Identifying Aflatoxin Exposure Risk from Supplemental Feeding of Deer

Aflatoxins are toxic secondary metabolites of fungal species, including Aspergillus flavus and Aspergillus parasiticus, which contaminate crops and feed (Marasas and Nelson 1987; Thompson and Henke 2000). Mass wildlife mortality events have occurred from consumption of aflatoxin-contaminated crops (Robinson et al. 1982; Cornish and Nettles 1999). The US Food and Drug Administration (FDA) limits aflatoxins within the human food supply chain (FDA 1979). Historically, corn with aflatoxin levels above the limit was sold to feed and bait wildlife (Fischer et al. 1995; Schweitzer et al. 2001).

Birds are especially susceptible to aflatoxins, particularly smaller birds (Moore et al. 2019). Daily diets ≥200 parts per billion (ppb) decreased weight, damaged livers, and increased mortality in Wild Turkeys (Meleagris gallopavo; Quist et al. 2000; Rauber et al. 2007). Northern Cardinals (Cardinalis cardinalis) showed decreased immune function with a single dose as low as 25 ppb (Moore et al. 2019). White-tailed deer (Odocoileus virginianus) developed liver damage when diets contained ≥800 ppb (Quist et al. 1997).

Supplemental feeding (hereafter: feeding) is when humans provide food for wildlife to improve body condition, compensate in times of scarcity, increase observations, or for other reasons (Inslerman et al. 2006). It is especially popular among deer hunters in the southeastern US, practiced by 89% of Arkansas private-land hunters (Zellers 2019) and approximately half of Mississippi hunting clubs (W. McKinley pers. comm.). Aflatoxins have been reported in wildlife feeders in Texas and Oklahoma (Oberheu and Dabbert 2001) and in bags of corn sold for wildlife feeding in the US areas of Georgia (Schweitzer et al. 2001), the Carolinas (Fischer et al. 1995), and Texas (Dunham et al. 2017).

Despite the popularity of feeding and the risk of aflatoxin contamination, there are still unknowns, including how risk differs by feed type and season. Past studies tended to focus on corn (Dunham et al. 2017), despite the variety of feeds used by hunters, most notably protein pellets (Bartoskewitz et al. 2003). Past studies also tended to sample during hunting season (October–February; Oberheu and Dabbert 2001), when feeding is more common but the climate is less hospitable to fungal growth (Sanchis and Magan 2004).

To address these gaps, we tested for aflatoxins in bulk/bagged wildlife feed, deer feeders, and corn piles with environmental exposure over a 10-d period. We also monitored wildlife visitation to feeders. Identifying aflatoxin contamination risk from supplemental feeding and comparing summer and hunting season feeding could inform managers and reduce risk of aflatoxicosis in wildlife.

We collected 100-g samples from bagged and bulk feed from six states, deer feeders in year-round use on 17 properties in Mississippi, and corn piles over time during May–January 2019 and 2020 (Fig. 1). Because aflatoxins are not uniformly distributed in feed (Dunham et al. 2017) or feeders (Newman et al. 2019), all samples consisted of multiple subsamples from throughout the feeder, feed bag, or feed pile. We kept all samples refrigerated until submission to the Mississippi State Chemical Laboratory, which used an enzyme-linked immunosorbent assay with a 5 ppb detection limit to detect aflatoxins.

The southeastern US, where we sampled, has a subtropical climate with long, humid summers and short, mild winters. Mississippi averages 140.2 cm of rain annually, evenly distributed, 26.5 C in the summer and 6.8 C in the winter (National Oceanic and Atmospheric Administration 2021).

Feeders were located in various habitat types (closed canopy pines, thinned pines, and bottomland hardwoods) and soil types (Black-land Prairie, Interior Flatlands, Lower Coastal Plain, Lower Thin Loess, Upper Coastal Plain, Upper Thick Loess, Upper Thin Loess; Kushla and Oldham 2017). We classified feeders as spin, gravity, or trough, and feed as corn, protein, or blend (Supplementary Material Table S1).

We collected feed samples from various retailers (Table S2), sampling whole corn, protein pellets, and wildlife feed blends (Table S3). We conducted environmental exposure trials by leaving corn in forests during July and November 2020. We placed 10 piles of approximately 2 kg of corn on the ground and protected them with exclusion cages (42×33×5 cm) secured with rebar. We collected samples on the day of placement and after 1, 3, 5, 8, and 10 d of environmental exposure.

We monitored wildlife visitation for 7 d at 67 feeder sites for a total of 469 camera nights. We secured two camera traps (Bushnell Trophy Cam HD Vital V3 Game Camera, Bushnell, Overland Park, Kansas, USA) 4–5 m from the feeder and set them to take one photograph per trigger event with a 25-min interval and a scanning photograph each hour. We recorded the species and number of individuals in each photograph.

