Exercise Immunology

Michael Gleeson, Nicolette Bishop, Neil Walsh, Michael Gleeson, Nicolette Bishop, Neil Walsh

• Editore: Routledge
• Anno: 2013
• ISBN:
  9781136455858
• Edizione: 1
• Argomento: Medizin
• Categoria: Immunologie

Exercise immunology is an important, emerging sub-discipline within exercise physiology, concerned with the relationship between exercise, immune function and infection risk. This book offers a comprehensive, up-to-date and evidence-based introduction to exercise immunology, including the physiological and molecular mechanisms that determine immune function and the implications for health and performance in sport and everyday life.

Written by a team of leading exercise physiologists, the book describes the characteristics of the immune system and how its components are organised to form an immune response. It explains the physiological basis of the relationship between stress, physical activity, immune function and infection risk, and identifies the ways in which exercise and nutrition interact with immune function in athletes and non-athletes. The book shows students how to evaluate the strengths and limitations of the evidence linking physical activity, immune system integrity and health, and explains why exercise is associated with anti-inflammatory effects that are potentially beneficial to long-term health.

Every chapter includes useful features, such as clear summaries, definitions of key terms, discussions of seminal research studies and practical guidelines for athletes on ways to minimise infection risk, with additional learning resources available on a companion website. This is an essential textbook for any course on exercise immunology or advanced exercise physiology.

1 The influence of exercise on infection risk

Nicolette C. Bishop

LEARNING OBJECTIVES

After studying this chapter, you should be able to:

•appreciate the different agents that cause common infections;

•appraise the J-shaped model of upper respiratory tract infection risk and exercise volume;

•appreciate the strengths and limitations of the methods used to measure the incidence of infection;

•evaluate the evidence concerning the effect of single bouts of prolonged exercise and intensive endurance training on infection risk;

•appreciate the influence of airway inflammation on symptoms of respiratory infection;

•evaluate the evidence concerning the effect of regular moderate exercise on infection risk compared with a sedentary lifestyle;

•appreciate other factors that influence symptoms of infection.

INTRODUCTION

Acute upper respiratory tract infections (URTI, such as coughs and colds, influenza, sinusitis, tonsillitis, other throat infections and middle ear infections) are among the most common illnesses experienced at all ages. These infections lead to absence from school and work, visits to general practitioners (GPs) and heavy use of ‘over the counter’ medicines. In the UK, respiratory infections account for approximately 5–5 million GP visits annually and an estimated cost of £170 million (Health Protection Agency, 2005). These infections are also one of the main reported causes of illness in athletes. For example, analysis of the 126 reported illnesses in athletes competing at the World Athletics Championships in Daegu, South Korea, in 2011 revealed that 40% of illnesses affected the upper respiratory tract, with confirmed infection in around 20% of cases (Alonso et al. 2012). Other than sickness associated with exercise-induced dehydration (12%), gastroenteritis/diarrhoea was the next most common illness reported (10%).

CAUSES OF INFECTIONS

Pathogens are disease-causing microorganisms and these can include viruses, bacteria, fungi and parasites. The causes of upper respiratory infections can be viral or bacterial and they are most commonly transmitted by airborne droplets or nasal secretions. Pathogens involved as causal agents in respiratory infections can lead to a wide variety of illnesses (Table 1.1) and many are seasonal in their activity, tending to have higher levels during the winter months.

Table 1.1 Examples of common causes of upper respiratory tract infections

INFECTION	ORGANISM
Common cold/cough	Rhinovirus, Coronavirus, adenovirus, influenza A and B, human parainfluenza virus, enterovirus
Middle ear infection (otitis media)	Rhinovirus and other common-cold viruses, Streptococcus pneumoniae, Haemophilus influenza, Moraxella catarrhalis
Sinusitis	Rhinovirus, Coronavirus, enterovirus, adenovirus, influenza A and B, S. pneumoniae, H. influenza, M. catarrhalis
Sore Throat	Adenovirus, influenza virus, rhinovirus, Epstein-Barr virus, cytomegalovirus, S. pyogenes, Mycoplasma pneumoniae

Note: Viral causes are in plain text; bacterial causes are in italics.

A virus is a microscopic organism that cannot replicate or express its genes without a host living cell. Viruses invade living cells and use the nucleic acid material that they contain to replicate themselves. A single virus particle (virion) is in and of itself essentially inert. It lacks the necessary components that cells need to reproduce. When a virus infects a cell, it marshals the cell's enzymes and much of the cellular machinery to replicate; viral replication produces many progeny that, when complete, leave the host cell to infect other cells in the organism.

Viruses may contain double-stranded DNA, double-stranded RNA, single-stranded DNA or single-stranded RNA. The type of genetic material found in a particular virus and the exact nature of what happens after a host is infected varies depending on the nature of the virus. Double-stranded DNA viruses typically must enter the host cell's nucleus before they can replicate. Single-stranded RNA viruses, however, replicate mainly in the host cell's cytoplasm. Once a virus infects its host and the viral progeny components are produced by the host's cellular machinery, the assembly of the viral capsid (the protein coat that envelopes viral genetic material) is usually a non-enzymatic, spontaneous process.

The virus may mutate during the process of its replication; this ability to change slightly in each infected person is the reason why treating viruses can be difficult. Viruses are responsible for several common diseases in humans, not least upper respiratory infections (Table 1.1). For example, there are more than 200 different viruses that can cause the common cold, most of which are rhinoviruses but other viruses such as coronaviruses and adenoviruses can also be responsible.

Bacteria are single-celled microscopic organisms that are larger than viruses and lack nuclei and other organised cell structures. As you can see in Table 1.1, several bacterial species are capable of causing respiratory infections (pathogenic bacteria), yet most are non-infectious and many play critical roles; for example, bacteria in the gut aid digestion. Bacteria come in a variety of shapes (e.g. rod-like, spheres and spirals) and sizes. Bacteria are usually classified as Gram-positive or Gram-negative, based on a basic microbiological staining procedure called the Gram stain, which reveals the presence (Gram-negative) or absence (Gram-positive) of an outer membrane. For example, the streptococcal species of bacteria are rod-like gram positive bacteria and, as outlined in Table 1.1, pathogenic species of streptococci (Streptococcus pyogenes, S. pneumoniae) are causes of sore throats, ear infections and sinusitis.

Gastroenteritis/diarrhoea is worthy of mention here, not least because it was the second most common infectious illness reported in athletes attending both the 2009 and 2011 World Championships (Alonso et al. 2010, 2012). The major cause of viral gastroenteritis in adults is norovirus (although other viruses such as adenoviruses can also be responsible). Close contact with infected individuals and poor personal hygiene, in particular not washing hands after using the toilet, is the major route of transmission. Bacterial causes (mainly through poor food hygiene practices or competing in polluted water) include bacteria of the Campylobacter species, Escherichia coli and Salmonella. Travellers' diarrhoea is a term used to refer to gastroenteritis that is acquired when travelling abroad and this can affect athletes travelling to competition. Travellers' diarrhoea is usually the result of drinking infected water or eating infected foods. It is caused by a range of different bacteria or, less commonly, parasites such as Cryptosporidium and Giardia intestinalis. These parasites are microorganisms that live in the gut and derive nutrients from the host, causing symptoms such as abdominal pain, vomiting and diarrhoea.

IS THERE A J-SHAPED RELATIONSHIP BETWEEN EXERCISE TRAINING LOAD AND INFECTION RISK?

The amount of exercise that a person does has an effect on respiratory infection incidence. It has been suggested that the relationship between exercise intensity/volume and susceptibility to URTI is J-shape...