Wildlife Diseases Notes
Wildlife Diseases
Wildlife diseases can affect humans and domestic livestock, impact wildlife populations, and indicate environmental issues like overpopulation.
Why We Care About Wildlife Diseases
Zoonoses: Wildlife carries diseases that can affect humans, such as rabies, West Nile Virus, and avian influenza.
Impact on Livestock: Wildlife can transmit diseases to domestic livestock, including bovine tuberculosis and pseudorabies.
Effects on Wildlife Populations:
Game species: Diseases like anthrax (in deer) and avian cholera (in waterfowl) can affect game species.
Threatened/Endangered (T/E) species: Canine distemper can impact T/E species.
Overpopulation Symptoms: Diseases and conditions like deer malnutrition/parasitism syndrome can be indicative of overpopulation.
Vocabulary
Pathogen: A disease-causing agent.
Zoonotic: A human disease that can be carried by animals.
Reservoir: A pool of infected animals that transmit pathogens to others, often without showing symptoms themselves.
Vector: An animal (often an arthropod) that transmits pathogens among wildlife.
Epidemiology: The study of pathogens and their progress through a population.
Incubation Period: The variable length of time between infection with a pathogen and the appearance of clinical effects.
Endemic: An area where a disease occurs naturally or regularly.
Pathogens
Prions:
Aberrant proteins that can spread within and between animals.
Cause transmissible spongiform encephalopathies (TSEs).
Viruses:
Packets of DNA or RNA with a protein coat.
Require host cells to reproduce.
Bacteria:
Prokaryotic, unicellular organisms.
Can be pathogenic or benign.
Protozoans:
Eukaryotic, unicellular organisms.
Can be pathogenic or benign.
Internal Parasites:
Mostly worms, such as tapeworms and roundworms.
External Parasites:
Mostly arthropods, such as insects, ticks, and mites.
Transmissible Spongiform Encephalopathies (TSEs)
Caused by misfolded prions.
Examples:
Scrapie
Bovine Spongiform Encephalopathy (BSE; "Mad Cow Disease")
Creutzfeldt-Jakob disease (CJD)
Variant Creutzfeldt-Jakob disease (vCJD)
Kuru
Chronic Wasting Disease (CWD)
Prions
Very resistant to destruction.
Boiling, autoclaving, heat sterilization, and cooking do not denature prions.
Irradiation does not denature prions.
Can be denatured/destroyed with bleach, acid baths, and protease treatments.
TSEs/Prions may be aerosolized and transmitted via bodily liquids.
Prions from some TSEs (CWD) may persist in the soil for decades.
Smaller than the smallest known virus by 100 times.
Do not contain nucleic acids (DNA or RNA), so vaccines are ineffective.
Can reproduce/repeat on their own, becoming infectious and transmissible.
Two general types:
PrP-sen: "sensitive"; normal (not misfolded).
PrP-res: "resistant"; resistant to being broken down and is the disease-causing form.
When PrP-sen comes into direct contact with PrP-res, they are converted to PrP-res, causing a chain reaction.
Misfolding Prions and Clinical Signs
PrP-res proteins stick together and form long chains called "amyloid fibers."
Amyloid fibers are toxic to cells and kill them.
Cells called Astrocytes digest dead neurons, leaving holes and amyloid fibers in the brain.
Causes encephalopathy, hence Transmissible Spongiform Encephalopathy (TSE).
Organs Affected by Specific TSEs
Thalamus: Fatal Familial Insomnia
Cerebral cortex: Classic CJD
Brain stem: Scrapie, BSE, and Chronic Wasting Disease
Cerebellum: Kuru
Organs Affected by Chronic Wasting Disease (CWD)
Brain: Prions attack neurons, causing degeneration, teeth grinding, drooping head, muscle weakness, and weight loss.
Eyes: Prions accumulate in the soft matter.
Spinal Cord: Prions can accumulate, causing weakness, abnormal behavior, and loss of coordination.
Spleen: Prion accumulation occurs.
Lymph Nodes: Accumulation erodes the immune system.
Scrapie
Fatal, degenerative TSE disease affecting sheep and goats.
