What Age Does CWD Occur? Understanding Infection and Symptom Onset in Cervids

While Chronic Wasting Disease (CWD) can infect susceptible animals from a young age, the onset of clinical symptoms typically occurs years later, primarily in adult cervids. The question of what age does CWD occur is complex, differentiating between early infection and the eventual manifestation of visible illness.

Quick Summary

Cervids can become infected with the CWD prion from as young as six months old or even in utero, but most symptomatic cases are observed in adults aged 3 to 7 years old. The long, silent incubation period means that infection happens well before outward signs of the progressive and fatal disease appear.

Key Points

• Infection Starts Early: Cervids can be infected with the CWD prion from as young as 6 months, and even in the womb, but show no immediate signs of illness.

• Symptoms Emerge in Adults: Due to a long incubation period (18-24+ months), most visible symptoms of CWD appear in adult animals, typically between 3 and 7 years of age.

• Prevalence Varies with Age: The cumulative risk of exposure means that older deer, especially males, have higher CWD prevalence rates compared to younger animals.

• Young, Healthy Carriers: Younger, seemingly healthy animals can be infected and actively shed infectious prions into the environment for months or years, contributing to the spread of the disease.

• Always Fatal Outcome: Regardless of the age of onset, CWD is a progressive neurological disease that is always fatal to infected cervids.

• Silent Transmission Risk: The extended incubation period complicates management, as a significant number of infected, yet asymptomatic, animals can exist within a population, silently spreading the disease.

CWD's Long Incubation Period

Chronic Wasting Disease is a transmissible spongiform encephalopathy (TSE) that results from the accumulation of misfolded prion proteins in the central nervous system of cervids, such as deer, elk, and moose. Unlike many diseases with a short incubation period, CWD has a lengthy and variable one. It can take 18 to 24 months, and often much longer, from the initial exposure to the point where an infected animal starts to show outward signs of illness. This long latency period is the primary reason why clinical signs are most commonly reported in adult animals, even though infection can occur much earlier.

Early Infection: CWD in Fawns and Younger Cervids

Research has confirmed that CWD infection can begin in fawns and even prenatally. Studies have shown that fawns as young as five to six months have tested positive for the infectious prion protein, even without showing any clinical signs. Further research indicates that vertical transmission from an infected doe to her fetus can occur, meaning that some animals are exposed and infected before birth. These younger cervids, though infected, can appear perfectly healthy for extended periods while unknowingly shedding infectious prions into the environment through saliva, urine, and feces. This silent shedding is a critical component of how the disease spreads and persists within a population.

Age-Related Differences in CWD Prevalence

While CWD can affect all age groups, prevalence rates are not uniform across the population. Data from multiple endemic areas show distinct demographic patterns:

• Higher Prevalence in Adults: The probability of CWD infection generally increases with age, reflecting the cumulative risk of exposure over time. As animals live longer, their chances of coming into contact with contaminated soil, water, or other infected cervids increase.
• Sex Bias in Adult Males: Adult males, particularly mature bucks, consistently show a higher prevalence of CWD compared to adult females. This disparity is attributed to differences in behavior. Older males typically have larger home ranges and exhibit more intense social behaviors, such as increased contact during the mating season (the rut), which increases their risk of direct and indirect exposure.
• Delayed Mortality in Older Animals: While older animals are more likely to be infected, they can also show symptoms earlier and progress faster than younger animals due to the cumulative effects of the disease. However, the disease is 100% fatal for all ages once clinical signs fully manifest.

Clinical Signs: Age and Symptom Progression

The symptoms of CWD are progressive and become more pronounced as the disease advances. The specific signs can vary, but generally follow a pattern:

• Subtle Early Changes: Behavioral changes often appear first and can be subtle. These might include listlessness, depression, decreased interaction with other animals, and a loss of fear of humans. These signs can be easily missed in the wild and are often not dramatic.
• Progressive Wasting: The most consistent and defining sign of CWD is a long-term, progressive weight loss, or 'wasting' of the body condition. An infected animal will continue to lose weight despite eating, a stark indicator that the disease has taken hold.
• Advanced Neurological Signs: In the late stages, infected animals develop severe neurological issues. These include stumbling, lack of coordination, a wide stance, excessive salivation, and droopy ears. Difficulty swallowing often leads to aspiration pneumonia, a common cause of death in terminally ill animals.

Comparison of CWD in Young vs. Old Cervids

Feature	Young Cervids (e.g., Fawns, Yearlings)	Adult Cervids (3+ years)
Infection Risk	Vulnerable to early exposure through vertical transmission or from the environment.	Cumulative risk of exposure increases with age, making infection more likely.
Symptom Onset	Long, asymptomatic incubation period. Clinical signs are rare before 15-18 months of age.	Most likely age group to show clinical symptoms due to the long incubation period.
Disease Prevalence	Lower than adult animals in most populations, but detectable in a small percentage.	Highest prevalence rates, especially among males, due to social behaviors.
Behavioral Changes	Generally appear healthy and normal, acting as silent carriers.	Exhibit subtle behavioral changes that progress to severe neurological issues.
Disease Outcome	Always fatal once clinical disease manifests, just like in adults.	Invariably fatal, with symptom severity worsening over time.

Implications for Wildlife and Public Health

The age at which CWD occurs has significant implications for both wildlife management and public health. Because infected cervids can appear completely healthy for years, standard surveillance relying solely on visual observation is insufficient. This highlights the need for consistent testing of harvested animals of all ages, as recommended by wildlife agencies. For example, the CDC provides valuable information on how CWD impacts animal populations and what steps are necessary for monitoring the disease. The high prevalence among older males, who are often targeted by hunters, means that managing harvest strategies can be a key component of mitigating disease spread. Understanding the disease's progression across age groups is crucial for effective monitoring and control strategies. Ultimately, CWD represents a persistent and complex challenge for wildlife authorities, and its age-dependent nature is a core part of its epidemiology.

Centers for Disease Control and Prevention: Chronic Wasting Disease in Animals

Frequently Asked Questions

At what age do most deer get infected with CWD?

Most deer are thought to get infected with CWD later in life, as the cumulative risk of exposure increases with age. However, infection can occur very early, sometimes before birth, and remain undetected for years during the long incubation period.

Can young deer or fawns get CWD?

Yes, fawns and other young deer can become infected with CWD. In-utero transmission has been documented, and some studies have found CWD prions in animals as young as a few months old.

Why do older male deer have a higher rate of CWD infection?

Older males often have a higher prevalence of CWD due to behavioral factors. They typically have larger home ranges and have more physical contact with other deer, particularly during the mating season, increasing their exposure risk.

What is the incubation period for CWD?

The incubation period for CWD is long, often taking 18 to 24 months, and sometimes longer, before an infected animal shows any clinical signs. This long, asymptomatic period is a major challenge for disease management.

What are the first signs of CWD, and at what age do they appear?

The first clinical signs of CWD are often subtle behavioral changes, such as listlessness or a lack of fear of humans. These and other more severe symptoms like wasting are typically seen in adult deer, usually aged 3 to 7, rather than in younger animals.

Does CWD affect older deer more severely?

While CWD is always fatal, older deer are more likely to have progressed further in the disease and may show more advanced symptoms. The disease's severity progresses with time, and older animals have had more time for the prions to accumulate in their systems.

If a young deer is healthy, does that mean it doesn't have CWD?

No, a healthy-looking appearance does not guarantee a deer is free of CWD. Because of the long incubation period, an infected animal can appear completely normal for years. The only way to confirm or rule out CWD is through lab testing of tissue samples.