Disease

60% of emerging infectious disease events since 1940-2008 zoonotic

• 72% of these disease events originated in wild animals

wildlife disease

• impairment of normal functions

• Domestic or wild animals may serve as reservoirs or vectors of pathogens that affect other animals, species, or humans

reservoir

• hold on to it, cannot transport

vector

• contains and transmits

disease categories (7)

• Infectious-

  • bacteria, viruses, rickettsiae (louse borne typhus), fungi

• Parasitic-

  • external and internal parasites (macro and micro)

• Toxic-

  • pollution or excesses of toxic substances in environment

• Physiological-

  • may be common in confined populations (i.e. zoos)

• Nutritional-

  • related to diet excesses or deficiencies

• Congenital-

  • birth defect (could indicate inbreeding)

• Degenerative-

  • may be common in confined populations (i.e. zoos)

means of disease diagnosis (5)

• Histopathology—

  • via tissues or lesion sampling

• Toxicology—

  • via organs, fat, blood or stomach contents

• Microbiology—

  • via organs, tissue, swabs, lesions or body fluids

• Parasitology—

  • via external, internal or blood parasite identification

• Hematology & Serology—

  • via blood sampling

population factors affecting parasite prevalence (4)

• Sex

  • Parasites may be more prevalent in one sex or another

  • May be due to positive correlation of infection with weight

• Age

  • Older individuals have higher infection rates

• Dominance

  • May go either way, depending on level of stress and dominance level

• Superspreaders

  • some individuals are more likely to spread disease than others

parasites: inter-group factors (4)

• Territoriality—multiple possible impacts

  • Territory defense can increase exposure to parasites

  • Energetically costly, high testosterone levels—can suppress immune system

  • Territory owner may have best resources—lower parasite load

  • Non-territory holders may “float”—greater exposure

• Group size and population density

  • Large group size/high population density may facilitate transmission of parasites or pathogen

• Severity of infectious diseases 

  • may be density dependent

  • Avian cholera in waterfowl on National Wildlife Refuges

• Population genetics 

  • also a tool in determining possible connectivity between subpopulations

case study: bison vs cattle

• Bison suspected of being reservoir for brucellosis (bacterial) in Yellowstone NP

• 50 % of bison test positive, but show no clinical signs

• In livestock

  • Causes abortions

  • Reduced milk production

  • Infertility and other reproductive disorders

• zoonotic

• No effective treatment

• Vaccine 65-75% effective in cattle, but lower in bison

• Ranchers want animals that leave the park killed, 

  • no evidence that brucellosis can be transmitted from bison to cattle

disease threats: habitat loss and predmentation

• Wetlands –drained in past to control mosquito population and malaria in US

  • Loss of wetland flora and fauna resulted

• Habitat loss (of all kinds) are forcing species into fragmented habitats that are small, often of poor quality

  • Causing stress

• Avian TB in the environment

  • remains in soil for years

disease and wildlife management

• Disease may be as much a part of management as habitat, food, & water

• Parasite loads may be indicators of population health 

• Public relations an important part of wildlife disease management 

disease as a natural phenomena

• Not all disease cause for alarm

• Disease may influence hunting seasons

  • Squirrels may have botfly infections and hunters may not wish to harvest

    • Solution--delayed hunting season

  • Rabbits may have tularemia 

    • Zoonotic-humans may become infected

    • Delay hunting until after freezing kills ticks

• natural disease control

• Disease is often controlled by predation—weaker individuals killed

• Populations without predation may have increased levels of parasites 

  • Due to intra-species transmission, rather than transmission through an intermediate host

• When populations are controlled, may only experience enzootic (low, but constant) levels of disease

When population densities get too high, may result in epizootic (outbreak, epidemic) levels of disease

enzootic

low, but constant

•  hosts

  • reservoir

  • resistant to disease

    • deer mice

    • grasshopper mice

    • kangaroo rats

epizootic

outbreak, epidemic

• hosts

  • amplifying

  • highly susceptible

    • prairie dogs

    • ground squirrels

Avian botulism kills 100,000+ waterfowl in CA & UT each year

• Microenvironment theory revolves around the bacteria perpetuating itself and being transmitted via maggots

