Our study has extended the date for the earliest case of chytridiomycosis in wild amphibians by 23 years. The next earliest case outside South Africa was found in Rana clamitans from Saint-Pierre-de-Wakefield, Québec, Canada, in 1961 (22). After the case in Canada, the earliest cases from other countries follow sequentially over a period of 38 years from 1961 to 1999 (Figure 3).
X. laevis in the wild does not show clinical signs, nor has it experienced any sudden die-offs. Moreover, only subclinical chytrid infections have been observed among captive colonies of X. laevis (26,27). A frog of a related species, X. tropicalis, died in captivity from chytridiomycosis, it was suspected of having contracted the fungus from X. laevis (27). An ideal host for transmission of chytridiomycosis through international translocation would be a species of amphibian that does not become diseased or die from the infection; hence, X. laevis could take on the role of a natural carrier.
The sudden appearance of chytridiomycosis can best be explained by the hypothesis that B. dendrobatidis was recently introduced into new regions and subsequently infected novel host species (1). Dispersal of B. dendrobatidis between countries is most likely by the global transportation of amphibians (1,2,23,28,29). The World Organization for Animal Health has recently placed amphibian chytridiomycosis on the Wildlife Diseases List in recognition of this risk. If Africa is the source of B. dendrobatidis, a feasible route of dissemination by infected amphibians needs to be identified. Some members of the family Pipidae have been exported, in particular Hymenochirus curtipes and X. laevis, to North America and Europe (30).
In terms of a most likely candidate for spread from Africa, the number of frogs and geographic dissemination favor X. laevis. Soon after discovery of the pregnancy assay for humans in 1934 (30), enormous quantities of the species were caught in the wild in southern Africa and exported around the world. The pregnancy assay is based on the principle that ovulation in X. laevis is induced by injection with urine from pregnant women because of high levels of gonadotropic hormones in the urine (31,32). X. laevis was selected as the most suitable amphibian for investigating the mechanism of the mating reflex because of the relative ease with which the animal can be maintained in captivity (33). For 34 years, the trade in X. laevis in South Africa was controlled by the then Cape of Good Hope Inland Fisheries Department (Western Cape Nature Conservation Board) at the Jonkershoek Fish Hatchery. As an indication of the numbers involved in this trade, 10,866 frogs were distributed in 1949, of which 3,803 (35 percent) were exported, and of the 20,942 frogs distributed in 1970, a total of 4,950 (24 percent) were shipped abroad (34,35). After the introduction of nonbiologic pregnancy tests, X. laevis became important as a model for the scientific study of immunity and later embryology and molecular biology. X. laevis could have carried the disease globally, particularly if the prevalence was similar to that seen in wild-caught X. laevis today. In the importing country, escaped frogs, the water they lived in (36), or both, could have come into contact with local amphibian species, and subsequent transmission of the disease could have followed. The establishment of feral populations of X. laevis in Ascension Island, the United Kingdom, the United States, and Chile in 1944, 1962, the 1960s, and 1985 (37), respectively, show that transmission could have become ongoing if these feral populations were infected.
Although we have demonstrated that B. dendrobatidis was in southern Africa since 1938, our studies provide no indication regarding whether this region was the original source within Africa. B. dendrobatidis has been found in wild frogs in Kenya and in frogs (X. tropicalis and X. laevis) wild-caught in Western Africa and detected after importation into the United States (12,26,27,38), which indicates that B. dendrobatidis is widely disseminated in Africa. Xenopus consists of 17 species that are found in sub-Saharan Africa, with a varying degree of sympatry between species (17). The overlap in the distribution and, in some cases, the sharing of habitats could facilitate transmission of B. dendrobatidis between these species. This finding would imply that chytridiomycosis could have originated elsewhere in Africa and spread within multiple host-region combinations. More detailed historical studies of archived African amphibians may indicate whether B. dendrobatidis was originally present in a small area of Africa from which it emerged to occupy large areas of the continent. Until the deficit in distribution data and comparative genetic studies is remedied, locating the source of the origin of B. dendrobatidis within Africa remains speculative. The relationship appears to have coevolved within an anuran host, and the opportunity to disseminate across the globe existed for B. dendrobatidis in southern Africa.
