Noninfectious diseases of wildlife

There are a considerable number of noninfectious diseases of wildlife (Fairbrother et al. 1996). Some are chemical problems that have been identified for an extended period, such as lead poisoning (plumbism) and mercury poisoning, whereas oth­ers have come to the attention of wildlife disease biologists more recently, such as oil pollution, cholinesterase-inhibiting pesticides, organochlo- rine pesticides, and polychlorinated biphenyls.

Plumbism

causal agent Lead (Pb) has long been rec­ognized as a toxic heavy metal for living organ­isms (Fisher et al. 2006). In nature, lead most commonly occurs as a sulfide in the mineral galena (Locke and Thomas 1996). Lead gener­ally is not soluble in hard, basic water, but it is somewhat soluble in softer, acidic fluids, includ­ing the gizzards of waterfowl or stomachs of raptors (Locke and Thomas 1996), and this is where it has become a significant problem.

host and distribution Since the seminal paper by Bellrose (1959) evaluating lead poison­ing among North American waterfowl, over 130 species of animals, including waterfowl, upland game birds, raptors, and reptiles, have been reported as exposed to or killed by ingest­ing lead shot or bullet fragments; other sources of lead such as lead sinkers (Scheuhammer and Norris 1995, Scheuhammer 2009), paint chips (Sileo and Fefer 1987), or mine tailings (Sileo et al. 2001) can cause lead poisoning, but are less significant sources than ingested lead shot or bullet fragments in North America (Locke and Thomas 1996).

reservoir and transmission For water­fowl in North America, the primary means of exposure to lead is by ingesting lead shotgun pellets, which are ingested as grit or incidentally accompany a food item (Locke and Thomas 1996). In many waterfowl hunting areas, lead pellets can build to very high concentrations on the wetlands (Guitart et al. 1994). On being swallowed, the pellets lodge in the gizzard or stomach and are subjected to acidic conditions and grinding that dissolve the lead and allow it to be absorbed into the blood (Locke and Thomas 1996).

For raptors, ingested lead from ammunition fragments in prey or carrion is the primary cause of plumbism, causing ongoing problems among bald eagles (Haliaeetus Ieucocephalus) (Pattee and Hennes 1983, Gill and Langelier 1994, Redig et al. 2009) and California condors (Gymnogyps californianus) (Chesley et al. 2009).

Ingested shot also is an important cause of lead poisoning for galliforms, gruiforms, and a variety of other species (Fisher et al. 2006, Tranel and Kimmel 2009). These and other avian taxa are exposed when feeding in shot-over areas, including shooting ranges (Pain et al. 2009). Although both ingestion and inhalation are entry routes of lead for humans, virtually all cases in wildlife involve ingestion (Fisher et al. 2006).

clinical effects and diagnosis Lead is a nonspecific poison affecting all body systems (Fisher et al. 2006). However, one key effect is that lead interferes in the forma­tion of two enzymes involved in the produc­tion of hemoglobin: delta-aminolevulinic acid dehydratase (ALAD) and heme synthetase (Locke and Thomas 1996).

Birds can die from acute intoxication if large amounts of lead are ingested, involving very few obvious signs or symptoms.

Birds with plumbism also suffer behavioral changes and impaired reproduction (Elder 1954, Buerger et al. 1986, Locke and Thomas 1996). The weakened condition and behavioral changes of affected birds make them more susceptible to other types of mortality (Scheuhammer and Norris 1996), including predation, starvation (Fisher et al. 2006), and other diseases (Trust et al. 1990, Rocke and Samuel 1991).

Lead toxicity can vary with numbers of pellets ingested as well as diet. Waterfowl ingesting hard seeds such as corn are more vulnerable to plumbism than those with a diet of soft leafy plants (Jordan and Bellrose 1951).

Many of the signs for raptors and other terrestrial birds are similar to those described for waterfowl; they include weakness, depression, leg paralysis, convulsions, distention of the pro­ventriculus, green watery feces, and weight loss (Locke and Thomas 1996, Fisher et al. 2006).

A diagnosis of lead poisoning in a bird car­cass is based on a combination of postmortem findings and chemical analysis of body tissues. Diagnosis is supported by lead pellets in the ventriculus, anemia, decreases in ALAD (Beyer et al. 2005), and significant increased levels of lead in blood, liver, kidneys, and bone (Locke and Thomas 1996, Pain 1996, Pain et al. 2009).

population effects Impacts of lead on wildlife include decreased survival and impaired reproduction in populations.

The shift to nontoxic shot resulted in reduced plumbism among wild migrating waterfowl. Gizzards from 44 to 71% of 16,651 samples taken in 1996 and 1997 from the Mississippi Flyway contained only nontoxic shot, and the authors estimated that the nontoxic shot reduced mortality from plumbism among mallards by 64% (Anderson et al. 2000).

Mortality from lead poisoning is of greatest concern in long-lived, slow-breeding species such as raptors (Fisher et al. 2006). Lead poi­soning in bald eagles was a significant basis for the initial banning of lead shot in the United States; about 10-15% of recorded post-fledgling mortality in bald and golden eagles (Aquila chrysaetos) was attributed to ingesting lead ammunition fragments from prey animals (Scheuhammer and Norris 1996). In one study, the prevalence of lead poisoning did not change significantly among bald eagles after lead was banned (Kramer and Redig 1997, Redig et al. 2009); however, this study was based on birds submitted to a raptor center rather than on a representative sample of healthy birds from the environment.

special problems The biological founda­tion for the serious impact of lead on waterfowl, raptors, and other species is well established. However, implementing effective conversion of lead shot to nontoxic substitutes has been complicated by complex societal and political factors (Friend et al. 2009); a thoughtful his­tory and summary of these issues is presented by Friend et al. (2009).

Even though the use of lead shot in waterfowl hunting has been banned in the United States since 1991, lead concentrations among waterfowl that may have significant population effects con­tinue to be documented (Matz and Flint 2009).

An additional topic of concern is the potential human health hazard of eating lead-poisoned game (Johansen et al.

control In North America, implementing nontoxic shot in and around waterfowl hunt­ing areas has been the major approach to reducing lead poisoning among waterfowl, rap­tors, and other species; overall, this approach has led to significant declines in plumbism mortality among wildlife (Anderson et al. 2000, Friend et al. 2009, Scheuhammer 2009). The nontoxic shot includes steel, tungsten, bismuth, and other metals, either alone or combined in alloys or poly­mers. Internationally, regulations on lead ammu­nition are established in at least 29 countries; however, these regulations vary widely, from par­tial, voluntary restrictions on the use of lead shot to a total ban on use and import of lead ammuni­tion (Avery and Watson 2009). Although a variety of other approaches have been used to reduce lead poisoning, such as voluntary change to nontoxic shot, plowing sediments to remove surface shot, providing grit and useful food, and treatment of affected birds (Mudge 1992), the only demonstra­bly effective solution has been a ban on hunting or a legislative shift to nontoxic shot (Scheuhammer and Norris 1995, Fisher et al. 2006).

Treatment of small numbers of lead-poisoned birds, especially birds of special concern, can be done with metal chelating agents, primarily calcium disodium ethylenediaminetretraacetate (Ca-EDTA) (Murase et al. 1992, Redig 1992).