Modest Proposal

An0nYm0Us UsEr nobody at vox.xs4all.nl
Mon Oct 3 16:17:32 EST 1994


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I am forwarding some recent postings from bionet.population-bio that show that
the Modest Proposal isn't as far-fetched as some people on this newsgroup seem
to think:

(forwarded text follows)

Dear networkers,

Recently, there has been much debate on this list on viral-vectored
immunocontraception. In the last issue or Journal of Applied Ecology, N.D.
Barlow discusses this issue as a mean of controlling possums in New
Zealand.
I think that such methods of population control are interesting, but the
risk of spread to a non-targetted species should always be kept in mind. If
this is put in practice for mammals such as rabbits, foxes or possums,
would mean that applying to humans will eventually occur.

It seems that this issue is not as much 'only theoretical' or fiction, as
some have thought it is...


Here the summary and two references from this paper :

Predicting the effect of a novel vertebrate biocontrol agent: a model for
viral-vectored immunocontraception of New Zealand possums

N.D. BARLOW
Biological Control Croup, AgResearch, Canterhury Agriculture & Science
Centre, PO Box 60, Lincoln, New Zealand

Journal of Applied Ecology (1994) 31, 454-462

Summary

1. A model is described for predicting the outcome of biological control of
New Zealand possums, using viral-vectored immunocontraception based on a
sexually transmitted herpes-type vector.
2. The model shows that success is possible in ecological terms, and
identifies the probable circumstances under which it is achievable. These
provide targets which a genetically modified virus must meet, aiding in the
quest for suitable vectors and appropriate genetic modifications to them.
3. In particular, the female reproductive system rather than the male's
should be targeted, contact rate (i.e. number of potentially infectious
contacts) per possum carrying the virus (i.e. infected, not infectious)
must be in the order of one or more per year and at least 75% of females
carrying the virus must be sterile at mating. Achieving this incidence is
critical, with only slightly lower values exerting a disproportionately
lower effect on possum densities.
4. Spatial aggregation of the viral vector and the existence of a recovered
and immune class of possums would both reduce substantially the impact of
the control agent, but the presence of even limited vector-induced
mortality would dramatically enhance it.
5. Immunocontraception is likely to confer a selective advantage on the
engineered virus by allowing multiple matings for an affected female. This
will raise the contact rate and prevalence, and allow the vector to compete
successfully with any existing wild strains.
6. Given an appropriate mechanism of action on the reproductive system,
viral vectored immunocontraception of this kind would, if successful, offer
a uniquely acceptable control for a vertebrate pest such as the possum,
being humane, species specific, cost-effective and environmentally benign.
7. Even if immunocontraception caused only limited suppression of an
otherwise uncontrolled population, it could contribute to successful
integrated control by greatly reducing the need for conventional poisoning
operations. Such integrated control also reduces possum densities more
rapidly than would occur with immunocontraception alone.

References

Bradley, M.P. & Reed, K.C. (1990) Fertility control of
animal populations: a prospective study of gonads,
gametes, genes. Proceedings of the Fertility Control in
Wildlife Conference. Melbourne, November 199O.

Tyndale-Biscoe, H. & Jackson, R. (1990) Viral-vectored
immunocontraception: a new concept in biological
control of wild animals. Proceedings of the Fertility
Control in Wildlife Conference, Melbourne, November
199O.



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In article <Pine.A32.3.90.941001130103.194017B-100000 at acc.wuacc.edu>,
zzdecell at ACC.WUACC.EDU (decelles paul) writes:
> Aside from Mr. Nobody's application of virus techniques for population
> control for humans, I also wonder about using these methods in general.
> I'm not familiar with new zealand possums, but the possums we have here
> hardly ever rise to the level of a pest species. Maybe an occasional
> nuisance but hardly one that would merit advanced biotechnology to
> control.
>
> Is it worth applying this quick techno-fix to these critters or is there
> some massive overriding public health need to control possums in this way?
> Please, perhaps you or someone else from down below can enlighten us.
>
> Dr. Paul Decelles

To expand on my previous aside about possums in NZ (although I'd describe
myself as a microbial ecologist / modeller, so it's not really my field)...

I guess possums were introduced for a fur trade, our only native mammal is a
small (and now threatened) bat.  They are insidiously and rapidly destroying
our native forests (the bits we haven't burnt down, that is).  Insidious and
rapid isn't a contradiction in terms, they're targetting more palatable
species
(generally the same ones our native birds depend on), so the forest doesn't
disappear, but aerial views show rapidly spreading death of certain species.
They have also recently been observed predating native bird chicks, and are
considered a potential vector for bovine TB (this may actually be a Good
Thing,
if money (cows) instead of just the environment is involved, something may
actually get done).

Shooting is not particularly effective, bounty systems naturally target areas
where high kills are easiest, as opposed to areas most needing relief.
Trapping has I think been successful on some small off shore islands (or
maybe
that was rats), but on the mainland I think they're breeding too fast for
that
to help.  There is a controversial but apparently effective program with
aerial
drops of the poison 1080 (sodium monofluoroacetate).  It's controversial
because it can kill native birds, but the numbers involved seem very low, and
destruction of habitat (by possums) must surely be the greater threat to
birds.
Increased rates of successful bird breeding (many of our native birds are
threatened) have been reported in areas targetted with 1080, although this
effect probably isn't highly replicated yet.

That leaves biocontrol.  Koala chlamydia has been tested unsuccesfully, but
there are other potential sources of diseases that might prove useful.
there's
also a long term project to develop an oral vaccine to make females reject
sperm.  And this other program to genetically modify a possum disease to
spread
this immunity to sperm.  Unfortunately it isn't a quick techno-fix, it's all
very much in early development stages at present.

We also have an unpleasant synergistic (sp?) relationship between rabbits and
an introduced weed the rabbits wont eat, between them they're wiping out our
native grassland ecosystems.  In general NZ's geographic isolation has
resulted
in a naive, or ancient, animal population that is very vulnerable to
introduced
mammals.  Many of our birds are flightless, our substitute for a mouse was a
giant insect that can't compete with rats, the stoats etc. released to remedy
the rabbits have more impact on the native birds, and of course our giant
landsnails and other unusual old form macro invertebrates are severly
depleted.

Hopefully if anyone want's any more information a real animal ecologist will
contribute.  Somehow I doubt the term 'extinction' has ever been applied to a
bacterium :-)

Cheers -Terry Brown

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