Scientists have long pondered how the bubonic plague bacterium, Yersinia pestis, can cause both stable, enzootic diseases in rodents and sudden, deadly disease outbreaks that decimate the same rodent populations. A new study concludes that the difference could be related to the fleas that transmit Y. pestis between animals. Fleas with just one early-stage infection are not sufficient to initiate epizootic disease among most wild rodents and promote a more stable enzootic state, according to this week’s publication in PLOS Pathogens by Joseph Hinnebusch of the National Institute of Allergy and Infections Diseases Rocky Mountain Study Laboratories, USA and Colleagues.
The plague primarily affects rodents, including rats, mice, gerbils, squirrels, marmots, and prairie dogs. The bacteria circulate within these host populations through several species of flea vectors. Fleas can transmit Y. pestis at various stages after an infectious blood meal; Transmission can occur as early as the next meal, a phenomenon known as early-stage transmission. Later, “blockage-dependent transmission” occurs after Y. pestis forms a bacterial biofilm in the flea’s digestive system, blocking the flow of an incoming meal and causing blood to flow back into the bite site after mixing with the biofilm.
In the new study, the researchers empirically evaluated for the first time the relative efficiency of the different phases of transmission by individual fleas. Cohorts of a ground squirrel flea, Oropsylla montana, were infected by feeding on mouse or rat blood infected with Y. pestis. The transmission efficiency of individual O. montana fleas was then measured over a four-week period, and the researchers created models to show how this efficiency would translate to the spread of Y. pestis through a rodent population.
Results showed that blockage-dependent transmission is much more efficient than early-stage transmission in terms of transmission probability, number of bacteria transmitted, and ability to drive an animal disease outbreak. The models showed that transmission in the early phase could only trigger an animal disease in naïve, very susceptible host populations and with high flea loads. In addition, the small dose of bacteria typically transmitted early in transmission can ‘immunize’ many individuals, promoting an enzootic state.
“Our models suggest that exposure of most wild rodents to sublethal, immunizing doses of Y. pestis transmitted during the early phase can mitigate the rapid spread of animal diseases by reducing the number of susceptible individuals in the population ‘ say the authors. “In many situations, early-stage transmission may be more important in maintaining the enzootic state than in promoting animal disease.”
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