A RETROSPECTIVE STUDY OF CAUSES OF SKIN LESIONS IN WILD TURKEYS (MELEAGRIS GALLOPAVO) IN THE EASTERN USA, 1975–2013

Skin lesions of Wild Turkeys (Meleagris gallopavo) often are grossly apparent to wildlife biologists, hunters, and concerned citizens and are a common reason that samples are submitted for diagnostic examination. However, to our knowledge, there are no comprehensive studies that have characterized the causes of skin lesions in Wild Turkeys. Because the gross lesions of most dermatologic diseases of turkeys appear remarkably similar, evaluation of subtle differences in disease patterns, such as anatomic location or seasonal trends, may provide insight into probable etiologies.

In addition to general interest in Wild Turkey diseases, specific concerns have arisen after the first detection of lymphoproliferative disease virus (LPDV) in Wild Turkeys during 2009 (Allison et al. 2014). Lymphoproliferative disease virus is a retrovirus that can induce lymphoid neoplasms in wild and domestic turkeys. Historically, LPDV had only been described in domestic turkeys in Europe and Israel. However, in their report, Allison et al. (2014) described naturally occurring cases of LPDV-associated lymphoid neoplasms in Wild Turkeys in the US, and cutaneous tumors were identified in a subset of these cases. This discovery has increased awareness of LPDV as a potential cause of morbidity and mortality in Wild Turkeys and generated concerns regarding the frequency of these viral-induced tumors involving the skin.

The purpose of this retrospective study is to characterize the causes of skin lesions in Wild Turkeys in the eastern US, with a focus on identifying epidemiologic patterns of specific diseases that may be useful for ante- or postmortem evaluation of affected birds.

Diagnostic reports from all Wild Turkey samples submitted to the Southeastern Cooperative Wildlife Disease Study (SCWDS), Athens, Georgia, for postmortem examination between 1 January 1975 and 31 December 2013 were reviewed. Case reports included in this study were from examinations of whole carcasses and tissue samples submitted for microscopic examination. Ancillary diagnostic testing in a subset of the cases included bacterial, fungal, and viral cultures and molecular testing. Because reports did not always differentiate between whole carcasses and tissue samples, this distinction was not made in this dataset. Individual cases were reviewed for the presence or absence of skin lesions, and cases with skin lesions were further evaluated to determine the etiology, gross and microscopic anatomic distribution, results of ancillary testing, and presence of concurrent disease. On the basis of all diagnostic findings, cases with skin lesions were categorized into one of the following six etiologic groups: 1) avian pox, 2) bacterial dermatitis, 3) fungal dermatitis, 4) ectoparasitism-related dermatitis, 5) neoplasia, and 6) other (including those cases that did not fit the inclusion criteria of the other categories). Inclusion into the avian pox category was based on the presence of characteristic histologic lesions (epidermal hyperplasia, swollen keratinocytes, hyperkeratosis, the presence of intracytoplasmic inclusion bodies, or a combination of factors), the detection of avian poxvirus DNA in skin lesions, or both. Cases were categorized as bacterial dermatitis or fungal dermatitis on the basis of the microscopic presence of bacterial colonies or fungal organisms, respectively, within skin lesions, the culture of bacteria or fungi from affected tissues, or both, in the absence of pathologic changes consistent with other etiologic agents. Cases were categorized as ectoparasitism-related dermatitis on the basis of the identification of ectoparasites in association with skin lesions and the absence of pathologic changes consistent with other etiologic agents. Cases were categorized as neoplasia on the basis of histologic identification and characterization of tumors in the skin. The oncogenic retroviruses LPDV and reticuloendotheliosis virus (REV) were detected in cases with cutaneous neoplasms by PCR as previously described (Allison et al. 2014).

The age, sex, location, and date of mortality were based on information provided in the diagnostic report; however, the methods employed are briefly described in the upcoming text. Wild Turkeys included in this study were categorized as hatch year (<1 yr old) or adults (>1 yr old) on the basis of primary feather and retrice characteristics (Pelham and Dickson 1992). Sex of the birds was determined by identification of gonads on postmortem examination or, for cases in which the whole bird was not submitted, as per the submitter. Date of death was also recorded.

