Evaluation of Phi6 Persistence and Suitability as an Enveloped Virus Surrogate

Understanding the survival and fate of viruses, such as SARS-CoV-2 in water is important to enable us to model and predict exposure risks through contact with wastewater, environmental water systems and drinking water. In particular, it will be necessary to understand how variable factors such as temperature, pH, salinity, organic matter concentration, or other matrix variabilities may impact virus survival.

However, the ability to undertake studies required to understand these factors is often hindered by the pathogenic nature of the virus. Typically, laboratories will require certified biosafety controls (Biosafety-Level 3 or 4), which are not commonly available to environmental and water engineering laboratories.

A common way to manage this problem is by the identification and validation of alternative non-pathogenic (or much less pathogenic) viruses to study as “surrogate viruses” for the pathogenic virus of interest. In order to be an effective model, a surrogate virus would need to exhibit reliably similar behaviour to the virus of interest, in system being investigated.

SARS-Cov-2 is an enveloped virus, and this enveloped structure is known to have a significant impact on the survival and fate of the virus in water and the environment, generally. Thus suitable surrogate viruses are likely to also be other enveloped viruses.

The reference to this blog post is a study reported in 2017 by Associate Professor Kyle Bibby and colleagues at the University of Pittsburgh in Pennsylvania, USA. In this study, the enveloped bacteriophage Phi6 (ϕ6) was evaluated as a surrogate for enveloped waterborne viruses. We have previously reported a study that used ϕ6 to assess the survivability, partitioning, and recovery of enveloped viruses in untreated municipal wastewater.

The results reported by Bibby et al., demonstrated that the predicted T90 (time for 90% inactivation) of ϕ6 under 12 investigated conditions varied from 24 min to 117 days depending on temperature, biological activity, and aqueous media composition. This observed ϕ6 persistence was then compared with reported persistence from other enveloped viruses from published scientific literature.

These experiments revealed that the apparent suitability of ϕ6 as an enveloped virus surrogate was dependent on the temperature and composition of the media tested. Of the evaluated viruses, 33%, including all conditions considered, had T90 values outside of the 95% confidence interval for ϕ6.

In other words, ϕ6 was not a particularly reliable surrogate since it did not exhibit behaviour that was particularly similar to the other enveloped viruses across a range of conditions. This outcome was largely a consequence of the wide variability of environmental persistence of enveloped viruses generally.

The authors concluded that ϕ6 cannot be considered to be a universally conservative surrogate for enveloped viruses in aqueous media. They stated that “ultimately, these results highlight the variability of enveloped virus persistence in the environment and the value of working with the virus of interest for environmental persistence studies”.

REFERENCE:

Aquino de Carvalho, N., Stachler, E. N., Cimabue, N. and Bibby, K. (2017) Evaluation of Phi6 Persistence and Suitability as an Enveloped Virus Surrogate. Environ. Sci. Technol., 51(15), 8692-8700.

https://doi.org/10.1021/acs.est.7b01296

Published by Stuart Khan

Professor of Civil & Environmental Engineering, University of New South Wales

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