FNR24150 -- Week 15 Notes

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

Microparasites

Bacteria, viruses, protists, fungi
Infected v. uninfected

1. Really the most important thing

Reproduce very quickly inside host
1. Macroparasites

Helminths (trematodes, nematodes, cestodes, acanthocephalans)
Arthropods (ticks, copepods, lice, insects, mites, fleas, etc.) & leaches
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

Batrachochytrium dendrobatidis (fungus)
Ranaviruses (virus)
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

aquatic snails

Picked up from infected bird feces
Rediae, cercariae
Asexual reproduction
1. Amphibs

Metacercariae
1. Waterfowl

Adults
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