Read Part I here.
Where did chytrid fungus come from? That, as it turns out, is a long (and still debated) story.
In the 1970s, in Queensland on Australia’s eastern coast, frogs began to disappear. First the Southern day frog, last-seen in 1975. Then the Southern gastric brooding frog, gone by 1979. Three other species followed: the cascade tree frog, the giant barred frog, and Fleay’s barred frog, each one experiencing a 90% population decrease, plummeting towards extinction. By 1985 the wave of disappearances had swept north: the Northern gastric brooding frog and the Eungella torrent frog disappearing by January 1985. A survey team found isolated remnant populations, hidden high up on hillsides in March 1985, but these were gone by June.
Amphibian researchers were alarmed, but had no idea of the cause. No bodies were being found; the frogs were simply…gone. By the early 90s, six more species had vanished. Researchers began to propose hypotheses. Perhaps it was a water-borne virus – all of the disappearing species were stream-dwelling, tropical forest frogs. But scientists lacked the forensic evidence needed for proof: with no corpses, they were relying on conjecture. In 1993, they caught a break. The amphibian population near Cookstown, Queensland, crashed dramatically. This time researchers were prepared. Samples were taken, and in 1998, Dr. Lee Berger identified the chytrid fungus as the source of amphibian plague. The identification helped, but it was too late by then to stop the spread in Australia. Retrospective analysis of dead frogs from Western Australia found that the fungus had been present there since at least 1985. The plague continued to sweep across the continent, and today chytrid fungus is found everywhere except the dry Northern Territory.
This pattern has repeated itself four times since, for a total of five epidemic fronts: Australia, Central America, the Sierra Nevadas, the northern tip of South America, and the Pyrenees. In each of these disparate locations, waves of infection have spread outwards from a sole epicentre, devastating each regions native amphibian population. While the locations are different, the effect is the same, and, as it turns out, so is the source. When scientists sequenced the genome of the fungus taken from each epidemic front, they found no differences. All of the fronts had a singular source, an amphibian patient zero.
Epidemiology, the study of patterns of disease in animal or human populations, is medical detective work. An epidemiologist gets to the scene of the crime often long after it has been committed: disease has spread, mortality is high, and waves of infection rampage through the population. From the endpoint, they trace their way backwards, walking in the path of the disease, hoping to understand where it came from. For an epizootic – an emerging, animal-borne – disease like chytrid (or, in humans: swine flu, avian flu, West Nile Virus, Marburg, Ebola, the list goes on), this means locating the reservoir.
A reservoir is an animal species that coexists uneasily with a pathogen. It is the pathogen’s natural home and resting place: the virus, bacteria, or fungi can live in the blood or body of its host, or on its surface, without causing rapid host death. The pathogen is likely to still harm the host in some way, but its virulence is low: it is the common cold, rather than pneumonia – annoying, but generally not life-threatening. But sometimes the reservoir comes in contact with another, closely related, species, and the pathogen switches hosts. In epidemiology, this host switching is called “spillover”, and can have devastating consequences.
When a pathogen spills over, it enters a new host species that has no immune response to its new bedfellow. Virulence increases; the effects of pathogen – minor in its regular host – are amplified by the lack of immune response. This amplification means that in a new host, rather than being a nuisance, the pathogen may become lethal. This is common in other epizootic disease. In rhesus macaques, the Herpes B virus is a nuisance, causing sores around the mouth – when transmitted to humans, through bites, feces, or spit, it amplifies, killing 60-70% of the people it infects.
The same, so researchers thought, might true of chytrid fungus. It must have an amphibian reservoir, one or more species which it can parasitize without killing. And finding that reservoir might be key to understanding the spread of the fungus, stopping it, and – maybe- curing it. A possible reservoir was found, almost by accident, right under researcher’s noses.
