Variation in microparasite free-living survival and indirect transmission can modulate the intensity of emerging outbreaks

Virulence and transmission biology of the widespread, ecologically important pathogen of zooplankton, Spirobacillus cienkowskii

Spirobacillus cienkowskii (Spirobacillus, hereafter) is a widely distributed bacterial pathogen that has significant impacts on the population dynamics of zooplankton (Daphnia spp.), particularly in months when Daphnia are asexually reproducing. However, little is known about Spirobacillus' virulence, transmission mode, and dynamics. As a result, we cannot explain the dynamics of Spirobacillus epidemics in nature or use Spirobacillus as a model pathogen, despite Daphnia's tractability as a model host. Here, we work to fill these knowledge gaps experimentally. We found that Spirobacillus is among the most virulent of Daphnia pathogens, killing its host within a week and reducing host fecundity. We further found that Spirobacillus did not transmit horizontally among hosts unless the host died or was destroyed (i.e., it is an "obligate killer"). In experiments aimed at quantifying the dynamics of horizontal transmission among asexually reproducing Daphnia, we demonstrated that Spirobacillus transmits poorly in the laboratory. In mesocosms, Spirobacillus failed to generate epidemics; in experiments wherein individual Daphnia were exposed, Spirobacillus' transmission success was low. In the (limited) set of conditions we considered, Spirobacillus' transmission success did not change with host density or pathogen dose and declined following environmental incubation. Finally, we conducted a field survey of Spirobacillus' prevalence within egg cases (ephippia) made by sexually reproducing Daphnia. We found Spirobacillus DNA in ~40% of ephippia, suggesting that, in addition to transmitting horizontally among asexually reproducing Daphnia, Spirobacillus may transmit vertically from sexually reproducing Daphnia. Our work fills critical gaps in the biology of Spirobacillus and illuminates new hypotheses vis-à-vis its life history.

IMPORTANCE

Spirobacillus cienkowskii is a bacterial pathogen of zooplankton, first described in the 19th century and recently placed in a new family of bacteria, the Silvanigrellaceae. Spirobacillus causes large epidemics in lake zooplankton populations and increases the probability that zooplankton will be eaten by predators. However, little is known about how Spirobacillus transmits among hosts, to what extent it reduces host survival and reproduction (i.e., how virulent it is), and what role virulence plays in Spirobacillus' life cycle. Here, we experimentally quantified Spirobacillus' virulence and showed that Spirobacillus must kill its host to transmit horizontally. We also found evidence that Spirobacillus may transmit vertically via Daphnia's seed-like egg sacks. Our work will help scientists to (i) understand Spirobacillus epidemics, (ii) use Spirobacillus as a model pathogen for the study of host-parasite interactions, and (iii) better understand the unusual group of bacteria to which Spirobacillus belongs.

Evolutionary Invasion Analysis of Modern Epidemics Highlights the Context-Dependence of Virulence Evolution

Models are often employed to integrate knowledge about epidemics across scales and simulate disease dynamics. While these approaches have played a central role in studying the mechanics underlying epidemics, we lack ways to reliably predict how the relationship between virulence (the harm to hosts caused by an infection) and transmission will evolve in certain virus-host contexts. In this study, we invoke evolutionary invasion analysis-a method used to identify the evolution of uninvadable strategies in dynamical systems-to examine how the virulence-transmission dichotomy can evolve in models of virus infections defined by different natural histories. We reveal peculiar patterns of virulence evolution between epidemics with different disease natural histories (SARS-CoV-2 and hepatitis C virus). We discuss the findings with regards to the public health implications of predicting virus evolution, and in broader theoretical canon involving virulence evolution in host-parasite systems.

Heterogeneity in network structure switches the dominant transmission mode of infectious diseases

Several recent emerging diseases have exhibited both sexual and nonsexual transmission modes (Ebola, Zika, and mpox). In the recent mpox outbreaks, transmission through sexual contacts appears to be the dominant mode of transmission. Motivated by this, we use an SIR-like model to argue that an initially dominant sexual transmission mode can be overtaken by casual transmission at later stages, even if the basic casual reproduction number is less than one. Our results highlight the risk of intervention designs which are informed only by the early dynamics of the disease.