  • Manage water levels to keep the cycle from starting—keep water levels constant & avoid shallow edges

disease and population dynamics

• Epizootics (e.g. rabies, sylvatic plague, etc.) easily documented

  • role in population control unclear

• Metapopulations 

  • may also have disease sources and sinks within them

• Disease control may be density-dependent

  • But it is hard to demonstrate clear linkage with other factors that also come into play (nutrition, etc.)

red grouse

• Periodic and rapid declines attributed to a parasitic nematode

• Population density correlates with infection rate

• However, another explanation is that the bird’s population exceeds carrying capacity of the habitat

  • Birds begin to starve, and the nematode is fatal to those birds in a weakened state

amphibian malformation and disease (4 causes)

• Amphibians suffering from deformities, diseases and population declines 

• Four major causes:

  • Injuries from predators

  • Fluke

  • Nutritional deficiencies

  • Contaminants 

• Iridoviruses (ranavirus)

• Endangered Wyoming toad and the boreal toad infected with chytrid fungus (Chytridomycosis)

hawaii’s birds and endangered species

• avian malaria

• Introduction of the mosquito (Culex pipiens) in 1826 resulted in extinction of a large number of Hawaii’s endemic birds from avian malaria—only safe above 600m elevation

• Current threats, include

  • Clearing

  • Feral animals (pigs, goats, cats, etc.) remain a threat

  • Introductions of 50 birds, 18 mammals, hundreds of plants

  • Climate change—increase elevation reached by avian malaria

• Hawaii represents .2% of US land mass, but has 25% of its endangered species & 72% of the US’  recorded historic extinctions

deer epizootic hemorrhagic disease EHD

• discovered in 1955

  • —most important endemic infectious disease of white-tailed deer in the southeast

  • Symptoms: disorientation, lethargy, bleeding from oral-nasal cavity, oral ulcers, swelling and excessive salivation, respiratory distress, weakness, altered gait

  • Hemorrhages in liver, heart, kidneys, lungs, gastrointestinal tract & spleen—failure of blood-clotting ability

• Die within 3-36 hours after symptoms develop

• Midges are disease vector—outbreaks stop with first frost—climate change may make this more of a northern climate issue

disease and biological controls: rabbits in Australia

• 1950 to control the rabbits via the introduction of the pox virus, Myxomatosis (spread by mosquitos, but harmless to native wildlife)

  • Initial rabbit reductions approached 90%, but resistant individuals flourished

• Next, rabbit hemorrhagic disease (RHD) escaped during trials on a neighboring island to Australia

  • Fortunately, it is host specific and reduced rabbits by 90% and continues to kill juveniles

  • Now spread to 40 countries threatening domestic and wild rabbits

vaccination by gnat

• Desert bighorn susceptible to blue tongue (virus) from domestic sheep

• Vaccine available, but how do you vaccinate a wild population?

• Gnats a vector for blue tongue

  • If gnats immunized with the vaccine, they are as effective as syringes in immunizing the wild population

When do pathogens become epizootic?

• Relationship between number of potential hosts and the number of individuals in a population who have contracted the pathogen and survived

• Also related to density of hosts

• Metapopulations of host may also lead to source and sink populations of the pathogen

six factors that drive viral emergence

• physical environmental factors

• microbe and virus

• ecological factors

  • envi bad, disease will spread

• social, political, and economic factors

  • poor ppl

• human and animal contact

• genetic and biological factors

west nile virus

• Discovered in Uganda in 1937

• 1999-appeared in humans (encephalitis) in New York City

• Large numbers of crows died (corvids particularly susceptible to virus), as well as in zoos in New York and Philadelphia (Humboldt penguin)

Appears to have been introduced via birds (migratory or imported) and continues to be spread by mosquitoes from bird to bird and from birds to humans

