Written by: Pierce Young

Wildlife managers across North America are increasingly concerned about the impacts of disease on deer populations. Two of the most significant diseases affecting white-tailed deer are Hemorrhagic Disease (HD) and Chronic Wasting Disease (CWD). While both are serious, they differ greatly in how they affect deer populations, how they spread, and their long-term consequences.

Hemorrhagic Disease (HD)

HD is a seasonal and cyclical disease caused by viruses in the Orbivirus genus (specifically epizootic hemorrhagic disease virus and bluetongue virus). It is primarily spread by biting midges (Culicoides spp.), making vector transmission a key component. HD typically results in explosive outbreaks in areas where deer populations have low immunity during late summer and early fall. Large impacts occur more often in Northern and Midwestern states with deer populations that infrequently encounter strains of this disease, compared to Southern states where deer frequently encounter the virus and have built immunities to most strains.

Key characteristics of HD:
  • Localized outbreaks with acute, heavy mortality—in areas where deer have not built up a previous immunity.
  • Short-term population declines that usually recover, then proceed to have antibodies.
  • No environmental persistence—once an outbreak ends, the virus does not remain in the environment.
  • Affected populations often develop some level of immunity, and recover.

Chronic Wasting Disease (CWD)

CWD, on the other hand, is a slow-moving, always-present threat. It is a prion disease (misfolding of proteins; not a virus or bacteria) that affects the nervous system, leading to cryptic, long-term mortality. It spreads through multiple transmission routes, including direct contact and environmental contamination (e.g., through saliva, urine, feces, or carcasses).

Key characteristics of CWD:
  • Year-round, continuous transmission.
  • Environmental persistence—prions remain infectious and build-up in the environment for years.
  • No known recovery or immunity in infected animals.
  • Causes gradual but sustained population declines by reducing survival and reproduction rates over time.

Population Impact Differences

  • HD often causes short-term, high-impact mortality, followed by recovery once the outbreak ends.
  • CWD causes a long-term, low-level decline as prevalence increases and survival rates decrease, that spreads slowly across the landscape, with no natural end to the outbreak.

Conclusion

While both diseases are concerning, CWD poses a far greater long-term threat to deer populations due to its environmental persistence, lack of recovery, and ongoing transmission. Understanding these differences is crucial for wildlife managers developing surveillance and control strategies to protect healthy deer populations.

For more information about CWD in Mississippi visit

www.mdwfp.com/cwd