To determine whether histopathology alone was sufficient for diagnosing avian pox, during 2012–13, samples from 48 Wild Turkeys with skin lesions were screened for avian poxvirus using PCR, including 20 suspected avian pox cases and 28 cases in which avian pox was not suspected on the basis of microscopic lesions (as defined earlier). Molecular testing was performed by targeting two genomic regions: the 4b core protein gene and the FWPV 002/259-CNPV 005/324 protein gene. Primers targeting the protein gene FWPV 002/259-CNPV 005/324 (5′-CGAKRAACCAGTAGAAAA-3′ and 5′-ATGATMGTAGAGAAAATAGC-3′) were based on a conserved stretch in the inverted terminal repeat region between the prototype fowlpox (GenBank AF198100) and canarypox (GenBank AY318871) viruses. These primers were used to verify the poxvirus strains identified by the 4b core protein gene sequences. Primers targeting the 4b core protein gene have been described (Lüschow et al. 2004).

Skin lesions were identified in 208 of 660 (32%) Wild Turkeys examined at SCWDS between 1975 and 2013. Of these 208 submissions with skin lesions, 199 (96%) had complete records and were included in this study.

When evaluated by decade throughout the study period there was a significant increase in the occurrence of skin lesions (P=0.034), avian pox (P=0.040), and neoplasia (P=0.040) over time. Among the 199 cases in which skin lesions were identified, avian pox was the most common cause (66%, 131/199), followed by bacterial dermatitis (22%, 44/199), ectoparasitism-related dermatitis (3%, 6/199), fungal dermatitis (2.5%, 5/199), and neoplasia (2.0%, 4/199; Table 1). The remaining nine cases (4.5%) did not fit into one of the diagnostic categories listed earlier. Overall, skin lesions were nearly evenly distributed between males (49%, 98/199) and females (47%, 93/199); sex was not determined in eight cases (4%). Adults were overrepresented among cases with skin lesions (86%, 171/199). Wild Turkey cases with skin lesions were submitted throughout the year but exhibited a bimodal distribution, peaking in April and October (Fig. 1).

Avian pox was the primary cause of skin lesions in Wild Turkey cases during the study period. Cases occurred over a wide geographic area encompassing 17 states (Alabama, Connecticut, Delaware, Florida, Georgia, Indiana, Kansas, Louisiana, Maine, Missouri, Mississippi, North Carolina, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia). The differences between the occurrence of avian pox in males and females (P=1.0, Fisher's exact test) or between adult and hatch year birds (P=0.431, Fisher's exact test) was not significant. The occurrence of avian pox varied significantly throughout the year (P=0.002, chi-square=10.06), being lowest in April and peaking in October (Fig. 1). Skin lesions observed in the fall and winter were significantly more likely to be caused by avian pox (odds ratio [OR] 2.684; 95% confidence limits [CL] 1.422, 5.068) than by other etiologies.

In avian pox cases, gross lesions almost always involved unfeathered portions of the head and neck (94%, 124/131; Table 1). Cases in which skin lesions involved the head and neck, or were restricted to the head and neck, were significantly more likely to be caused by avian pox than other etiologies (OR 18.1; 95% CL 6.508, 50.32, and OR 2.676; 95% CL 1.416, 5.059, respectively). Gross lesions in avian pox cases consisted of raised, exophytic, ulcerated or crusted skin nodules and frequently occluded the ears and eyes or involved the commissures of the beak (Fig. 2a, b). Diphtheritic lesions involving the mucosa of the oral cavity and esophagus occurred in roughly half of all avian pox cases (49%, 64/131) and were not observed in any other category of skin lesion. Avian pox lesions occurred less frequently over the legs and feet and only occasionally involved the feathered skin (Table 1). There were two avian pox cases with exclusively oral lesions, one case each that involved the oral cavity and the feathered skin or feet, and one case with lesions only on the feet and legs.

