Introduction

Deep or invasive mycoses occur when internal organs are infected, they can either be localised i.e. in a mycotic rumenitis or disseminated where several organs are infected.

Although all fungi originate from the environment, they can be broadly grouped according to their virulence.

1. Thermal dimorphic fungi. In this group are the most virulent of the fungi and include Coccidioides immitis, Paracoccidiodes brasiliensis, Blastomyces dermatitis and Histoplasma capsulatum. Endemic in the Americas.

2. The pathogenic yeasts, Cryptococcus and Candida.

3. Pathogenic opportunistic septate moulds

4. Pathogenic mucormycota

Opportunistic infections common in Australia include cryptococcosis caused by the yeasts Cryptococcus neoformans and C. gattii; Candidiasis caused by Candida species and; Aspergillosis due to Aspergillus species. Animals more prone to these infections are those that are immune-compromised, are treated with antibiotics and have a breed predisposition. Hospital (nosocomial) infections due to contaminated intravenous preparations have also been recorded. Pneumocystis carinii, a yeast (was classified as a protozoa), which is a normal inhabitant of the upper respiratory tract occasionally causes opportunistic pulmonary infections in immune-compromised dogs and horses and those with a breed disposition.

1. Thermal dimorphic fungi (brief no details)

Where endemic, pathogenic thermal dimorphic fungi such as Histoplasma capsulatum or Blastomyces dermatitidis will cause infections in healthy people and animals. For example, histoplasmosis is linked to aerosol exposure to bird and bat droppings present in moist, slightly acidic soils. It is found in river valleys and is endemic along the Ohio and Mississippi river valleys in the USA. Internationally, cases are RARE, and have been recorded in North and Central America, eastern and southern Europe, parts of Africa, eastern Asia and Australia. Outbreaks of histoplasmosis and blastomycosis are associated with construction and renovation activities that disturb soil. It usually occurs as an atypical pneumonia, in the acute form appearing in epidemics with prostration, fever and respiratory symptoms. Although infection is more common in humans, cases in horses, dogs, cattle and other mammals are recorded. Blastomycosis is common in dogs and resembles systemic mycoses in dogs caused by Cryptococcus and Aspergillus terreus.

Coccidiomycosis and Paracoccidiomycosis (Don’t need to know)

(Found only in humans, coccidioidomycosis (Valley fever) is caused by the dimorphic fungus Coccidioides immitis and C. posadasii that grows in the soil of Southwestern USA, Mexico and Central and South America. This fungus produces arthroconidia (conidia produced by fragmentation of a hyphus) that become air-borne during conditions that disturb the soil. <1% of immunocompetent affected people suffer dissemination but in these cases mortality is high. 40% of primary infections are symptomatic, usually presenting with respiratory tract symptoms plus fever, with pleuritic pain. There may be arthralgia with swelling (often knees and ankles).

the pathogenic yeasts

cryptococcosis (KNOW)

Cryptococcosis is a common systemic yeast infection in Australia where the yeast is found in high numbers in bird droppings and the dead bark of trees i.e. the Red river gum tree. Although all mammals are susceptible, the most serious infections are more common in felines, koalas, dogs and parrots. It is caused by the yeasts: Cryptococcus neoformans and Cryptococcus gattii. Whereas both yeasts have caused infections in most animals species, including people, koalas have only been infected with Cryptococcus gattii.

Pathogenesis

Infections generally occur via wounds and the respiratory tract where a primary focus of infection occurs. The fungus will then disseminate throughout the body and could infect any organ, however, it has a predilection for the central nervous system. Interestingly cats are very susceptible to the central nervous system being affected and in dogs and parrots, the infection tends to by multisystemic without CNS involvement. Although this yeast has a number of virulence factors, the predominant virulence factor is its ability to produce a very thick polysaccharide capsule which is not readily recognised by the immune system of the host. Thus there is usually a weak inflammatory response and affected tissues take on a pale gelatinous appearance.

Virulence factors

1. Capsule (NB): A polysaccharide capsule prevents effective opsonization, is toxic to macrophages and decreases inflammatory response
2. Melanin and mannitol: Reduce the toxicity of hydroxy radicals and superoxides present in the phagolysosome
3. Phospholipase: Aids survival within macrophages
4. Enables spread of the yeast from respiratory tract to central nervous system
5. Sialic acids: Present in cell wall, direct complement proteins towards degradative pathway

The disease is diagnosed by the presence of spherical, budding yeasts with a thick polysaccharide capsule, forming microcolonies in affected tissue.

The yeast is easily cultured on a number of agars and at 37C and 30C. Its speciation is done by qPCR. A lateral flow assay is available that will detect the capsular antigens in blood or CSF.

