Digitally-coloured SEM of Ebola particles budding from the cell surface. Image Credit: National Institute of Allergy and Infectious Diseases (NIAID), 2014.
Digitally-coloured SEM of Ebola particles budding from the cell surface. Image Credit: National Institute of Allergy and Infectious Diseases (NIAID), 2014.

On October 19th, 2014, Dr. Ian Crozier was discharged from Emory University Hospital after a near-fatal encounter with Ebola, which he contracted while caring for patients in Sierra Leone. Despite a lingering fatigue, his doctors were confident that he would make a full recovery. It therefore came as a shock when, almost two months later, Dr. Crozier was readmitted with severe uveitis threatening his sight, the inside of his afflicted eye teeming with Ebola. With additional treatment, Dr. Crozier’s symptoms receded and his eyesight returned. However, he is not the only survivor to show signs of a post-Ebola syndrome. Following their apparent recovery and release from hospital, survivors have reported a litany of symptoms including joint and muscle pains, chronic headaches, difficulty hearing, fatigue, and vision problems, leading many to wonder what risks are posed by the virus that may be lurking in their bodies.

Contained within Dr. Crozier’s eye, the Ebola virus had no chance of being transmitted to a new host through casual contact. However, the eye is not the only site where Ebola can persist. The recent death of a Liberian woman, whose only connection with Ebola was unprotected sex with a survivor, highlights the possibility that Ebola can persist in the testes and be transmitted sexually. Comparative sequence analysis confirmed that the virus isolated from her body and her partner’s semen showed similarities at key points, suggesting that the Ebola was transmitted sexually.

A new mode of transmission for Ebola could change the rules of host-pathogen interaction, applying very different selection pressures on both sides of the equation. Dr. Crozier hopes that his case will help doctors “change the natural history of the disease for survivors” and reduce the damage caused by symptoms of post-Ebola syndrome. But could we go even further and try to direct the evolution of an originally deadly virus towards benignity?

The adaptive trade-off hypothesis, originally proposed by Anderson and May (1982), predicts that virulence will reduce with the length of time that the host-pathogen interaction progresses, eventually reaching an equilibrium where mortality is reduced and transmission is maximized. This belief is inextricably tied to the assumption that virulence has negative consequences for the pathogen. Myxomatosis has become the poster child for this type of equilibrium after a hypervirulent strain of the Myxoma virus was released in Australia in 1950 to control a bunny boom. The virus was tracked and researchers noted that virulence decreased in subsequent years. Moreover, the majority of transmissions were determined to come from rabbits that survived longer, providing a distinct survival benefit for virus strains with reduced virulence.

Cholera outbreaks occurring around South America in the 1990’s offered an opportunity to examine the effect of social infrastructure on pathogenicity, providing further support for the adaptive trade-off hypothesis. Researchers observed that areas with poor water sanitation were susceptible to cholera outbreaks caused by Vibrio cholera strains with high virulence. By contrast, cholera strains with lower virulence were found to cause outbreaks in countries with better water sanitation.

While the studies on cholera suggest that simply reducing ease of transmission favours the evolution of less virulent disease, other research has not yielded the same clear results. Subjecting bacteriophages to conditions restricting their transmission only modestly decreases their virulence, and areas with good mosquito control programs are not rewarded with less virulent vector-borne illnesses. Moreover, many of the models and experiments that predict this type of virulence evolution use highly artificial systems with extreme conditions that don’t account for the complexity and variability found in nature. In a world where different pathogens have to coexist within the same host, and where each host has different levels of resistance, nutrition, and social support, the evolution of pathogen virulence cannot be explained as a simple trade-off with transmission.

The largest Ebola outbreak in history has yielded the largest pool of survivors to date and as the epidemic in West Africa ebbs, focus has shifted away from the dangerous acute infection to the emergence of a post-Ebola syndrome. Even if it proves too complicated to direct Ebola’s virulence in a specific direction, this syndrome can still offer insight into the virus’ evolution in the human host. Anticipation of the changes in symptomatology and modes of transmission will aid in the design of more effective care strategies for Ebola patients.


References

1. Alizon, S., Hurford, A., Mideo, N., van Baalen, M. (2009). Virulence evolution and the trade-off hypothesis. Journal of Evolutionary Biology. 22:245-259.

2. Christie, A., et al. (May 8, 2015). Possible Sexual Transmission of Ebola Virus – Liberia 2015. CDC Morbidity and Mortality Weekly Report.

3. Grady, D. (May 7, 2015). After Nearly Claiming his Life, Ebola Lurked in a Doctor’s Eye. The New York Times.

4. Kerr, P.J. (2012). Myxomatosis in Australia and Europe: a model for emerging infectious diseases. Antiviral Research. 93: 387-415.

5. Messenger, S.L., Molineux, I.J., Bull, J.J. (2000). Virulence evolution in a virus obeys a trade-off. Proceedings of the Royal Society of London. 266:397-404.

6. Turner, PE., Cooper, V.S., Lenski, R.E. (1998). Tradeoff between horizontal and vertical modes of transmission in bacterial plasmids. Evolution. 52: 315-329.

7. Zimmer, C. (2003). Taming Pathogens: An Elegant Idea, but Does it Work? Science. 300: 1362-1364.

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