The Epidemiological Signature of Pathogen Populations That Vary in the Relationship between Free-Living Parasite Survival and Virulence

Abstract

The relationship between parasite virulence and transmission is a pillar of evolutionary theory that has implications for public health. Part of this canon involves the idea that virulence and free-living survival (a key component of transmission) may have different relationships in different host–parasite systems. Most examinations of the evolution of virulence-transmission relationships—Theoretical or empirical in nature—Tend to focus on the evolution of virulence, with transmission being a secondary consideration. Even within transmission studies, the focus on free-living survival is a smaller subset, though recent studies have examined its importance in the ecology of infectious diseases. Few studies have examined the epidemic-scale consequences of variation in survival across different virulence–survival relationships. In this study, we utilize a mathematical model motivated by aspects of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) natural history to investigate how evolutionary changes in survival may influence several aspects of disease dynamics at the epidemiological scale. Across virulence–survival relationships (where these traits are either positively or negatively correlated), we found that small changes (5% above and below the nominal value) in survival can have a meaningful effect on certain outbreak features, including R0, and on the size of the infectious peak in the population. These results highlight the importance of properly understanding the mechanistic relationship between virulence and parasite survival, as the evolution of increased survival across different relationships with virulence may have considerably different epidemiological signatures.

SARS-CoV-2 on Ocular Surfaces in a Cohort of Patients With COVID-19 From the Lombardy Region, Italy

“…This virus has high human transmission via airways but has a medium virulence. 19 Therefore, many people test positive for the presence of the virus without any signs of disease. SARS-CoV-2 RNA has been found in the nasopharyngeal tract and bronchial drainage, 1 in saliva, 20 in tears, [21][22][23][24] in urine, 25 and in feces 26 but not in seminal fluids.…”

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

“…We argue that there is a twofold advantage to understanding the biology of off-host transmission-stage survival in the external environment [ 18 ] and suggest investigating it in other parasites as well. First, theoretical models show that the survival rate of transmission stages can strongly influence the epidemiology of disease and the evolution of its virulence [ 77 , 78 ]. This holds true for human parasites, our livestock and agricultural plants, and wildlife conservation.…”

The roles of environmental variation and parasite survival in virulence–transmission relationships

“…Hypotheses for ETP virulence-transmission relationships include the 'sit-and-wait' [76,77] and the 'Curse of the Pharaoh' hypotheses [78], which state that extended persistence in the off-host environment releases parasites from the constraint of host survival, allowing the evolution of high virulence. Many theoretical studies have explored evolutionary relationships between virulence and parasite longevity [70,74,[78][79][80][81][82] where virulence is more likely to increase when its evolution is independent of free-living survival or other transmission traits.…”

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