Communicable Diseases

A Global Perspective

Roger Webber

• Publisher: CABI
• ISBN:
  9781780647449
• Edition: 5
• Topic: Medicina
• Subtopic: Enfermedades y alergias

Completely updated and revised, and now published in its fifth edition with an integrated eBook, this prestigious and best-selling text has continued to provide an essential overview of the subject for over 20 years. A comprehensive yet synoptic account of diseases and their modes of transmission, the book covers epidemiology, control strategies, notifications and regulations, as well as an overview of the major diseases established, new and emerging. Concise so that key information can be found at a glance. Numerous clear diagrams, bullet points and tables for rapid review and learning. Integrated eBook for online use or download to a desktop, tablet or smartphone.Communicable Diseases continues to provide an essential resource for all those in public health and medical science, and for healthcare workers needing a comprehensive yet concise practical text.

1 Elements of Communicable Diseases

1.1 What Are Communicable Diseases?

A communicable disease is an illness that is transmitted from a person, animal, or inanimate source to another person either directly, with the assistance of an intermediate host or by a vector. Communicable diseases cover a wider range than the person-to-person transmission of infectious diseases; they include the parasitic diseases, infections transmitted by a vector, the zoonoses and all the transmissible diseases.

Communicable diseases present in an epidemic or endemic form. An epidemic is the introduction of a new infection or the presence of an illness in excess of normal expectancy. This can be seasonal, such as with influenza, or when the number of susceptible persons is sufficient for a new epidemic to take place, e.g. measles. Any unknown infection will be epidemic when first introduced. An endemic disease is constantly present in a geographical area or population group, e.g. malaria is found in tropical countries and endemic in the adult population.

Communicable diseases are dependent on the person being susceptible to infection, so children meeting an infection for the first time or communities that have been isolated from the infection are more liable to become infected. They are found particularly in conditions that encourage transmission, such as overcrowding or poor hygiene, so are more common in developing countries. They are invariably associated with poverty.

Epidemic diseases devastate whole populations, as when measles ravaged Fiji in 1875, killing a quarter of the population, both adults and children. Populations then have to start again from the survivors to recover their former strength. These are essentially young and growing populations.

With endemic diseases it is children that are particularly vulnerable, so there is a high birth rate to compensate. With so many young people in the population, non-communicable diseases are uncommon, but as people live longer then they become more frequent. Non-communicable diseases such as coronary heart disease (CHD) are the main problem of older aged populations as seen in the Western world.

This division between the developed and developing world is purely artificial where diseases are concerned. When the plague, or ‘Black Death’ as it was known, spread across Europe it caused as much devastation as when communicable diseases were introduced to newly found nations by Western explorers and missionaries. The population started again from the survivors, as it has had to do in developing countries. Just over 100 years ago, measles was as serious a cause of childhood death in large European cities as it is today in developing countries without well-organized vaccination programmes. A tropical environment is more favourable to many diseases than the cooler temperate regions, but even here such tropical diseases as malaria were once common in Europe. There is nothing new or different about these artificially divided parts of the world except for the resources that each is able to devote to the improvement of their health. Communicable diseases could be reduced to manageable problems if enough resources, both in financial and educational terms, could be spent on them.

The difference between communicable and non-communicable diseases used to be quite clear-cut. Where there was an organism that was transmitted, it was communicable; otherwise the disease was classified as non-communicable. However, this strict boundary is becoming less well defined as new suspect organisms are discovered, or diseases, by their very nature, suggest a communicable origin. Various cancers are good examples; the link between hepatitis B virus (HBV, Section 14.11) and hepatocellular cancer is well established, and is now being prevented by routine vaccination. Epstein–Barr virus (EBV) seems to be a pathogenic factor in Burkitt’s lymphoma, but there is also a causal relationship with malaria, so controlling malaria (Section 15.6) in Africa and Papua New Guinea, where this tumour is found, could have a double benefit. EBV might also have a causal effect in non-Hodgkin’s lymphoma and nasopharyngeal cancer. Kaposi’s sarcoma may well be transmitted by the sexual route, as shown by the number of people with it who acquire human immunodeficiency virus (HIV) infection via sexual transmission as compared with those becoming infected from blood transmission, in which the tumour occurs only rarely. The trematode worms Schistosoma haematobium (Section 11.1) and Clonorchis sinensis (Section 9.5) are causative factors in bladder cancer and cholangiocarcinoma, respectively, so their control as communicable diseases will also reduce cancer incidence. Helicobacter pylori, an organism that thrives in gastric secretions, is probably a causative factor in gastric cancer. The commonest cancer with a communicable cause is cancer of the cervix, due to infection with the human papilloma virus (HPV, Section 14.9). Prevention of this infection by vaccination offers the greatest hope of reducing this important cause of female mortality.

