We analyzed 69 eastern Tennessee wildlife samples for Baylisascaris spp. during 2011. The prevalence of Baylisascaris spp. in raccoons (Procyon lotor) was 16% (8/49), an increase compared to previous surveys in this region. One Virginia opossum (Didelphis virginiana) had eggs in its feces, indicating that opossums can play a role in Baylisascaris spp. transmission.
Genus Baylisascaris, in the order Ascaridida, is comprised of 10 species that use various definitive hosts. The life history and transmission, geographic distribution, clinical signs, and wildlife population implications of Baylisascaris spp. have been well reviewed by Kazacos (2001, 2016). To date, the majority of research has been devoted to Baylisascaris procyonis due to the documented cases of visceral, ocular, and neural larva migrans in various hosts including humans. Cases of neural larva migrans in humans can be severe and may result in death (Lee et al. 2010; Hernandez et al. 2013; Kazacos 2016). Since the 1970s, over 2,400 cases of B. procyonis-associated encephalitis in nonhuman animals and 25 human cases have been documented in the US and Canada (Hernandez et al. 2013; Kazacos 2016). In a recent study, 7% of asymptomatic people tested positive for B. procyonis antibodies, indicating that subclinical infections can occur (Weinstein 2017).
In Tennessee, there have been three previous surveys of B. procyonis. The first reported a prevalence of 7.5% (19/253) and was conducted throughout Tennessee from 1976–77 (Bafundo et al. 1980). A later survey, undertaken from 1980–85 in northwestern Tennessee and southwestern Kentucky, detected a 3.4% (5/145) prevalence (Smith et al. 1985). Lastly, Souza et al. (2009) found a 12.7% (15/118) prevalence in a 2007 survey in eastern Tennessee, indicating a substantial increase from previous studies.
To date, Baylisascaris has not been documented in free-ranging Virginia opossums (Didelphis virginiana), despite overlapping populations of opossums and infected raccoons as well as results from experiments indicating that opossums fed Baylisascaris-infected tissue yielded intestinal nematode infections (Kazacos 2001; Gardner and Sunquist 2003). In addition, opossums have been known to ingest raccoon feces, providing an alternative explanation for B. procyonis eggs in their digestive tracts (Page et al. 1999).
During 2011, 69 meso-mammal carcasses or fecal samples from raccoons, skunks (Mephitis mephitis), opossums, bobcats (Lynx rufus), and coyotes (Canis latrans) were opportunistically collected from 11 counties in eastern Tennessee. Stomach, intestinal, and colon contents from each carcass were carefully examined. Recovered nematodes were enumerated and identified to genus based on morphologic characteristics (Kazacos 2001; Zajac and Conboy 2012). Fecal samples were examined for Baylisascaris spp. eggs by centrifugal fecal flotation using Sheather's sugar solution (Zajac and Conboy 2012) followed by microscopic examination.
The prevalence of Baylisascaris spp. was calculated for all host species with a 95% binomial confidence interval. The B. procyonis prevalence in raccoons in our survey was compared to the three aforementioned published studies performed in Tennessee using an analysis of variance test. P-values less than 0.05 were considered statistically significant.
Out of 69 animals examined in this study, 10 (15%; 95% confidence interval [CI] 8–21%) had evidence of Baylisascaris (Table 1). Of the 49 raccoon samples examined, 8 (16%; 95% CI 6–27%) were positive for B. procyonis. One out of 11 (9%; 95% CI 0–26%) skunks had Baylisascaris columnaris and one out of 6 (17%; 95% CI 0–47%) opossums had Baylisascaris eggs in its feces. None of the bobcats or coyotes had evidence of Baylisascaris infection. Seventy-two percent of the Baylisascaris spp.-positive animals originated from one of Tennessee's most populous regions, consisting of the two adjacent counties of Knox and Anderson (Fig. 1; see Supplementary Material Table 1).
The 16% prevalence of B. procyonis in raccoons detected in this survey was significantly higher than the 7.5% prevalence determined in 1976–77 (Bafundo et al. 1980; P<0.0001) and 3.4% in 1980–81 (Smith et al. 1985; P<0.0001). The apparent increase in B. procyonis prevalence from 12.7% in 2007 (Souza et al. 2009) to 16% in our study was not statistically significant (P=0.539).
The results of our survey confirmed previous experimental studies suggesting that opossums could shed Baylisascaris eggs (Gardner and Sunquist 2003). Given that no Baylisascaris nematodes were observed on necropsy of the opossum with eggs present in its fecal material, it suggests the opossum inadvertently ingested the Baylisascaris eggs from contaminated materials. Although we did not attempt to larvate these eggs to determine their viability following passage through the opossum gastrointestinal tract, the ability of ascarid eggs to survive harsh environmental conditions strongly suggests that the eggs would be viable. These findings suggest that opossums should be further investigated for their potential in spreading Baylisascaris spp. eggs.
This survey supports previous data that indicate the prevalence of Baylisascaris spp. in Tennessee raccoons is increasing. Of particular interest is the higher prevalence of Baylisascaris spp. in human-dense areas. Ogdee et al. (2017) disclosed that a single raccoon infected with B. procyonis could shed eggs on approximately 0.03 ha/yr, further emphasizing the potential public health risk of Baylisascaris spp. Steps to minimize Baylisascaris spp. transmission to humans include discouraging peridomestic inhabitations of raccoons, skunks, and opossums at farms and homes by storing all pet and livestock food in secure containers and covering sand boxes, brush, and compost piles to prevent animal denning or defecation (Kazacos 2016). The neurotropism and pathogenicity of B. procyonis warrants continued surveillance of wildlife reservoirs, increased investigation into the role of the opossum in transmission, and education of the public on efforts to minimize transmission and human infection of the parasite. This is particularly important given that B. procyonis eggs can remain infectious for at least 2 yr on all soil types in regions with hot, dry summer climates similar to Tennessee (Ogdee et al. 2016).
Supplementary material for this article is online at http://dx.doi.org/10.7589/2017-12-303.