A joint IFISC (UIB-CSIC) and University of Buenos Aires project analyses how spatial distribution affects underwater epidemics involving immobile hosts. Published in the Royal Society Open Science journal, the study builds on a previous epidemiological model developed by an interdisciplinary collaboration between IFISC (UIB-CSIC), IMEDEA (UIB-CSIC), LIMIA (IRFAP) and INAGEA, as published in Ecological Modelling. This new model reproduces epizootics: epidemics that simultaneously attack a large number of individuals of one or several animal species, factoring in the state of the parasites acting as infectious agents. The authors validate the model with data from the noble pen shell (Pinna nobilis) epidemic, transmitted between individuals by the parasite Haplosporidium pinnae, which has decimated this particular mollusc population in the Mediterranean Sea. The new article explores the model’s properties in a more realistic scenario where pathogen transmission is conditioned by host spatial distribution and parasite mobility, as well as the stochastic effects that occur in small systems.
When studying transmission in epidemics (such as Covid-19), the so-called SIR model is the most widely used. In this approach, all individuals in the population are divided into three categories that give the model its name: S- Susceptible to infection, I- Infected and R- Removed (those who cannot be reinfected either because they have acquired immunity or died). These three elements suffice to describe many epidemics of contact diseases, where infected individuals cannot be reinfected (such as chickenpox). Marine epidemics, however, involve subtler processes, especially when the host is unable to move because it is a sessile organism and the disease needs the assistance of another organism or mechanism to infect new individuals. The authors of the study propose an evolution of the classical SIR model by adding a new state—P— to represent parasitic concentration in marine environments. The SIRP model, alongside similar recently published models, is a new development since previous models only considered the host state. Infection in sessile hosts (such as noble pen shells) does not occur by direct contact but, rather, by production and excretion of parasites by infected individuals that healthy (Susceptible) hosts absorb through filtration. Parasites are produced and excreted into the marine environment, where they remain infectious until they die or are absorbed by hosts. In other words, in parasite-borne marine diseases, parasites have a dual role: not only are they agents that induce infection but also act as vectors that transmit the disease from an infected immobile host to a susceptible one.
Including an additional parameter makes the model more complex in terms of analytical processing, although the authors demonstrate that the SIRP model can be simplified to an SIR model under certain conditions, representing an original contribution of the study. The model was validated with available experimental data for the recent noble pen shell (Pinna nobilis) epidemic caused by the Haplosporidium pinnae parasite, and demonstrated that the reduced SIR model is able to fit the data. Thus, even if species suffering epidemics cannot move (such as the noble pen shell) and contagion is due to parasites, under certain conditions it is possible to study transmission as if by direct contact. Once the model was validated with experimental data, the researchers analysed the impact of spatial transmission. They found that parasite mobility affects epidemiological conditions: greater mobility facilitates transmission between specimens and leads to more severe epidemics with more rapid extinctions. A future goal will be to analyse the actual spatial distributions of noble pen shells.
The authors conclude that this novel approach may be useful for understanding emerging diseases in shellfish species with a high economic and ecological value. Parasite-borne pathogens are responsible for some of the most significant and consistent marine disease epizootics on record, and are considered to be the main epidemics of concern for the global seafood industry.
Àlex Giménez-Romero, Amalia Grau, Iris E. Hendriks, Manuel A. Matías (2021). Modelling parasite-produced marine diseases: The case of the mass mortality event of Pinna nobilis, Ecological Modelling, 459. https://doi.org/10.1016/j.ecolmodel.2021.109705
Àlex Giménez-Romero, Federico Vázquez, Cristóbal López, Manuel A. Matías (2022). Spatial effects in parasite-induced marine diseases of immobile hosts. R. Soc. Open Sci., 9, 212023. https://doi.org/10.1098/rsos.212023
Photo: Arnaud Abadie