Strongyloides sp. has been identified as a cause of high fawn mortality in several New York state captive white-tailed deer (WTD) herds during the last 4 years. Fawns from these herds submitted to the Cornell Animal Health Diagnostic Center (AHDC) had histories of acute onset of decreased appetite, dull attitude, weakness, and sometimes diarrhea at approximately 8-14 days of age. While a subset of these fawns had diarrhea prior to death, it was not a consistent clinical sign. Producers reported rapid deterioration and death of these fawns within 12-24 hours of the onset of clinical signs, despite supportive care with antimicrobials, anti-inflammatories, oral feeding and IV fluid therapy. Fawn mortality rates of greater than 50% were reported in affected herds. Post-mortem examinations revealed parasitic enteropathy with coiled nematodes within the proximal duodenum and high numbers of Strongyloides sp. eggs identified in feces or colon contents. Concurrent bronchopneumonia was also described post mortem in some cases. PCR analysis of adult female worms from one affected herd identified Strongyloides vituli. This species has also been identified in adult wildlife WTD submissions suffering from weakness and weight loss. Ancillary diagnostic testing performed to rule out other differentials in these fawns were negative, including diagnostics for Salmonella sp., rotavirus, beta coronavirus, pathogenic E. coli and bovine viral diarrhea virus.

Strongyloides papillosus hyperinfection has been previously described by Forrester et al. in 1974 as a cause of high fawn mortality in one captive WTD herd in Florida. Age of onset, clinical signs and post-mortem findings were similar to those described in NY herds where S. vituli has been diagnosed. Strongyloides papillosus and S. vituli are morphologically indistinguishable and require differentiation through polymerase chain reaction, a molecular diagnostic modality that was not available during the Forrester et al investigation. Proposed routes of fawn exposure to Strongyloides larvae include intra-uterine infections, trans-mammary transmission via colostrum or milk from the doe and percutaneous infection through exposure to infectious L3 larvae in the environment. Fawn mortality declined in NY herds following introduction of oral or pour-on macrocyclic lactone anthelminthic therapy, and by minimizing over-crowding and creating dry housing.

Strongyloides are slender nematode parasites with a worldwide geographic distribution and predilection for the small intestine of ruminants. Domesticated and wild ruminants of North America are known hosts of Strongyloides, including cattle, goats, sheep and cervids. Juvenile ruminants are most susceptible to heavy infection with Strongyloides upon first exposure but develop acquired immunity to future Strongyloides infections. Apparently healthy animals tolerate infection with low Strongyloides loads. High fecal egg counts in neonatal and juvenile ruminants result in clinical disease, with a more common presentation of chronic ill thrift with diarrhea, rather than the acute onset described in these NY fawns. Strongyloides infection often occurs during summer months following periods of high heat and humidity, as these conditions facilitate rapid environmental replication and lead to an increased risk of host exposure to infectious L3 larvae. The prepatent period to egg shedding is approximately 8-11 days in the ruminant host and the modified Wisconsin double centrifugation floatation method is a reliable means for identifying and quantifying Strongyloides eggs per gram in feces. The Strongyloides egg in ruminant hosts can be differentiated from other strongyle-type eggs because they are larvated, measuring 40 to 50 μm in length. Larval culture followed by PCR assay confirms the species of Strongyloides.

We are still learning best practices for the diagnosis, treatment, and prevention of Strongyloides sp. infection in captive WTD. Producers experiencing mortality in fawns are encouraged to work with their herd veterinarian to submit fecal samples for fecal egg counts from affected WTD or perform post-mortem examination through the Anatomic Pathology service at the Cornell AHDC. 

View the fact sheet on Strongyloides and a detailed description of recent outbreaks of sudden death in weaned heifers on NY dairies associated with S. papillosus.