Parasitic Diseases of Wild Birds provides thorough coverage of major parasite groups affecting wild bird species. Broken into four sections covering protozoa, helminths, leeches, and arthropod parasites, this volume provides reviews of the history, disease, epizootiology, pathology, and population impacts caused by parasitic disease. Taking a unique approach that focuses on the effects of the parasites on the host, Parasitic Diseases of Wild Birds fills a unique niche in animal health literature.

"This is an excellent text that contains a tremendous amount of information useful for both wildlife biologists and veterinarians." - Journal of the American Veterinary Medical Association, June 2009

"Overall this very reasonably-priced book is quite well done. I recommend this book to any veterinarian working with wild birds or wild-caught birds including zoo specimens or veterinary students with an interest in such matters. Wildlife biologists, wildlife rehabilitators and wildlife centers should also find this book useful. University parasitologists should also find an interest in this new offering." -Veterinary Information Network

Parasitic Diseases of Wild Birds provides thorough coverage of major parasite groups affecting wild bird species. Broken into four sections covering protozoa, heminths, leeches, and arthropod parasites, this volume will provide reviews of the history, disease, epizootiology, pathology, and population impacts caused by parasitic disease. Taking a unique approach that focuses on the effects of the parasites on the host, Parasitic Diseases of Wild Birds will fill a unique niche in animal health literature.

Parasitic Diseases of Wild Birds provides thorough coverage of major parasite groups affecting wild bird species. Broken into four sections covering protozoa, heminths, leeches, and arthropod parasites, this volume will provide reviews of the history, disease, epizootiology, pathology, and population impacts caused by parasitic disease. Taking a unique approach that focuses on the effects of the parasites on the host, Parasitic Diseases of Wild Birds will fill a unique niche in animal health literature.

Carter T. Atkinson, Ph.D., is a research microbiologist with the U. S. Geological Survey, Pacific Island Ecosystems Research Center. His research focuses on protozoan parasites of vertebrates, particularly birds, with a recent emphasis on the effects of introduced avian malaria on Hawaiian forest birds.

Nancy J. Thomas, DVM, MS, Diplomate, American College of Veterinary Pathologists, is an endangered species specialist for the U. S. Geological Survey, National Wildlife Health Center. She has more than 20 years of experience in diagnostic pathology and research on wild bird diseases.

D. Bruce Hunter, DVM, MSC, is a professor in the Department of Pathobiology, Ontario Veterinary College, University of Guelph. His specialty encompasses avian, fur-bearing, and wildlife pathology and ecosystem health. His research interests have included infectious diseases in commercial poultry, West Nile Virus in owls, and diseases of mink.

Parasitic Diseases of Wild Birds

John Wiley & Sons

Chapter One

Gary A. Wobeser

Parasitism has been defined in many ways, but in terms of wildlife disease, it is usually taken to mean an obligatory trophic association between individuals of two species in which one (the parasite) derives its food from a living organism of the other species (the host). An individual host bird can be viewed as an island of habitat that provides resources for parasites, with the parasites deriving benefits while the host is harmed or bears some cost. Parasitism is common in nature; for example, Price (1980) estimated that half of all animal taxa are parasitic. Parasitism is ubiquitous in wild birds and individual birds are affected by many different parasites during their lifetime, but our understanding of the parasites that occur in wild birds is fragmentary.

Moore and Clayton (1997) concluded that the majority of parasites of wild birds have yet to be described taxonomically. Some groups, such as blood-inhabiting protozoa (the hematozoa), have been studied widely, perhaps because of the ease with which blood can be collected from living birds, while little is known about other groups such as intestinal flagellates. But even within the hematozoa, species diversity has probably been greatly underestimated (Bensch et al. 2007). Similarly, more is known about the effects of arthropod ectoparasites than about the effect of protozoa and helminths on birds, and cavity nesting birds have been studied more extensively than most other species because of the relative ease in capturing, examining, and following these birds.

Studying parasitism in wild birds is subject to a number of constraints that make working with disease in any free-ranging species more difficult than studying humans or domestic animals. These include:

inadequate baseline information about the host species. Knowledge of avian life history traits is rudimentary (Zera and Harshman 2001), and so one often must extrapolate from other species and collect information about the basic biology of the host while trying to understand a host-parasite relationship;

difficulty in quantifying factors related to disease. It is impossible to assess the significance of a parasite for a population without the ability to calculate basic epidemiological proportions such as prevalence, incidence, morbidity, and mortality rates. The number of individuals affected by a parasite (the numerator for such calculations) is usually difficult to determine and the population at risk (the denominator) rarely can be measured adequately;

the need to consider the long-term effect of a parasite in wild birds. This may be very difficult, even when the number affected and the population at risk can be determined. If a disease, such as coccidiosis, occurs in a flock of chickens and 15% die, the significance of the disease is that 15% fewer chickens go to market. However, a similar 15% loss in a wild bird population might result in more resources per capita for the remaining birds, leading to reduced mortality from other factors and/or improved reproduction. The potential for compensation or other delayed effects may be very important in assessing the impact of a parasite on wild birds at the population level;

