Chytrid Fungus
WRITTEN BY D.KNIGHT
History
Chytridomycosis, Chytrid fungus for short is caused by the bacteria Batrachochytrium dendrobatidis.
Chytrid was first present in the 1960s, and may have caused the amphibian population crashes from the 1970s onwards. In 1993 It was found amongst dead and dying frogs in Queensland, and has been devastating amphibian populations ever since. One third of the world's 6,260 amphibian species are globally threatened or extinct. The primary threat to their survival is still habitat destruction, which impacts 61 percent of known amphibian species. But climate change and the deadly chytrid fungus could potentially take the lead over the next century—or at least make things much, much worse for amphibians. The origins of the fungus are unknown, but there is a high chance the spread was caused back in the 1930s when African clawed frogs were exported to produce frog-based pregnancy tests. To date there is no know cure to treat entire habbitats, however individuals can be treated and cured in captivity. This much is clear: frogs are dying.
Biology
Batrachochytrium dendrobatidis (Bd), is an aquatic fungus that is a very highly virulent fungus in Vertebrates, most of all amphibians. The fungus has been found worldwide, with over 350 species affected, across all continents, except Antarctica. Chytridiomycosis is found in a range of species and habitats including lakes, natural forests, riparian zones and water courses. The fungus prefers lower temperatures which may explain the high precedence of the fungus in high elevations in the tropics. In culture conditions optimum growth amongst zoospores occurred at 23°C, whilst slower at 28°C. The fungus cannot stand, and dies at temperatures above 86°C . Rohr looked at how climate change could be affecting the ability of amphibians to fight the fungus. When then frogs were suddenly moved from the hot to cooler temperature, they ended up with 25 percent increase of the fungus than if they were kept at the cooler temperature. Rohr suspects that the frogs' ability to secrete fungus-fighting skin secretions is temperature-sensitive. Rohr and his colleagues brought back frogs and the fungus to the lab, were Cuban tree frogs were raised in Styrofoam tanks at either 15 or 25 degrees for four weeks. After four weeks they were then infected with chytrid fungus, and to confirm Rohrs theory’s the fungus grew better and killed more frogs when the temperatures were cooler.
The effects
The disease has been a big factor in amphibian declines, due to its highly contagious nature and its aggressive effects on the host. Chytrid takes a fast acting course, starting with the amphibian’s first encounter with the zoospores. Then the amphibian’s skin quickly gives rise to sporangia, which produce more zoospores. The fungus then progresses when the new zoospores re-infect the host. Once the fungus has taken hold of the host morphological changes occur including reddening of the ventral skin, convulsions with extension of hind limbs, sloughing of the superficial epidermis of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small ulcers or haemorrhage. The amphibian’s behaviour then changes and the host may become lethargic, fail to find shelter, lose the ability to righting its self and adopt an abnormal posture. Death then usually occurs in 2-3 weeks, but the cause of death is unknown. It may be due to physical changes, affecting respiration, or the fungus may give off a toxin. Tadpoles are not affected, due to their lack of Keratin, however once the tadpole starts to morph into froglets Keratin becomes present, and the fungus can then take hold and start to effect the first Keratinised parts, the mouth. Infected amphibians have been known to still attempt to breed and these off spring may well already be infected, however evidence on this is still rather vague.
History
Chytridomycosis, Chytrid fungus for short is caused by the bacteria Batrachochytrium dendrobatidis.
Chytrid was first present in the 1960s, and may have caused the amphibian population crashes from the 1970s onwards. In 1993 It was found amongst dead and dying frogs in Queensland, and has been devastating amphibian populations ever since. One third of the world's 6,260 amphibian species are globally threatened or extinct. The primary threat to their survival is still habitat destruction, which impacts 61 percent of known amphibian species. But climate change and the deadly chytrid fungus could potentially take the lead over the next century—or at least make things much, much worse for amphibians. The origins of the fungus are unknown, but there is a high chance the spread was caused back in the 1930s when African clawed frogs were exported to produce frog-based pregnancy tests. To date there is no know cure to treat entire habbitats, however individuals can be treated and cured in captivity. This much is clear: frogs are dying.
Biology
Batrachochytrium dendrobatidis (Bd), is an aquatic fungus that is a very highly virulent fungus in Vertebrates, most of all amphibians. The fungus has been found worldwide, with over 350 species affected, across all continents, except Antarctica. Chytridiomycosis is found in a range of species and habitats including lakes, natural forests, riparian zones and water courses. The fungus prefers lower temperatures which may explain the high precedence of the fungus in high elevations in the tropics. In culture conditions optimum growth amongst zoospores occurred at 23°C, whilst slower at 28°C. The fungus cannot stand, and dies at temperatures above 86°C . Rohr looked at how climate change could be affecting the ability of amphibians to fight the fungus. When then frogs were suddenly moved from the hot to cooler temperature, they ended up with 25 percent increase of the fungus than if they were kept at the cooler temperature. Rohr suspects that the frogs' ability to secrete fungus-fighting skin secretions is temperature-sensitive. Rohr and his colleagues brought back frogs and the fungus to the lab, were Cuban tree frogs were raised in Styrofoam tanks at either 15 or 25 degrees for four weeks. After four weeks they were then infected with chytrid fungus, and to confirm Rohrs theory’s the fungus grew better and killed more frogs when the temperatures were cooler.
