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Samples taken from the lesions tested positive for the disease. Courtesy Dr. Grace Zhang

The most common fungal disease in captive reptiles is commonly called yellow fungus disease (YFD) because it often causes a yellow to yellow-brown skin color. This disease is also called CANV, after the fungus thought to cause it, Chrysosporium anamorph of Nannizziopsis vriesii. It is now known that the most commonly diagnosed cause of YFD in captive bearded dragons (Pogona vitticeps) is Nannizziopsis guarroi. Fungal disease should be among the list of potential causes of disease for any reptile with skin problems.

Bearded dragons with YFD often show up with small yellow to brown, red, grey, or black crusts on the surface of a few scales that, in time, get larger. These crusts often appear on the face, lips, and front legs, but can appear anywhere on the body. These crusts or spots eventually turn dark and become thickened or rough.

Occasionally, an early sign of YFD is a bad shed that leaves behind dull scales with a roughened appearance. In some dragons, it may result in a shedding storm, where the dragon seems to be shedding all the time. As the disease progresses, it may become an internal infection and lead to poor appetite, weight loss, and death. YFD is contagious and can spread from one dragon to another, both by direct contact between animals and through a contaminated enclosure. If you have a YFD-positive dragon, any cage mate needs to be watched closely for signs of disease.

YFD is associated with stress and is often seen in dragons kept under crowded, unsanitary conditions. It’s often seen in those dragons that are raised in groups, shipped in groups, or displayed for sale in groups. It is also thought that injured toenails and minor bite wounds associated with dragons under crowded conditions result in damage to the skin, which provides an opening for the fungus to infect the dragon.

Sanitation may be compromised with high populations of lizards in the same cage, which in turn increases the likelihood of a lizard being exposed to the fungus. Furthermore, crowding causes stress, which suppresses the immune response, so a lizard may not be able to fight off even a low level of the fungus. But because the YFD is a primary pathogen (it can cause disease even in healthy reptiles with good husbandry), any dragon can get it.

Affected Species

This article focuses on YFD in bearded dragons, but many reptile species have been diagnosed with similar fungal diseases. It is important to understand that there are many different species of fungus that can cause disease in reptiles, although you certainly don’t have to memorize them, which include: 

• Nine species of Nannizziopsis are associated with fungal disease in chameleons, geckos (such as leopard geckos), tegus, iguanas, and crocodiles.
  
  
• Paranannizziopsis includes four species that infect snakes, lizards, and tuataras.
  
  
• Ophidiomyces, with the single species Ophidiomyces ophiodiicola, has been diagnosed in over 60 snake species.
  
  
• Nannizziopsi guarroi, the main causal agent of yellow fungus disease in captive bearded dragons (Pogona vitticeps).

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This is the same bearded dragon as the previous image. Courtesy of Dr. Tania Burrows

Diagnosis

Your veterinarian will start by taking a thorough medical history and give your lizard a physical examination. YFD cannot be distinguished from a variety of other diseases just by looking, including other fungal diseases, bacterial diseases, some viruses, and things like burns. Your veterinarian will likely recommend some tests to determine if your dragon has YFD.

Available Tests

• Biopsy: Your veterinarian will anesthetize your dragon for a short period of time and get a few small tissue samples that will be looked at under the microscope to look for the fungus. Additional testing will be needed to determine which fungus it is.
• Cytology: Your veterinarian will collect some crust material, put it on a slide, and stain it. The samples will be examined under a microscope to look for the fungus. Additional testing will be needed to determine which fungus it is.
• Fungal Culture:  If a fungus is suspected or confirmed, a swab can be taken and sent for a fungal culture. One of the benefits of taking a culture is that it allows your veterinarian to determine which drugs may be most effective for treating the fungus present. 
• Panfungal PCR: Your veterinarian will collect a sample and test for the presence of fungal DNA. This test generally requires that the DNA undergo evaluation to determine which fungus is present.
• qPCR: This test can detect DNA from all species of Nannizziopsis known to infect reptiles. The quantitative component of the assay can help with the evaluation of treatment success, using multiple cutaneous swab samples to show a reduction of DNA fungus present, and so indicating treatment success.
• Next Generation Sequencing (NGS): Samples collected by your veterinarian can be evaluated and the DNA sequenced to determine the species of fungus present.

Your veterinarian may suggest bloodwork, X-rays, and/or ultrasound to better assess your reptile’s health and rule out a serious internal illness caused by the fungus infecting the dragon’s deeper tissues or organs.

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Diagnosis confirmed with histology. Courtesy Dr. Rachel Williams

Treatment

YFD is contagious to other dragons, and so the infected dragon should be kept in a separate cage from all other reptiles in the collection. Your veterinarian will discuss how to prevent transmission of the infection from the infected dragon to others.

Any areas of the skin with YFD changes (called lesions) should be cleaned up or removed in a process called debridement because the dead skin contains fungus that is out of reach of the antifungal medication given to the dragon. Systemic antifungal drugs (given by injection or orally) are always indicated, as a YFD infection often extends beyond the skin. Treatments are typically continued for two weeks after all spots have disappeared.

Your veterinarian will suggest treatment with antifungal drugs for your infected dragon and may choose one or a combination of the following drugs: Voriconazole, amphotericin B, terbinafine, and/or itraconazole. He may also recommend treating any skin biopsy sites or lesions with topical medication, once or twice a day.

Your veterinarian will also discuss how to disinfect your dragon’s enclosure. Studies have shown that effective disinfectants against N. guarroi are 2% dilution of commercially available bleach with a 10-minute contact time and a 10% dilution of commercially available bleach with a 2-minute contact time.

Prevention

Isolate
If you have a dragon with YFD, any cage mate needs to be watched closely for signs of disease. You should consider separating the cage mates to reduce the risk of cross-infection from the infected one to the seemingly healthy one. Another reason to separate them is to reduce competition between them. Sometimes, there are subtle social interactions that create stress and immunosuppression and make cage mates more susceptible to infections. Other exposed reptiles should have thorough veterinary exams, and any skin problems should be checked for YFD.

Quarantine
Any new dragon to the collection that arrives with a skin problem or that develops a skin problem while in quarantine should not be placed in the main collection released until checked for YFD.

Proper Hygiene and Sanitation
Proper sanitation and hygiene are key risk reducers. As with other infectious diseases, a reptile is better able to resist infection if it is fed a proper diet and given excellent husbandry (i.e., a thermal gradient, appropriate lighting, a suitable humidity range, hiding spots). Avoid purchasing reptiles kept under crowded conditions, particularly where there are obvious skin lesions on other specimens sharing the same container.

Prognosis and Outcome

YFD is often fatal, and the treatment is usually for a long period of time. The best chance for a successful cure is when the disease is diagnosed and treated by your veterinarian early on in the disease process.

Zoonotic Potential

The fungus that causes YFD can only grow at temperatures lower than that of the human body, and so the potential for infection from your dragon to you is very low or non-existent.