Chapter 1
General Concepts in the Ecology of Invertebrate Diseases

Ann E. Hajek1 and David I. Shapiro-Ilan2

1Department of Entomology,, Cornell University, Ithaca, NY, USA

2USDA-ARS, SEA,, SE Fruit and Tree Nut Research Unit, Byron, GA, USA

1.1 Introduction

With the advent of molecular methods, new species of pathogens and parasites are constantly being described, and as these new species are found, we are learning more about the ecology of new invertebrate diseases, as well as diseases known for many years. Parasitism is a specific and common life-history strategy, and understanding the activity of parasites is central to community and population ecology (Bonsall, 2004). Parasitism of invertebrate hosts also has practical sides, because diseases can help to control insects in an environmentally friendly manner, but we also need to understand the ecology of diseases killing beneficial invertebrates, ranging from pollinators to clams and shrimp, in order to protect managed populations.

The ecology of invertebrate diseases is often referred to as the epizootiology of invertebrate diseases; the word epizootiology is similar to the term epidemiology but refers specifically to "the science of causes and forms of the mass phenomenon of disease at all levels of intensity in an animal population" (Fuxa and Tanada, 1987). The ecology of animal diseases, with emphasis on vertebrates, has been treated in an edited book on disease ecology (Hudson et al., 2002), followed by books emphasizing community and ecosystem ecology (Collinge and Ray, 2006; Hatcher and Dunn, 2011; Ostfeld et al., 2014).

Disease ecology with an emphasis on invertebrates was first addressed by Steinhaus (1949), specifically in relation to insects, and the treatment of this subject developed depth and breadth with the publication of an edited volume by Fuxa and Tanada (1987). Around this time, Anderson and May (1981, 1982) created models to investigate factors driving the development of disease epizootics, with at least one system involving epizootics caused by a granulovirus in a forest-defoliating lepidopteran (Anderson and May, 1980). Today, studies of the ecology of invertebrate diseases are commonly conducted, often to understand the ecology underpinning control of invertebrate pests by pathogens, or to understand protection from pathogens for invertebrates valued by humans. In addition, ecological studies of invertebrate diseases are used to build theoretical insights into the causes and dynamics of all diseases. With the wealth of knowledge that has accumulated since the last synthesis on the ecology of invertebrate diseases in 1987, it is high time to pull together information on this subject. We are also broadening the focus of this book to include the ecology of diseases of all invertebrates and not only insects. Therefore, the hosts included in this book range from pest insects like grasshoppers and caterpillars to valued insects like bees, along with marine and soil invertebrates that are important to humans or ecosystems.

In this chapter, we will present and define the basic concepts on which this field of study is built. Concepts that will be defined will be consistent with definitions in the online glossary published by the Society for Invertebrate Pathology (Onstad et al., 2006).

1.1.1 What Is Disease?

There are numerous definitions for disease, but we consider disease to be a departure from the state of health or normality. Of course, this creates a very broad characterization, including multitudes of causes. However, this book will focus on infectious diseases, which are those diseases caused by living organisms. Invertebrates are also hosts to many noninfectious diseases, of which physical and chemical injuries, genetic diseases, and cancers are a few examples. An example of noninfectious disease impacting insects can occur due to exposure to pesticides. Noninfectious diseases are, however, outside of the material being covered in this book. Descriptive treatments of noninfectious diseases of a diversity of invertebrates can be found in Lewbart (2012) and Sparks (1972).

Returning to disease being a departure from health, this can be much more difficult to determine for invertebrates than for higher vertebrates. Diseases of invertebrates that cause subacute effects and which do not kill hosts could very well be regularly occurring at low prevalence but going undetected. Perhaps recent studies demonstrating the diversity of previously undetected and unrecognized pathogens occurring in honey bee (Apis mellifera) colonies that do not die (see Chapter 14) indicate that departures from health being caused by a diversity of parasites acting together can be quite common. The most frequent way that invertebrate diseases are recognized is due to the death of hosts, so emphasis in this field has been on acute diseases. However, in recent years, investigations have included the impact of disease on host fitness, both for pathogens causing chronic diseases and for acute pathogens, after infection and before host death.

The living organisms causing infectious diseases are parasites, which are organisms that live at a host's expense. This is a very successful life-history strategy as it has been estimated that the majority of species on earth have parasitic lifestyles (Price, 1980; Zimmer, 2000). Pathogens are defined as microorganisms capable of producing disease under normal conditions of host resistance and rarely living in close association with a host without producing some level of disease. Simply put, pathogens can be thought of as microscopic parasites. In this book, the main groups of pathogens that will be covered are viruses, bacteria, fungi, nematodes, and protists. Although many nematodes are not microscopic, the genera that constitute a special group - entomopathogenic nematodes - kill their hosts with the aid of symbiotic bacteria; these nematodes have traditionally been included and studied within the discipline of invertebrate pathology, and thus we include them as well. With the great diversity of pathogens and hosts being covered, of course examples will be missed. In addition, this is presently an expanding field of study, as new species of pathogens are constantly being found. One good example of this is the number of different pathogens that infect honey bees, with numerous examples discovered in recent years. We will focus on representatives from the diversity of pathogens and hosts for which we know the most about disease ecology.

Diseases may be chronic or acute (or somewhere in between). Chronic diseases are of a long duration, and thus the host is expected to survive a relatively long time before expiring, or to die of other causes before the disease can become fatal. Acute diseases of invertebrates are often of a short duration; host mortality or maximum severity is expected to occur within a relatively short time after infection. Certain pathogen groups, such as entomopathogenic nematodes (Heterorhabditidae and Steinernematidae) and their symbiotic bacteria tend to cause acute disease, whereas others, such as most Microsporidia, tend to cause chronic diseases. Within other pathogen groups, chronic versus acute diseases vary among combinations of pathogens and hosts.

1.1.2 Terminology and Measurements

Accurate use of terminology is critical to effective communication in science. In pathology, a number of terms have been used with variable meanings in the literature (Shapiro-Ilan et al., 2005). The terminology used in this book is supported by a widely accepted glossary by Onstad et al. (2006), which is based on an earlier glossary by Steinhaus and Martignoni (1970). Thus, we refer the reader to Onstad et al. (2006) as a reference for definitions that may not be spelled out in this chapter or other chapters within this book. Nonetheless, some of the more common terms in invertebrate pathology are defined and discussed in this section.

1.1.2.1 Prevalence/Incidence

Prevalence and incidence are examples of terms that have been variably defined in the literature. Prevalence refers to the total number or proportion of disease cases in a population at a given time. For example, if a survey of 10 000 pecan weevil (Curculio caryae) larvae in a population indicates that 4000 of the individuals are infected with the fungus Beauveria bassiana, then the prevalence rate is 40%. In contrast, incidence is the number or proportion of new cases of a disease within a population during a specific period of time. For example, in a given week, if 200 C. caryae larvae were found to be infected with B. bassiana within a population of 10 000 larvae, then the incidence rate would 2% for that week. Both terms are important for quantifying infection and disease levels over space and time. The difference lies in incidence emphasizing only new cases versus prevalence including both new and old cases. Therefore, incidence may be more useful in predicting risk or spread of disease within a certain timeframe, while prevalence provides an assessment of the full impact of a disease on a population at a given time.

1.1.2.2 Pathogenicity/Virulence

The terminology of pathogenicity and virulence has also been variably defined in the literature. However, definitions of pathogenicity and virulence in the field of invertebrate pathology have been largely consistent over time, and are in agreement with definitions found in the fields of human pathology and microbiology (Shapiro-Ilan et al., 2005). Pathogenicity is the quality...