Histopathologic features of avian pox are characteristic, but can vary with severity, chronicity, and presence of secondary infections. Microscopic lesions include epidermal hyperplasia, swollen keratinocytes (ballooning degeneration), and hyperkeratosis (van Riper and Forrester 2007). The presence of distinctive 15–30-μm, eosinophilic, intracytoplasmic inclusion bodies (Bollinger bodies) was noted in nearly three quarters of all avian pox cases (68%, 89/131; Fig. 2c, d) but was not included as a required criterion for avian pox diagnosis because of variable degrees of tissue preservation and inherent differences in reporting by individuals over the study period. Epidermal ulceration, necrosis, serocellular crusting, and heterophilic to lymphoplasmacytic dermatitis and panniculitis also were observed to varying degrees (Fig. 2c, d). Secondary bacterial and fungal infections occurred in 45% (59/131) of the cases with avian pox on the basis of microscopic identification of predominantly coccoid (or mixed morphologic) bacterial populations or fungal agents within superficial lesions.

The majority of avian pox diagnoses in this study were based on identification of characteristic microscopic lesions (82%, 107/131). In the targeted study of 48 cases with skin lesions tested for avian pox by PCR during 2012–13, all 20 samples diagnosed as avian pox on the basis of microscopic lesions were PCR positive and the 28 samples without microscopic avian pox lesions were negative. Avian poxvirus was not detected in any of the 12 cases diagnosed as bacterial dermatitis on the basis of microscopic findings. Sequence analysis of the 20 PCR-positive samples genetically identified the viruses as fowlpox virus. Additionally, a small subset of four poxvirus cases in Wild Turkeys recovered between 2006 and 2010 were historically analyzed by PCR and sequencing. Although fowlpox virus was again identified in the majority of these cases, a single case of canarypox virus was identified. Avipoxvirus sequences obtained from Wild Turkeys have been deposited in GenBank under the accession numbers KR259105–KR259128.

Bacterial dermatitis was the second most frequent cause of skin lesions in Wild Turkeys (Table 1). The difference between the occurrence of bacterial dermatitis in males and females (P=0.860, Fisher's exact test) or between adult and hatch year birds (P=0.372, Fisher's exact test) was not significant. The occurrence of bacterial dermatitis varied significantly throughout the year (P=0.005, chi-square=13.08), with a peak occurring in April (Fig. 1). Skin lesions observed in the spring and summer months were significantly more likely to be caused by bacteria (OR 3.298; 95% CL 1.589, 6.844) than other etiologies.

Gross lesions of bacterial dermatitis did not exhibit an anatomic predilection for unfeathered skin as was observed with avian pox (Table 1). However, cases with skin lesions restricted to the feet were more likely to be caused by bacteria than other etiologies (OR 11.92; 95% CL 2.313, 61.44). Grossly, the bacterial dermatitis cases varied from diffuse ulcerative lesions to proliferative lesions similar to those observed with avian pox (Fig. 2e, f). Histopathologic lesions occasionally shared some microscopic features with those of pox (epidermal hyperplasia) but lacked characteristic features of Bollinger bodies and ballooning degeneration. Epidermal hyperplasia was less severe to absent compared with avian pox lesions, and ulceration was a more common microscopic finding in bacterial dermatitis cases (Fig. 2g). Bacteria were identified within lesions microscopically (Fig. 2h). Of the 44 bacterial dermatitis cases, 29 were submitted for routine aerobic and anaerobic bacterial culture. The most frequently isolated organisms included commensal skin microflora such as Staphylococcus spp. (n=16) and Streptococcus spp. (n=2) or common environmental bacteria such as Escherichia coli (n=7), Enterococcus spp. (n=6), Bacillus spp. (n=4), Enterobacter spp. (n=3), Citrobacter spp. (n=2), and a Corynebacterium-like agent (n=2).

