11 - Fungal disease
Impacts on human and animal health
Social and economic effects (eg. crop pathogens, ringworm)
Deep mycoses added to WHO neglected tropical disease list
Changing due to emergence of new diseases
1970s onwards chytridiomycosis - greatest disease driven loss of biodiversity ever
2007 white nose syndrome of bats
Ecosystem effects
Mycotoxins
Natural chemicals produced by fungi that can have adverse effects on the health of animals
Eg. aflatoxins
Affect animal health via plants
Crop losses lead to poor nutrition, starvation
Direct poisoning - potent effects, target organ often liver
Various species of fungi involved, in specific conditions (eg. poorly managed fermentation with silage)
Feeding spoiled haylage can lead to a range of disease including hypersensitivity, allergic pulmonary disease, kidney, liver damage, CNS alterations, seizures, death
Mycoses
Superficial mycoses (dermatophytosis)
Eg. ringworm:
Fungal infection of keratinised tissue (skin, nails, fur, hair, claws)
Tinea, Trichophyton, Microsporum
Each species has a preferred host:
Geophilic - soil
Anthropophilic - humans
Zoophilic - other animals
Zoonosis
Dermatophytes
Growth and sporulation forms
Cause: ringworm, tinea corporis, capitis, pedis
Parasitic cycle - humans
Saprophytic cycle - in soil
Microsporum canis
Common fungal infection in cats and dogs (zoonotic)
Difficult to manage in multi pet households
Extensive hair loss - due to fungi digesting keratin
Clinical signs - multifocal alopecia, crusting, scaling, pruritus
Diagnosis:
Tape impression smears
Woods lamp
Culture hair plucks
Treatment
Topical treatment - antifungal washes, griseofulvin (toxicity)
Environmental decontamination
Trichophyton mentagrophytes
Guinea pigs
Some genetic variants possess potential of human to human transmission
Trichophyton verrucosum
In livestock
Economic losses - hide / leather industry
Persistent chronic infection - favours cold dark conditions
Malassezia
14 species of yeast like fungi
Linked with inciting factors - chronic infection, allergic infections or endocrine disease
Common in skin fold dermatitis
Tape impression smears
Deep mycoses
Eg. Histoplasmosis, Sporotrichosis, Cryptococcosis
Many ascomycetes are dimorphic - can grow in hyphal or yeast form - depends on the environment
Candida auris
Causes serious infections
Often resistant to medicines - multiple drug resistance
Complicated treatment - easily misidentified as other Candida species
Commonly acquired in hospitals by patients with a weakened immune system
Dimorphism and Histoplasma spp.
Stealth pathogen due to immune evasion (Histoplasma)
Interdependent phases of survival
Host, pathogen, environment
It is a neglected disease impacting on equine health and human livelihoods
Histoplasma transmission and pathogenesis
In humans - usually in respiratory pneumonia form
Horses - debilitation, lung and ocular disease
Diverse genome
Limited info going in
Sporothrix schenckii
Emerging in South America
Zoonotic infection with feline and human infections
Deep cutaneous infection
Cryptococcus gattii
Associated with trees or soil around trees
Humans and animals can become infected after inhaling airborne, dehydrated yeast cells or spores
Travel through respiratory tract and enter lungs of the host
Small size of the yeast and/or spores allows them to become lodged deep in the lung tissue
Environment inside the host signals for C. gattii to transform into its yeast form and then the cells grow trick capsules to protect themselves
The yeasts then divide and multiply by budding
After infecting the lungs, C. gatti can travel through the bloodstream
Either on their own or within a macrophage cells - to infect other areas of the body, typically the CNS
Cryptococcal cell mediated damage to host
Diagnosis
→ Halo effect around yeast cell - Indian ink stain of CSF
→ Chocolate coloured colonies on bird seed agar
→ mucoid colonies on SAB
Cryptococcal antigen lateral flow assay
Fungal disease treatment and prevention
Vaccine candidates are sparse
