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|title=A Novel Representation of Terms Related to Infectious Disease Epidemiology for Epidemic Modeling: The Apollo Structured Vocabulary and Pre-existing Representations
|pdfUrl=https://ceur-ws.org/Vol-1327/icbo2014_paper_30.pdf
|volume=Vol-1327
|dblpUrl=https://dblp.org/rec/conf/icbo/BrochhausenHLBM14
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==A Novel Representation of Terms Related to Infectious Disease Epidemiology for Epidemic Modeling: The Apollo Structured Vocabulary and Pre-existing Representations==
ICBO 2014 Proceedings A novel representation of terms related to infectious disease epidemiology for epidemic modeling The Apollo Structured Vocabulary and pre-existing representations Mathias Brochhausen, Josh Hanna Shawn T. Brown Pittsburgh Supercomputing Center Division of Biomedical Informatics Carnegie Mellon University University of Arkansas for Medical Sciences Pittsburgh, USA Little Rock, USA mbrochhausen@uams.edu Michael M. Wagner, John D. Levander, Nicholas E. William R. Hogan Millett Department of Biomedical Informatics Department of Health Outcomes and Policy University of Pittsburgh University of Florida Pittsburgh, USA Gainesville, USA Abstract—The Apollo Structured Vocabulary (Apollo-SV) is a infection transmission, populations of hosts, interventions, and Web Ontology Language 2 (OWL 2) representation of terms the outcomes of infections. Using this input information— related to epidemic simulation. We are developing Apollo-SV by which we refer to as an infectious disease scenario—a ontological analysis of the information used and created by simulator’s algorithm computes the progression of one or more epidemic simulators and the entities this information is about. infections in one or more populations over time, under zero or A key finding of our analysis is that the input of an epidemic more interventions. The result of this computation—the output simulator is properly understood as (1) a representation of an of the simulator—is information on which decision makers can ecosystem at simulator time zero, (2) information about base policy or decisions about disease control. infectious diseases of interest in the ecosystem, and (3) information about plans to control the diseases. This insight is At present, each simulator uses its own representation of reflected in the scope of Apollo-SV, which includes terms from its input and output information. For example, the FRED the domains of both infectious disease epidemiology and simulator, version 2.0.1 [1] refers to the duration of school population biology. closure 2 as ‘school_closure_period’, whereas FluTE version We also found that some definitions in the Infectious Disease 1.15 [2] refers to it as ‘schoolclosuredays’. The differences Ontology (IDO), including ‘infection’, ‘infection acquisition’, make it difficult to compare simulators and re-use machine ‘infectious disease’, ‘pathogen’, and ‘host’, were not compatible readable information. For example, Halloran et al. spent 6 with the meanings of the terms as used in epidemic simulation; months creating a comparative study of three simulators [3]. thus, we created new definitions of these terms. Our analysis of epidemic simulators—which are To address this problem, we are developing machine- mathematical models of phenomena studied by infectious disease interpretable representations for the input and outputs of DTMs epidemiology—afforded several advantages that likely explain and promulgating their adoption as de facto standards. The key why we discovered limitations of IDO. As a result, we goal of the standards is to enable an analyst to specify the same recommend that development of biomedical ontologies intended infectious disease scenario exactly once, and run the scenario for reuse consider the perspective of the overlapping biological on multiple simulators with no additional effort. science(s) involved. Apollo-SV is freely available at: In this paper, we describe one element of our proposed http://purl.obolibrary.org/obo/apollo_sv.owl. standards—the Apollo Structured Vocabulary (Apollo-SV). Apollo-SV is an OWL 2 representation of terms related to Keywords—disease transmission models, epidemic simulators, epidemic simulation. The other two elements are an XML biomedical ontology, infectious disease epidemiology Schema Document (XSD), which defines the syntax for simulator input, and a database schema that defines the I. INTRODUCTION representation of simulator output. Apollo-SV defines the The science and practice of infectious disease epidemiology, terminology used in the XSD and database schema. These like climate science, is increasingly reliant on computational elements are described in Wagner et al. [4] simulation. The simulators—known as epidemic simulators or more generally disease transmission models (DTMs)1—require machine-interpretable information about pathogens, rates of 2 Closing schools is one infectious disease control strategy that simulators study for the control of influenza epidemics. The duration 1 DTMs also model endemic infections such as malaria. of the closure is the length of time during which schools are closed. 