Neoplasms accounted for a small percentage of skin lesions overall (2.0%, 4/199). The difference between the occurrence of neoplasia in males and females (P=0.469, Fisher's exact test), between adult and hatch year birds (P=0.332, Fisher's exact test), or across the seasons (P=0.311, chi-square test) was not significant. All four cases of neoplasia involving the skin were lymphoid neoplasms, and all had gross lesions involving the head and neck. Three of the turkeys also had lesions that involved the legs, and one of those turkeys also had lesions on the feet. Gross lesions consisted of smooth to ulcerated or crusted nodules and were indistinguishable from avian pox (Fig. 2i, j). Histopathologically, the dermis and subcutis were markedly expanded to effaced by sheets of pleomorphic lymphocytes, lymphoblasts, and plasma cells, with compression and multifocal ulceration of the overlying epidermis (Fig. 2k, l). Large areas of necrosis and serocellular crusting were present superficially and throughout the dermis in most cases. Two of these cases tested positive for LPDV proviral DNA and negative for REV by PCR, one case tested positive for both LPDV and REV by PCR, and one was not tested for either of these viruses.

Sporadic cases of fungal dermatitis occurred infrequently (2.5%, 5/199). The difference between the occurrence of fungal dermatitis in males and females (P=0.369, Fisher's exact test), between adult and hatch year birds (P=1.0, Fisher's exact test), or across seasons (P=0.535, chi-square test) was not significant. Skin lesions associated with fungal infection did not exhibit any anatomical predilection. Cases of fungal dermatitis were grossly indistinguishable from bacterial dermatitis. Microscopically, lesions typically were confined to the epidermis and superficial dermis, and were predominantly ulcerative and heterophilic with intralesional fungal hyphae. Samples from two of the five cases in which fungal organisms were identified microscopically were submitted for fungal culture. A Penicillium sp. was isolated from one bird, and no agents were isolated from the other.

Ectoparasites were occasional incidental findings in Wild Turkeys and rarely the primary etiology of skin lesions (3%, 6/199). The difference between the occurrence of ectoparasitism-related dermatitis in males and females (P=0.435, Fisher's exact test), between adult and hatch year birds (P=1.0, Fisher's exact test), or across seasons (P=0.434, chi-square test) was not significant. A single case each of dermatitis was associated with infestations, with scaly leg mites (Knemidocoptes mutans) affecting only the feet, fowl cyst mites (Laminosioptes cysticola) affecting only the feet, and chiggers (Neoschongastia americanum) affecting only the feathered skin. Three cases of severe louse infestations occurred, and the following species were identified: Chelopistes meleagridis, Menacanthus stramineus, and Lipeurus caponis. Microscopic lesions in cases of ectoparasitism-related dermatitis consisted of epidermal hyperplasia and hyperkeratosis with variable infiltrates of lymphocytes, plasma cells, macrophages, and heterophils. Feather lice also were reported as incidental findings in 14 Wild Turkeys.

Nine cases of skin lesions were categorized as “other” that either did not fit within the previously described categories or did not have a confirmed etiology. These included one case each of follicular cysts, epidermal cysts, dry gangrene, and trauma and five cases in which the etiology of skin lesions was not determined.

Skin lesions were present in approximately one third of all Wild Turkey samples submitted to SCWDS from 1975 to 2013. As a review of diagnostic submissions, these results may not reflect the true incidence or relative importance of skin lesions in Wild Turkeys. A significant increase occurred in the proportion of cases with skin lesions over the study period, which could reflect changing tendencies in detection and submission of overt cases for evaluation or an actual increase in the incidence of skin lesions in Wild Turkeys over time. Because the total population of affected versus unaffected turkeys is unknown, this distinction cannot be made. The high frequency of skin lesions in Wild Turkeys submitted to the SCWDS diagnostic service likely is due to the visibility of such lesions as well as the awareness of wildlife biologists and citizens regarding specific disease entities like avian pox.

The most common anatomic distribution of skin lesions in this study involved the head and neck. Several factors likely contribute to this observed finding, including the vulnerability of unfeathered skin to mechanical trauma and insect bites, the increased visibility of lesions on unfeathered skin of the head and neck, and the potential for lesions in this location to impair vision, hearing, and prehension of food, thereby leading to debilitation or death and increasing the likelihood of submission for postmortem examination. Unfortunately, inconsistencies in reporting prohibited determining whether skin lesions were a primary cause of debilitation or death in individual cases.

