INTRODUCTION

The emergence, or more often the re-emergence, of disease-causing organisms is a continuing threat to mankind either directly by impacting on human health, or indirectly through the effects they may have on wild and domesticated animals. Virus infections can cause disease outbreaks of plague dimensions in humans and animals, with serious social, economic and environmental consequences. In the past, diseases such as smallpox, measles, polio and influenza have caused devastation in the human population. Smallpox has the distinction of being the only virus disease to have been eradicated globally while the others mentioned above are, or can be, effectively controlled by vaccination.

Almost every year, new viruses emerge which threaten the lives of humans and animals. In particular, members of the family Paramyxoviridae, a group that includes viruses causing some of the most virulent diseases of animals, pose significant medical, economic and ecological threats (Chant et al., 1998; Renshaw et al., 2000). Of the many new paramyxoviruses that have emerged in the past decade, the so-called ‘equine morbillivirus’ received a high profile in the mid-1990s because it caused the deaths of several racehorses and two human fatalities in Australia (Murray et al., 1995). The virus was initially grouped with the morbilliviruses because of a distant sequence relationship with members of the group, but it is now considered to belong to a new, as yet unnamed, genus in the Paramyxoviridae. It has been renamed Hendra virus, after the location of the first outbreak. In 1999, another newly discovered paramyxovirus, Nipah virus, caused a calamitous outbreak of disease in pigs in Malaysia.

INTRODUCTION

The emergence of human immunodeficiency virus type 1 (HIV-1) in the human population is one of nature's most recent and highly successful adaptive radiations. This microbe illustrates Darwinian natural selection at a fast-forward pace. In the West, HIV readily exploited various niches of our late 20th century lifestyles, including air travel, narcotics dependence and steamy, promiscuous bath houses (Shilts, 1987). Yet it is wreaking most havoc among the world's poorest and underprivileged communities. HIV/AIDS presents a fascinating but frightening danse macabre of sex, drugs and death, with no end in sight to the burgeoning pandemic.

Of course, one does not need to be a microbiologist to know about AIDS. The disease was first recognized when small clusters of young homosexual men in American cities were reported to suffer rare opportunistic infections (OIs) (Pneumocystis carinii pneumonia, cytomegalovirus retinitis) and Kaposi's sarcoma (Centers for Disease Control, 1981). The underlying immune deficiency was soon shown to involve a selective depletion of CD4- positive, T-helper lymphocytes (Gottlieb et al., 1981). Initially, it was not clear whether ‘gay compromise syndrome’ as it was then named was transmissible or was related to non-infectious lifestyle factors. By early 1982, however, reports of AIDS in recipients of blood transfusions and pooled clotting factors, as well as among injecting drug users, indicated that an infectious agent was to blame. The appearance of AIDS in African countries and in Haiti suggested that the unknown pathogen was already widespread. In the past 20 years, the causative agent, HIV, has rampaged across the globe, with an estimated 53 million infected people among whom 19 million have died (Piot, 2000; UNAIDS, 2000b).

INTRODUCTION

Filoviruses cause fulminant haemorrhagic fever in humans and non-human primates, killing up to 90 % of the infected patients. Since the discovery of Marburg virus (MBGV) in 1967 and the emergence of Ebola virus (EBOV), its better known cousin, a few years later, these infections have therefore been a matter of high public and scientific concern. Although it is clear from the recorded history of filovirus outbreaks that all of them have so far been self-limiting and that the total number of human infections hitherto documented scarcely exceeds a thousand cases, EBOV by now ranges among the most ill-famed human viruses. For a long time, research on filoviruses has been impeded by their high pathogenicity, but with the advent of recombinant DNA technology our knowledge of the genome structures and the replication strategies of these agents has increased significantly.

PATHOPHYSIOLOGY OF FILOVIRUS INFECTIONS

The pathophysiological changes that make filovirus infections so devastating are just beginning to be unravelled. Pathogenesis in fatal infections in human and non-human primates is similar, suggesting the primate system is a reasonable model for studying filovirus haemorrhagic fever (Simpson et al., 1968; Murphy et al., 1971; Ellis et al., 1978; Fisher- Hoch et al., 1985; Ryabchikova et al., 1994). Clinical and biochemical findings support the anatomical observations of extensive liver involvement, renal damage, changes in vascular permeability, including endothelial damage, and activation of the clotting cascade. The visceral organ necrosis is a consequence of virus replication in parenchymal cells. However, no organ, not even the liver, shows sufficient damage to account for death.