Long-term antifungals are used to treat this systemic disease, with the treatment of choice being either fluconazole or voriconazole. These are both azole drugs with a good CNS penetration.

2. candidiasis

Candida species not only can cause superficial infections, they have a predilection for the gastrointestinal tract whehere they cause gastrointestinal disease as well as at times they invade internal tissues causing a systemic mycosis. For details on this disease, please refer to the Chapter on “Superficial Mycoses in Animals”.

the pathogenic moulds

A number of moulds are known to cause opportunistic infections in animals. Most of these fungi have a tropism for blood vessels. This often leads to thrombosis of the affected blood vessels resulting in ischaemic necrosis.

The low oxygen tension in tissues limits fungal growth to hyphae  or their resistant forms. Fungal infections in warm-blooded animals are restricted to those fungi that have the ability to grow at body temperatures of 35C or above. A large number of fungal species have the ability to cause disease, Only the more common causes will be discussed in this chapter.

aspergillosis

Aspergillus species are ubiquitous moulds found in the environment. They are often associated with decomposing plant material. Due to their high reproductive capacity, they are able to produce countless, fine conidia that easily become air-borne and are inhaled.

They grow easily on fungal media such as Sabouraud’s dextrose agar with fluffy growth present after 48 hours of incubation and anamorphic reproductive structures present after 3 days of incubation. Many of the pathogenic Aspergillus species will not only grow at room temperature but also at 42°C. They can be identified into different groups by the gross colour and microscopic appearance of their fruiting structures. Occasionally teleomorphic reproductive structures will occur when growing Aspergillus.

The pictures below show the primary identification structures of Aspergillus species

Diseases in animals caused by Aspergillus species include the following:

1. Respiratory tract infections: Nasal aspergillosis,  guttural pouch mycoses of horses, fungal pneumonia and air-sacculitis in birds
2. Systemic mycoses in dogs
3. Abortions in livestock and horses (See section on mucormycota)

Respiratory tract infections

Aspergillus fumigatus is the most common cause of sinoaspergillosis, an upper respiratory tract mycoses in long nosed dogs and horses and air-sacculitis and pneumonia in chicks. Note that other moulds have also been implicated in this disease. Long-nosed dogs and horses often present with a chronic nasal discharge or nose bleed as the fungi will invade the mucosa of the nose, turbinates and head sinuses causing inflammation and bone lysis. On endoscopic examination (green) fungal plagues may be seen on the nasal mucosa.

Not only do horses suffer from sinonasal aspergillosis but the fungus can lodge in the guttural pouches, causing vasculitis and fungal plaques = GUTTURAL POUCH MYCOSIS. The dorsal pouch is close to the internal carotid artery and cranial nerves. The fungi can extend into the internal or branches of the external carotid arteries and cause severe and repeated bouts of epistaxis {nose bleed). Extension to the cranial  and sympathetic nerves results in dysphagia (a poor prognostic sign), Horner syndrome and displacement of the soft palate.

Rarely the fungi will descend to the lungs causing mycotic granulomas within the respiratory tree.

Asergillus fumigatus is easily cultured in the laboratory and will readily produce anamorphic reproductive structures in any culture media. The identification of this fungus in the laboratory is shown in the pictures below

2. Systemic mycoses in dogs

Rarely dog develop a systemic mycosis with the offending fungus entering via wounds or the gastrointestinal tract.  Rarely the respiratory tract is involved. Although all breeds of dogs can suffer from systemic mycoses due to a variety of fungi, disseminated Aspergillus terreus infection is more common in middle-aged German Shepherd Dogs. In dogs, the fungi will often infect joints, bones, especially the vertebral column (radiograph of discospondylitis in the lumbar vertebrae of a GSD below), the pancreas and kidney. The clinical signs of a systemic mycosis may include localised signs such as joint pain or renal failure, the most common clinical sign is a non-specific fever. Since the clinical signs are non diagnostic,   the presence of fungi in the urine is highly indicative of a systemic fungal infection with renal involvement. The long-term prognosis for this disease is poor. Antifungal therapy is usually administered for at least 6 months and often for the lifespan of the dog.

In endemic regions of the USA,  dogs can be infected with the thermal dimorphic yeast Blastomyces dermatitidis causing similar disease to systemic aspergillosis. This infection is rare outside USA.

Air sacculitis in birds

3.  Pathogenic mucormycota

Members of the Mucormycota such as Mucor, Absidia and Rhizopus commonly infect and decompose stored grains and grasses. Thus they are often causes of disease in intensively reared livestock. These fungi are able to survive and proliferate in an acidic rumen in animals suffering from a sub-clinical rumen acidosis due to grain overload. Aspergillus species can also cause these infections.

