parasite, infects another, the host, and the parasite does some mea- sure of harm to the host while itself ...... Website of the World Federation of Parasitologists. http://www. wfpnet.org/tab_home.php. Windsor DA (1998) Most of the species on ...
TheIntroduction Challenge of An Parasite Control to Parasitism I don’t mind a parasite. I object to a cut-rate one. Rick Blaine in the movie Casablanca tm & © Turner Entertainment Co.
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In This Chapter Building an Understanding of the Basics of Parasitism Hosts—Essential Lifelines for Parasites
arasitism is the mode of existence in which one organism, a Appreciating Parasitism’s parasite, infects another, the host, and the parasite does some meaPlace in Nature sure of harm to the host while itself deriving a benefit. Parasitism is not rare—as we shall see in this book, parasitism is one of the most common lifestyles on earth. As such, the study of parasitism can teach us a great deal about life in general. Parasitism can be studied at many different levels. For instance, parasites in the aggregate pose formidable problems for human health and well-being. Over 500,000 African children still die of malaria every year. Along with the parasites that cause malaria, there are many other parasites that further jeopardize the health of people, especially those living in disadvantaged conditions (Figure 1.1). A greater understanding of the biology of the responsible organisms could lead to development of new vaccines, drugs, or control strategies to abolish these long-standing scourges of humanity. Parallel considerations apply to veterinary medicine or production of food plants because parasites are also a constant menace to these endeavors. Some livestock parasites are resistant to virtually all known drugs and many of our essential food plants are shockingly uniform in their genetic makeup and thus vulnerable to diseases caused by newly emerging parasites of plants. Other reasons to study parasitology are more subtle yet still important. For example, we are currently experiencing crashes of honeybee (Apis mellifera) populations in the Northern Hemisphere (Box 1.1). This situation is worrisome both because honeybees are an indicator of the overall health of the ecosystems in which we live and because they are indispensible pollinators of many of the plants we rely on for food. Many interacting factors, including parasite infections, lie behind these mysterious crashes in bee populations and are worthy of the attention of inquiring biologists. In general, the ongoing loss of biodiversity, in some cases aided and abetted by parasites, provides another strong motivation to know more about parasites and their biology. In the context of conservation biology, it is important to Species names highlighted in red are keep in mind that parasites represent as much as half of all species and, as we included in the Rogues’ Gallery, starting shall see, are valuable and imperiled forms of biodiversity in their own right. on page 429. The efforts of ecologists and evolutionary biologists have in recent years brought into much clearer focus the extent to which parasites have influ 1 enced our world. Parasites play key roles in food webs and energy flow within
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Chapter 1: An Introduction to Parasitism
Figure 1.1 Parasites take a toll on human health. A man suffering from the Concepts in Parasitology Figure: 01.01 disfiguring effects of filariasis, likely caused by Wuchereria bancrofti. A chronic and debilitating infection like this can impair physical and mental health.
ecosystems, and they have long acted as powerful selective agents influencing the population dynamics, competitive interactions, likelihood of predation, and mating behavior of the hosts they occupy, including humans. Revelation of the impact of parasitism in both ecological and evolutionary time frames is an exciting, and very much ongoing endeavor. Furthermore, it has a practical component, in helping us understand, for example, the underlying factors leading to emergence of new parasite problems, whether they afflict food plants, honeybees, lions, cattle, or humans. Most students find parasites to be intrinsically fascinating and this book hopes to capture that interest. Whether the focus is on a parasite gene that encodes a new vaccine target or on images taken from satellites that help gauge the global responses of parasites to climate change, parasites have much to offer in improving our understanding of life on Earth. In order to understand the concepts relating to parasites and the interfaces between the study of parasitism and several prominent biological disciplines, such as biodiversity studies, immunology, ecology, evolution, conservation biology, and disease control, that are the topics of the chapters to follow, the purpose of this chapter is to introduce the basic vocabulary needed to pursue the study of parasitism. This chapter also describes some of the nuances in definitions of parasitism and explores the boundaries between parasitism and other common intimate associations.
1.1 BUILDING AN UNDERSTANDING OF THE BASICS OF PARASITISM Parasites live in or on their hosts and cause them harm There is no single distinctive feature that cleanly separates all parasites from all nonparasites. Brooks and McLennan (1993) have said that “the only unambiguous definition is that parasites are those organisms studied by
BOX 1.1
Parasites and the Decline of Honeybees The European, Western, or Common honeybee Apis mellifera (Figure 1.1A) is the most commonly domesticated honeybee species. It plays a vital role as a pollinator of both wild plants and important crop plants and in the production of hive products such as honey. Since late 2006, A. mellifera has suffered declines in its populations, especially in the Northern Hemisphere, a phenomenon called colony collapse disorder that is marked by the widespread and sudden disappearance of honeybee workers from colonies. Honeybees and their health are instructive for parasitologists because they show that parasites can have farreaching and unexpected impacts, that different kinds of parasites may have unforeseen synergistic effects, and that introductions of parasites into new
environments often have disastrous consequences. Furthermore, they show that it is often not straightforward to ascribe a particular malady like colony collapse disorder to a single cause. One of the players in this story is the infamous Varroa mite (Varroa destructor) (Figure 1.1B), a mite that attaches to the bee and sucks the bee’s blood (hemolymph), weakening its host as it does so. In the process, the mite may also facilitate transmission of a range of RNA viruses such as the deformed wing virus (DWV) directly into the hemolymph of the honeybee. This bypasses more traditional routes of viral transmission, such as via oral or sexual contact. Although the mite alone has been implicated in collapse of colonies, the one–two punch of mite and virus have been considered to be
major contributors to widespread advent of colony collapse disorder. In this case the mite serves as a host in which DWV can increase in abundance and persist for long periods of time. Studies in Hawaii where DWV and other viruses were present but mites were previously absent have shown that introduction of the mite has had the effect of increasing the prevalence of DWV from about 10% to 100% (Figure 1.1C). Furthermore, the titer, or amount, of the virus increased in individual bees by a millionfold and the genetic diversity of DWV decreased to a single strain. These trends suggest that the introduction of the mite has greatly favored transmission of a single highly contagious variant of DWV that is welladapted to the Varroa mite, a partnership that poses grave threats for honeybees throughout the world.
BUILDING AN UNDERSTANDING OF THE BASICS OF PARASITISM
Before concluding that colony collapse disorder can be attributed solely to Varroa mites, the honey bee viruses they habor, or both, note that many other causes also have been proposed including use of pesticides on crop plants that honeybees visit as sources of nectar and pollen and overall habitat degradation. Clearly a variety of stressors may be involved that interact with each other in complex and as yet imperfectly
A
References
understood ways, unfortunately with considerable detriment to honeybees. The situation is further complicated by the use of chemicals called miticides to control Varroa mites, with attendant increases in mite populations resistant to the chemicals. Some feral honeybee populations appear to be developing increased resistance to mite attacks, offering hope for a brighter future for honeybees.
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Henry M, Beguin M, Requier F et al (2012) A common pesticide decreases foraging success and survival in honey bees. Science 336:348– 350 (doi:10.1126/science.1215039). Martin SJ, Highfield AC, Brettell L et al (2012) Global Honey Bee Viral Landscape Altered by a Parasitic Mite. Science 336:1304–1306 (doi: 10.1126/science.1220941).
1011 1010
B
Number of DWV copies per bee
109 108 107 106 105 104 1000 100
DWV prevalence DWV strain diversity Year
2009
2009
2009
2010
2010
Location
Kauai
Maui
Big Is. Big Is. Big Is.
Oahu
0
0
Varroa exposure time (yr)
0
2009