Fluid distribution problems and platelet abnormalities are dominant clinical manifestations, reflecting damage of endothelial cells and decrease of platelets.

‘Dabei kommt natürlich alles darauf an, dass wir für die betreffenden Seuchen ganz zuverlässige Methoden zum Nachweis der Krankheitserreger besitzen. Insbesondere gilt dies für diejenigen Kranken, welche sich im frühesten Stadium der Krankheit befinden und für die sogenannten Bazillenträger, weil diese beiden Kategorien von Kranken erfahrungsgemäβ am meisten zur Verschleppung der Seuchen beitragen. Auf jeden Fall bildet eine sichere Diagnose gewissermaβen den Schlüssel für die moderne Seuchenbekämpfung.’ [‘Of course it all depends on the availability of reliable methods to detect the causative agents of the pestilence in question. This is particularly true for those patients in very early stages of the disease and for the so-called “carriers”, as experience tells us that these two categories of patients are the main contributors when it comes to the spread of the pestilence. In any case, a reliable diagnosis is the key for the modern battle against pestilence.’]

PROLOGUE

Unlike related viruses infecting both man and animals (for example, influenza virus and rabies virus), in which zoonotic pathways are well established, Borna virus infections of man have only been reported recently and their possible zoonotic transmission remains unknown.

INTRODUCTION

There was an unprecedented global resurgence of epidemic dengue fever (DF) in the waning years of the 20th century. Both the viruses and the mosquito vectors expanded their geographic distribution, and there was a dramatic increase in the frequency of epidemic dengue, followed by the emergence of the severe form of disease, dengue haemorrhagic fever (DHF), in most tropical regions of the world (Gubler, 1988, 1997, 1998; Halstead, 1992). An estimated 3 billion people currently live in areas at risk for dengue infection (World Health Organization, 1999) (Fig. 1). Each year there are an estimated 50–100 million cases of the mild disease, DF, and several hundred thousand cases of the severe form of disease, DHF, depending on the epidemic activity that particular year. The case fatality rate of DHF cases averages about 5% (World Health Organization, 1999). In 2001, epidemic DF/DHF is one of the most important emergent tropical infectious diseases in the world, primarily affecting the urban centres of tropical developing countries. This chapter briefly reviews the changing epidemiology of dengue, discusses the factors responsible for the recent resurgence, and reviews options for prevention and control.

NATURAL HISTORY

DF/DHF is caused by infection with dengue viruses. There are four closely related virus serotypes, DEN-1, DEN-2, DEN-3 and DEN-4, which show extensive cross-reactivity in serologic tests, but do not provide cross-protective immunity (World Health Organization, 1997; Gubler, 1998). Thus persons living in an endemic area can be infected with three and probably all four of the dengue serotypes during their lifetime.

INTRODUCTION

The last century ended with heightened awareness of an impending influenza pandemic, i.e. a major worldwide epidemic caused by the emergence of a novel influenza A subtype against which the population has little or no immunity. The ‘bird ‘flu’ (H5N1) outbreak in Hong Kong in 1997, which claimed the lives of six of its 18 victims, was terminated at the end of the year by the slaughter of the poultry in the live bird markets, which removed the source of infection (Claas et al., 1998; Subbarao et al., 1998; Bender et al., 1999). Shortly after, in late 1998 in southern China and in early 1999 in Hong Kong, cases of mild influenza caused by another influenza A subtype, H9N2, indicated that the repopulated bird markets still harboured potential human pathogens (Peiris et al., 1999; Lin et al., 2000). Most of these infections resulted from avian-to-human transmission and there was little evidence of human-to-human spread; neither virus became established (as yet) in the human population (Bridges et al., 2000; Katz et al., 1999). The high mortality associated with influenza A H5N1 infection (of both chickens and people) raised the spectre of a recurrence of a 1918–1919-like pandemic which is estimated to have claimed in excess of 40 million lives worldwide (Crosby, 1989).