21 ICBO 2014 Proceedings II. METHODS In accordance with the Foundry principle of orthogonality, We developed Apollo-SV for use in a set of Web services which stipulates that a given term is defined only once across designed to improve access to epidemic simulators. We begin all ontologies, we search for and import pre-existing this section with an overview of these services, then detail our ontological representations into Apollo-SV. Besides importing methods to develop Apollo-SV, including conformance to entire ontologies, we import selected classes, individuals, and OBO Foundry Principles, methods to ensure validity for its properties using the Minimum Information to Reference an intended use, and multi-disciplinary development. External Ontology Term (MIREOT) [9] Protégé plugin that we developed [10]. The Apollo Web Services: Briefly, the Apollo Web Services are a set of Web services designed to allow a publicly available, We also adhere to Foundry naming conventions [11]. We Web-based, end-user application to access multiple epidemic edit our terms to (1) avoid connectives ('and', 'or'), (2) prefer simulators through requests to a single Broker service (Fig. 1). singular nouns, (3) avoid the use of negations, and (4) avoid catch-all terms such as "Unknown x". In Figure 1, the Simple End User Application (SEUA) creates an infectious disease scenario for simulation, encoded To help link the OWL file with the XSD, we create a in an XML document that conforms to the Apollo XSD, which Unique Apollo Label (UAL) for classes in Apollo-SV. The uses terminology defined by Apollo-SV. The SEUA invokes UAL is the exact XSD type or attribute name to which the the runSimulation() method of the Broker service with the class in Apollo-SV corresponds, for example, infectious disease scenario. The Broker service invokes the InfectiousDisease and basicReproductionNumber. Translator service, which translates the infectious disease Analysis of simulators’ input and output files, and scenario into the native terminology and syntax of the documentation: We analyzed the input and output files of four requested simulator(s). epidemic simulators. We also analyzed documentation, such as user guides and published papers. We reviewed terms that we extracted from these resources with the developers of the simulators to identify relevant but missing terms, to discover synonymy among terms, and to detect and resolve ambiguity. Validation by representation in XSD message syntax: We further refine our OWL DL representations by using the terms in the XSD representation as it progressively expands to be able to represent the input of four simulators. Validation by automatic translation: The process of developing the mappings from the XSD and Apollo-SV terms to the native language of the simulators identifies additional issues with Apollo-SV that we feed back into our analysis. Validation by implementation in a user application: The SEUA exposes Apollo-SV definitions and elucidations in tool tips that appear when the mouse hovers over a term. This view Fig. 1. The relationships of Apollo components and epidemic simulators. Apollo-SV defines the terminology used in Apollo XSD, which specifies the identifies problems with elucidations by placing them into the message syntax for the Web services [1]. The SEUA calls the Broker service context of an end-user configuring a simulator, and wanting to to configure simulators (messages passed along blue arrows) and to access understand what is meant by a term. simulator output (messages passed along red arrows). The Translator service translates Apollo messages to/from native simulator input/output. The SEUA Public release: To encourage adoption of Apollo-SV and to is available at http://research.rods.pitt.edu; the XSD is available at: allow external scientific review, comments, and requests for http://research.rods.pitt.edu/apollo-types_2.0.2.xsd. Purple ovals represent additions, we make Apollo-SV publicly available at Apollo standards; blue ovals represent Apollo-developed software that use the http://purl.obolibrary.org/obo/apollo_sv.owl, a permanent URL Apollo Web services; and red ovals represent entities interacting with Apollo. (PURL). We also ensure that Apollo-SV is easily accessible for Upper ontology: We import Basic Formal Ontology browsing and download at the Web-based Ontobee portal: (http://www.ifomis.org/bfo/1.1) into Apollo-SV as its upper http://www.ontobee.org/browser/index.php?o=APOLLO_SV. ontology [5]. The issue tracker and under-development version of Apollo- SV are located at our Google Code site. The PURL to the Conformance with OBO Foundry principles: We followed the development version of Apollo-SV is principles of the OBO Foundry in implementing Apollo-SV [6, http://purl.obolibrary.org/obo/apollo_sv/dev/apollo_sv.owl. 7]. Thus we release it in a common format, OWL 2 [8]. Multi-disciplinary development: The team developing Apollo- In accordance with Foundry principles, we write a textual SV comprises personnel with backgrounds in simulator definition for every term that we create. Because formal development, disease surveillance, medicine, biomedical ontological textual definitions often use the technical language informatics, medical terminologies, ontological engineering, of ontologists, we created an elucidation annotation for classes artificial intelligence, and formal logic. All these individuals, in Apollo-SV. The elucidation restates the definition in including a simulator developer (author SB), have been language more familiar to subject matter experts. We also axiomatize Apollo-SV terms wherever possible (e.g., Fig. 2-5). 