Out of the 'host' box: extreme off-host conditions alter the infectivity and virulence of a parasitic bacterium

Disease agents play an important role in the ecology and life history of wild and cultivated populations and communities. While most studies focus on the adaptation of parasites to their hosts, the adaptation of free-living parasite stages to their external (off-host) environment may tell us a lot about the factors that shape the distribution of parasites. Pasteuria ramosa is an endoparasitic bacterium of the water flea Daphnia with a wide geographical distribution. Its transmission stages rest outside of the host and thus experience varying environmental regimes. We examined the life history of P. ramosa populations from four environmental conditions (i.e. groups of habitats): the factorial combinations of summer-dry water bodies or not, and winter-freeze water bodies or not. Our goal was to examine how the combination of winter temperature and summer dryness affects the parasite's ability to attach to its host and to infect it. We subjected samples of the four groups of habitats to temperatures of 20, 33, 46 and 60°C in dry and wet conditions, and exposed a susceptible clone of Daphnia magna to the treated spores. We found that spores which had undergone desiccation endured higher temperatures better than spores kept wet, both regarding attachment and subsequent infection. Furthermore, spores treated with heightened temperatures were much less infective and virulent. Even under high temperatures (60°C), exposed spores from all populations were able to attach to the host cuticle, albeit they were unable to establish infection. Our work highlights the sensitivity of a host-free resting stage of a bacterial parasite to the external environment. Long heatwaves and harsh summers, which are becoming more frequent owing to recent climate changes, may therefore pose a problem for parasite survival. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Environmental transmission of Pseudogymnoascus destructans to hibernating little brown bats

Pathogens with persistent environmental stages can have devastating effects on wildlife communities. White-nose syndrome (WNS), caused by the fungus Pseudogymnoascus destructans, has caused widespread declines in bat populations of North America. In 2009, during the early stages of the WNS investigation and before molecular techniques had been developed to readily detect P. destructans in environmental samples, we initiated this study to assess whether P. destructans can persist in the hibernaculum environment in the absence of its conclusive bat host and cause infections in naive bats. We transferred little brown bats (Myotis lucifugus) from an unaffected winter colony in northwest Wisconsin to two P. destructans contaminated hibernacula in Vermont where native bats had been excluded. Infection with P. destructans was apparent on some bats within 8 weeks following the introduction of unexposed bats to these environments, and mortality from WNS was confirmed by histopathology at both sites 14 weeks following introduction. These results indicate that environmental exposure to P. destructans is sufficient to cause the infection and mortality associated with WNS in naive bats, which increases the probability of winter colony extirpation and complicates conservation efforts.

Assessing the impact of adherence to Non-pharmaceutical interventions and indirect transmission on the dynamics of COVID-19: a mathematical modelling study

Adherence to public health policies such as the non-pharmaceutical interventions implemented against COVID-19 plays a major role in reducing infections and controlling the spread of the diseases. In addition, understanding the transmission dynamics of the disease is also important in order to make and implement efficient public health policies. In this paper, we developed an SEIR-type compartmental model to assess the impact of adherence to COVID-19 non-pharmaceutical interventions and indirect transmission on the dynamics of the disease. Our model considers both direct and indirect transmission routes and stratifies the population into two groups: those that adhere to COVID-19 non-pharmaceutical interventions (NPIs) and those that do not adhere to the NPIs. We compute the control reproduction number and the final epidemic size relation for our model and study the effect of different parameters of the model on these quantities. Our results show that there is a significant benefit in adhering to the COVID-19 NPIs.

Inactivation of SARS-CoV-2 on surfaces and in solution with Virusend (TX-10), a novel disinfectant

Until an effective vaccine against SARS-CoV-2 is available on a widespread scale, the control of the COVID-19 pandemic is reliant upon effective pandemic control measures. The ability of SARS-CoV-2 to remain viable on surfaces and in aerosols, means indirect contact transmission can occur and there is an opportunity to reduce transmission using effective disinfectants in public and communal spaces. Virusend (TX-10), a novel disinfectant, has been developed as a highly effective disinfectant against a range of microbial agents. Here we investigate the ability of Virusend to inactivate SARS-CoV-2. Using surface and solution inactivation assays, we show that Virusend is able to reduce SARS-CoV-2 viral titre by 4 log10 p.f.u. ml-1 within 1 min of contact. Ensuring disinfectants are highly effective against SARS-CoV-2 is important in eliminating environmental sources of the virus to control the COVID-19 pandemic.