Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Epidemiological model of Microsporidia

A model of N.bombi epidemiology has been developed using a classical susceptible-infected scheme. This approach is very powerful even though no explicit genetics is taken into account and individual traits neither vary in the host nor in the parasite population.

The general structure of the model reflect parasite transmission events as well as host and parasite mortality rates. Groups of individuals within a colony are classified as healthy (susceptible) or infected. In addition, there are several distinct classes: larvae (L), pupae (P), workers (W), young queens (Q) and males (M). Furthermore, spores of the parasite (Nosema) are contained outside hosts in a reservoir inside the colony and a reservoir outside the colony (i.e. in the field). Colonies in a population are connected by horizontal transmission of the parasite and produce successive generations of hibernating queens.

Transmission events are modeled in form of the mass action principle. At a certain time (year), an infection enters the population with an infected queen (that may have migrated from somewhere else: in the example below, this is in year 3). An infected queen has a certain chance to found a colony (for the initial queen it is assumed that colony is established so that the infection enters the population). Hence, in later years, all infected queens can give raise to a colony but not all of them will succeed as determined by a stochastic filter.

Once the infected colony grows, the parasite will spread from the mother queen to her larvae and workers. In turn, infected workers will spread the parasite into the field reservoir, from where it can be picked up by a worker from another colony. In this way, the parasite spreads within and among colonies as the season progresses. At the end of the season, a certain number of the daughter queens that are produced in the population will be infected. They over-winter with the same probability as the non-infected ones (according to our previous unpublished results).

Hence, the model describes, both, an epidemic within a typical colony and an epidemic among colonies over years.The model is still based on a number of estimated parameters because conclusive quantified data is lacking for parasite transmission within and between colonies. The ambition of the project was to integrate the mathematical model with the recommendations fror disease control in breeding facilities. As the model still needs further refinements this has not been possible.

Nevertheless, the Pollinator Parasites project has laid the foundation for a mathematical model describing the relationships between the host and the parasite.

This model will be published in scientific media and developed into a tool that will be further refind and exploited. Undoubtedly, a model that captures the host-parasite adaptions is of wide interest not only to bumble bee breeders, but to scientists working on pests and diseases in pollinating social insects. This model represents a major step forward for the implementation of epidemiological theory into host-parasite relations in social insects.

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