What is a disease?

• “Any deviation from, or impairement of the normal structure or function of any part, organ, or system of the living animal or plant body”

• Clinical signs may be morphological, physiological, or behavioural

  • Morbidity, mortality, reduced growth, malnutrition, developmental, mental, activity level, appearance, etc.

Major types of diseases

• Infectious

  • Contagious v. non-contagious

• Genetic

• Nutritional & physiological

  • Scurvy

• Toxicological

  • Biotic v. non-biotic

    • Biotic: toxic algal blooms

• Traumatic

  • Losing a limb

• Mental Illness

• Combinations

Agents of infectious disease

1. Microparasites

   1. Bacteria, viruses, protists, fungi

   2. Infected v. uninfected

      1. Really the most important thing

   3. Reproduce very quickly inside host

2. Macroparasites

   1. Helminths (trematodes, nematodes, cestodes, acanthocephalans)

   2. Arthropods (ticks, copepods, lice, insects, mites, fleas, etc.) & leaches

   3. Intensity-dependent pathology

      1. The more macroparasites you have, the greater the impact they have on you

Modes of transmission

• Direct

  • Horizontal

    • Sneezing, coughing, etc.

  • Vertical

    • Parent to offspring

  • Single host

  • Multi-host

• Indirect

  • Complex life cycle

    • Needs multiple hosts

  • Vector-borne

• All parasites want to maximize transmission

Parasites and extinction

• Parasites rarely drive their hosts to extinction

  • Transmission limited at low population sizes

  • Similar to predator-prey cycles

• Several factors can help drive disease-induced extinction

  • Small populations (inbreeding, stochasticity)

  • Frequency-dependent transmission (STDs)

  • Multiple host species

    • If parasite is a generalist

  • Environmental reservoirs or persistence

Amphibian parasites

• Host to a diversity of parasites

  • Viruses

  • bacteria

  • Fungi

  • Water mold

  • Trematodes

• Most are relatively benign and the individual can clear the infection or live with it

3 Parasites of Conservation Concern

1. Batrachochytrium dendrobatidis (fungus)

2. Ranaviruses (virus)

3. Ribeiroia ondatrae (trematode)

• Linked to mortality events, severe pathology (malformations), or extinctions

• Relatively new to science (1996)

• Appear to be increasing in prevalence in amphib populations either locally or globally

Chytrid fungus

• Batrachochytrium dendrobatidis (Bd)

• Non-hyphal parasitic fungus (aquatic)

• Only chytrid species pathogenic to vertebrates

• Attacks keratinized tissue

  • Larvae: only the mouthparts have keratin

  • Adults: Keratin found throughout skin

• Life cycle

  • Zoospores

    • Floating around in water

    • Once contact is made, forms a cyst right under skin

    • Cyst continues to grow and eventually pops like a pimple

    • Goes on to infect more skin

Field signs of Bd infection

• Lethargy and paralysis

• Sloughing of skin and skin lesions

• Time to death highly variable (18-48 days)

• Infected individuals may appear healthy

• Some species live with infection

  • May serve as reservoirs for other spp. to get infected from

• Loss of pigmentation in mouth parts of larvae

Cause of Bd mortality

• Impaired osmoregulation (suspected primary cause)

  • Decreased water uptake and ion exchange (sloughed skin)

  • Altered electrolyte/solute levels (decrease in calcium interferes with muscle activity)

    • Lethargy and paralysis

• Impaired cutaneous respiration

Why is the fungus emerging?

• 2 hypotheses

  • Native pathogen

    • Increasing in prevalence due to some environmental change (climate change)

    • Increased amphib stress

  • Novel introduction

    • Pathogen somehow got introduced to a new area

    • Encountering hosts that have never evolved to deal with this pathogen

    • Africa → Central America, Australia, US

    • 

    • 

Bd Distribution

• Present in most well sampled areas

• While detected globally, the outcome of infection varies widely among regions

• 

Bd: Patterns of susceptibility

• Species vary widely in susceptibility to infection and disease outcomes

  • Extinctions have occurred

  • Frogs appear to be more susceptible than salamanders

  • Use of aquatic habitats increases chances of exposure

  • Some species act as reservoirs

Amphibian ranaviruses

• Infect ectothermic vertebrates (herps and fish)

• 6 total spp.

• 3 spp. are known to infect herps (>15 isolates)

• All 3 ranaviruses linked to amphibian die-offs

  • Ambystoma tigrinum virus (ATV)

  • Bohle iridovirus (BIV)

  • Frog virus 3 (FV3)

• Characteristics

  • dsDNA (double-stranded DNA)

  • 3x smaller than bacteria

  • Icosahedral shape (20 sides)

  • Paracrystalline arrays

Organ Destruction

• 3 primary organs: liver, spleen, and kidney

  • 

• Cell death occurs within 6-9 hrs

• 

• 

• Fast progression

  • 1-3 days signs, 3-7 days mortality

Persistence and transmission

• Concerns typically evaporate along with bodies of water

• Becomes a concern again when ponds form and adults congregate to breed

  • No vertical transmission; horizontal instead

Ranavirus: Patterns of susceptibility

• Species vary widely in susceptibility to infection and disease outcomes

  • Population declines have occurred

  • FV3 → frogs; ATV → salamanders

  • Ranids highly susceptible to FV3

    • Wood frogs, tree frogs, etc.

  • Fast developing larvae = poorer immune functions

  • Some species may act as reservoirs

Parasite Infection → Deformed Frogs (Trematoda)

• Ribeiroia ondatrae (Trematoda)

• Parasitic fluke (Plathyelminthes)

• Targets limb tissue

• 

Cycle of deformity

1. aquatic snails

   1. Picked up from infected bird feces

   2. Rediae, cercariae

   3. Asexual reproduction

2. Amphibs

   1. Metacercariae

3. Waterfowl

   1. Adults

   2. Sexual reproduction

• Extra legs serve to make predation easier (to complete parasite life cycle)

• 

Human Influences

• Factors that influence snail abundance should be linked to parasite load in amphibs and malformations

• Eutrophication

  • Ecosystem response to the addition of artificial or natural substances, such as nitrates and phosphates

  • Bottom-up effect

    • Algal (plant) population growth increases due to the addition of fertilizer

    • Higher snail abundance → more infected snails → More cercariae per infected snail → more infection in amphibians