Re: H1N5 (and Bakker's virus extinction hypothesis) now H5N1

[Dann Pigdon <dannj@alphalink.com.au>]

On Tue, 16 May 2006 19:57:09 +0200, Tommy Tyrberg wrote
> At 04:36 2006-05-15, you wrote:
> 
> >This is of no surprise, since apparently the fleas that spread BP to humans
> >only do so below a certain temperature threshold. Above that threshold the
> >fleas can continue to feed off rats with no trouble. Below a certain
> >temperature, the plague pathogen causes clotting in the gut of the flea,
> >causing it to slowly starve to death no matter how much it tries to feed. 
In
> >this situation the flea is more likely to feed off any animal (rather than
> >their usual target species) out of sheer desperation. Healthy fleas usually
> >prefer feeding on only one specific host species.
> >
> >So the lower the temperature gets, the more likely infected fleas are to 
try
> >feeding on human blood (or so the theory goes).
> 
> No, normally rat fleas only abandon the rats when they die...

Or when the flea's proventriculus becomes blocked due to the action of the 
Y.pestis bacterium at low temperatures. Fleas themselves are usually immune 
to the bacteria since their body temperature is around 26 degrees C (the 
bacteria only become virrulent at 37 degrees C, once inside a mammalian 
host). However Y.pestis can only incubate in a rat flea if the air 
temperature is at least 21 degrees C for about 2 weeks. Hence it seems that 
each plague outbreak in colder climates was introduced from elsewhere, rather 
than being endemic to the affected region. Once inside a warm-bodied host, 
however, the bacteria can multiply with abandon and spread via direct or 
indirect contact.

Apparently some African rats manage to survive with plague-infected fleas. If 
those rats get accidently transported to colder climates (the ivory trade has 
been blamed for some Roman plague outbreaks), the bacteria start to clot in 
the rat fleas' guts, so no matter how much they try to feed they still 
gradually starve to death. No doubt they abandon their original hosts and try 
to feed from a succession of local rats (which lack immunity). As those all 
die off, the fleas may try to feed off other species in desperation (such as 
humans). Generally peaking, each species of flea prefers only one species of 
host. For them to try to feed off of another host is not their usual 
behaviour.

Or it may be that at certain temperatures, fleas simply don't transmit the 
bacteria even to their rat hosts. Maybe the blocking of the flea's 
proventriculus increased the chances of it infecting a host (an attempt to 
ingest blood with a blocked gut may cause it to regurgitate blood while 
feeding). That's just a guess on my part though. Is there an epidemiologist 
in the house?


>  temperature is not the key factor. However there is very good 
> evidence that bubonic plague in Europe almost always occurred in the 
> autumn, though nobody knows why.

Decreasing air temperatures perhaps... 

> That there was a very large volcanic eruption (or possibly, but more 
> unlikely, an impact) in the mid 6th century is well documented. The 
> dimming of the sun is actually mentioned by Prokopios, who also is 
> our main source for the Justinian plague. It is also mentioned in 
> Chinese sources and a possible link to the plague has been discussed 
> before, but it has proven difficult to find a causal link.

Such dimming of the sun due to constant cloud/dust cover tends to decrease 
temperatures... :)

Regardless of the specific factors involved in the spread of bubonic plague 
to humans, I think the thing that is relevant to dinosaurs is that most 
pandemics tend to occur when a pathogen leaves its usual host for something 
new (which has no prior immunity). What factors could have lead to such a 
senario at the end of the Cretaceous? It would seem that most major faunal 
exchanges occured well before the end-Maastrichtian (tyrannosaurs, 
hadrosaurs, ceratopians, etc from Asia to N.America is one example), and they 
didn't result in a global dinosaur pandemic (although extinctions of a few 
local species almost certainly occured).


--
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Dann Pigdon
GIS / Archaeologist         http://heretichides.soffiles.com
Melbourne, Australia        http://www.geocities.com/dannsdinosaurs
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