First recognized over 250 years ago in Great Britain and Western Europe.
Reported throughout the world.
Largely under control.
In 2022, 21,600 samples collected, 491 positives.
In 2023, 10,000+ collected, no positives.
Bovine Spongiform Encephalopathy (BSE; Mad Cow Disease)
Progressive neurological disorder (TSE) of cattle caused by a prion.
First infections occurred in the 1970s.
Likely originated from feeding cattle meat-and-bone meal containing BSE-infected products from spontaneous BSE cases or scrapie-infected sheep products.
Spread throughout the UK cattle industry by feeding rendered, prion-infected, bovine meat-and-bone meal to young calves.
Mad Cow Disease (BSE) Epizootic in the United Kingdom
Peaked in January 1993 at almost 1,000 new cases per week.
Annual numbers of BSE cases in the UK have since dropped:
2 cases in 2015
11 cases in 2010
225 cases in 2005
1,443 cases in 2000
14,562 cases in 1995
Cumulatively, through the end of 2015, more than 184,500 cases of BSE had been confirmed in the UK in over 35,000 herds.
Current Control in the US:
Severe restrictions on the importation of live ruminants (cattle, sheep, and goats) and certain ruminant products from countries where BSE was known to exist.
Creutzfeldt-Jakob Disease (CJD)
Several rare fatal diseases of humans are also TSEs.
CJD occurs worldwide.
Three major categories of CJD:
Sporadic CJD: No known risk factors. Most common type (at least 85% of cases).
Hereditary CJD: Family history and/or positive genetic mutation test (5-10% of cases in the US).
Acquired CJD: Transmitted by exposure to brain or nervous system tissue, usually through medical procedures (<1% of cases since 1920).
vCJD (variant CJD): Discovered in 1996, thought to have resulted from the BSE (mad cow) outbreak in Europe.
Abnormal prion proteins aggregate, leading to neuron loss and other brain damage.
Kuru
A very rare disease, a TSE.
Caused by an infectious protein (prion) found in contaminated human brain tissue.
Causes brain and nervous system changes similar to Creutzfeldt-Jakob disease.
Found among people from New Guinea who practiced cannibalism, eating the brains of dead people as part of a funeral ritual.
Practice stopped in 1960.
Cases reported for many years afterward due to the long incubation period (up to 50 years).
Was the start of an epidemic of kuru among the Fore people; at its peak in the 1950s, killed up to 2% of the tribe each year, with 1000 concurrent cases in PNG in 1957.
Epidemiological surveillance and studies on human genetics and past mortuary practices are ongoing in the kuru-affected region.
Chronic Wasting Disease (CWD)
Transmissible spongiform encephalopathy (TSE) disease that affects deer and elk.
Similar to Mad Cow disease in cattle and scrapie in sheep.
No evidence CWD affects humans…yet….
Fatal, with a long incubation time (>1 year).
Generalized wasting syndrome.
Endemic to Colorado/Wyoming but recently spread to WI, NY, TX, TN, AL, etc.
Species Susceptible to CWD (as of 2024)
Mule deer
Black-tailed deer
White-tailed deer
Elk
Moose
Reindeer/caribou
Red deer
Sika deer
CWD Spread
28 states & 4 Canadian provinces.
Still rapidly expanding.
Newest detection: Alabama in early January 2022.
CWD Transmission and Environmental Factors
Shedding:
Nasal secretions
Saliva
Antler velvet
Skin
Blood (open wound/lesion)
Birthing matter
Milk
Urine and feces
Agent Transport:
Cervid host movement
Water
Dust
Scavengers
Predators
Insects
Environmental Hot Spots:
Carcasses
Scrapes/rubs
Mineral licks
Wintering areas
Captive facilities
Soil Binding:
Immobilization
Altered infectivity
Altered stability
Variance with soil type
Environmental Degradation:
Mineral oxidation
Heat/dessication
Freeze/thaw
Biodegradation
Uptake:
Direct contact: Saliva/mucus
Indirect contact: Soil/water/vegetation/surfaces
Ruminant gut passage
Intestinal uptake
Oral ingestion
Oral lesion uptake
Nasal inhalation and uptake
CWD Signs and Symptoms
Severe emaciation & dehydration.
