Theileriosis in Multiple Neonatal White-tailed Deer (Odocoileus virginianus) in Delaware, USA

Theileria spp. are protozoal parasites of domestic and wild ungulates (Wood et al. 2013). First documented by Kreier et al. (1962), Theileria cervi is the only species reported in white-tailed deer (Odocoileus virginianus). Lone star ticks (Amblyomma americanum) are the definitive biologic vector of T. cervi, and deer are a major host of lone star ticks (Kuttler et al. 1967; Hazen-Karr et al. 1987). After transmission by ticks, T. cervi replicates in white blood cells (WBCs) and then infects red blood cells (Kuttler et al. 1967). Replication in WBCs is a short portion of the life cycle, and organisms are infrequently observed on histology (Wood et al. 2013). A diagnosis of theileriosis is often achieved by examining Giemsa-stained whole blood smears for identification of intraerythrocytic parasites (Yabsley et al. 2005; Wood et al. 2013). Infections are often subclinical; however, disease and mortality can occur in animals with a severe parasite burden or in conjunction with malnutrition or other diseases (Yabsley et al. 2005). Neonates are particularly prone to immunodeficiency and emaciation, thus predisposing this age group to developing clinical infections. Clinical signs are primarily due to the intraerythrocytic stage of the parasite and include icterus, lethargy, weight loss, and death. Infection of red blood cells leads to anemia and subsequent organ ischemia (decreased blood flow). Clinical symptoms of theileriosis in fawns may increase mortality risk through a reduced ability to avoid predation. Furthermore, physiologically stressed fawns may vocalize more readily than healthy fawns (Chitwood et al. 2014), providing an auditory cue to predators.

We captured 79 white-tailed deer neonates as part of an investigation into fawn mortality in southern Delaware, US (38°43′N, 75°22′W) during May–July 2017. We located fawns using vaginal implant transmitters (Bowman and Jacobson 1998) and ground searches using forward-looking infrared technology (Scout III, FLIR Systems, Wilsonville, Oregon, USA). We affixed each fawn with a 68-g expandable, very high frequency radio collar with a 2-h mortality sensor (M4210, Advanced Telemetry Systems, Isanti, Minnesota, USA). We determined age of fawns not captured at the birth site with a combination of behavior, hoof development, and umbilicus condition (Haugen and Speake 1958; Sams et al. 1996). Age at capture ranged from 0 d to 14 d (mean=3.4 d, SD=3.1 d). We monitored fawns twice daily until they were 30 d old, once daily from 31 d to 60 d old, three times per week from 61 d to 90 d old, and once weekly thereafter. Upon detection of a mortality signal, we recovered and refrigerated all carcasses. The University of Delaware Institutional Animal Care and Use Committee approved all trapping and handling procedures (no. 1288).

Of the 79 captured fawns, 25 fawns (32%) were found deceased and submitted for postmortem examination to the Pennsylvania Animal Diagnostic Laboratory System pathology laboratory at The University of Pennsylvania (Philadelphia, Pennsylvania, USA). Overall mortality rates for white-tailed deer fawns vary widely throughout their range, but the mortality rate we observed was comparable to previous research in the mid-Atlantic region (Vreeland et al. 2004). A total of 21 carcasses had sufficiently intact viscera for complete postmortem evaluation. Postmortem examinations consisted of gross inspection of all viscera, removal of the viscera, and collection of tissue samples for histology. After fixation and routine processing, we stained tissues with H&E, and a board-certified veterinary pathologist examined the glass slides. The most common contributing factor to mortality was emaciation (10/21). All fawns had numerous nymphal and adult arthropod parasites attached to the skin, predominantly lone star ticks. Yellowing of the tissues (icterus or jaundice) was apparent in three fawns. Theileria spp. organisms or evidence of recent infection was identified in 6/21 (29%) fawns. Histopathology of the liver from four fawns identified enlarged WBCs with serpentine multilobulated nuclei and abundant cytoplasm distended by innumerable 1–2-μm-diameter round basophilic parasitic organisms (meront stage; Fig. 1). In three of these fawns, WBCs containing the organisms were ruptured, with release of the merozoite stage into the surrounding liver tissue. Free organisms were associated with multifocal random acute neutrophilic liver inflammation and necrosis. Two additional fawns had similar foci of necrosis and inflammation, suggesting recent infection, although organisms were not identified. A Giemsa stain revealed pinpoint basophilic parasites within circulating erythrocytes (piroplasms; Fig. 2). Given the unique morphology of the enlarged WBCs and various parasitic stages, we diagnosed the fawns with T. cervi in accordance with previous descriptions (Kreier et al. 1962; Telford and Forrester 1991; Wood et al. 2013). We did not perform additional diagnostic tests (polymerase chain reaction, blood smear analysis, or serology) because of a lack of adequate samples (fresh whole blood) and the availability of tests for formalin-fixed tissue.

Of the six infected fawns in this report, three fawns had icterus and acute centrilobular hepatic necrosis (evidence of anemia and hepatic ischemia), suggesting a clinically active infection. All of these fawns had concurrent disease (severe pneumonia or emaciation) that likely contributed to their mortality, which occurred at ages 69, 37, and 16 d. In the remaining three fawns, identification of Theileria spp. organisms was an incidental finding, and death was attributed to another cause (oral necrobacillosis, oral and ruminal necrobacillosis with a perforated rumen ulcer, and emaciation with no liver damage) at ages 26, 25, and 9 d. The infected fawns appeared to be located randomly throughout the study area, and we observed no spatial correlation or home range overlap. Mean age at mortality for fawns not diagnosed with theileriosis was 30.5 d (SD=12.6 d).

Theileriosis in white-tailed deer by T. cervi is historically more common in the southeastern region of the US (Davidson et al. 1983), where up to 50% of deer in enzootic areas may be infected, although clinical cases are rare (Robinson et al. 1967; Wood et al. 2013). Recent expansions in lone star tick abundance and range (Springer et al. 2014, 2015) may increase the frequency of clinical cases as more T. cervi–naïve populations are exposed to the parasite (Wood et al. 2013), with the potential for increased morbidity and mortality in white-tailed deer neonates.

We acknowledge J. Rogerson and E. Boyd with the Delaware Division of Fish and Wildlife. The Wildlife and Sportfish Restoration grant from the US Fish and Wildlife Services under award F15AF00929 and the Delaware Department of Natural Resources and Environmental Control provided major funding. The US Department of Agriculture National Institute of Food and Agriculture, Hatch project DEL00712 and McIntire Stennis DEL00672, also supported this work. We thank Assistant Editor M. Miller and an anonymous reviewer for comments that greatly improved this manuscript.