Canine distemper virus is a member of the genus Morbillivirus in the family Paramyxoviridae. Canine distemper has been recorded in domestic dogs for centuries. It is now recognized as a worldwide problem of carnivores and has the second highest fatality rate of any infectious disease, after rabies, in domestic dogs. The importance of this disease in nondomestic animals has become evident with vaccine-induced infections in a variety of species and large-scale epidemics in captive and free-ranging felids. To date, canine distemper has been reported in all families of terrestrial carnivores: Canidae, Felidae, Hyaenidae, Mustelidae, Procyonidae, Ursidae, and Viverridae. Veterinarians, including those working with nondomestic carnivores, should be familiar with the clinical signs, diagnosis, and clinical management of this disease.
INTRODUCTION
Canine distemper (CD) is the most important worldwide infectious disease of domestic dogs (Canis familiaris), and its fatality rate is second only to that of rabies.122 CD is caused by canine distemper virus (CDV), first isolated by Carré in 1905.30 Clinical distemper has been known for centuries.76 In recent years, vaccine-induced infections have occurred in a variety of species,26–28,63,74,88,94,121,123 as have large-scale epidemics in felids.14,113 Canine distemper virus may have the most far reaching implications of any infectious agent for susceptible free-living and captive carnivores.93 The discoveries of related viruses, such as phocine and delphine morbilliviruses,103,127 and CDV's similarity to the measles virus suggest viral mutability and a zoonotic potential for CDV.
HOST RANGE
Although CD occurs worldwide in carnivores, much of its natural history is unknown. Evidence of CDV infection has been reported in all families of terrestrial carnivores: Canidae, Felidae, Hyaenidae, Mustelidae, Procyonidae, Ursidae, and Viverridae.9,24,45,54,61,83,93,97,110 The red or lesser panda (Ailurus fulgens) and giant panda (Ailuropoda melanoleuca) have been included in the Ursidae for this review.100,101,134
Morbidity and mortality vary greatly in carnivores. The case fatality rate of domestic ferrets (Mustela putorius furo) approaches 100%,39 whereas 50–70% of infected domestic dogs may remain asymptomatic carriers.59 Fatal CDV infections have also been experimentally induced in suids and primates.13,86,135 Natural cases of CDV-induced fatal encephalitis have been documented in a Japanese macaque (Macaca fuscata)136 and collared peccaries (Tayassu tajacu).11 Within the U.S., regular epidemics occur in free-ranging raccoons (Procyon lotor), a species that may play a role in CD epidemiology in domestic dogs in that region.67,89,114 Conversely, in regions of the world where CDV vaccination is not widely employed (e.g., parts of Africa), domestic dogs may serve as a reservoir for free-ranging wildlife.3,113
ETIOLOGIC AGENT, TRANSMISSION, AND PATHOGENESIS
Canine distemper virus, a relatively large (150–250 nm) single-stranded RNA virus with a lipoprotein envelope, is a morbillivirus in the family Paramyxoviridae.59,122 Three other well-known diseases are caused by members of the Morbillivirus genus: measles in primates, rinderpest in artiodactylids, and peste des petits ruminants in small ruminants. Three recently discovered viruses, phocine distemper virus in seals and cetacean morbilliviruses in porpoises and dolphins, also belong to this genus.103,127
The major mode of CDV transmission is through aerosolization of respiratory exudate containing virus, although other body excretions and secretions (e.g., urine) can result in infection in susceptible hosts if aerosolized. Canine distemper is highly contagious, and viral shedding may follow infection for 60–90 days.59 Transplacental infection has been documented in domestic dogs.77 The epidemiologic role of vertical transmission in CD and whether or not such transmission can occur in nondomestic species are unknown. Although usually short-lived in the environment, the virus can survive at lower temperatures (e.g., 48 hr at 25°C and 14 days at 5°C)117 and may be transmitted either by direct contact or by fomites.