This book presents the proceedings of a meeting ‘New Challenges to Health: The Threat of Virus Infection’ organized by the Society for General Microbiology at Heriot- Watt University, Edinburgh, in March 2001. The purpose of the meeting was to review the continuing threat of viruses (and prions) to human and animal health. Although several virus diseases have been controlled by vaccination (such as polio, measles, mumps, rubella and yellow fever) and one (smallpox) has been eradicated, viruses remain a potent threat to human and animal health due to their ability to evolve and adapt rapidly. For viruses such as influenza and HIV, the ability to undergo rapid antigenic variation enables them to evade existing immunity and cause disease. Viruses may adapt to new situations, such as changes in the population density of human, animal or insect hosts, or the presence of substantial numbers of immunosuppressed individuals, and cause disease where hitherto they were unable to do so. Rapid virus evolution also enables virus strains to arise that are resistant to existing antiviral drugs. The book starts with a consideration of the mathematical modelling of epidemic virus infections, and the surveillance and emergence of virus infections. There follow chapters that review the molecular and cell biological mechanisms by which viruses and prions may induce disease. These include influenza and HIV, the devastating haemorrhagic diseases caused by Ebola and Marburg viruses, prion diseases such as BSE and variant CJD, certain psychiatric illness, and how the emergence of drug-resistant strains of virus poses a major problem for antiviral chemotherapy.

INTRODUCTION

The history, ecology and genetics of the European rabbit and the two viruses, myxoma virus and calicivirus, used to control the rabbit in Australia offer a useful set of parallels and insights into the behaviour of such systems. In this review, the process of invasion of Australia by the rabbit and the consequent invasions of the Australian rabbit population by myxoma virus and a calicivirus are described.

THE RABBIT

The European rabbit (Oryctolagus cuniculus L.) evolved in what is now the Iberian Peninsula and southern France in Europe. During classic antiquity, mariners moved rabbits to islands in the Mediterranean. The species was then spread by human agencies across much of western Europe and, finally, during the age of European colonization of other parts of the world, populations were established on hundreds of islands and several continents (Thompson & King, 1994). The rabbit arrived in Australia from England with the first European settlers in 1788. Over the next 70 years, rabbits were repeatedly brought to Australia and on occasion either escaped or were released into the wild. While some of these populations survived and a few spread slowly, they did not thrive (Rolls, 1984; Stodart & Parer, 1988).

Invasion

In 1859, this pattern changed with the arrival in Australia of a shipment of 13 rabbits from near Glastonbury in England. These were taken to Barwon Park, a property near Geelong in Victoria, where they were cosseted as their numbers built up. Eventually they escaped or were released from their enclosures. They were also distributed as gifts by the owner to his friends and associates.

Aims and objectives

This exercise is designed to demonstrate:

1. The external morphology of the adult stage of the cestode parasite, Hymenolepis diminuta.

2. The morphology of the metacestode stage (cysticercoid) of H. diminuta.

3. The distribution of tapeworms in the intestine.

Introduction

Many parasites use more than a single host species during the course of their life cycle. Often the larval stages develop in one host and the adults in another. Thus, the parasite must be transmitted from one host to the next in order to complete its development and the most common relationship between hosts exploited by parasites for transmission, particularly those living in the intestine as adults, is that involved in the predator-prey food chain. For example, some tapeworms develop as larvae in small flour beetles, Tribolium confusum, but parasitise rodents such as mice and rats as adults. Rodents are essentially grain feeders, but will consume insects also and therefore by depending on two host species, both of which live in close association with cereals and grain, the tapeworms ensure that their transmission cycle is completed successfully.

This exercise is based on the tapeworm Hymenolepis diminuta (Fig. 1.14.1), which develops successfully to the adult stage in the rat. In nature it is a parasite of wild rodents, but some isolates have been maintained in laboratory animals for many decades. Adult worms often achieve a length of over 80 cm. The worms can therefore be dissected from the rat intestine and handled with relative ease to show the outer structure of tapeworms, movement and general body organisation.

Aims and objectives

This exercise is designed to demonstrate:

1. An important humoral immune killing mechanism against Trypanosoma brucei.

2. Binding of trypanosomes to macrophages mediated by specific antibodies, the first stage in the process of opsonisation.

3. Antigenic variation by T. brucei as a mechanism of evasion of specific antibodies.

Introduction

T. brucei is one of three parasite species that collectively cause Nagana, one of the most important diseases of livestock in sub-Saharan Africa. The other two species are T. congolense and T. vivax. There are three sub-species of T. brucei, two of which, T. b. gambiense and T. b. rhodesiense, cause sleeping sickness in humans. Currently, sleeping sickness is predicted to cause 300000–500000 new cases per year. Infection is normally fatal in the absence of chemotherapy but most infected people will have no access to the relevant drugs.