22 ICBO 2014 Proceedings actively engaged in review of Apollo-SV, and their feedback TABLE II. CLASSES IN APOLLO-SV BY SUBDOMAIN guides design decisions. Domain Classes in Apollo-SV III. RESULTS Infectious disease Infection Infection acquisition epidemiology Pathogen Host Overall, Apollo-SV has 594 classes: 287 that we created Latent period Infectious period new in Apollo-SV and 307 that we imported: 57 via MIREOT Contaminated thing Contamination acquisition (Table I) and 250 from entire ontologies. The number of Contamination imported classes is artificially high because the import of entire Infectious disease Basic reproduction ontologies brings classes into Apollo-SV we do not require. scenario number Transmission Transmission probability TABLE I. RE-USE OF CLASSES AND OBJECT PROPERTIES FROM PRE- coefficient EXISTING ONTOLOGIES IN APOLLO-SV VIA MIREOT. Disease Infectious disease control Ontology (by Classes Object Total transmission model strategy OBO Foundry Properties Susceptible Exposed population namespace) population Infectious Resistant population UBERON 7 1 8 population OMRSE 26 7 33 Population biology Ecosystem Biotic ecosystem GO 1 0 1 OGMS 11 0 11 Abiotic ecosystem Community OBI 9 5 14 Population Population census IDO 3 7 10 Population Abiotic ecosystem census Totals 57 20 77 infection and immunity census The core classes in Apollo-SV represent key entities of interest to infectious disease epidemiology and population abnormality to immunocompetent organisms of the same biology (Table II). Throughout the course of developing the Species as the host (the organism corresponding to the Apollo standard, we reached the conclusion that the input to an extended organism) through transmission of a member or epidemic simulator is properly understood as a representation offspring of a member of the infectious agent population. of an ecosystem at simulator time zero, with additional However, epidemic simulators represent infection as a information about infectious diseases and planned or ongoing process, because that is how ‘infection’ is defined in infectious interventions to control them. This conclusion motivates the disease epidemiology. For example, [13, 14] define ‘infection’ inclusion in Apollo-SV of terms from population biology. In as the invasion of a host organism's tissue by pathogens, the turn, the ecosystem viewpoint heavily influenced our definitions of key terms in infectious disease epidemiology. multiplication of those pathogens, and the reaction of the host’s tissue(s) to the pathogens and the toxins they produce. At present, Apollo-SV and the XSD enable configuration of three epidemic simulators with the same infectious disease Also, the IDO definition requires that an infection cause scenario in the SEUA. We are piloting a fourth simulator. They clinical abnormality in an individual of a particular species. are (1) a compartmental model developed by authors MMW, However, infectious disease epidemiology recognizes the NEM, and JDL (disease agnostic); (2) the FRED model existence of species that do not experience clinical developed by the University of Pittsburgh Public Health abnormalities when infected with a particular pathogen. The Dynamics Laboratory in collaboration with the Pittsburgh importance in epidemic simulation is that members of species Supercomputing Center and the School of Computer Science at that can experience clinical abnormalities when infected can Carnegie Mellon University, University of Pittsburgh and acquire infection with the pathogen from a ‘carrier’ species. Imperial College (influenza A in humans); and (3) the FluTE model developed by the University of Washington and Fred Apollo-SV defines ‘infection’ as: A reproduction of a Hutchinson Cancer Research Center in Seattle and the Los pathogen in (a part of) the tissue of an organism from another Alamos National Laboratories (influenza A in humans). species (Fig. 2). With respect to Foundry orthogonality, we attempted to This biologically-grounded definition recognizes that two reuse IDO’s definitions of ‘infection’, ‘pathogen’, ‘host’, but species are interacting and—from the pathogen species point of had to create new definitions (and thus new representations) for view—infection is a process of reproduction. The definition them in Apollo-SV as discussed in the following sections. only requires reproduction of one species within the tissues of an individual (organism) from another species. A. Infection B. Infection Acquisition (reformulation of Transmission IDO defines infection as a material entity that is: Process) A part of an extended organism that itself has as part a IDO imports two definitions of ‘transmission process’ from population of one or more infectious agents and that is (1) the Transmission Ontology: clinically abnormal in virtue of the presence of this infectious agent population, or (2) has a disposition to bring clinical 1. A process that is the means during which the pathogen is transmitted directly or indirectly from its natural reservoir, a susceptible host or source to a new host. 23 ICBO 2014 Proceedings 