Natural CD pathogenesis in domestic dogs has been well characterized and may be similar in nondomestic species. A systemic infection with viremia is often present. Central nervous system (CNS) involvement is variable and dependent on the host's immune response. Within 24 hr of entering the respiratory tracts, virus spreads in macrophages via local lymphatics to tonsils and bronchial lymph nodes. Replication of the virus occurs in the tonsils and bronchial lymph nodes 2–4 days postinfection; concurrently, low numbers of CDV-infected mononuclear cells are found in other lymphoid organs. Within 4–6 days, virus proliferates widely in lymphoid organs (e.g., spleen, mesenteric lymph nodes, Kupffer's cells in the liver, and the lamina propria of the stomach and small intestine) and spreads, probably through blood, to epithelial and CNS tissues within 8–9 days of infection. The pathogenesis within 9–14 days depends on the humoral and cell-mediated host immune response. Dogs with adequate antibody titers and cell-mediated cytotoxicity will clear the virus from most tissues with no clinical signs, whereas dogs with a poor immune response experience viral spread to many tissues. Dogs with an intermediate cell-mediated response and delayed humoral response will have most virus in epithelial tissues cleared as antibody titers rise. In these latter dogs, delayed CNS signs and hyperkeratosis of the foot pads (“hard pad disease”) may result when virus persists in uveal, neural, and integumental tissues.7,59
SUSCEPTIBILITY/CLINICAL SIGNS
Clinical signs of CD are influenced by virus strain virulence, environmental conditions, host age and immune status, and the infected species' identity. In all species, the respiratory, gastrointestinal, integumentary, and CNS systems are most commonly affected. Diphasic fever and general malaise are often associated with viremia. Infections, probably secondary to leukopenia, are common and may complicate the clinical course.
Canidae
Up to 70% of infections in domestic dogs may be subclinical.59 Mild illness, with nonspecific listlessness, partial anorexia, fever, and upper respiratory tract infection, may occur. However, the acute generalized form has a high mortality rate in domestic dogs, with a 14–18-day incubation period and an initial transient fever and leukopenia 4–7 days postinfection. Clinical signs in acute generalized CD are related to the respiratory and gastrointestinal systems and include conjunctivitis, pneumonia, diarrhea (often hemorrhagic), anorexia, and severe dehydration. A neurologic manifestation of CD may occur 1–3 wk after recovery from acute generalized infection.7,125 Additionally, neurologic distemper can occur in dogs of any age that had no or mild systemic signs and may manifest as chronic progressive neurologic dysfunction in older dogs (usually over 6 yr of age). Neurologic complications depend on viral distribution in the CNS and may include hyperesthesia, cervical rigidity, seizures, cerebellar and vestibular signs, and paraparesis or tetraparesis with sensory ataxia. Myoclonus, the involuntary twitching of muscles in a forceful simultaneous contraction (often leading to “chewing gum” fits), is also highly suggestive of CD.23 Additional signs of CDV in the domestic dog include digital hyperkeratosis (hard pads) and optic neuritis, chorioretinitis, and uveitis.51 In young dogs, juvenile cellulitis and metaphyseal bone lesions and irregularities to the surface of teeth due to enamel hypoplasia may be evident.18,42,84
Nondomestic canid species vary in CDV susceptibility, although clinical signs in acute generalized disease often resemble those described for the domestic dog. Natural and/or vaccine-induced CDV-associated disease has been documented in African wild dogs (Lycaon pictus),1,2,4,45,87,126 Australian dingos (Canis dingo),16 South American bush dogs (Speothos venaticus),88 maned wolves (Chrysocyon brachyurus),123 bat-eared foxes (Otocyon megalotis),68,90 kit foxes (Vulpes macrotis macrotis),16 raccoon dogs (Nyctereutes procyonoides),16,82 coyotes (Canis latrans),38,52,53 red foxes (Vulpes vulpes),16,80 and gray foxes (Urocyon cinereargenteus).16,63,67 CD-related mortalities have also been suspected in free-ranging wolf (Canis lupus) pups71 and fennec foxes (Fennecus zerda).93 All canids may be susceptible to CDV.
