(--note-- This post was originally written on November 16th--the conference was held Nov. 5-7)
I've never been one to turn down a party, so how could I possibly have resisted three days of presentations about a fungal disease of amphibians? Well, I couldn't, and last week's conference in Tempe, Arizona, titled “Amphibian Declines and Chytridiomycosis: Translating Science Into Urgent Action
,” was a splendid opportunity to be brought up to snuff on what's being called the most destructive epidemic ever known. Most of the papers were very interesting, and I found my mind wandering only a couple of times during the entire event. The objective was to discuss the latest research and to lay out a plan of action. I could type all day about the stuff I learned, but I think we'd all be best off if I limited this to a synopsis of the current state of knowledge.
Chytridiomycosis is caused by the chytrid fungus Batrachochytrium dendrobatidis
, a monotypic genus described in 1999 by Joan Longcore, Allan Pessier and Donald Nichols. It has been variously referred to as Chytridiomycosis (cumbersome), Chytrid (inaccurate, since it refers to the entire family), and Bd
(the fungus' binomial acronym). One of a handful of decisions established at last week's event was that in common conversation the disease should be called “Bd
,” to avoid confusion among the lay public. Previously known chytrid fungi feed on decaying organic matter, plants and invertebrates, but Bd attacks the epidermal cells of amphibians. All adult frogs appear to be susceptible to some degree, although a number of species never seem to get sick. Bd
has been isolated from several salamanders, some of which appear to be able to act as asymptomatic carriers, while for others the fungus is lethal. Experimental attempts to infect sirens, amphiumae, and a number of fish species have so far proved fruitless, and the only known case of Bd in caecilians involved two dozen recently confiscated Typhlonectes
sp., 13 of which tested positive. Since histological assays were never performed on these animals, it isn't known with certainty that they weren't merely living in infected water.
Bd seems to need keratinized skin to grow on, and in larval amphibians it can only attack the mouthparts of tadpoles and the toes of salamanders. These larval infections seem to be innocuous until the animals metamorphose. Infected susceptible species usually die 2-3 weeks after transformation. The primary symptoms are behavioral changes including lethargy, tetanic spasms, postural changes and poor righting reflexes, as well as skin changes, including acanthosis, edema, keratinization, ulceration, erosion, vacuolation, discoloration and excessive shedding. Toe tips of infected frogs are often matted with unshed skin. Exactly how Bd
kills its hosts is still unknown. There is evidence that electrolytic balance is affected, ultimately causing heart failure [UPDATE: Since writing this, the electrolytic balance theory has been confirmed
]. Other possible causes are dehydration, mycotoxin and secondary infections.(UPDATE: Thanks to Dr. Joyce Longcore, who emailed me with the following remarks: "If double line represents frog skin, the Bd thallus is completely within a single cell, with, perhaps a single rhizoid going deeper.See Berger et al. 2005 (Pdf available on Rick Speare’s Amphibian Disease site) for her life cycle. This (and your diagram) represent what occurs on agar. On frog skin, the zoospore cyst probably produces a germ tube that grows into the layer of epidermal cells that still has intact nuclei (see TEM photos in Berger et al.) The nucleus enters through the germ tube and the zoosporangium develops within a host cell in that epidermal layer, which gradually moves closer to the skin surface as skin layers are shed and zoosporangia develop. By the time the sporangium is mature the host cell is near the surface and the discharge papilla grows so that the zoospores are released to the surface, as shown by your 'mature zoosporangium'. 'Colonial thalli' are not a part of the life cycle but two- or more parted zoosporangia are an alternate form of development, perhaps developing from zoospores that received more than one nucleus during the cleavage within the mature zoosporangium. Your mature zoosporangium could show zoospores emerging and settling down near the first, now empty zoosporangium, because, you are right—zoosporangia do tend to occur in groups. Although zoospores can swim, they seldom move far by flagellar activity.We do not yet have good TEM evidence about germ tube development or the number and complexity of the rhizoidal system when within amphibian skin.")
The infective stage of Bd is a flagellated, free-swimming zoospore. Upon settling on a host, it probably forms germ tube, which carries the nuclear material into developing epidermal cells. Within these keratinizing cells the developing fungus may form sparse rhizoids. This organism (thallus), ultimately grows into the mature stage, the zoosporangium, within new zoospores are formed. During this time the infected host epidermal cell nears the skin surface and the discharge tube of the fungus penetrates the cell membrane to release zoospores to the exterior. Zoospores, although motile, seldom swim far and often settle near the original infection, thus leading to infections occurring in groups in lightly infected animals. Reinfection from these zoospores is an important aspect of making an infection lethal. As many as 4.4 million zoospores can be shed from an infected animal each day. Bd prefers temperatures between 4 and 27 degrees C. above this range, virulence diminishes markedly, and temperatures over 30 degrees for more than a day or two appear to kill the fungus.
The earliest known Bd isolates came from an African Clawed Frog or “Platanna” (Xenopus laevius
) collected in 1938, just as human pregnancy tests using Platannas were being accepted as the most effective available, beginning a huge increase in the global exportation of the species. Today these frogs are found in many alien waters; a couple of years ago, I nearly caught one in a southern California stream. Although they're no longer exploited for pregnancy tests, Platannas are still popular lab animals, and since 1998, nearly 10,000 of them were exported from Africa, all of them wild-caught. Bd occurs in wild, trasported and feral populations of X. laevius
, which never become symptomatic. Today's conventional model of Bd spread begins with an endemic African fungus living peacefully on the pelts of rather resistant local frogs, and being introduced to other continents via Platannas. Further, more local spreading was done by other frogs—in North America, the resistant Bullfrog (Rana catesbeiana
) was probably an important vector. In a number of regions, biologists actually watched and monitored spreads as they were happening, and the puzzle of global Bd
dispersal is quickly being put together. Another accomplishment of last week's meetings was the fusion of the two major global Bd
mapping projects. The resultant group plans to have an interactive website up soon, where researchers from all over the world can input their data. Many aspects of the disease's movements are still hard to understand. It spread down Central America in classic epidemic form, but in other places its trajectory has been surprising, sometimes tearing through uninhabited country far faster than frogs can move.
The entire Bd
genome has been sequenced, and much has been gleaned from DNA evidence. The extremely low worldwide genetic diversity points to an organism whose dispersal began very recently. There is good genetic evidence that Bd
can reproduce sexually, although this has never been observed in chytrid fungi. If such a thing is possible, rapid evolution will likely make fighting the epidemic that much more complicated a task.
In further installment(s) I will discuss specific effects on global frog populations, and what has been, should be, and could be done to minimize the effects of this important disease.
_____________________upper: TABLE MOUNTAIN GHOST FROGS (2004) acrylic diptych 20" x 15"; 20" x 15"
center: Bd Life Cycle Diagram ink on paper 5" x 7"
lower: RETICULATED GLASS FROG (1998) acrylic 7" x 7"