If X. laevis did carry B. dendrobatidis out of Africa as we propose, other amphibian species subsequently could have distributed it between and within countries. The American bullfrog, Rana catesbeiana, has been proposed as an important vector, mainly through international trade as a food item, but also within countries as populations established for the food trade escape and spread (29). The earliest current record for the occurrence of chytridiomycosis in R. catesbeiana is 1978 in South Carolina (38), 40 years after the first record in southern Africa, but details on the intensity of the search for chytridiomycosis in archived bullfrogs are not available. The transmission of chytridiomycosis globally may involve a series of key steps: 1) occurrence of B. dendrobatidis in an amphibian vector in southern Africa that is relatively resistant to disease (X. laevis), 2) sudden rise in 1935 of export trade in this vector because of technologic advances (Xenopus pregnancy test), 3) escape of the pathogen from the exported Xenopus to establish new foci in other countries (possibly expedited in some countries by establishment of feral populations of X. laevis), 4) transmission into other vector amphibians (food and pet trade), and 5) further transmission to other countries along different trade routes in key amphibian vectors that move in high numbers and become established in commercial populations and closely interact with wild frogs, which likely leads to feral populations (food frogs R. catesbeiana). Spread through native amphibian populations with epidemic disease in some species could have occurred at any point after B. dendrobatidis entered a naïve native species.
We have provided epidemiologic evidence that Africa is the origin of the amphibian chytrid fungus. Support for six of the seven criteria proposed for the source of B. dendrobatidis has been demonstrated: 1) the major host (X. laevis) shows minimal or no apparent clinical effects, 2) site of the earliest global occurrence (1938), 3) this date precedes any amphibian declines in pristine areas, 4) the prevalence in the source host or hosts (Xenopus spp.) has been stable over time, 5) no geographic spreading pattern could be observed over time, and 6) a feasible means of global dissemination exists via the international trade in wild-caught X. laevis, which commenced in 1935 and continues today. Criterion 7, greater genetic diversity of B. dendrobatidis at the source, has not been investigated. A low level of genetic variation was shown for 35 strains of B. dendrobatidis and suggested that B. dendrobatidis was a recently emerged clone (39). The strains had been collected in North America, Australia, Panama, and Africa from wild and captive amphibians. Three strains isolated from captive X. tropicalis in United States had been imported from Ghana. Although these showed no significant differences from the U.S. strains (39), their assignment to Africa assumes no cross-infection had occurred within the importing facility. Future work on the genetic diversity of B. dendrobatidis in Africa compared with strains from regions outside Africa will add weight to the hypothesis if greater genetic diversity is found in African strains.
Originally posted by rbmorrisNice work Morris.
Our study has extended the date for the earliest case of chytridiomycosis in wild amphibians by 23 years. The next earliest case outside South Africa was found in Rana clamitans from Saint-Pierre-de-Wakefield, Québec, Canada, in 1961 (22). After the case in Canada, the earliest cases from other countries follow sequentially over a period of 38 years from 19 ...[text shortened]... rica will add weight to the hypothesis if greater genetic diversity is found in African strains.