Felidae
Subclinical CDV infection in domestic cats (Felis catus) has been demonstrated experimentally,13 and CD disease has been reported sporadically in nondomestic felids.22,35,48,58,108,124 During a 1992 CD epidemic among 74 large captive felids, infection was histopathologically confirmed in African lion (Panthera leo), tiger (Panthera tigris), leopard (Panthera pardus), and jaguar (Panthera onca).14 Forty-seven percent of these cats became ill and 23% died, with signs of gastrointestinal, respiratory, and CNS disease. Sixty percent of ill cats manifested CNS disease with or without preceding gastrointestinal and respiratory disease. Generalized seizure activity, the most common neurologic abnormality, usually culminated in acute death. With rare exceptions, animals that experienced mild disease and recovered had high CDV neutralizing antibody titers, whereas those that died or were euthanatized had low or no detectable titers. Hyperkeratosis of the foot pads did not appear in any case. Histopathologic lesions identified in the lungs and the CNS differed from those in canid species (see pathology section). Only members of the genus Panthera died, however, several mountain lions (Felis concolor) demonstrated vague gastrointestinal or respiratory system signs. One clinically healthy mountain lion was shown by a CDV neutralization test to have seroconverted during the epidemic. Disease was not identified in such small felid species as bobcat (Felis rufus), serval (Felis serval), and margay (Felis wiedii) in this collection. A serologic survey of several other private and zoologic collections revealed CDV neutralizing antibody in a variety of healthy cats, some with a past history of gastrointestinal or respiratory system disease.14
In 1994, a multispecies CD epidemic of unknown origin in the Serengeti ecosystem affected 30% of a population of 3,000 African lions.65,96,113 Many of the affected lions were emaciated, but the most frequent clinical manifestations were neurologic, including grand mal seizures and myoclonus.113 Up to 50% of lions with clinical signs may have died. Domestic dogs (up to 30,000, many of which were unvaccinated) could have transmitted the virus to spotted hyenas (Crocuta crocuta), which in turn may have transmitted the disease to lions.29,61,113 Serengeti lions are nomadic and could distribute the virus over a large range.
Hyaenidae
Fatal CD has been documented in captive hyenas and in free-ranging Serengeti hyenas.3,21,61,97,111 However, a retrospective study involving free-ranging spotted hyenas in the Masai Mara, Kenya, showed a significant rise in CDV antibodies without clinical signs or increased mortality during a period of high domestic dog mortality associated with a CD epidemic.3
Mustelidae
Mustelids are among the species most susceptible to CDV disease, and the clinical presentation is similar to that seen in domestic dogs, with some exceptions. Domestic ferrets and black-footed ferrets (Mustela nigripes) are highly susceptible to natural CDV infection and have a fatality rate close to a 100%.10,19,39 Fatal vaccine-induced disease has also been documented in both species.28,55 In addition to ocular and nasal discharge, diarrhea, anorexia, seizures, and myoclonus, black-footed ferrets often have severe hyperkeratosis of the foot pads, whole body erythema, and chin and groin rash with associated pruritus.28,133 All mustelids are probably susceptible to clinical CD. There are reports of CD in American badgers (Taxidea taxus),15,56 striped skunk (Mephitis mephitits),41 European mink (Mustela lutreola) and American mink (Mustela vison),94,107,121 Eurasian badgers (Meles meles),16 and European otters (Lutra lutra).54,116
Procyonidae
Natural CDV infections in raccoons37,67,75,89,112,114 and vaccine-induced infections in kinkajous (Potos flavus)74 have been documented. All procyonids are probably susceptible to CDV infection, with clinical presentations resembling those in domestic dogs.47,91,105 Cystitis with pyuria is common,91,105 and jaundice is sometimes associated with CDV infection in raccoons.75 Canine distemper virus is endemic in some North American raccoon populations,37,47,67,73,89,114 so this species may be a reservoir for nondomestic zoo animals and domestic dogs. Additionally, CD must be differentiated from rabies in individual raccoons with neurologic signs.
Ursidae
Many ursids are susceptible to CDV infection on the basis of serologic surveys,34,43,49,83,85 and clinical CD has most commonly been documented in red pandas and giant pandas,26,27,46,69,110 although one report of clinical disease in polar bears (Ursus maritimus) and Tremarctos ornatus neonates has been published.115 In a serologic study of polar bears in Alaska and Russia, 35.6% of 191 samples collected from 186 bears were positive for morbillivirus antibodies on the basis of the CDV microtiter neutralization test.49 In a separate serologic study conducted in Alaska, 14% of 480 of grizzly bears (Ursus arctos horribilis) were seropositive, whereas none of the 40 black bears (Ursus americanus) tested had antibody titers.34 Serum neutralization tests with the Onderstepoort CDV strain resulted in seroprevalences of 16% (2/12) and 36% (4/11) in captive and free-ranging Marsican brown bears (Ursus arctos marsicanus), respectively, in Italy.85 A seroprevalence study of free-ranging Florida black bears (Ursus americanus floridanus) found 8% of 66 bears seropositive for CDV antibodies.43
Clinical signs of CD, whether from natural exposure or vaccine induction, may be similar in red pandas and domestic dogs,26,27,69,118 but some differences have been described.46 Signs include purulent oculonasal discharge, anorexia, diarrhea, ascending paresis, and, in some cases, terminal seizures and coma.
