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emerging diseases

Heikki Henttonen msehah at niini.metla.fi
Fri Oct 27 07:45:53 EST 1995

In article <199510241940.PAA12431 at gold.muskoka.com>, bmlsseng at MUSKOKA.COM (Dennis Denomy) says:
>These viruses have been around for years, but it is only when humans begin
>to intrude into new places, or the natural environment is altered, that
>these viruses leave their hosts and mix with the human population. 

Hantaviruses, for example, leave the host every time when the host is 
urinating. It has nothing to do with humans. It is the abundance of 
infected hosts which to a great extent determines the spread to humans. 
It should be kept in mind that environmental changes, due  whatever 
reasons, favor some host species but can as well disfavor other ones. 
So, due to environmental change some hidden viruses can become more common
when the host ecology changes, but some, now common viruses may as well 
disappear with their host. It seems that emerging diseases due to ecological 
damage is one of the slogans of the day.

 I have
>some evidence that this has already occurred in many areas:  Guanarito
>outbreak in Venezuala in 1989 in a rural community beginning to clear a
>forest;  Rift Valley Fever outbreak in 1987 after damming of Senegal River
>in Mauritania;  Rift Valley Fever outbreak in 1970 after Aswan Dam was built
>and Lake Nasser formed;  Hantavirus outbreak in New Mexico, Colorado and
>Nevada in 1993 after a rodent population exploded. 

Rodents increased due to the  weather conditions contributing to their 
food source. It had nothing to do with ecological damage. As far as I 
have heard, the local native people have known HPS disease for a long 
time; obviously these epidemics have occurred also earlier.

 Also Machupo, Junin,
>Sabia, Puumala 

The host of Puumala is the bank vole Clethrionomys glareolus, the most 
common rodent species in any forested habitats in Europe. It has regular 
density fluctuations (cycles) in northern Europe but more stable dynamics 
in central and western Europe. These differences in dynamics are reflected
in the pattern of human disease: regular epidemics in north coinciding
with rodent peaks but rare epidemics in more southern latitudes. All this
is natural, only reflecting the differences in rodent dynamics related to
the structure of animal communities and abiotic conditions. In fact, we 
have predicted that if climate change will ameliorate conditions in 
northern Europe,  the rodent dynamics will become more stable, and our 
hantavirus disease becomes less common. 
What I wish to emphasize is that many of the emerging diseases have 
probably had their dynamics for long, but we have only recently learned 
to diagnoze these diseases. To clump all emerging diseases with ecological
damage is not correct, even though some of them certainly have 
benefitted from human acticities favoring certain host species. It is also 
true that increasing environmental change in areas where the number of
potential hosts is the greatest, e.g. in the tropics, may release new
pathogens. Also changes in human behavior, partly due to population 
increase, sometimes because of war conditions, make people more 
susceptible. But, Nature is changing all the time, both due to man but also 
without him, and so do the animals species and their pathogens. 
Identification of the host species of "emerging diseases" and 
good ecological knowledge about the hosts helps us to understand also the
emerging diseases.

and Dengue Fever.  However, were there any similar reasons
>for the outbreaks of Ebola (Reston, Zaire and Sudan), Marburg and Lassa
>Fever, or do these hemorrhagic fevers just come out of nowhere?  

Nothing in nature comes from nowhere. 

Heikki Henttonen

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