Hi,
All good points. Of course I knew about botulinum toxicity and that it
could affect birds but I hadn't realized the extent or its importance.
The recent post to Birding Aus set me looking further. One reference
which I found quite interesting was "Field Manual of Wildlife Diseases"
published by the USGS.
http://www.nwhc.usgs.gov/publications/field_manual/
It has lots of interesting information and some of it supports your
suggestions, with regard to human activity and effects, in the first
paragraph.
The selective pressure must now be quite high (at least in recent
times). The reference indicates that die offs of waterbirds due to
botulinum toxicity of the order of hundreds of thousands to millions are
relatively common in North America, and that it is the major cause of
death in waders. Perhaps we will see an increase in
resistance/insensitivity as birds evolve.
Cheers
Andrew
On 10/02/2012 5:02 AM, Stephen Ambrose wrote:
I accidentally posted this response to Andrew Hobbs yesterday, instead of to
the whole Birding-aus list.
Cheers,
Stephen
Stephen Ambrose
Ryde NSW
-----Original Message-----
Sent: Thursday, 9 February 2012 12:27 PM
To: 'Andrew Hobbs'
Subject: FW: [Birding-Aus] Botulism poisoning in waterfowl
Andrew,
Putting more thought into your initial question, perhaps in the past there
hasn't been a need for waterfowl to develop a resistance to the Clostridium
botulinum neurotoxins. Before human reclamation of wetlands for agriculture
and urban development there were probably sufficient wetland areas for
waterfowl to retreat to in order to avoid localised Clostridium outbreaks.
Therefore, they displayed behavioural, rather than physiological/anatomical,
adaptations to neurotoxin outbreaks. But with widespread reclamation of
wetlands, waterfowl populations now occur at higher concentrations on
retained wetlands, their droppings and nutrient runoff from surrounding
agricultural and urban landscapes provide added nutrients for the bacterium,
thus allowing the bacterial colonies to grow. And there are fewer wetland
areas to which waterfowl can retreat to avoid Clostridium outbreaks. So
maybe large-scale exposure to neurotoxins by waterfowl is a relatively
recent phenomenon in evolutionary terms and there has not been enough time
for waterfowl to develop a physiological/anatomical immunity.
Additionally, there is always the danger of assuming that a species or
population has the evolutionary (genetic) capacity to develop an immunity or
insensitivity to neurotoxins. But maybe that genetic capacity cannot be
expressed within the evolutionary history of a species or population. For
instance, why have many small mammals not developed a resistance to the
venom of predatory snakes? Is it because they have not had enough time
evolutionarily to develop that resistance, or is it just genetically
impossible for them to evolve that physiological/anatomical immunity and the
best line of defence is to evolve behaviours that avoid venomous predators?
Anyway, just some initial thoughts which you may or may not like to
accept...
Cheers,
Stephen
Stephen Ambrose
Ryde NSW
-----Original Message-----
From:
On Behalf Of Stephen
Ambrose
Sent: Thursday, 9 February 2012 9:12 AM
To: 'Andrew Hobbs'
Cc:
Subject: Re: [Birding-Aus] Botulism poisoning in waterfowl
Hi Andrew,
The question I would be posing in the light of your query is how recently
evolved are the current molecular structures of the neurotoxins of the
Clostridium botulinum? Perhaps Clostridium botulinum is a recently evolved
bacterium (in the evolutionary history of waterfowl) and/or its neurotoxins
are recently evolved proteins. If so, perhaps there has not been enough time
for ducks and swans to evolve a mechanism of resistance to the neurotoxins.
Stephen Ambrose
Ryde NSW
-----Original Message-----
From:
On Behalf Of Andrew Hobbs
Sent: Wednesday, 8 February 2012 6:28 PM
Cc:
Subject: Re: [Birding-Aus] Botulism poisoning in waterfowl
On 8/02/2012 1:39 PM, Carl Clifford wrote:
Andrew,
The problem with Botulism is, that the cause of death is not from
infection by Clostridium botulinum, but from neurotoxins produced by
the bacteria. The botulinum toxins have LD50s of less than 100
nanograms per Kg. Hydrogen Cyanide has an LD50 of only 1-3g/Kg
In Thomas, Hunter and Atkinson's "Infectious Diseases of Wild Birds",
P. 405, they state that natural immunity does not play a significant
role in Botulism, because "The toxin is so poisonous that the amount
required to immunise an animal is much higher than the lethal dose."
They do say that it has been observed that naturally occurring
antibodies to several botulinum neurotoxins were found in several
carrion eating species of birds.
I wasn't actually thinking of acquired immunity. I was thinking more of
evolved immunity. Botulinum toxin (a group of 7 or 8 individual toxins
from /Clostridium botulinum and other related species./) and Tetanus
toxin (from /Clostridium tetani //) /are all proteins, and target
membrane proteins in their action. So mutants resistant to the toxins
should be possible. Indeed while humans, mice etc are very sensitive to
tetanus toxins, dogs, rats, some birds, snakes and amphibians are much
less sensitive and turtles apparently are insensitive to the toxin. Also
there is a range of sensitivities to botulinum toxins in humans with
some humans being insensitive (Schiavo et al Physiol RevJanuary 2000
vol. 80 no. 2 717-766 ). So it should be eminently feasible for ducks
and swans to have evolved immunity or insensitivity to the toxins too.
I suspect that such ducks and swans have been ferreting around in the
ooze for a long time so the question is still, why are they affected,
and has it changed recently. (however you define recently).
Cheers
Andrew
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