As research on chytrid fungus progressed in the early 2000s, scientists in Africa noticed something. The fungus was not uncommon in some species of African frogs, and the frogs appeared to carry it mostly asymptomatically. Che Weldon, at the time a PhD student at North-West University in South Africa, decided to investigate. Using library specimens of African clawed frogs (scientist’s libraries are sometimes creepy and filled with dead animals rather than books) dating between 1879 and 1999, he tested for the presence of chytrid. And he found it. In a short paper published in Emerging Infectious Diseases in 2004, Weldon and his colleagues reported their results: chytrid fungus had been found in 2.7% of their samples, and that number had not changed across years, or seasons, or region, since 1938. “Chytridiomycosis”, they reported, “was a stable endemic infection in southern Africa for 23 years before any positive specimen was found outside Africa”.
So how did the frogs get out? Pregnancy tests. In 1934, researchers discovered something that indigenous Africans had known for centuries – African clawed frogs were an extremely accurate pregnancy test. When a woman becomes pregnant, there is an immediate hormonal response. After the egg is fertilized, her body begins to produce steadily increasing levels of progesterone. Progesterone prevents menstruation, allowing the fertilized egg the time to become implanted in the uterine lining. The hormone is also present in African clawed frogs – and in them, it triggers the immediate production of eggs.
So here’s how the oldest pregnancy test in the world works. You think you’re pregnant. The pharmacy is a three-day walk away (or still non-existent), so you grab the nearest African clawed frog. Then, just like a modern test, you put it between your legs and pee on it. If you’re pregnant, the increased levels of progesterone present in your urine will trigger the development of eggs on the back of the frog. Congratulations!
Before this response could be obtained through synthetic chemicals, African clawed frogs were exported around the world, and harvested for use in labs for pregnancy assays. This, Weldon and company argued, provided an ideal opportunity for the spread of chytrid fungus across the globe.
A few years ago, a friend called me and I asked if I wanted some frogs. “Sure,” I said, “why not?”. A few days later, she dropped by my apartment with a small aquarium, and a bucket full of African clawed frogs, Xenopus laevis. The African clawed frog is a primitive species, living its life completely in the water (unlike most frogs, which are semi-terrestrial). It’s characterized by the claws on its hindfeet, and by the lateral line system running down its body. Lateral line systems are ancient nerve systems found in many aquatic animals, and used to detect vibrations in the water. They have the appearance of making clawed frogs look like they’ve been dissected and then sewn up – I affectionately called them my FrankenFrogs.
Along with their use as a cheap home-pregnancy kit, African clawed frogs are incredibly popular in research labs; my new family was rescued from the discard bucket of a genetics lab on campus. They grow quickly and can regenerate limbs to some degree, and in terms of both genetics and development they are fairly similar to humans, making them useful as model organisms. This also makes them wide spread. They’re traded between labs, and shipped around the world. They’re also aggressive colonizers, and adapt easily to new environments: more than one lab or pet population has escaped and settled into a new home-away-from-home (including a population living in a pond in San Francisco’s Golden Gate Park).
But they might also hold hope for a cure. Clawed frogs become infected, but are asymptomatic: the fungus seems not to affect them. As it turns out, when they detect an infection their body mounts an immune response too – but rather than leading to hyperkeratosis they’ve opted for a different form of defense. Many frogs have granular glands on the surface of their skin (better known as poison glands, or, “that thing that might get me high if lick it”). These sacs are filled with a variety of different peptides: short chains of amino acids, generally used as signalling devices in the body – sort of like text messages, “Hey, gonads, turn off the testosterone, we’ve got enough here”. But in some species of frogs, including African clawed frogs, these peptides have antimicrobial functions, which may serve to isolate the site of a chytrid infection, and prevent it from spreading over the animal’s whole body.
This discovery has led to hope that a synthetic version could be created as a sort of antimicrobial wash, which could be administered to infected populations. Additional research, along similar lines, has discovered a bacteria found in soil and on the skin of some salamanders, Janthinobacterium lividum, which can protect amphibians against chytrid infection – the bacteria eat the zoospores before they can settle. Research is ongoing, but between these two hopes, and other solutions, your grandkids may still be able to see frogs in the wild.