Introduction
This is a continuation of the Chapter on Superficial mycoses.
Mycotic keratitis (KNOW)
Mycotic keratitis (keratomycosis) is common in horses and occasionally dogs. This is due to the fact that the horse has a large prominent eye and when grazing the eye is subject to trauma via grass awns and other objects. Thus fungi commonly found on grasses may either cause a very superficial infection (melting ulcer) or deeper-seated infection (stromal abscess) in horses. Fungal and inflammatory proteases result in erosion and ulceration of the cornea. This condition is very painful and if not treated in time, the eye can become permanently blind. Keratomycosis is suspected when there there is corneal pitting, erosions and ulcers. Ulcers or other trauma to the cornea can be revealed by fluorescein staining, where they will shine apple green in a blue light. Superficial punctate lesions on the cornea can be revealed by Rose Bengal stain.
The suspect diagnosis is confirmed by cytology that reveals the presence of fungal structures. Culture to identify the fungal species and provide a antifungal susceptibility result can also be carried out.
Note that many fungal species are involved in this infection, the most common being the dematiaceous fungi (brown pigmented) such as Curvularia species and unpigmented fungi such as Fusarium species. Mycoses caused by the brown pigmented fungi are sometimes called phaeohypomycosis.
Fungal diseases of wildlife
Like domesticated animals, wildlife are also susceptible to fungal infections. It must be noted, however, that poikilothermic animals. like fish. whose body temperature is close to that of the environment which is the source of the fungi.
A few diseases are highlighted in this section. You will only ever be asked on diseases of wildlife in Australia. Thus only 2 diseases shown below could be asked, namely chytridiomycosis and yellow fungus disease.
I have included white nose syndrome in microbats and snake fungal disease both common in the USA to indicate the effect that fungal disease can have on a immunological naive population. This is one of the reasons why border biosecurity is critical.
Chytridiomycosis in frogs: Chytridiomycota
Chytrids are primative aquatic fungi that produce zoospores (motile). They are mainly saprobes. Importance:
- Orpinomyces joyonii : cellulose decomposers in the rumen
- Synchytrium endobioticum: potato wart disease
- Batrachochytrium dendrobatidis (Bd): Chytridiomycosis of amphibians
Batrachochytrium dendrobatidis and B. salamandrivorans, commonly named the Chrytid fungi, are the causes of a devastating, world-wide skin disease of at least 700 species of amphibians (frogs, toads, newts, salamanders, and recently zebra fish) which infects the outer layers of the of the skin resulting in skin thickening. This decreases the amphibian’s ability to exchange electrolytes and maintain hydration. Death is often due to heart failure caused by hyperkalaemia. The disease can be difficult to eradicate, but combined environmental and tadpole local antifungal therapy using dips has been proven to be effective for localised disease i.e. infected ponds, aquariums. The find out more about this disease a link is provided.
The Chytridiomycota have both a teleomorphic and anamorphic stage of their life cycle where motile zoospores are produced.
However, only the anamorphic lifecycle is recognised in Bd.
Yellow fungus disease of reptiles
Nannizziopsis guarroi is a keratinolytic and contagious fungus that causes a superficial dermatitis in cold-blooded animals. This fungus has restricted growth at 37°C and thus tends not to affect warm-blooded animals. Initially the fungus will cause focal necrotic lesions that are crusty. In bearded dragons the lesions are often yellow. The infection will then spread over the skin and as the skin is extensively damaged it will spread to the subcutis causing pyogranulomatous lesions and from there can spread into the deeper tissues and systemically. Systemic treatment has proven to be difficult as most of the triazoles are toxic in reptiles. Once a day treatment with voriconazole (10 mg/kg) is considered to be the least toxic and most effective. Good infection control should also be able to exclude the fungus.
White nose syndrome in microbats of the USA (I will not ask this disease)
Since 2006 over 6 billion of the USA micro-bat populations have died after contracting white-nose syndrome which is caused by a white (hypomycete), psychophilic (cold-loving) mould called Pseudogymnoascus destructans. This fungus originated from Europe first infecting bats in the Eastern States north of New York City. The fungus has now spread throughout USA. To see a timeline map go to https://www.whitenosesyndrome.org/. The fungus is also widely distributed in Europe and Asia, however, the micro-bats in these areas appear resistant to disease.
Whilst the bats are hibernating, it infects the skin causing the bats to increase their metabolic rate excreting greater quantities of CO2, this causes them to lose fluids, electrolytes (hyperkalaemia) and utilise fats. They wake up to get water but cannot replenish their electrolytes. Deaths occur when they cannot make it to Spring and result from hypotonic dehydration and starvation.
Tracing of the movement of the fungus has proven difficult as all USA populations belong to a single genotype. However, the genotyping of the genes coding for the capsid proteins of a double-stranded RNA Partivirus that specifically infects P. destructans allows this fungus to be traced geographically.
Could this fungus enter Australia? Due to the resistance of fungal conidia, it is believed that cavers may introduce this fungus. Areas at risk are those parts of Australia where the year-round temperature in the caves are below 20°C, at latitudes less than 32°. These conditions are met in parts of South Australia and Victoria. Tasmania does not have any cave dwelling bats.
Snake fungal disease (No need to know)
Snake fungal disease caused by Ophidiomyces ophiodiicola infects over 30 species of wild snakes in eastern and Midwestern USA, and Canada causing eye and skin lesions that can spread to the underlying musculature leading to emaciation and ultimately death. This fungus has also been found in grass snakes in the UK.
END OF CHAPTER