5. Emerging virus infections

Arboviruses are generally transmitted by small biting insects such as mosquitoes. These insects are not just passive carriers of the viruses, but are required by the viruses to complete their life cycles. So arboviruses cannot pass directly from one victim to another except rarely by transfusion of contaminated blood or transplantation of an infected organ. Nevertheless, these viruses are spreading widely, causing large epidemics in humans and domestic animals, with increasing morbidity, mortality, and severe economic implications. ‘Emerging virus infections: arthropod-transmitted viruses’ describes the yellow fever, Zika, Chikungunya, West Nile, dengue fever, and Rift Valley viruses. As the emergence and re-emergence of viruses is increasing, vaccines to protect both humans and domestic animals and to prevent virus spread are urgently needed.

1. What are viruses?

‘What are viruses?’ introduces viruses and their structure. Martinus Beijerinck, in 1898, was the first to coin the term ‘virus’, and invention of the electron microscope in the late 1930s greatly enhanced virus identification. Viruses are not cells, but obligate parasites that must infect a cell and use its organelles in order to reproduce. They carry either DNA or RNA, and have a protein coat called a capsid. The whole structure is called a virion. Viruses have a high mutation rate, which helps them to survive and boost their resistance to antiviral drugs. The molecular clock technique to track a virus’s history is also explained.

Introduction

The Introduction outlines the structure of this VSI. The first two chapters introduce viruses, their structure and diversity, how they live, and their effects. Then the constant battle between viruses and the immune system of the infected individual is outlined, followed by chapters about infection by emerging viruses, epidemic viruses, pandemic viruses, and those that persist in the body for a lifetime. Later chapters look at how our knowledge of viruses has advanced through the ages and how the recent molecular revolution has enhanced our ability to isolate new viruses and to diagnose and treat virus infections. The final chapter speculates about how humans and viruses might interact in the future.

7. Persistent viruses

‘Persistent viruses’ explains how some viruses have evolved strategies for overcoming immune mechanisms and survive inside their host for prolonged periods, even for a lifetime. Evasion strategies encompass three basic manoeuvres: finding a niche in which to hide from immune attack, manipulating immune processes to benefit the virus, and outwitting immune defences by mutating rapidly. Most persistent viruses have evolved to cause mild or even asymptomatic infections, since a life-threatening disease would not only be detrimental to the host, but would also deprive the virus of its home. The herpesvirus family, retrovirus family, HIV-1, and hepatitis viruses are described in detail.

3. Kill or be killed

‘Kill or be killed’ shows how viruses survive—they must reproduce before the host either dies or its immune system recognizes and eliminates them. The transmission routes of viruses such as flu, measles, common cold, herpes simplex virus, HIV, Epstein–Barr, and hepatitis B are discussed. How do we fight viruses? All living organisms have defences against invading viruses. Vertebrates, and possibly some invertebrates, are immune to re-infection by the same virus. Another protective mechanism, used by plants, but also by insects and other animal species, is gene silencing by RNA interference. The human immune response is explained, discussing the role of lymphocytes and immunopathology, where the immune response may actually harm the body.

9. Turning the tables

How are diseases such as smallpox eradicated? ‘Turning the tables’ outlines the development of vaccination programmes including Edward Jenner’s research on smallpox in the late 18th century and Louis Pasteur’s rabies vaccine. In order to prevent virus spread, at least 80 per cent of a population must be vaccinated. The difference between live and inactivated vaccines is explained using the case of polio. The ethical debate surrounding vaccination is also discussed along with antiviral drugs, recombinant subunit viral vaccines, the difficulties of producing a vaccine for HIV, and the invention of the polymerase chain reaction in the 1980s and human genome sequencing, which have revolutionized viral diagnosis.

6. Epidemics and pandemics

Once an acute emerging virus is successfully established in a population, it generally settles into a mode of cyclical epidemics during which many susceptible people are infected and become immune to further attack. When most are immune, the virus moves on, only returning when a new susceptible population has emerged, generally consisting of those born since the last epidemic. ‘Epidemics and pandemics’ considers historical epidemics of viruses, vaccination programmes, and the spread of viruses from one continent to another—pandemics. Airborne viruses mainly cause respiratory illnesses, like flu, the common cold, or pneumonia, while those transmitted by faecal–oral contamination, like rotaviruses and noroviruses, cause intestinal upsets with nausea, vomiting, and diarrhoea.

10. Viruses past, present, and future

‘Viruses past, present, and future’ looks at the changing pattern of virus infections through the ages and speculates about how humans and viruses might interact in the future. Viruses, such as smallpox and yellow fever, have debilitated populations across the globe throughout history. New research technologies will allow a greater number of ‘new’ viruses to be identified, leading to preventive vaccines and novel treatments. However, we may also face potential man-made virus threats from biological weapons of mass destruction to the unwitting promotion of pathogenic viruses, possibly through xenotransplantation, immune-suppressed chemotherapy patients, or viruses escaping from laboratories due to poor safety measures.

8. Tumour viruses

One in three people develop cancer during their lives and 10–20 per cent of human cancers are linked to viruses. ‘Tumour viruses’ shows how viral oncogenes have been instrumental in uncovering the molecular mechanisms involved in cancer development. Tumours develop when a single cell in an organism is released from the usual constraints that regulate its growth and then replicates unchecked. This rogue cell then produces a mass of similar cells, forming a tumour (or cancer) that invades the surrounding tissues and may spread from its original site. The tumour-causing behaviour of oncogenic retroviruses, human T lymphotropic virus, herpesviruses, hepatitis viruses, and papilloma viruses are all considered.

4. Emerging virus infections

The term ‘emerging virus infection’ refers to both the emergence of an infectious disease caused by a virus that is new to the species it infects, and to a re-emerging infection, meaning that the disease is increasing in frequency, either in its traditional geographic location or in a new area. ‘Emerging viruses: vertebrate-transmitted viruses’ discusses examples such as swine flu, bird flu, Ebola virus, HIV, and the SARS and MERS coronaviruses. The differing patterns of emerging virus outbreaks depend on viral factors, including incubation period, disease manifestations and method of spread, and important host factors like living conditions, propensity to travel, and the success of any preventive measures.