The rumen wall is invaded resulting in a full wall thickness punctate necrosis due to fungal invasion of the arterioles resulting in a vasculitis. The fungi can also enter the bloodstream this way reaching the gravid uterus to infect the placenta, ultimately resulting in abortion of a starved foetus.

Mares may also abort due to a fungal infection, often an Aspergillus species, however, the fungus tends to invade the uterus and then the placenta via a partial open cervix (ascending infection).

Unusual fungal infections

Avian gastric yeast disease

Macrorhabdus ornithogaster or avian gastric yeast is a yeast that infects the proventriculus and ventriculus of many bird species. Some birds such as show budgerigars, cockatiels and finches can develop chronic disease losing weight over time. Birds appear to be eating normally but don’t ingest their food with the crop of the bird often being empty. This gigantic gram-positive rod/filament is found in mucosal or crop scrapings. Barium meal radiographs show an enlarged proventriculus with a narrow hour glass constriction between the proventriculus and ventriculus. Treatment involves the use of amhotericin B or nystatin, both polyene antifungals. Acidification of the gastrointestinal tract using vinegar or a Lactobacillus probiotic may reduce mucosal colonisation of Macrorhabdus.

Pneumocysticosis

Pneumocystis carinii a yeast colonising the airways of animals is a rare cause of interstitial pneumonia in especially rats. Other strains of P. carinii can cause pneumonia in immunocompromised animals including foals, such as Arabian foals with combined immunodeficiency syndrome and certain breeds of dogs including the Cavalier King Charles spaniel, Yorkshire terrier and miniature Dachshunds. Dogs suffering from chronic demodicosis or distemper may be prone to pneumocysticosis. Diagnosis is based on the presence of non-staining yeast cysts (looks foamy) in tracheal or bronchial wash fluids. PCR detection of P. carinii specific DNA is also possible. Trimethoprim sulphonamide is used to treat this disease. Corticosteroids are also administered to improve lung function. Prognosis is, however, guarded.

Treatment of fungal infections

The decision to treat will be based upon the disease prognosis with cutaneous infections carrying a good prognosis and systemic/deep disseminated infections a poor prognosis. Dependent on the type of mycoses, treatment can be topical or parenteral.  When treating deep or systemic dissemination mycoses, it is often necessary to place the animal on very long-term therapy with regular monitoring for clinical and mycological recovery.

Where possible surgical resection should be used for cutaneous and sub-cutaneous granulomas i.e. mycetomas and “kunkers”. Make sure that skin lesions are removed with wide margins.

Four classes of antifungal agents are available:

1. Polyenes such as amphotericin B. These are broad-spectrum fungicidal drugs that bind to ergosterol in the fungal cell membrane and disrupt the membrane. This drug is highly nephrotoxic which can exacerbate problems with dogs that are already in renal failure due to fungal infections. Lipid formulations of this drug are less toxic. A topical polyene that is effective against gastrointestinal candidiasis and mycotic keratitis is nystatin.
2. Pyrimidines such as 5-fluorocytosine which is a pyrimidine analogue that inhibits DNA and protein synthesis. It is a narrow-spectrum antifungal that is most used for yeast infections. It is generally not used in animals. In humans, it is used in combination with amphotericin B for life-threatening mycoses.
3. Azoles. These are broad-spectrum fungistatic drugs that inhibit ergosterol synthesis by inhibition of cytochrome P-450-dependent 14a-lanosterol demethylation. They are safer than many of the other classes and are commonly used in veterinary medicine. Common drugs include 1^st generation such as ketoconazole (Nizoral), itraconazole (Sporanox) and fluconazole (Diflucan). With the advent of antifungal resistance second generation drugs such as voriconazole (Vfend) and posaconazole (Noxafil) have been developed. Both fluconazole and voriconazole can be used for CNS infections whereas itraconazole and posaconazole are used for deep mycoses that don’t have CNS involvement.
4. Echinocandins. Casofungin, micafungin and anidulafungin are narrow-spectrum fungicidal drugs that target cell wall synthesis by inhibiting β-1,3-d-glucan synthesis. They are considered effective against Candida and Aspergillus species. They are not currently used in animals.
5. Terbinafine  is a synthetic allylamine antifungal. It is highly lipophilic in nature and tends to accumulate in skin, nails, and fatty tissues. Terbinafine inhibits ergosterol synthesis by inhibiting the fungal squalene monooxygenase, an enzyme that is part of the fungal cell wall synthesis pathway. It acts synergistically with the azole antifungals and can be applied topically to treat superficial infections and systemically.

END OF CHAPTER