Canine distemper virus can be fatal to captive giant pandas110 and can also affect captive red pandas.110 In China, CDV antibody titers were detected in one of five captive and one of three recently rescued giant pandas.83
Viverridae
Two viverrid species may develop CD, the binturong (Arctictis binturong)31,60,64 and the masked palm civet (Paguma larvata).81 There are recent anecdotal reports of vaccine-induced CD in captive binturong (R. J. Montali, unpubl. data). In the free-ranging masked palm civet, clinical signs included dehydration, dyspnea, serous oculonasal discharge, diarrhea, local alopecia, and convulsions.81
DIAGNOSTIC PROCEDURES
Antemortem
In domestic dogs, acute generalized CD infection is often diagnosed by clinical signs in animals not previously vaccinated. In nondomestic species, CD may be suspected on the basis of clinical signs but must be differentiated from such other diseases with respiratory, neurologic, and/or gastrointestinal manifestations as rabies, feline panleukopenia, toxoplasmosis, canine parvovirus, lead poisoning, and bacterial enteritides. Digital, nasal, and eyelid hyperkeratoses, common in infected ferrets and mink,39,107 are highly suggestive of infection. In raccoons, foxes, and ferrets, jaundice associated with CDV infection is occasional and unique.24,75 Absolute lymphopenia, thrombocytopenia, regenerative anemia, decreased albumin, and increased α- and γ-globulin concentrations may be present.59,122 Low numbers of CDV inclusions may be detected in the cytoplasm (and occasionally nuclei) of stained peripheral blood cells, especially lymphocytes. Inclusion bodies may also be detected in smears prepared from conjunctival scrapings. However, inclusion bodies are unlikely to be present in either the blood or conjunctival scrapings outside of the acute phase of infection. Interstitial or alveolar lung patterns on thoracic radiographs also support a diagnosis. Central spinal fluid (CSF) may show increased protein (>25 mg/dl) and cell count (>10 cells/µl with a predominance of lymphocytes) and increased pressure associated with inflammation. Increased anti-CDV antibody in the CSF is definitive evidence of neurologic CDV infection71 unless the blood–brain barrier has been disrupted because CDV-specific IgG is not present in the CSF of vaccinated dogs.
Serologic tests are often unrewarding in clinical CD because most nondomestic animals die before antibody titers are measurable. However, paired sera (10–14 days apart) can be tested by viral neutralization12 or the indirect fluorescent antibody test for a four-fold rise in antibody titer.6 Enzyme-linked immunosorbent assays have been developed to detect serum IgG and IgM antibodies to CDV99,109,129,130 and CDV antigen.20,109 Detection of IgM indicates recent CDV infection unless the animal was vaccinated within 3 wk of the test. The detection of IgG is more ambiguous and can indicate either vaccination or infection.
Immunohistocytochemistry is also useful in diagnosing CD.17,78 Immunofluorescence is usually performed on cytologic smears prepared from conjunctival, tonsilar, genital, or respiratory epithelium. Other tissues in which virus may be detected antemortem with immunocytology are blood and buffy coat smears, CSF, skin, and foot pads. Viral persistence at these sites ranges from a few days postinfection in buffy coat smears to greater than 60 days in skin and foot pads.7,59
It is difficult to isolate CDV by routine cell culture. Virus isolation is most successful by direct cultivation of target tissues of lymphocytes and macrophages from the infected host.10 In cultures with no cytopathic effects after 48–72 hr, fluorescent antibody tests can detect CDV.7,10 Polymerase chain reaction should be considered for the antemortem detection and differentiation of CD.62
Postmortem
Lesions of CDV infection are similar in nondomestic carnivores and in domestic dogs.24,44,107 The most significant gross lesions are pneumonia, depletion of lymphopoietic organs, and hyperkeratosis of the nose, foot pads, and eyelids. In uncomplicated CDV infection, the only consistent pathologic finding is thymic atrophy. Common histologic findings are hyperkeratosis of the nose, foot pads, and eyelids; eosinophilic inclusion bodies in many organs (most commonly cytoplasmic but occasionally intranuclear in the CNS, urinary bladder, and bronchial epithelium); lymphoid depletion; diffuse interstitial pneumonia; and perivascular lymphoplasmacytic infiltration in areas of demyelination and neuronal degeneration of the CNS. Syncytial giant cells in the lungs and CNS white matter, anterior uvea, and lymph nodes may also be present.