😉
Put on your Yamaka
Its time for Chanukah
So much funnaka
To celebrate Chanukah
Chanukah is the festival of lights
Instead of one day of presents
We get eight crazy nights
When you feel like the only kid in town
Without a Christmas tree
Here's a new list of people who are Jewish
Just like you and me
Winona Ryder,
Drinks Manischewitz wine
Then spins a Dreidle with Ralph Lauren and Calvin Klein
Guess who gives and receives
Loads of Chanukah toys
The girls from Veruca Salt and all three Beastie Boys
Lenny Kravitz is half Jewish,
Courtney Love is half too
Put them together
What a funky bad ass Jew
We got Harvey Keitel
And flash dancer Jennifer Beals
Yasmine Bleeth from Baywatch is Jewish
And yes her boobs are real
Put on that yarmulka
Its time for Chanukah
2 time Ocsar winning Dustin Hoffmanaka
celebrates Chanukah
O.J. Simpson
Still not a Jew
But guess who is,
The guy who does the voice for Scooby Doo
Bob Dylan was born a Jew
Then he wasn't
but now he's back,
Mary Tyler Moore's husband is Jewish
'Cause we're pretty good in the sack.
Guess who got bar-mitzvahed
On the PGA tour
No I'm not talking about Tiger Woods
I'm talkin' about Mr. Happy Gilmore.
So many Jews are in the show biz
Bruce Springsteen isn't Jewish
But my mother thinks he is.
Tell the world-amanaka
It's time to celebrate Chanukah
It's not pronounced Ch-nakah
The C is silent in Chanukah
So read your hooked on phonica
Get drunk in Tijuanaka
If you really really wannaka
Have a happy happy happy happy Chanukah!
Two-dimensional, stratified shear flow over a ridge is considered. The finite-amplitude disturbances are steady and hydrostatic, and solutions are derived from the Boussinesq from the Long's equation. Two limiting solutions are examined; viz., 1) the case of marginal or neutral static stability and 2) the case of infinite static stability either at or above the lower boundary. The former case is associated with a critical point for the horizontal flow velocity, u=0; an infinite value of u accompanies the latter case. The conditions for neutral static stability that have been derived for uniform upstream flow conditions are shown to apply to the case when both the upstream static stability N¯(z¯ ) and the horizontal velocity u¯(z¯ ) are nonuniform in the vertical direction z¯. Upstream variations of N¯(z¯ ) and u¯(z¯ ) cannot be specified arbitrarily if the relative vorticity vanishes at some point either at the ridge or in the airstream above. An unbounded solution, u = ∞, of Long's equation will occur unless the condition [N¯−2(u¯−2/2)z¯]z¯ < 1 is satisfied. The physical interpretation of this constraint on the upstream flow is provided. It is also noted that the same condition has been derived by Abarbanel et al. as a sufficient condition for the nonlinear stability of a stratified shear flow to three-dimensional distrurbances. However, the physical relationship between these two model results has not been established.
Originally posted by IM4Y2NVMy name is Takeshi Hikari and I favor my Japanese heritage. I am happy to be here because I can meet with other intellectuals and have a good game of chess with them.
to be here at RHP.
I'm G and I like to play chess and chat while I have no moves to make.
Who are you and what do you like?
🙂
My name, in addition, means Brave Light in my native tongue.
Originally posted by rbmorrisI love it when you talk Physics!
Two-dimensional, stratified shear flow over a ridge is considered. The finite-amplitude disturbances are steady and hydrostatic, and solutions are derived from the Boussinesq from the Long's equation. Two limiting solutions are examined; viz., 1) the case of marginal or neutral static stability and 2) the case of infinite static stability either at or above ...[text shortened]... . However, the physical relationship between these two model results has not been established.
Tell me more!
Originally posted by SamuraiWarriorDomo arigato, Mr. Roboto, Mata ah-oo hima de Domo arigato
My name is Takeshi Hikari and I favor my Japanese heritage. I am happy to be here because I can meet with other intellectuals and have a good game of chess with them.
My name, in addition, means Brave Light in my native tongue.
Originally posted by SamuraiWarriorGreetings Takeshi! Glad to have you here.
My name is Takeshi Hikari and I favor my Japanese heritage. I am happy to be here because I can meet with other intellectuals and have a good game of chess with them.
My name, in addition, means Brave Light in my native tongue.