We compared prevalence of aflatoxins in summer feeders, hunting season (October–February) feeders, and hunting season bagged/bulk feed using a chi-square test (chisq.test, R Core Team 2020). We evaluated aflatoxin production from environmental exposure using a repeated measures model with a zero-inflated Poisson distribution. Aflatoxin level was the dependent variable, days since placed (DAY) and SEASON (summer and hunting season) were independent variables, and site was a random effect (glmmTMB, R Core Team 2020). We treated nondetections of aflatoxins as zeros.

Overall, we sampled 73 feeders during summers and 34 feeders and 64 bagged/bulk feed sources during hunting seasons. We detected aflatoxins in 3/73 summer feeders (4.1%, mean±SD, x̄=58±71 ppb, range 9–139 ppb), 2/34 hunting season feeders (5.9%, x̄=60±1 ppb, range 59–61 ppb), and 7/64 hunting season feed samples (10.9%, x̄=13±8 ppb, range 5–23 ppb; Fig. 2). Aflatoxin prevalence did not differ between summer feeders, hunting season feeders, and hunting season feed (χ²= 2.5, P=0.28).

Aflatoxins levels in corn exposed to environmental contamination were significantly affected by DAY (P<0.0001) and SEASON (P<0.0001). In summer, the first day of detection was day 5 (Fig. 3). On day 10, all samples were positive (100%, x̄=2,220.5±541.3 ppb, range 1,115–3,619 ppb). However, the only positive during the hunting season was collected on day 1 (35 ppb; Fig. 4). Average temperatures and humidity during environmental exposure trials were 25 C and 86% in the summer and 12.4 C and 75% during hunting season.

Of the >35 species photographed, the most common species detected were raccoons (Procyon lotor; x̄=15.3±2.5/d, mean±SD), white-tailed deer (6.5±1.0), Virginia opossums (Didelphis virginiana; 0.9±0.2), and wild pigs (Sus scrofa; 0.5±0.2; Fig. 5). Of all feeder sites, 48% were visited by songbirds, 34% by Wild Turkeys, and 3% by Northern Bobwhites (Colinus virginianus).

We found lower aflatoxin prevalence than in previous studies (Brown and Cooper 2006; Dunham et al. 2017), which may indicate industry improvements. However, aflatoxin levels in summer feeders (≤139 ppb) may still be detrimental to avian health (Quist et al. 2000; Moore et al. 2019). Summer temperatures were ideal for aflatoxin production (25–33 C) and slightly below optimum for A. flavus growth (35–38 C; Marasas and Nelson 1987; Sanchis and Magan 2004). In contrast, hunting season weather conditions were primarily outside the optimal ranges for both. This may explain why summer environmental-exposure samples (i.e., corn piles) had consistently greater levels and an overall greater prevalence of aflatoxins compared to hunting season samples. Though this suggests that summer feeding, especially when feed is on the ground, is a higher risk than hunting season feeding, summer and hunting season feeders did not differ by prevalence or average positive level of aflatoxins. This may be because feeders protect feed from environmental exposure.

Similarly to past studies (Campbell et al. 2013; Bowman et al. 2015), we documented numerous species visiting deer feeders. This includes Wild Turkeys, supporting concerns that declining Wild Turkey populations in the Southeast may be related to aflatoxin-contaminated deer corn (Balkcom et al. 2017; Butler and Godwin 2017). Such aflatoxicosis mortalities may be less visible when they occur in the forest as compared to in croplands. Moreover, aflatoxicosis can cause population declines through decreased reproduction, reduced growth rates, and immunosuppression (Monson et al. 2015).

Given low prevalence, we were unable to compare aflatoxin risk by feed or feeder type despite evidence that feed characteristics affect aflatoxin production (Dale 2011). Future research is needed on this relationship to inform best management practices for feeding.

This publication is a contribution of the Forest and Wildlife Research Center, Mississippi State University, and the work was supported with McIntire-Stennis Funds. This work was also supported by the Mississippi Department of Wildlife, Fisheries, and Parks and US Fish and Wildlife Service (Pittman-Robertson) Federal Aid in Wildlife Restoration funding. The project would not have been possible without help from numerous landowners. We thank our collaborator, B. Navarre, and technicians L. Resop, H. Deasy, C. Deignan, H. Miller, A. Porter, and K. Smith. No Institutional Animal Care and Use Committee compliance was required for this research.

Supplementary material for this article is online at http://dx.doi.org/10.7589/JWD-D-21-00052.