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Event: 1939

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Viral infection and host-to-host transmission, proliferated

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Viral infection, proliferated

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
viral release from host cell increased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Sars-CoV-2 IFN-I antiviral antagonism leading to infection proliferation AdverseOutcome Arthur Author (send email) Under development: Not open for comment. Do not cite Under Development


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
mink Mustela lutreola High NCBI
cat Felis catus High NCBI
rhesus macaque Macaca mulatta High NCBI
dog Canis lupus familiaris Moderate NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
All life stages High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Unspecific High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Much is now understood in terms of human-to-human COVID-19 transmission. Coronaviruses, as with many other respiratory viruses, are transmitted primarily through respiratory droplets, but can also spread through aerosols, fecal-oral transmission, or contact with contaminated surfaces (Harrison et al. 2020). Respiratory droplets and aerosols containing the virus are generated through an infected person coughing, sneezing or talking, and enter the secondary host system through upper and lower respiratory tissues, with the lung being the primary tropism. Barriers to transmission in place worldwide include social distancing, face shields, cloth masks, frequent hand washing, and surface disinfection (Harrison et al. 2020).

Vaccination is the standard strategy for reducing or eliminating viral disease transmission, symptoms, and mortality in humans, and in some cases domesticated animals. However, the weight of evidence indicates that the reservoir species (bats in the case of betacoronaviruses) and potential intermediate hosts are wildlife, and different control measures will be required to prevent future spillover. Indeed, the intermediate host of the SARS-CoV-2 virus has yet to be identified (Delahay et al. 2021). This key event is therefore focused primarily on the species of potential concern, exposure and transmission routes across species, and the conditions indicative of or conducive toward cross-species spillover of zoonoses or infectious viral diseases of animal origin.

Species of Potential Concern

The reservoir host for SARS-CoV-2-like viruses is believed to be the bat.

Exposure and Transmission Routes

SARS-CoV-2-infected media (respiratory droplets, bodily fluids, tissues, feces): Exposure routes are the pathway into the body of the virus shed from an infected reservoir host animal to the intermediate host, or either type of host animal to humans. These routes may include inhalation, oral, or through broken skin or mucosal membranes (e.g., eyes, nostrils) after touching contaminated media or surfaces and then touching the face (Harrison et al. 2020). Animals may transfer saliva or nasal discharge directly through facial contact, licking or biting. Transmission occurs through these routes when the virus reaches a tissue with cells that allow entry and replication.

Spillover Conditions

Conditions that allow for exposure and transmission across species:

  • Close proximity of animal communities (bats to potential intermediate hosts; wildlife to domestic animal farms).
  • Direct human contact with wildlife (Johnson et al. 2015), including:
    • Zoos, wildlife farms, domesticated animal farms, feeding and animal care;
    • Hunting and dressing wild game;
    • Cleaning of storage buildings, barns, or other structures that may be used by wildlife for shelter, breeding, or feeding, with potential for feces or other contamination (CDC, 2021);
    • Wet markets where live animals or bush meat are traded;
    • Research facilities that express viruses from wild samples in cell culture, that house potential host species, or that collect and store bodily fluid or tissue samples.
  • Virus isolated from animal species shows genomic similarity to the human virus, but also high host plasticity to be capable of cross-species viral immune evasion and replication (Johnson et al. 2015).

Similar host genetics. Spillover species and new host species share genetic similarity in the components of the cell entry, immune system and replication machinery (Warren et al. 2019). That is, the virus can enter the cell and evade the virus detection and immediate systemic type I interferon (IFN) response to allow replication and generation of viral load in both species. The viral proteins must be capable of interacting with the appropriate cellular proteins in either species. The most studied and considered indicative of infectability is the ACE2 and other cell entry proteins.

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Either the virus or antibodies can be detected with available tests. Active infection can be detected through PCR tests from nasal swab, oropharyngeal swab, rectal swab or saliva samples that indicate the quantity and/or presence of the virus. Antibodies can be detected in blood using various assays including immunofluorescence.

ELISA, Indirect immunofluorescence assay (IIFA) for antibodies (Schlottau et al. 2020; Freuling et al. 2020)

Virus neutralization test (VNT) for antibodies (Schlottau et al. 2020; Freuling et al. 2020)

Quantitative reverse transcription PCR (qRT-PCR) for viral load (log10 genome copies) (Freuling et al. 2020)

Titration (Tissue culture infectious dose where 50% of infected cells display cytopathic effect [TCID50 assay]: levels of infectious virus, or viral titre) (Freuling et al. 2020)

Virus-specific immunoglobulin characterization (Freuling et al. 2020)

SARS-CoV-2 spike protein neutralizing antibodies in saliva from animals that developed serum antibodies (Freuling et al. 2020)

Serum sample, autopsy, histopathology for tissue lesions (Schlottau et al. 2020; Freuling et al. 2020)

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

Homo sapiens

Broad mammalian host range based on spike protein tropism for and binding to ACE2 (Conceicao et al. 2020; Wu et al. 2020) and cross-species ACE2 structural analysis (Damas et al. 2020). Some literature found on non-human hosts indicates that NSPs and accessory proteins can interact in a similar manner with bird (chicken) and other mammal proteins in the IFN-I pathway (Moustaqil et al. 2021; Rui et al. 2021).

Evidence for Perturbation by Stressor

Regulatory Significance of the Adverse Outcome

An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP. For KEs that are designated as an AO, one additional field of information (regulatory significance of the AO) should be completed, to the extent feasible. If the KE is being described is not an AO, simply indicate “not an AO” in this section.A key criterion for defining an AO is its relevance for regulatory decision-making (i.e., it corresponds to an accepted protection goal or common apical endpoint in an established regulatory guideline study). For example, in humans this may constitute increased risk of disease-related pathology in a particular organ or organ system in an individual or in either the entire or a specified subset of the population. In wildlife, this will most often be an outcome of demographic significance that has meaning in terms of estimates of population sustainability. Given this consideration, in addition to describing the biological state associated with the AO, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to describe regulatory examples using this AO. More help


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help

Under construction

Freuling CM, Breithaupt A, Müller T, et al. 2020. Susceptibility of Raccoon Dogs for Experimental SARS-CoV-2 Infection. Emerging Infectious Diseases. 26(12):2982-2985. doi:10.3201/eid2612.203733.

CDC, 2021.

Conceicao et al. 2020. The SARS-CoV-2 Spike protein has a broad tropism for mammalian ACE2 proteins. PLoS Biol 18(12): e3001016.

Damas et al. 2020. Broad host range of SARS-CoV-2 predicted by comparative and structural analysis of ACE2 in vertebrates. PNAS vol. 117 no. 36:22311–22322 

Moustaqil et al. 2021. SARS-CoV-2 proteases PLpro and 3CLpro cleave IRF3 and critical modulators of inflammatory pathways (NLRP12 and TAB1): implications for disease presentation across species, Emerging Microbes & Infections, 10:1, 178-195.

Rui et al. 2021. Unique and complementary suppression of cGAS-STING and RNA sensing-triggered innate immune responses by SARS-CoV-2 proteins. Sig Transduct Target Ther 6, 123.

Wu et al. 2020. Broad host range of SARS-CoV-2 and the molecular basis for SARS-CoV-2 binding to cat ACE2. Cell Discovery 6:68.