Reply: Epidemiologic studies need asymptomatic controls
Authors: S.L. Becker, F. Chappuis, K. Polman, E.K. N'Goran, L. von Müller, J. Utzinger
Journal: Clinical Microbiology and Infection (2015)

We thank Dubourg and Fenollar [[1]Dubourg G. Fenollar F. Epidemiologic studies need asymptomatic controls.Clin Microbiol Infect. 2015; 21: e51-e52Abstract Full Text Full Text PDF Scopus (14) Google Scholar] for their interest in our work pertaining to the complex aetiology of persistent diarrhoea in the tropics and the challenges of differential diagnosis [2Becker S.L. Vogt J. Knopp S. Panning M. Warhurst D.C. Polman K. et al.Persistent digestive disorders in the tropics: causative infectious pathogens and reference diagnostic tests.BMC Infect Dis. 2013; 13: 37Crossref PubMed Scopus (63) Google Scholar, 3Becker S.L. Chatigre J.K. Gohou J.P. Coulibaly J.T. Leuppi R. Polman K. et al.Combined stool-based multiplex PCR and microscopy for enhanced pathogen detection in patients with persistent diarrhoea and asymptomatic controls from Côte d'Ivoire.Clin Microbiol Infect. 2015; 21: 591.e1-591.e10Abstract Full Text Full Text PDF Scopus (51) Google Scholar]. We agree that the inclusion of asymptomatic controls in epidemiological studies is pivotal to deepen our understanding of the aetiology of infectious diseases, which in turn will guide patient management and public health action. Three additional points are offered for consideration. First, it is important to note that the epidemiology of infectious diseases varies depending on the social–ecological context [4Heesterbeek H. Anderson R.M. Andreasen V. Bansa S. De Angelis D. Dye C. et al.Modeling infectious disease dynamics in the complex landscape of global health.Science. 2015; 347 (aaa4339)Crossref PubMed Scopus (365) Google Scholar, 5Utzinger J. N’Goran E.K. Caffrey C.R. Keiser J. From innovation to application: social–ecological context, diagnostics, drugs and integrated control of schistosomiasis.Acta Trop. 2011; 120: S121-S137Crossref PubMed Scopus (171) Google Scholar]. Hence, there is a need to carefully validate whether insights gained in typical Western settings pertaining to the aetiology and differential diagnosis of infectious disease syndromes may readily be transferred to resource-constrained settings of tropical and subtropical areas. For example, a recent study has shown considerable variation in the pathogenesis and antimicrobial resistance patterns of Staphylococcus aureus isolates giving rise to skin and soft tissue infections (SSTIs) across continents, and the authors have called for the development and implementation of region-specific recommendations for an adequate clinical management [[6]Nurjadi D. Friedrich-Jänicke B. Schäfer J. Van Genderen P.J.J. Goorhuis A. Perignon A. et al.Skin and soft tissue infections in intercontinental travellers and the import of multi-resistant Staphylococcus aureus to Europe.Clin Microbiol Infect. 2015; 21: 567.e1-567.e10Abstract Full Text Full Text PDF Scopus (66) Google Scholar]. We believe that a syndrome-specific approach to investigate common clinical problems in low- and middle-income countries is more appropriate than studies or interventions targeted at single pathogens. Apart from SSTIs, fever of unknown origin, respiratory tract infections, neurological disorders and diarrhoeal diseases are frequently encountered in the tropics, and much needs to be done to elucidate their aetiology and the impact on public health in resource-constrained settings. An international research consortium with the acronym NIDIAG (http://nidiag.org) is currently conducting multicentric studies in Africa and Asia to investigate the aetiology, symptomatology and clinical management of three clinical syndromes, i.e. (i) persistent digestive disorders (defined a persistent diarrhoea ≥14 days in individuals aged >12 months and/or persistent abdominal pain in children and adolescents younger than 18 years) [[2]Becker S.L. Vogt J. Knopp S. Panning M. Warhurst D.C. Polman K. et al.Persistent digestive disorders in the tropics: causative infectious pathogens and reference diagnostic tests.BMC Infect Dis. 2013; 13: 37Crossref PubMed Scopus (63) Google Scholar]; (ii) persistent fever (≥7 days) [[7]Yansouni C.P. Bottieau E. Chappuis F. Phoba M.F. Lunguya O. Ifeka B.B. et al.Rapid diagnostic tests for a coordinated approach to fever syndromes in low-resource settings.Clin Infect Dis. 2012; 55: 610-611Crossref PubMed Scopus (17) Google Scholar]; and (iii) neurological disorders [[8]Yansouni C.P. Bottieau E. Lutumba P. Lutumba P. Winkler A.S. Lynen L. et al.Rapid diagnostic tests for neurological infections in central Africa.Lancet Infect Dis. 2013; 13: 546-558Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar]. Insights gained during these studies will be utilised to develop evidence-based diagnosis–treatment algorithms that will improve patient management at the peripheral healthcare level. Second, Dubourg and Fenollar emphasise that the setting- specificity of infectious diseases calls for the inclusion of healthy controls whenever the causative agents of communicable diseases