In contrast to histopathologic lesions identified in the domestic dog, lungs of large felids may show diffuse alveolar type 2 cell hyperplasia with intracytoplasmic and intranuclear viral inclusion bodies.14 These cells were strongly positive for CDV antigens by immunohistocytochemical staining. This cellular response appears to be unique to large felids.14 Additionally, feline brain histopathology may lack the typical canid pattern of demyelination with astrocytosis and vascular cuffing. Most cats have had mild, patchy CNS lesions compared with those of canids.
The lung, liver, lymph nodes, brain, and spleen of any dead animal with suspected CDV infection should be collected for viral isolation and/or PCR. Immunohistocytochemistry on formalin-fixed tissues provides definitive evidence of CDV infection.17,78 Vaccine virus can be differentiated from street virus by differential cell culture on the basis of different target cell susceptibility.10
CLINICAL MANAGEMENT
Canine distemper is best prevented by vaccination.32 Currently, the vaccines commercially available in North America contain modified live CDV that is tissue culture adapted, primate tissue Vero cells adapted, or egg adapted or a canarypox-vectored CDV, in combination with modified live virus (MLV) canine adenovirus type 2, canine coronavirus, canine parainfluenza virus, and canine parvovirus.32,95,104 MLV vaccines derived from egg-adapted and primate tissue strains of virus have generally been safer than canine tissue culture–adapted strains for nondomestic species; most vaccine-induced CDV infections result from the latter. The Fromm-D™ vaccine (Solvay, Mendota Heights, Minnesota 55120, USA), containing an egg-adapted strain and taken off the market in 1994, was labeled for use in ferrets and safe in many nondomestic species.66,131 The USDA-approved Fervac-D™ (United Vaccines, Inc., Madison, Wisconsin 53744, USA), an egg-adapted strain containing vaccine for use in ferrets, has induced disease in red pandas94 and anaphylaxis in some mustelids (notably ferrets) and viverrids (R. J. Montali, unpubl. data). It should not be used in these species nor, perhaps, in other exotic carnivores. Galaxy-D™ (Solvay, currently being manufactured by Fort Dodge, Overland Park, Kansas 50501, USA for Schering-Plough, Union, New Jersey 07083, USA), a MLV tissue culture–adapted strain with simian cell substrate, has shown promise in nondomestic species and may be safe for use in some canids, procyonids, and wolf species.105 Studies by the ad hoc American Association of Zoo Veterinarian's CDV subcommittee with Galaxy-D™ showed it to be relatively safe and efficacious in maned wolves and hybrid black-footed ferret × Siberian polecat.95 The use of multivalent vaccines containing CDV, such as Galaxy-6-MPH-L (Solvay), should be discouraged, at least in some nondomestic species, because of possible immunosuppression and clinical disease brought about by other MLV components.94,121
An experimental killed vaccine, prepared with B-propriolactone–inactivated Onderstepoort strain, has good safety but variable efficacy when used in both domestic and nondomestic species,95 but unfortunately it is available only in limited amounts to selected institutions holding such species as SSP-managed red pandas and black-footed ferrets. A subunit vaccine with CDV immune-stimulating complexes protected domestic dogs experimentally infected with CDV40 and harbor seals (Phoca vitulina) exposed to phocid distemper virus-1128 and may be useful for nondomestic species. Recombinant vaccines with vaccina,72,120 canarypox,104,120,132 and fowlpox72 vectors with either CDV104,120,132 or rinderpest virus72 antigens have been experimentally tested in ferrets and dogs. The recombinant canarypox-vectored CDV vaccine (Merial, Ltd., Inc., Athens, Georgia 30601, USA) has been safe and efficacious and lacked interference from other canine vaccine components in initial trials and may soon be available.104,132 However, it is presently only licensed and available as a polyvalent product104 and cannot be recommended at this time for use in nondomestic species. A monovalent form of this canarypox-vectored CDV vaccine is being sought.95
Vaccination schedules for nondomestic species are based on recommendations for the domestic dog.6,32 Domestic dogs that received colostrum as neonates should be vaccinated every 3–4 wk between 6 and 16 wk of age. Colostrum-deprived neonates should be given two vaccinations administered on a 3–4 wk interval and starting at 2 wk of age because maternal antibodies acquired in utero should be absent by 4–6 wk of age.6
Data on maternal antibody interference with vaccination of raccoons and ferrets suggest that a final CDV vaccine should be administered at 18–20 wk of age in raccoons and after 10 wk of age in ferrets.57,105 This illustrates the importance of tailoring vaccination programs to a particular species' needs. Additionally, vaccination schedules may require modification during CD epidemics or periods of increased risk of exposure.