2. Suggested definition: A process by which a pathogen Apollo-SV defines ‘infection acquisition’ as: The passes from one host organism to a second host organism biological process of pathogen organism(s) entering (the body of the same Species. of) a host organism from a contagious host or a contaminated thing and reproducing using host resources. As with our definition of ‘infection’, this definition is biologically grounded and recognizes that from the pathogen species’ point of view, infection acquisition is the entry into a host and the beginning of reproduction there. Note that Apollo- SV’s definition of ‘contaminated thing’ is general and includes natural reservoirs, vector organisms that are not infected (a.k.a. mechanical vectors), and fomites like contaminated pencils. C. Host IDO defines ‘host’ as: An organism bearing a host role This definition is not sufficient in and of itself to understand what IDO refers to by ‘host’. It is also necessary to review its definitions of ‘host role’ and ‘extended organism’: 1. ‘Host role’: A role borne by an organism in virtue of the fact that its extended organism contains a material entity other than the organism. 2. ‘Extended organism’: An object aggregate consisting of an organism and all material entities located within the Fig. 2. Representation of the equivalent class axiom for ‘infection’ in organism, overlapping the organism, or occupying sites Apollo-SV. Boxes represent named classes, boxes with curved bases represent anonymous classes, arrows represent object properties. In the boxes is the formed in part by the organism. rdfs:label and the namespace of the source ontology, if different from Apollo- Under these definitions, any organism that has an artificial SV. Each arrow is labeled with the rdfs:label of the property it represents. joint, a penny in its gut, or an arrow through its chest is a host. The second, “suggested definition” erroneously restricts The fact that a person with a prosthetic knee is a “host” is transmission to occur only between two hosts of the same counterintuitive. This definition is too admissive for our use species. It is thus not usable in infectious disease epidemiology cases (and for clinical medicine, too): any foreign material or any other science that deals with cross-species transmission. entity inside the organism’s body renders the organism a host. The first definition of ‘transmission process’ has two major In addition, from the ontological perspective, we doubt problems. The first problem is that it is circular, defining there is any such entity as host role. First, according to BFO, a ‘transmission process’ in terms of a pathogen being role is manifested or realized in one or more processes. transmitted, with no definition of ‘transmitted.’ However, because there is no representation of the infection process in IDO, infection cannot be the realization. No other The second problem is an ontological one. It attributes to process in IDO suffices, either. If there is no process that one process the property of being the means by which realizes a role, then by definition of ‘role’, there is no role. something else happens. For example, assume droplet spread of infection from one host to another by a sneeze. This Apollo-SV defines host as: An organism that has as part definition equates the sneeze with the transmission process. some tissue that is the location of an infection (Fig. 3). That is, it says that only the sneeze exists, but it also has the We therefore distinguish pathogen and host based on which property of “having transmitted the pathogen”. However, one is reproducing inside tissue (pathogen) and which one is equating the sneeze to the transmission process is nonsensical the location of the reproduction (host). because for transmission to be complete, the second host must have an infection. But this infection will not begin for minutes D. Pathogen to hours after the sneeze is over. The sneeze cannot somehow IDO defines ‘pathogen’ as: A material entity with a extend itself in time until an infection is established, but pathogenic disposition. conversely not extend in time when no infection results. There exist two distinct processes: the sneeze and the transmission. Again, this definition requires the definitions of other terms to understand its meaning: We also had the insight that it is only the second host who acquires the infection that undergoes a change during the 1. ‘Pathogenic disposition’: A disposition to initiate process. Therefore, we chose to rename it ‘infection processes that result in a disorder. acquisition’. We recognize that we are diverging from standard terminology in the field, but anyone wishing to add an 2. ‘Disorder’: A material entity which is clinically alternative label to the infection acquisition class in Apollo-SV abnormal and part of an extended organism. Disorders could do so without changing the meaning of the class. are the physical basis of disease. 