Vaccination is an obvious alternative to reliance on drugs for prevention of infection and/or disease but the development of potential vaccines has been little explored in African trypanosomiasis because these parasites undergo antigenic variation. Each T. brucei parasite is enwrapped in a surface coat that is visible by electron microscopy. The coat is comprised of approximately 107 copies of a single molecular species of glycoprotein. It acts as a physical barrier preventing access to the underlying plasma membrane by components of non-specific immune responses that are lethal to uncoated parasites.

Aims and objectives

This exercise is designed to demonstrate:

1. The use of Enzyme Linked Immunosorbent Assay (ELISA) for the detection of antibody produced by experimental immunisation.

2. The induction of an immune response to a parasite antigen.

3. The change in antibody titre during an immunisation programme.

Introduction

One of the most widely used immunoassays in parasitology is the ELISA. This technique was developed in the 1970s as a quantitative and sensitive assay of antibodies during responses to parasitic infection, but can also be adapted to quantify circulating and faecal antigens. When dealing with parasitic infections, it is important to be able to detect the type of antibody response in terms of class and subclass, which may give some indication of resistance or susceptibility (Hagan et al., 1991). In addition, the ELISA is a useful tool for the detection of antibodies produced by experimental immunisation.

Fig. 4.6.1 shows the various components involved in a standard indirect ELISA. The antigen is bound onto a plastic microassay plate and any free sites are then blocked to prevent non-specific binding. The antiserum is added and, if any antigen-specific antibodies are present in the serum, they will bind to the plate. A second antibody, conjugated to an enzyme that recognises the first class of antibody, is then added. If the primary antibody recognised the antigen in the previous step, the secondary antibody will bind to the primary antibody.

Aims and objectives

This exercise is designed to demonstrate:

1. The biology of plant parasitic nematodes.

2. The complexity of the host-parasite relationship in these organisms.

Introduction

Plant parasitic nematodes are serious pests of agricultural crops throughout the world. The nematodes cause damage directly to the crop plant and may also act as vectors for plant viruses. The chemicals used to control plant parasitic nematodes are amongst the most toxic applied to the soil but, as yet, other ways of controlling these parasites are not commercially viable. Some plant parasitic nematodes are classified as migratory ectoparasites; these are soil dwellers which simply feed on roots with which they come into contact. Others, including the most economically important species (the root knot and cyst forming nematodes), are sedentary endoparasites and have far more complex life cycles.

An example of a sedentary endoparasite is the potato cyst nematode (PCN) Globodera pallida. This nematode has an intimate relationship with its host. Cysts, each of which contains 2-300 eggs, lie dormant in the soil until the second stage juveniles (J2) within the eggs (the moult from J1 to J2 occurs in the egg) are stimulated to hatch by the presence of potato root diffusates. After hatch, the J2 locates and enters the growing host root, usually just behind the zone of elongation. It then migrates intracellularly until it reaches the differentiating vascular cylinder tissue, where it sets up a feeding site.

Aims and objectives

This exercise is designed to demonstrate:

1. Parasite-induced behavioural modifications in their hosts.

2. Changes in behaviour at specific stages of infection.

3. The importance of critical evaluation of the experimental observations in terms of possible adaptive manipulation of the host.

Introduction

Many parasites with complex life cycles have adopted strategies that enhance their chances of transmission to the next host. Where no free-living stages exist, these tactics often involve a parasite-induced alteration in host behaviour that increases periods of contact with the next host. Helminths with life cycles that involve passive transfer between hosts make use of predator-prey associations (the food chain) for transmission. In many such cases, changes in host appearance or behaviour that may render them more vulnerable to predation have been described. For example, species of fresh-water shrimps infected with the cystacanth stages of acanthocephalan worms change in colour and behavioural patterns so that they are more conspicuous and spend more time in the vicinity of the precise duck, mammal or fish predator that acts as definitive host for that parasite species (see also exercise 7.5). Additional examples are reviewed by Hurd (1990).

Predation of the intermediate host will only result in successful transmission if the parasite stage is mature and infective. In some cases it has been shown that the intermediate host does not become more vulnerable to predation until the parasite has completed that stage of its development.