Loss of coordination, drooping of heads/ears, excessive salivation.
Long incubation time.
Ultimately fatal!
CWD in Texas
First case discovered in 2012 in free-ranging mule deer in West Texas.
First case in Texas white-tailed deer was found in a deer-breeding facility in 2015 during routine disease monitoring.
Map of positives (2018).
3 species found positive (white-tailed deer, mule deer, elk).
Texas has recorded 500+ confirmed cases, mostly discovered in captive deer breeding pens.
The Texas Parks and Wildlife (TPW) Commission adopted comprehensive CWD Management Rules on June 20, 2016.
Many other legislative actions have since taken place.
CWD Complications in Texas
Urban areas: human-wildlife conflict emerges.
Hunters.
30,000 Mule Deer; 757,000 White-tailed Deer (predicted 2023-2024)
Deer breeders.
980 deer breeders in Texas (September 2021).
$1.6 billion annual economic impact (2017).
https://tpwd.texas.gov/business/permits/land/wildlifemanagement/deerbreeder/
Texas Parks and Wildlife (TPWD) CWD Epidemiology
Amplification Factors:
Movement
Density
Cumulative contamination
Domestic Cycle
CWD susceptible species movement
Contaminated Feed?
Veterinary practices?
Reproductive practices?
Sylvatic Cycle
Environmental Contamination
Escapes
Fence line contact
Run-off
Scavengers
Invertebrates
Regulatory Authority for CWD in Texas
CWD is a reportable disease.
Texas Animal Health Commission (TAHC) has authority for reporting and tracking the disease in alternative livestock (elk, red deer, and sika deer).
TPWD has regulatory authority for free-ranging white-tailed deer and mule deer.
Both agencies share regulatory authority over captive deer held under Deer Breeder Permits.
Deer are owned by the public by law, but Texas is unique in that “private” practices are performed on a “public” entity.
Challenges with CWD Regulations
Recent cases where captive facilities are depopulated by TPWD as a disease management strategy.
Litigation and lawsuits from private citizens suing TPWD, the state of Texas, or individual employees.
Epizootic Hemorrhagic Disease (EHD) and Bluetongue Virus (BT)
Viral diseases affecting deer, sheep, and pronghorn.
Same symptoms, different diseases.
Most important disease affecting deer in most areas.
Causes rapid disease and up to 60% mortality.
Hemorrhage in many tissues (like Ebola in humans).
Endemic to the south, with periodic outbreaks farther north.
Southern deer can be resistant, especially in FL and TX.
Hemorrhagic Disease and Bluetongue
Causative agent(s): Epizootic hemorrhagic disease (EHD) viruses (2 serotypes) or bluetongue (BT) virus (5 serotypes).
Hosts: White-tailed deer, mule deer, pronghorn, elk, bighorn sheep, etc.
Signs & symptoms: Highly variable; swelling of the head/tongue/neck; fever; respiratory stress; reduced activity; often found near water sources.
Rabies
Viral disease affecting all mammals, including humans.
Acute neural disease that is always fatal.
Affects brain function; animals may be lethargic or manic (attacking anything).
Most important carriers in the U.S. are raccoons, foxes, skunks, and bats.
Raccoon rabies is spreading in the east, with a large effort to stop it.
Avian Cholera
Bacterial disease affecting waterfowl and some other birds.
Rapidly fatal in ducks and geese.
Rarely see sick birds, just dead ones.
Birds often in good body condition.
May decimate populations in concentrated areas (wintering or stopover grounds).
Signs include lethargy, convulsions, many dead birds very suddenly.
Hemorrhage on the heart and gizzard, spots on the liver.
Avian Botulism
Occurs naturally and is a leading cause of bird death, especially in migratory waterfowl.
Millions of birds die annually worldwide.
Caused when a virus infects type C Clostridium botulinum bacteria, causing the bacteria to produce a toxin.