the sample of wild birds available for study is usually biased by the method of collection and may not represent the actual state of nature. Depending on the method of collection, affected birds may be under- or overrepresented, even in groups collected by mass-capture methods (Sulzbach and Cooke 1978); and

the anonymity of wild birds, except for the small number marked by the researcher. For instance, although age is an important disease determinant, the age of wild birds often cannot be determined except to differentiate hatch-year from after-hatch-year birds. Individuals seldom can be traced back in time to discover previous exposure to disease agents or forward in time to discover their fate. Commonly used techniques such as retrospective and prospective case-control studies that are useful in human and veterinary epidemiology are impossible except in unusual circumstances, such as in birds with a high degree of nest site fidelity.

A fundamental feature of parasitism is that the presence of a parasite involves a cost to the host. The costs of parasitism may include:

loss of resources extracted by the parasite directly from the host, for example, loss of blood to blood-feeding ectoparasites;

competition between the parasite and the host for resources, as occurs with cestodes that absorb nutrients from the host's gut content;

costs to the host for defense against parasites. These may include foregoing resource-rich areas to avoid areas where parasites may be present, costs for grooming, moving away from parasites, or abandoning a nest, costs to develop and maintain innate and acquired resistance, and costs to activate these systems;

costs resulting from tissue injury related to the parasite. This may be direct damage caused by the parasite or, more often, injury from the inflammatory and immune response to the parasite. Some injuries may result in dysfunction, such as reduced mobility, reduced digestive efficiency, or increased loss of nutrients through intestinal or kidney injury, that interfere with obtaining or retaining resources;

costs related to improper development as a result of parasitism early in life (e.g., Spencer et al. 2005); and

costs to repair or replace damaged tissues.

The diversity of parasites and the variety of ways that they interact with hosts make it difficult to measure the cost of a single parasite species; to compare the relative cost of different parasites such as the lice, intestinal coccidia, and tracheal worms, all of which might be infecting a single host; or to understand how these parasites may interact with each other and with other environmental factors to affect a host population. The costs described above are related to resources, and particularly to energy ["the single common denominator of life"; "something that is absolutely essential and involved in every action large or small" (Odum 1993)]. Energy is a measure of the ability to do work and is a "currency" that can be used to consider the costs of all types of parasitism, at least conceptually if not quantitatively at this time. Four basic features must be considered when using energy as a currency to consider parasitism:

The supply of energy is limited. Most birds are unable to increase their intake of energy readily, and so they must function within a finite budget. In other words, a bird cannot use more energy than it can assimilate or has in storage.

The amount of energy available and accessible is not constant or uniform. The energy available to a bird varies with the time of year, weather, habitat conditions, and the number of competitors for that energy. Not all individuals in a population have equal access to the resources that are available; thus, within a group or population some birds may have abundant resources while others do not.

Use of energy for one purpose reduces the amount available for other uses. Most of the energy assimilated by a bird is used for maintenance, that is, keeping the body functioning, repaired, maintaining a high core temperature, avoiding predators, and defending against disease. Energy that remains can be used for production (growth and reproduction) or stored as fat for future use. If extra energy is used to defend against parasites or to repair tissue injured by parasites, the energy available for reproduction or growth is reduced. For instance, the cost of producing antibody to a novel antigen is equivalent to that of producing half an egg in female House Sparrows (Passer domesticus) (Martin et al. 2003) and mounting an immune response resulted in asymmetry of flight feathers in nestling Mountain Chickadees (Poecile gambeli) (Whitaker and Fair 2002). Conversely, increased reproductive effort may result in reduced ability to mount a defense against parasites (Deerenberg et al. 1997).

The need for energy for various purposes is highly variable among individuals and at different times of year.

Because an individual cannot maximize all life history traits simultaneously, life history theory suggests that a bird should adopt a strategy that optimizes energy use among resource-demanding activities, such as defense and reproduction, to maximize lifetime fitness. Ecologists use the term "trade-off" for this process of making physiological choices among competing needs for resources that should maximize the chances of an individual's genes being passed on to the next generation. Individuals that make the wrong choices are less successful or "fit," and this may provide a basis for genetic selection.

As a result of heterogeneity in both the supply of energy and the need for energy, the appropriate physiological trade-offs in relation to parasitism vary among individual birds and for different parasites, and the pattern of trade-offs is different seasonally and annually. For this reason, the reaction to parasites and the effects of parasitism must always be considered in terms of the context in which parasitism is occurring and of how the situation might influence resource trade-offs. For instance, during one season a bird may be in poor nutritional condition and need to direct all its available resources to simply staying alive, with little or no ability to mount an effective defense against parasites or to grow or reproduce. At another time of year the same bird may have ample resources to meet all needs, and so it can afford strong resistance to parasites and still be able to grow and reproduce effectively.