24 ICBO 2014 Proceedings Fig. 3. Representation of the equivalent class axiom for "host" in Apollo-SV. Fig. 4. Representation of the equivalent class axiom for "pathogen" in The graphical representation is analogous to Fig. 1. Apollo-SV. The graphical representation is analogous to Fig. 1. Thus per IDO any material that causes injury is a pathogen, We set a high priority on implementing Apollo-SV in a including the endotoxin of Clostridium difficile or an overdose Web service for three reasons. First, we wanted to demonstrate of acetaminophen. IDO does have an infectious agent class as a the capability to initialize multiple simulators with one subtype to pathogen that refers specifically to organisms that infectious disease scenario to motivate the adoption of Apollo. enter into a host cause injury. But this definition is not how In addition, we wanted to lower barriers to adoption by making infectious disease epidemiology uses the term ‘pathogen’. available a reference implementation. Lastly, implementation is the basis of our iterative development and refinement IDO also asserts pathogens must typically cause disease. process that ensures Apollo is production ready and flexible, However, attenuated poliovirus used in oral polio vaccines which also lowers the barriers to adoption. infects the gut mucosa of humans and thus is a pathogen (or infectious agent per IDO), but it causes disease in only one per A key insight from our iterative development of Apollo-SV 2.7 million first doses of vaccine. and XSD is that a simulator configuration is properly understood as a representation of an ecosystem at a particular Apollo-SV defines ‘pathogen’ as: A material entity that is time. This insight led us to include in Apollo-SV key terms the bearer of a disposition that, when realized, is realized as an from population biology, such as ‘ecosystem’ and ‘census’. infection (Fig. 4). Furthermore, it led us to our redefinition of ‘infection’, which E. Infectious disease was central to redefining other IDO terms. IDO defines ‘infectious disease’ as: A disease whose physical basis is an infectious disorder. Per IDO, infectious disorder is a subytpe of infection. However, we require a representation of infectious disease that is consistent with our definition of ‘infection’ as a process. But because IDO defines ‘infection’ and thus by inheritance ‘infectious disorder’ as a material entity, we could not reuse this definition of ‘infectious disease’. Apollo-SV defines ‘infectious disease’ as: A disease that inheres in a host and, when realized, is realized as a disease course that is causally preceded by an infection (Fig. 5). This definition is compatible with the OBO Foundry definition of disease, which is in the Ontology of General Medical Science (OGMS) [15]. We thus were able to reuse the OGMS definition of disease, in keeping with the Foundry principle of orthogonality. Fig. 5. Representation of the equivalent class axiom for "infectious disease" Note that the disease inheres only in the host. From the in Apollo-SV. The graphical representation is analogous to Fig. 1. pathogen’s perspective, there is no clinical abnormality (which is a necessary condition to meet the definition of disease in A key result of our development of Apollo-SV was that we OGMS). For the pathogen, infection is perfectly normal. could not reuse IDO definitions of ‘infection’, ‘host’, ‘pathogen’, and ‘infectious disease’, and thus we created the IV. DISCUSSION definitions presented here. This result was surprising because Apollo-SV version 2.0.1 is an ontology for use in representing we had anticipated reuse of IDO at the outset of Apollo-SV DTM input and output. It includes core terms from infectious development. Given that we did not expect this result, it is disease epidemiology and population biology. Apollo-SV worth considering the possible reasons behind it. currently supports the representation of infectious disease A key reason is that our concentration on how terms are scenarios that can be run on three epidemic simulators and the used in biological sciences—especially population biology— results of the simulations. exposed many issues. This focus differed fundamentally from 25 ICBO 2014 Proceedings IDO’s concentration on how the terms are used in clinical potential to generate the XSD from the ontology, a successful medicine. In particular, our focus led us to a requirement to strategy in other projects [18]. represent the process of infection as opposed to the steady- state, material-entity view of IDO. ACKNOWLEDGMENTS So what then led us to the perspective of population This work was funded by award R01GM101151 from the biology? We believe the reason we reached this perspective, as National Institute for General Medical Sciences (NIGMS). well as ontological clarity elsewhere in Apollo-SV, is that This paper does not represent the view of NIGMS. This work working with epidemic simulators quickly brought into view used the Protégé resource, which is supported by grant the key phenomena studied (that are also of relevance to GM10331601 from NIGMS. epidemic simulation) and their fundamental nature. These REFERENCES simulators are mathematical models in the field of infectious [1] J. J. Grefenstette, S. T. Brown, R. Rosenfeld, et al. FRED (A disease epidemiology. They have explicit ontological Framework for Reconstructing Epidemic Dynamics): an open-source commitments that have been rigorously vetted through peer software system for modeling infectious diseases and control strategies review of research using the models (as well as the models using census-based populations. 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