Nematodes immune to anthelm. ?

Dan Holdsworth drh92 at aber.ac.uk
Thu Jan 23 13:52:42 EST 1997


In article <5c3n9l$j4k$1 at mhadf.production.compuserve.com>, Paul
<100526.3050 at CompuServe.COM> writes:
:Can anyone tell me why nematodes are becoming immune to 
:anthelminphic drugs ?

Same reason that bacteria are becoming resistant to antibiotics;
same reason that insects become resistant to pesticides. 

Natural selection. 

Of course, it's not helped by ignorant fools thinking on the lines of
"Oh Bother, my drug has stopped working at dose x. Let's try dose 2x and
hope thatthat works".

All this sort of behaviour does is select for even MORE resistant individuals
in the population; what you should do when you encounter resistance is
think "Oh Bother, my drug isn't working. Hey, I'd better find out how this
drug works, then find another that works in a completely different way, and
then switch to the completely different one for a while".

I grant you the reasoning is a little complex; basically you don't want
resistant individuals to survive. The way it works is that usually, the 
wildtype of the pest (or whatever) is as fit as it can be. 

The resistant type is often slightly less fit, and in the absence of the 
pesticide would not be able to compete with the wildtype. The resistant
form may also be even more susceptible to other forms of attack than is the
wildtype.

So, once you see resistance, you MUST change the pesticide to something
with a VERY different mode of action, and eradicate the resistant pests if
you can. Otherwise, just keep changing pesticide from time to time. This
alters the selection pressure, and this strategy is the optimal one for
keeping resistance from occurring.

There is also one final trick that can be pulled on resistant animals. It
was founbd in the cotton growing industry that various boll-eating insects
were a problem. Cotton is unusual as a crop in that you can wash it when you
have harvested it, so it doesn't much matter what kind of vile concoction 
you spray with, you can remove it later.

Cotton pests have come to be so resistant to the pesticides, and so otherwise
crippled that they positively NEED the farmers to spray in order to kill 
their natural enemies. When biocontrol was tried on cotton a few years ago,
it was found to be a resounding success, since the pests seemed to have lost
a lot of the natural resistance to predation. Working from this, it seems
sensible to let the natural predators have a crack at the pest every so 
once in a while; in effect you would be using natural predators as just 
another form of pesticide.

I hope this helps somewhat.

It is by caffeine alone I set my mind in motion, it is by the beans of Java
that thoughts acquire speed, the hands acquire shaking, the shaking becomes a
warning, it is by caffeine alone I set my mind in motion.
Dan Holdsworth, drh92 at aber.ac.uk **SPAMMERS WILL BE FILTERED**



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