are studied. In our view, this suggestion is particularly important in studies that analyse samples stemming from body compartments that are not normally sterile, as, for example, the gastrointestinal and the respiratory tract, and for conditions that cannot be reliably diagnosed by well-validated and specific tests. With regard to the aforementioned NIDIAG study on persistent digestive disorders [[2]Becker S.L. Vogt J. Knopp S. Panning M. Warhurst D.C. Polman K. et al.Persistent digestive disorders in the tropics: causative infectious pathogens and reference diagnostic tests.BMC Infect Dis. 2013; 13: 37Crossref PubMed Scopus (63) Google Scholar], we have adopted a case–control approach. Case–control studies are less suitable than cohort studies to investigate causal relationships and are more prone to bias, but they nevertheless provide relevant scientific information on the association of certain pathogens with specific clinical symptoms [[9]Schulz K.F. Grimes D.A. Case–control studies: research in reverse.Lancet. 2002; 359: 431-434Abstract Full Text Full Text PDF PubMed Scopus (304) Google Scholar]. In the case of digestive disorders, our recent study from Côte d’Ivoire has brought to light that no clear link exists between most pathogens and the symptomatology of the infected individual and that multiple coinfections are the norm rather than the exception [[3]Becker S.L. Chatigre J.K. Gohou J.P. Coulibaly J.T. Leuppi R. Polman K. et al.Combined stool-based multiplex PCR and microscopy for enhanced pathogen detection in patients with persistent diarrhoea and asymptomatic controls from Côte d'Ivoire.Clin Microbiol Infect. 2015; 21: 591.e1-591.e10Abstract Full Text Full Text PDF Scopus (51) Google Scholar]. Future work should therefore not only focus on the assessment of a range of specific pathogens, but should also address more general perturbations of the intestinal microbiome. A recent study aimed to identify all bacterial and fungal microorganisms that could be cultured from faecal samples stemming from 347 individuals with or without diarrhoea in Senegal [[10]Samb-Ba B. Mazenot C. Gassama-Sow A. Dubourg G. Richet H. Hugon P. et al.MALDI-TOF identification of the human gut microbiome in people with and without diarrhea in Senegal.PLoS One. 2014; 9: e87419Crossref PubMed Scopus (40) Google Scholar]. Strikingly, the number of detected bacterial species was significantly lower in individuals with diarrhoea, a finding that could partially be explained by a decrease of commensal species that form part of the ‘normal’ intestinal gut flora. It is likely that the application of new diagnostic techniques such as metagenomics and culturomics will further our understanding of the complex pathogenesis of digestive diseases. Third, the routine use of molecular techniques such as polymerase chain reaction (PCR) assays in epidemiological studies investigating diarrhoeal diseases has generated unexpected and surprising results. For decades, several pathogens such as Shigella spp. were thought to be obligate pathogens, meaning that no asymptomatic colonisation would occur. However, through the application of highly sensitive stool-based multiplex PCR, we have now learned that a considerable proportion of healthy individuals may also be diagnosed with these organisms, particularly in settings of high endemicity. Prompted by this observation in PCR-based studies, Frickmann et al. [[11]Frickmann H. Schwarz N.G. Rakotozandrindrainy R. May J. Hagen R.M. PCR for enteric pathogens in high-prevalence settings. What does a positive signal tell us?.Infect Dis (Lond). 2015; 47: 491-498Crossref PubMed Scopus (35) Google Scholar] have raised the question, ‘What does a positive signal tell us?’ Thus far, no definite answer can be given, but it is likely that both asymptomatic colonisation and prolonged shedding of pathogen nucleic acids (e.g. after a resolved infection) contribute to the high rates of detected pathogens in PCR-based studies. Comprehensive comparative studies that concurrently use highly sensitive PCR tests, bacterial stool culture, parasite microscopy and immunological rapid diagnostic tests may help to develop standards for the clinical interpretation of PCR-positive tests in endemic settings. Taken together, and in view of our own data and recent experience, we agree with the proposal made by Dubourg and Fenollar that the uncharted epidemiology of many infectious diseases in tropical settings calls for the inclusion of asymptomatic controls. Furthermore, it is likely that the implementation of new, highly sensitive molecular diagnostics will further improve our understanding of the pathogenesis of important clinical syndromes in resource-constrained settings. We hope that this knowledge can be translated into simple, setting-specific clinical–diagnostic algorithms, such as those being developed by the NIDIAG consortium. This work is part of the NIDIAG European research network (Collaborative Project), supported by the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement 260260. All authors report no conflicts of interest relevant to this letter.