Young birds may have different priorities than adults and the sexes may have different strategies and trade-offs. For instance, Tschirren et al. (2003) suggested that a greater need for carotenoid-based coloration for signaling by male Great Tits (Parus major) might lead to a trade-off that results in reduced immunocompetence in males. Privileged individuals within the population, such as birds that possess a territory, may have a totally different context for trade-offs related to parasites than do the "have-nots" within the population. Changes in environmental conditions may change the context; for example, Blow Fly (Protocalliphora braueri) larvae had no effect on Sage Thrasher (Oreoscoptes montanus) nestling weight, size at fledging, or mean fledgling age, but in a year with cold wet weather, survival and fledging success were markedly reduced among parasitized birds compared to unparasitized birds (Howe 1992).

Knowledge of how trade-offs occur in relation to parasitism is fragmentary at this time and general rules about which activity (reproduction, growth, defense against predators or parasites) should take precedence for resources are likely subject to many exceptions. For instance, hosts may be selected to develop acquired immunity to only some of the disease agents that they encounter (Boots and Bowers 2004). While mounting a strong defensive response to parasites is likely a "good" thing generally, in some situations it may be adaptive to suppress the defensive response. This may be the case in nesting Common Eiders (Somateria mollissima). Female eiders do not feed during breeding and face severe resource restrictions while incubating. Birds that do not begin with adequate resources abandon their nest in order to survive.

Hanssen et al. (2004) immunized incubating female eiders with nonpathogenic antigens, including sheep red blood cells. Not surprisingly, the rate of successful immunization was not very good compared to what would be expected at other times of year. Under these circumstances, it appears that the appropriate choice for many eiders is to use their limited resources to survive and reproduce rather than to mount an immune response. A second part of the same study compared survival of birds that mounted an immune response to that of birds that did not produce antibodies. Both responding and nonresponding eiders had sufficient resources to complete reproduction; however, only about 27% of birds that produced antibody to sheep red blood cells returned to the colony in subsequent years, compared with approximately 72% of birds that did not produce antibody. Under these conditions, females that invested in an immune response "experienced considerably impaired long-term survival" compared to females that did not respond. This example also serves to illustrate that the effect of a trade-off on fitness may be delayed.

The cost to the host is not obvious for most parasites encountered in wild birds. It is only in a minority of situations, described elsewhere in this book, that parasitism is clearly associated with recognizable functional impairment of the host that we can characterize as disease. The apparently "benign" nature of many parasites could be because:

the effect of the parasites actually is so trivial as to be undetectable;

the cost is not trivial but it is tolerable; that is, the bird has sufficient resources to cover the costs without significant negative effects on other functions under conditions at the time the effect was measured;

the cost of parasitism is obscured by other more proximate regulatory factors such as predation and competition. Predation is thought to be a major factor in shaping the life history of birds (Zera and Harshman 2001) and parasitized prey may be taken disproportionately by predators (Temple 1987). In some situations the parasite benefits if the infected host is eaten by an appropriate predator (parasite-induced trophic transmission; Lafferty 1999). But infections in which there is no apparent benefit to the parasite may make animals more susceptible to predators, perhaps because of the pathology induced by the parasite. Hudson et al. (1992a) found that Red Grouse (Lagopus lagopus scotica) killed by predators were more heavily parasitized by the cecal nematode (Trichostrongylus tenuis) than were hunter-killed birds and that birds with many worms may emit more scent and, hence, be more vulnerable to mammalian predators. In some situations, increased vulnerability to predators may be related to energy trade-offs and reduced resources for predator vigilance or avoidance. For instance, Common Redshanks (Tringa totanus) that are energetically stressed (as might result from parasitism) respond by taking risks that increase the probability of predation (Quinn and Cresswell 2004). The interaction between predation and parasitism is undoubtedly complex. Navarro et al. (2004) found that House Sparrows exposed to potential predators (cat or owl) had reduced T-cell-mediated immune response and a higher prevalence and intensity of infection with Haemoproteus spp. than did sparrows exposed to nonthreatening animals (rabbit or pigeon), suggesting that even the threat of predation may alter trade-offs that influence parasitism. Although little is known about the effect of parasitism on intraspecific competition, this may be an important factor. For instance, male Greater Sage-Grouse (Centrocercus urophasianus) infested with lice are discriminated against for breeding (Spurrier et al. 1991). Females appear to recognize infected males by the occurrence of petechial hemorrhages on the air sacs and males infested with lice are shunned, and so their reproductive input to the population is minimal; that is, their fitness is very low and there is likely negative selection against their genotype. In a similar manner, male Red Grouse infected with T. tenuis may have difficulty defending a territory (Delahay et al. 1995). Consideration of interactions between parasitism and competition must also include competition among species that share parasites, such as the Ring-necked Pheasant (Phasianus colchicus) and Gray Partridge (Perdix perdix) that share Heterakis gallinarum, with asymmetrically severe effects on the partridge (Tompkins et al. 2001b); and

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