The effects
The disease has been a big factor in amphibian declines, due to its highly contagious nature and its aggressive effects on the host. Chytrid takes a fast acting course, starting with the amphibian’s first encounter with the zoospores. Then the amphibian’s skin quickly gives rise to sporangia, which produce more zoospores. The fungus then progresses when the new zoospores re-infect the host. Once the fungus has taken hold of the host morphological changes occur including reddening of the ventral skin, convulsions with extension of hind limbs, sloughing of the superficial epidermis of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small ulcers or haemorrhage. The amphibian’s behaviour then changes and the host may become lethargic, fail to find shelter, lose the ability to righting its self and adopt an abnormal posture. Death then usually occurs in 2-3 weeks, but the cause of death is unknown. It may be due to physical changes, affecting respiration, or the fungus may give off a toxin. Tadpoles are not affected, due to their lack of Keratin, however once the tadpole starts to morph into froglets Keratin becomes present, and the fungus can then take hold and start to effect the first Keratinised parts, the mouth. Infected amphibians have been known to still attempt to breed and these off spring may well already be infected, however evidence on this is still rather vague.
Chytrid in captivity
Captive amphibians are NOT SAFE from chytrid fungus, mortalities have been reported in private and zoo collections. Several different treatments have been used with varying degrees of success including various anti-fungal drugs and exposure to high temperatures. Probably the most readily available and cost-effective drug treatment is itraconazole, which the frog is bathed in for ten minutes each day for 10 days. However it is important that at least two treatments are carried out along with a repeat PCR test. The best way to avoid adding chytrid to your collection is to never bring in another amphibian that is wild caught or to add any amphibians from any other facility. The best way to operate is to do what any responsible keepers should already be doing , quarantine all in coming amphibians for at least eight weeks. Quarantine should vivariums should be in an isolated room away from all other amphibians and avoid using anything which will come into contact with your current collection. Whilst quarantining new amphibians a PCR test should be carried out and temperatures are maintained between 18 and 25°C in order for chytrid to be at its most active and therefore detectable. The incubation period for the disease can be 9-76 days, and most tropical amphibians will die in this time. Some animals can carry the infection without showing signs of disease so a steady routine testing for chytrid is vital for both new and current amphibians in your collection.
Swabbing
Swab the underside of the frog over the particularly focusing on the 'drink patch' (underside of the pelvic region involved in water uptake through the skin), the inner thighs of the hind legs and the underside of the digits on the hind feet - including the webbing between the toes. ©ZSL
Captive amphibians are NOT SAFE from chytrid fungus, mortalities have been reported in private and zoo collections. Several different treatments have been used with varying degrees of success including various anti-fungal drugs and exposure to high temperatures. Probably the most readily available and cost-effective drug treatment is itraconazole, which the frog is bathed in for ten minutes each day for 10 days. However it is important that at least two treatments are carried out along with a repeat PCR test. The best way to avoid adding chytrid to your collection is to never bring in another amphibian that is wild caught or to add any amphibians from any other facility. The best way to operate is to do what any responsible keepers should already be doing , quarantine all in coming amphibians for at least eight weeks. Quarantine should vivariums should be in an isolated room away from all other amphibians and avoid using anything which will come into contact with your current collection. Whilst quarantining new amphibians a PCR test should be carried out and temperatures are maintained between 18 and 25°C in order for chytrid to be at its most active and therefore detectable. The incubation period for the disease can be 9-76 days, and most tropical amphibians will die in this time. Some animals can carry the infection without showing signs of disease so a steady routine testing for chytrid is vital for both new and current amphibians in your collection.
Swabbing
Swab the underside of the frog over the particularly focusing on the 'drink patch' (underside of the pelvic region involved in water uptake through the skin), the inner thighs of the hind legs and the underside of the digits on the hind feet - including the webbing between the toes. ©ZSL
Testing
Tests are available by institutes and zoos. A real-time PCR test for Amphibian Chytrid Fungus is available from
Tests are available by institutes and zoos. A real-time PCR test for Amphibian Chytrid Fungus is available from
Diagnostic Laboratory,
Wildlife Epidemiology Zoological Society of London (ZSL) Welcome Building London NW1 4RY UK Email: [email protected] |
Wildlife Disease Laboratories
Institute for Conservation Research San Diego Zoo * Dr. Allan Pessier Email: [email protected] 619-231-1515, Ext 4510 |