Limited number of drugs and toxicity problems
Not suitable for environmental use
Fungal disease usually secondary to underlying disease / immunosusceptibility
Diagnostic methods - efficacy varies
Quarantine
Good hygiene and sanitation
Management in free-ranging wildlife if different from pets or farm animals
Antifungal target within fungal cell
Limited number of antifungal agents
Polyenes - disruption of fungal cell membrane binding to ergosterol
Eg. Amphotericin, nystatin
Azoles - interrupts enzymatic synthesis of ergosterol
Eg. Fluconazole, Itraconazole, Ketoconazole
Allylamines - inhibit fungal squalene - a key precursor to ergosterol
Eg. Butenafine, terbinafine
Echinocandins - cell lysis by disrupting fungal cell glucan synthesis resistance to osmotic forces
Eg. Caspofungin
Fungal infections of plants
Panama disease / banana wilt
Caused by Fusarium oxysporum
Resistant to fungicides
Control is limited to phytosanitary measures
2010 outbreak of the strain Tropical Race 4 - threatened the Cavendish banana (most popular cultivar)
Vectors - water, soils residues, replanting of suckers, farming tools and transport, leaf trash
Wheat blast
Caused by Magnaporthe oryzae
Spreads via infected seeds, crop residues, spores
Common in South America and Asia
Leaf rust
Caused by Puccinia triticina
Can lead to up to 20% yield loss
In temperate zones it is destructive on winter wheat because the pathogen overwinters
Most prevalent of all the wheat rust diseases
Emerging infectious diseases
Rapid worldwide mobility
Globalisation
Implications for international disease management
Humans, animals and pathogens move round globalised world
White nose syndrome of bats
Pseudogymnoascus destructans (geomyces desctructans)
Skin infection of bats
Originated in Eurasia
Identified in USA and Canada in 2006
5-6 million dead bats
Rapid spread, high mortality, population impact
Concerns:
Ecosystem services from bats to US agriculture - worth $4-50 billion per year
Bats eat insects
Pollination (chiropterophilous plants)
Seed dispersal
Pseudogymnoascus destructans
Previously known as Geomyces destructans
Fungus can also live as saprotroph in soil
So in animal host dies - no problem for fungus
Clinical signs and effects
Hibernation in mammals correlated with reduced immune response
Susceptibility factor
Bats hibernate for around 6 months each year
Fungus invades skin of hibernating bats
Destroys tissues
Wing tissue crucial for heat dissipation, water regulation, gas exchange, and blood pressure regulation
Bats overheat, use up food reserves too fast
Disrupts hydration and hibernation
Awake during hibernation, starve and die
Bat-to-bat transmission
Bats can recover if moved from hibernation to warm environment, given food and water; wing tissue regenerates
Not practical for typical US bat colonies of 104 to 105 bats
P. destructans in Europe
Found on bats but no mass mortality
Bats hibernate in smaller groups
Less extreme winters
Adaptation to co-exist with fungus?
Chytridiomycosis
Frogs and other amphibians
Chytridiomycota
Chytrid fungus
Motile zoospores - lives in water
Batrachochytrium dendrobatidis Bd
Frogs and toads
B. salamandrivorans Bsal
Salamanders and newts
Infects keratinised tissue
Skin in adults; limited effects on tadpoles
Regions affected: Australia, the Caribbean and North, Central and South America. Also present in Africa and Europe
Ecological emergency
Food chain birds / mammals
Key predator of mosquitoes therefore impacts on burden of dengue / malaria
Clinical signs and effects
Fungus invades the skin
Problems with respiration and regulation of body hydration
Chytridiomycosis life cycle
Transmission
Worldwide through trade in amphibians
Pharmaceutical industry - toads for pregnancy testing
Food
Pets
Spread since 1930s
Original source probably South Africa (Xenopus laevis)
Also possible pathogen hypervirulence evolved through recombination of previously isolated populations
Research in Institute of Integrative Biology into how it spreads within sites in Spain