CWD

• 1980s chronic wasting disease (a form of transmissible spongiform encephalopathies— “mad cow”) appeared in free-ranging deer and elk

• No cases of humans contracting CWD—but hunters warned 

• 2012 found in captive herd of deer in PA

• 2015 found in wild deer in PA

avian influenza

• Wild populations of birds may carry forms of avian influenza

  • More than 50% of human cases have been fatal

• Transfer to humans via pigs

  • pig acts as mixing vessel for bird flu

  • makes new strains

• Concern about the virus becoming drug resistant and resulting in a pandemic

  • Spanish flu of 1918 resulted in 50-100 million deaths worldwide

Rabbit hemorrhagic disease RHDV

• 1984-discovered in China

• Mortality rate up to 70%

  • Die with 12-36 hours

• Used in Australia to control invasive rabbit population

• Has spread to 40 countries

• Found in domestic rabbits in PA in 2022—see Disease Management Area to right

lyme disease

• Bacterium primarily transmitted by deer tick (black-legged tick) from one host to another

• White-footed mouse, raccoons, & striped skunks reservoir species

• dilution effect and lyme disease

  • The “dilution effect” implies that where species vary in susceptibility to infection by a pathogen, higher species diversity often leads to lower infection prevalence in hosts. For directly transmitted pathogens, non-host species may “dilute” infection directly and indirectly. 

Lyme disease is more readily transmitted when environmental degradation lowers species diversity

rabies

• If not immediately treated, 100% fatal in humans

• Rhabdovirus attacks brain and spinal cord

• All mammals susceptible

• Transmittable via bites or aerosol 

WNS

• White fungus found on head and skin of bats

  • First seen in 2007--NY

• Kills bats in hibernacula

  • Starve to death before spring

  • Cave bats most susceptible

snake fungal disease

• First seen 1990s

• Causes skin lesions

  • Most severe when coming out of hibernation

  • Fatal in eastern massasauga and high mortality in timber rattlesnake

• Source unknown—always present?, introduced?, pet trade?

Diseases of Concern to Field Biologists (2)

• Tularemia (rabbit fever)

  • Tick borne bacteria

  • Rabbits are terrestrial reservoir species, also found in beaver and muskrat

  • Contracted via handling, tick bite, or eating uncooked infected meat

  • No vaccine

  • Investigated in 1950s as biological weapon

  • Summer is greatest threat

• Histoplasmosis

  • Soil fungus associated with high concentrations of bat or bird guano

  • Inhaled—respiratory fungus can be fatal

  • Respirators should be worn

capture myopathy

• Or white muscle disease-syndrome

• Occurs during stress of capture

  • Death—quickly to weeks later

• Due to storage of lactic acid in muscles during anaerobic muscle exertion—changes pH of blood and affects heart, liver, lungs

disease and climate change

• Changes in mean temperature or climate variability can alter disease risk

  • Effects host-parasite behavior, development, fecundity, and mortality

• Sometimes conflicting (e.g.):

  • Warm temps. Accelerate invert development, but reduce lifespan

  • Challenge is to determine the net outcome

  • Compounded complexity due to interactions of species

• Increased variability in climate interferes with frog resistance to chytrid, but chytrid grows best in cold—situation complex!

Risks of climate change

• Climate warming could, in some cases, extirpate a pathogen

• Other cases, it may expand and could place species or whole communities at risk of extinction

• Polar regions are most sensitive to emerging parasitic diseases, but more knowledge is from tropical seas

  • Caribbean a “disease hot spot”—new pathogens causing major declines of corals (a keystone species)

Phenology and Species Movement Impacts on Disease Dynamics

• Range shifts may bring new diseases

  • Northward shift of oyster diseases

• Migratory species particularly sensitive to climate change

  • May lower parasite risk, when an animal can leave

  • Strenuous migrations weed out weak and sick—reduce transmission

• Milder winters may allow migratory species to stay put

• Milder winters may enhance parasite over-winter survival

  • Tularemia “rabbit sickness”