The botulism spore, or resting stage of the bacteria, is commonly found in wetland soils and can survive for years, withstanding freezing and drying.
Avian Influenza (Bird Flu)
Refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses.
These viruses naturally spread among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species.
Virus is present in their saliva, nasal secretions, and feces.
Susceptible birds become infected when they have contact with the virus as it is shed by infected birds.
Infection can also occur through contact with surfaces contaminated with the virus from infected birds.
Bird flu viruses do not normally infect humans, but sporadic human infections have occurred.
There is a growing number of cases. Likely the largest wild bird disease issue right now as of 2022.
Lyme Disease
Zoonotic disease caused by a bacterium.
Does not affect wildlife.
Causes flu-like symptoms and chronic joint pain in humans.
Vector is the black-legged (deer) tick.
Reservoir for the disease is in white-footed and deer mice.
Deer are hosts for the tick vector, not the bacterium.
Mange
Skin disease of wild canids caused by mites.
Especially severe in red foxes and coyotes.
Causes extensive hair loss, foul odor, and inflammation of the skin.
Death is possible due to secondary infection, emaciation, or hypothermia.
Can cause temporary skin irritation in humans.
Ranavirus
Ranaviruses infect amphibians, reptiles, and fish; however, susceptibility varies by species and across phylogenetic lineages.
For amphibians, the orders Anura and Caudata are affected, and natural infections have been documented in at least 72 species.
Ranaviruses have also been associated with disease in snakes, lizards, and chelonians and may be best known for die-offs in eastern box turtles.
Transmission is horizontal via direct contact, ingestion of the virus or infected animals, and water exposure.
Studies have shown that infection can begin within seconds of contact.
Disease (and deaths) may be established in as short as 3 days but also may take weeks and is likely dependent on host susceptibility.
White-Nose Syndrome (WNS)
The fungal disease killing bats in North America.
Research indicates the fungus that causes WNS is likely exotic, introduced from Europe.
Started in New York in 2006 and has spread to more than half of the United States and five Canadian provinces by August 2016.
Scientists predict some regional extinctions of bat species.
West Nile Virus (WNV)
The leading cause of mosquito-borne disease in the continental United States.
Most people infected with WNV do not feel sick.
1 in 5 humans develop a fever and other symptoms.
1 out of 150 humans develop a serious, sometimes fatal, illness.
It infects over 250 species of birds, but corvids (crows, blue jays, and ravens) are the most susceptible to dying from the disease.
Although tens of thousands of birds have died, and studies of some bird species show regional declines, few region-wide declines can be attributed to WNV.
Horses represent approximately 97% of all reported non-human mammalian cases of WNV.
Very little is known about other mammals.
Bot Fly Infestation
Parasitic larva of bot flies infect many species.
Generally does not cause significant harm.
Unsightly, leading to wasted meat.
Infect the skin of squirrels (warbles, wolves), rabbits, and small mammals.
Infect nasal passages of white-tailed deer.
Cutaneous Fibromas
More commonly called deer warts by hunters, are occasionally observed on harvested deer.
The fibromas are peeling, especially when they appear in large numbers.
"Grotesque" is a common description of these harmless skin blemishes.
Cannot be transferred to humans.
Very rarely cause harm to deer, only when disabling movement, eyesight, smell, etc.
Canine Distemper
A contagious and serious disease caused by a virus that attacks the respiratory, gastrointestinal, and nervous systems of carnivores.
Broad host range among North American carnivores, including mink, striped skunks, fishers, red and gray foxes, raccoons, coyotes, and weasels.
Most often become infected through airborne exposure (through sneezing or coughing) to the virus from an infected animal, but urine, saliva, etc., are also responsible.
Infected animals will develop watery to pus-like discharge from their eyes. They then develop fever, nasal discharge, coughing, lethargy, reduced appetite, and vomiting.
There is no cure, only supportive care, so preventative measures to reduce the risk of spreading the virus are key. This virus is nearly 100% fatal for mustelids.
Outbreaks of canine distemper may have a significant impact on local or regional populations of raccoons, gray foxes, and skunks in parts of the United States and on raccoons in Canada.