Avian pox was the primary cause of skin lesions in Wild Turkeys and was diagnosed in 66% of skin cases. Studies estimating prevalence of avian pox in Wild Turkey populations are few, often conducted on limited geographic ranges, and occasionally rely solely on the identification of suggestive gross lesions. The reported prevalence in these studies has been highly variable, ranging from 1.4% up to 67% (Lutz and Crawford 1987; Forrester 1991; Forrester and Spalding 2003). Population sampling methods (e.g., targeted surveillance or samples of convenience such as hunter-harvested animals) and diagnostic techniques (e.g., reliance on presence of gross lesions or confirmation by histopathology or molecular techniques) likely play a large role in the wide variation of estimated prevalence of avian pox. Reviews of diagnostic sample submissions such as this one are not suited for estimating prevalence but suggest that reliance on gross lesions for diagnosis of avian pox is insufficient.

Avian poxvirus refers to a group of epitheliotropic viruses within the Avipoxvirus genus of the family Poxviridae, which have a worldwide distribution and have been reported in 278 bird species in 23 orders (Bolte et al. 1999; van Riper and Forrester 2007). Avipoxviruses historically have been placed arbitrarily into nine species on the basis of the host of isolation (Francki et al. 1991). However, recent phylogenetic analysis of avipoxviruses suggests the existence of three main clades: clade A (fowlpox viruses), clade B (canarypox viruses), and clade C (psittacinepox viruses) (Gyuranecz et al. 2013). In this study, 23 of 24 (96%) avian poxvirus infections in Wild Turkeys in which sequencing was performed were attributed to fowlpox virus, and there was a single case of canarypox in a Wild Turkey from Florida. Gross and microscopic lesions in the turkey with canarypox were indistinguishable from those of birds infected with fowlpox virus. The significance of this finding and the prevalence of canarypox virus in the Wild Turkey population are currently unknown.

Avian poxvirus is transmitted mechanically, either by mosquito vectors or through direct contact with lesions or contaminated fomites (van Riper and Forrester 2007). In this study, poxvirus infections were observed throughout the year but were most common in the fall and winter, peaking in October (Fig. 1). Total Wild Turkey submissions also peaked in October, and the concurrent peak in avian pox cases may simply reflect increased submissions during this time, potentially because of increased observations of affected turkeys during fall hunting seasons.

Alternatively, the seasonal occurrence of avian pox lesions may follow annual peaks in the abundance of mosquito vectors, including Culex nigripalpus and Wyeomyia vanduzeei (Akey et al. 1981). A careful evaluation of the seasonal abundance and activity of vectors would be necessary to investigate their association with temporal patterns of avian pox occurrence. Although such investigation is beyond the scope of this study, a previous study in Florida reported similar peaks in seasonal occurrence of avian pox cases during the fall in association with peak vector abundance (Forrester 1991). Host density, seasonal behavioral changes, and seasonal variations in the age-class structure of populations may also contribute to trends in poxvirus infection (van Riper and Forrester 2007).

Consistent with previous reports (Forrester 1992), gross and microscopic lesions of avian poxvirus infections predominantly affected the unfeathered portions of the head and neck, whereas lesions were noted less frequently over the unfeathered portions of the legs and feet, as well as areas of feathered skin. Diphtheritic lesions involving the oral cavity and esophagus have been documented as a manifestation of poxvirus infection (wet pox), and characteristic histopathologic lesions are identical to those of cutaneous pox lesions (van Riper and Forrester 2007). Although microscopic findings in oral lesions were not consistently reported in these cases, diphtheritic lesions in the oral cavity occurred in approximately half of the turkey cases with cutaneous pox. Although other diseases of Wild Turkeys may grossly mimic those of diphtheritic pox lesions (e.g., trichomoniasis, candidiasis, capillariasis, and vitamin A deficiency), the presence of cutaneous lesions in conjunction with oral diphtheritic lesions is nearly pathognomonic for avian poxvirus infection in Wild Turkeys (van Riper and Forrester 2007). Our study found two cases that presented with oral diphtheritic pox lesions (confirmed microscopically) in the absence of cutaneous pox lesions. A single such case was also reported by Forrester (1992).