Equally intriguing is the possibility that atheroma has an infective cause or association. With arteriosclerosis being largely responsible for CHD and the major killer in Western countries, the possibility of preventing it by finding the infective causal agent is attractive. Chlamydia pneumoniae has been found within atheroma lesions, but not normal arteries, while cytomegalovirus (CMV) is able to infect the smooth muscle cells of arterial walls. The association of H. pylori and CHD now seems unlikely, but herpesvirus 1 could induce an endothelial cell response. The cause will probably be found to be multifactorial, but perhaps in time nearly all diseases will be shown to have a transmissible factor in their causation. Even road accidents – for which there seems no need to look for a predisposing cause in a communicable disease – might be made more likely to occur as a result of infection with toxoplasmosis (Section 17.5). Toxoplasma gondii, the causative organism, is also thought to be a causal factor in neurosis and schizophrenia, and recent work also suggests a correlation with brain cancer.

While avoiding infection is normally the best strategy, our obsession with cleanliness might be responsible for the increase in allergies, type 1 diabetes, inflammatory bowel disorders and multiple sclerosis in developed countries. These are found less commonly in developing countries where conditions of hygiene are poor and were rare in developed countries in the early part of the last century. It is thought that our obsession with cleanliness and using antibacterial products prevents minor infections that stimulate the immune system. The use of antibiotics, especially in children in the first 3 years of life, is an important cause. A little bit of dirt is good for us, and especially for young children when they are developing their immune response.

The key to any communicable disease is to think of it in terms of agent, transmission, host and environment. These components are illustrated in Fig. 1.1, which will be used as a framework in the description of this section. There needs to be a causative agent, which requires a means of transmission from one host to another, but the outcome of infection will be influenced by the environment in which the disease is transmitted and the response of the host.

Fig. 1.1. The interrelationship of the agent, transmission, host and environment.

1.2 The Agent

The agent can be an organism (virus, prion, bacteria, rickettsia, protozoa, helminth, fungus or arthropod) or a physical or chemical agent (toxin or poison). If it is an organism, the agent needs to multiply, find a means of transmission and survive.

1.2.1 Multiplication

Two methods of multiplication occur, sexual and asexual reproduction, which have different advantages. In asexual reproduction a succession of exact or almost exact replicas are produced, so that any natural selection will act on batches or strains, rather than on individuals. By contrast, sexual reproduction offers great scope for variety, both within the cells of the single organism and from one organism to another. This means that natural selection acts on individuals, and variations of vigour and adaptability occur.

There are different consequences of these two methods of reproduction. With asexual organisms the strain of the organism is either successful or unsuccessful in invading the host, whereas in sexually reproducing organisms certain individuals will succeed while others will not. In continuing its existence, only one organism of the asexual parasite requires to be transmitted, whereas with the sexually reproducing parasite, both male and female adults must meet before reproduction can take place. Some parasites seem to be at a tremendous disadvantage, e.g. the filarial worm Wuchereria bancrofti, in which both male and female individuals go through long migrations in the body to find an individual of the opposite sex, but despite all these problems this is one of the most successful of all parasites.

Whether the organism reproduces sexually or asexually is relevant in treatment and control. If a treatment is successful at destroying an asexually reproducing organism, then it will be successful against all the other individuals of that strain of the organism – unless a mutation occurs which renders the individual resistant to treatment, when this resistance will be conferred on all other organisms developing from this strain. In contrast, sexual reproduction produces individuals of different vigour, meaning that some individuals will succumb to treatment, while others will not. However, having two sexes can be a disadvantage for the organism in that methods of control can be devised which attack only one of the sexes, or designed to reduce the chance of individuals of each sex from meeting.

1.2.2 Survival

Parasitic agents survive by finding a suitable host within a certain time. They have been able to improve their chances of finding a new host or surviving in the environment by a number of different methods.

Reservoirs and parasite adaptability

A reservoir is a storage place for water but also serves as an appropriate term to describe a suitable place for storing agents of infection. Once an agent invades a host, there is normally a latent or waiting period while sufficient organisms are produced before the main attack is mounted. If the host survives but is not able to eliminate the invading organism, then the organism can continue to live and reproduce in the host.

The relationship between the parasite and host is one of continual challenge, what has been termed a biological arms race. When the parasite first attacks a new species, the host attempts to eliminate it, resulting in a severe reaction. In time, adaptation can occur, so that the reaction of the host diminishes and the adaptability of the parasite increases. The parasite is then able to live in the host with few ill effects (e.g. Trichuris trichiura), forming an established population, and continuing with minimal reaction from the host. The host then acts as a reservoir from which parasites attack new hosts of the sa...