population studied, 464 individuals (12.4%) were seronegative or susceptible to infection, whereas about 17.0% of individuals 1- to 19-years-old were susceptible to infection. Between the ages of 20 to 24 years, 8.4% were susceptible and between 25 to 29 years, 5.0% of persons were still susceptible to infection. CONCLUSIONS Socioeconomic level, density of population, crowding, and gender were not found as risk factors for susceptibility to VZV infection in adolescents and young adults. Low educational level was found as a risk factor for susceptibility. High proportions of adolescents and young adults in Mexico are susceptible to VZV infection and should receive special attention when designing vaccination programs.