Adults should be vaccinated twice, 3–4 wk apart. Modified live CDV vaccines induce long-lived immunity in domestic dogs50 and hybrid ferrets.131 Consequently, the risk of adverse events, including anaphylaxis, has raised questions about the need for annual vaccination of domestic animals.50 Yearly vaccine boosters may be advisable in nondomestic species for which data on antibody persistence postvaccination are lacking. Assessment of the immune response (e.g., IgG antibody levels) provides a way to assess the need for booster vaccination.130
Vaccination of nondomestic felids is not recommended by the Felid Taxon Advisory Group, although favorable results with a monovalent recombinant form of canarypox-vectored CDV vaccine (R. J. Montali, unpubl. data) may change this. Vaccination of free-ranging potential reservoir animals (e.g., domestic dogs in Africa and raccoons in urban North American zoos) for CDV and efforts to minimize their contact with captive and wild felids may decrease the risk for CDV infection in nondomestic felids.
All CDV vaccine use for nondomestic species is extra-label except for Fervac-D™ in ferrets and Distemink™ (United Vaccines, Inc., Madison, Wisconsin 53744, USA) in mink. Veterinarians should consider obtaining signed consent forms prior to vaccinating pet ferrets and pretreating with diphenhydramine to lessen the severity of anaphylactic reactions (R. A. Yates, unpubl. data). Currently, a monovalent canarypox-vectored CDV recombinant vaccine holds the most promise for general exotic carnivore protection against CDV if it becomes licensed for ferrets and widely available.
Canine distemper virus is extremely susceptible to ultraviolet light, heat, desiccation, and common disinfectants (e.g., formaldehyde, ether, chloroform, phenolic compounds, and quarternary ammonium compounds). It is short lived in the environment but can survive at low temperatures (e.g., 48 hr at 25°C and 14 days at 5°C) for extended periods.117 Good hygienic practices and the separation of potential virus-shedding animals from susceptible hosts should be instituted.
There is no specific therapy for animals with clinical CD. Nonspecific treatment is supportive and includes fluids, antibiotics (for secondary bacterial infections), and drugs to minimize CNS inflammation and seizure activity. The prognosis in acute generalized CD is often poor but depends on the virulence of the virus strain along with the identity, age, and immune status of the individual affected. Neurologic manifestations worsen the prognosis.
ZOONOTIC POTENTIAL/FUTURE DIRECTIONS
There is no definitive evidence of naturally acquired human CDV disease although asymptomatic experimental infection may occur.98 However, there may be a relationship between subacute sclerosing panencephalitis (SSPE), multiple sclerosis (MS), Paget's disease, and CDV infection. Dog ownership and clinical MS are statistically correlated,36 as are dog ownership and Paget's disease.102 However, there is no evidence of a causal relationship between CDV and MS.25 More evidence supports an association between measles with SSPE, although such an association is still theoretical.33,106 Nevertheless, with the new and emerging morbilliviruses, both CDV and viruses closely related to CDV in nonhuman primates,136 tayassuids,11 marine mammals,127 and nondomestic felids,14,113 there is concern about the mutability and changing epidemiology of CD. We must be cautious when working with animals potentially infected with morbilliviruses. Potential new vaccines, including DNA (plasmid) products against these agents,119 may hold great promise as safe and effective vaccines for exotic carnivores.
Acknowledgments
We thank Drs. Suzanne Kennedy-Stoskopf, Terry M. Norton, and Jean A. Paré for assistance with the manuscript.