Secondary bacterial and fungal infections were common in cases of avian pox. The nodular hyperkeratotic nature of cutaneous pox lesions makes them susceptible to trauma and creates an ideal microenvironment for proliferation of normal skin bacteria. Heavy bacterial or fungal contamination and resulting inflammation have the potential to mask lesions of avian pox; however, remnant poxviral inclusions typically can be observed even with extensive secondary infections. In the subset of skin lesions that were screened for avian poxvirus by PCR, agreement between histologic and molecular diagnoses was 100%, suggesting that histopathology alone is an efficient means to diagnose avian pox. However, for cases in which poor tissue preservation or secondary bacterial or fungal colonization might impede microscopic identification of poxvirus lesions, molecular diagnostics or virus isolation may serve as useful alternatives.

Only four cases of neoplasms involving the skin were identified in this study, and all were lymphoid in origin. Lymphoproliferative disease virus, an oncogenic retrovirus previously reported only in domestic turkeys in Europe and Israel, was recently identified from rare cases of lymphoid neoplasia in Wild Turkeys in the southeastern US (Allison et al. 2014; Thomas et al. 2015). Two cases of lymphoid neoplasms involving the skin were associated with LPDV in this review and are the same two cases with cutaneous lesions reported by Allison et al. (2014). Reticuloendotheliosis virus is another potential retroviral cause of lymphoid neoplasms in domestic turkeys and Wild Turkeys (Payne 1998). Although no cases of lymphoid neoplasia in this study were attributed solely to REV (i.e., a dual LPDV-REV infection was noted), and case reports of REV-associated neoplasia in Wild Turkeys did not have skin lesions, it remains a potential differential for cutaneous lymphoid neoplasms in Wild Turkeys (Ley et al. 1989; Hayes et al. 1992).

Other causes of skin lesions besides avian pox and neoplasia occurred in low numbers, with no age-related or sex-related trends, nor a predilection for anatomic location of lesions. A diagnosis of bacterial or fungal dermatitis was made by excluding other important causes of skin lesions in Wild Turkeys (such as avian pox), and the predominance of a single organism within the lesion as identified by microscopic evaluation or culture. Cases of bacterial and fungal dermatitis likely originate secondary to trauma. This hypothesis is supported by the wide variety of bacterial and fungal organisms identified, none of which were primary pathogens. Skin lesions observed in the spring and summer were more likely to be bacterial in origin than other etiologies, which may reflect a predisposition to trauma during times of courtship and mating.

The three species of feather lice observed in this study have been previously reported in Wild Turkeys (Lane et al. 2006) and typically are considered an incidental finding. However, three birds in this review were reported to have skin lesions attributed to heavy louse infestations. Single cases of scaly leg mite and fowl cyst mite were also observed in this study.

In conclusion, Wild Turkeys are susceptible to a number of diseases that result in skin lesions, and these are common reasons for submission to diagnostic laboratories. The response of skin to injury is often similar, regardless of etiology, and gross examination alone is often insufficient to determine the etiology of skin lesions accurately. However, the anatomic distribution and seasonal occurrence of cutaneous lesions may offer helpful clues to diagnosticians or biologists when conducting ante- or postmortem examinations of affected Wild Turkeys. Additionally, these data may have value for developing differential diagnoses in situations in which the turkey is not physically available for examination (e.g., trail cameras or pictures).

This study could not have been completed without assistance of the diagnosticians, technicians, and collaborators at SCWDS, past and present, and the numerous staff biologists and technicians affiliated with state and federal wildlife agencies that submitted the specimens. This study was made possible through continued financial support from the member states of the SCWDS provided by the Federal Aid to Wildlife Restoration Act (50 Stat. 917).