New Scientist - USA (2021-07-17)

42 | New Scientist | 17 July 2021

use, and it seemed likely that chytrid

came with them.

However, more recent evidence points to

a different origin. By studying the DNA of

chytrids, it has become clear that they have

been around for millions of years. Various

strains have long existed all around the world

and amphibians lived more or less happily

alongside their local strain – long exposure

enabling them to build up resistance and avoid

any potential harms caused by the fungus.

Amphibian apocalypse

We know that amphibians in South-East Asia

are generally more resistant to the most

dangerous strain of Bd, the one responsible for

the panzootic, than amphibians are elsewhere.

This suggests the panzootic strain originally

came from South-East Asia. “We believe

amphibians have been living with this

particular chytrid for at least 40 million

years,” says Fisher. “So, a very long time,

but only in one part of the planet.” Frogs

from this region were probably then exported,

both to be pets and for lab experiments.

This spread the South-East Asian strain of

Bd to places where local amphibians had no

resistance to it, and kicked off all the trouble.

The strain circulating most widely now

appears to be a highly virulent hybrid between

that initial Asian strain and another chytrid.

In 2019, yet another new strain of Bd was

discovered. There could be more. The parallels

with the covid-19 pandemic, where travel has

spread new virus variants around the world to

devastating effect, are clear.

This suggests a course of action that could

slow chytrid’s spread: tight controls or a ban

on the export of amphibians. Although chytrid

is found worldwide, there are pockets of clean

habitat. For instance, Madagascar, a hotspot of

amphibian diversity, is still thought to be free

of the fungus. Bsal, the chytrid that mostly

infects salamanders, seems to be confined to

Europe for the moment. The UK, US, Canada

and the EU all have legislation to control the

movement of salamanders. According to the

North American Bsal Task Force, as of 2020

there weren’t any cases of Bsal in North

Animal outbreaks

Amphibians aren’t the only animals battling a panzootic

disease – one that has spread around the world

PAST THREATS

Rinderpest This viral disease caused the deaths of

millions of cattle and buffalo around the world, resulting

in famines in the 20th century. Following the development

of a vaccine in 1960, there was an international effort to

get rid of rinderpest. It was declared eradicated in 2011.

PRESENT THREATS

Avian influenza One strain of “bird flu”, called H5N1,

is panzootic. It causes respiratory problems in birds,

including domestic poultry and has a high mortality rate.

It can also infect humans – 700 people are known to have

contracted it since 2003 and about 60 per cent have died.

White-nose syndrome This disease, which affects bats,

is caused by a fungus called Pseudogymnoascus

destructans. Millions of bats have died of it in North

America. It is also prevalent across Europe, although bats

there are thought to have developed some resistance.

Newcastle disease Another panzootic that affects birds,

including pigeons, the Newcastle disease virus is highly

contagious. So far, however, there have been no cases

of people catching it.

FUTURE THREATS

African swine fever Acute forms of this viral disease

cause fever, depression and loss of appetite in pigs.

It spreads easily and has recently been seen across

Africa, Asia and Europe. Some think it could go global.

Sarcoptic mange A skin infection reported in thousands

of individuals across almost 150 wild and domestic

mammal species. It is caused by the mite Sarcoptes

scabiei, which is also behind the itchy condition known

as scabies in people. Sarcoptic mange may be the newest

emerging panzootic in wildlife.

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Millions of

cattle were

killed by

Rinderpest