Controlling M Worm
Recently, there has been a lot of controversy about the use of
doramectin (trade name of Dectomax) vs. ivermectin (brand name of
Ivomec) for controlling meningeal worm.
This worm, sometimes called M worm or brain worm, is prevalent in the
eastern USA where both white tailed deer and a moist environment for
slugs/snails is prevalent. This worm, in order to be infective to
alpacas, MUST reside within snails/slugs for part its developmental
cycle. It is from the snail/slug slime trail that the infective M worm
arvae come from.
is sufficient. Rather, the issue must be to keep out migrating snails
and slugs (almost impossible) - or use a preventative drug treatment.
Remember that LOTS of areas have white tailed deer, but not all areas are conducive for slugs and snails. You need both and generally, east
of the Mississippi River is M worm territory for sure. There are also
a few areas west of there that are also moist (eastern Texas for
example) and have M worm issues.
The issue of rational worm prevention is not a static one; it changes as
we realize implications of our actions. Recently, there have been
changes in recommendations.
However, the implications are important. If you live in an area where M
worm is present, then not using preventive treatment is identical to not
using a monthly drug to prevent dog heartworms. Some owners will not
give the dog monthly heartworm medication and the will survive. Those
owners proudly proclaim that "drugs are not necessary to control
heartworm!" Sadly, both dogs and alpacas die every year as well
intentioned (but misguided) owners do not like to use "drugs" in their
For example, years ago when doramectin (brand name of Dectomax) came
out, I was all for it. Compared to ivermectin, it had a longer lasting
blood profile, caused less injection pain and had the potential for
lasting more than 30 days. Seemed great! It was a lot more expensive
than ivermectin, but the benefits seemed substantial.
I "converted" to the use of doramectin four or five years ago, but did
not think of the implications of inadvertently overexposing gut worms
to a wormer.
We have now come to realize that doramectin does NOT last longer than 30
days and the prolonged activity is not what we need to prevent M worm or
to minimize parasite resistance.
Here are some facts, interpretations and management implications on preventing M worm.
- Prevention of M worm uses an injected avermectin (class of drug that includes vermectin and doramectin) once per month.
- The dose of injected avermectin varies from one ml per 60 lbs to one ml per 100 lbs depending on the vet you ask.
-Injected preventive dose avermectins kill M worm ONLY when it is
migrating from gut to spinal cord. Once the parasite is in spinal
cord, the treatment must shift to other drugs (expensive).
-M worm travels from gut to spinal cord and transit time takes some
-M worm cannot reproduce in camelids.
-M worm cannot develop any resistance to an avermectin as M worm cannot
reproduce in camelids.
-Gut worms (not M worm) develop resistance to avermectins when they are
continuously exposed to the anti-parasitic wormer.
-Ivermectin has a very short plasma "half live" (less than two days);
doramectin has a very long "half life" many times that of ivermectin.
-Once a migrating M worm is dead, it doesn't need to be killed again.br> Interpretations:
M worm is especially vulnerable to an anti-parasitic drug as it travels
between gut and spinal cord. One use of an anti-parasitic would be to
expose this worm to a drug that kills it all the time. The problem
with that approach is that by continuously killing the M worm, you
expose other gut parasites to the continuous exposure to a drug that
directly fosters their resistance to those anti-parasitic drugs.
We are realizing that more and more alpacas and llamas are dying from
drug resistant gut worms/parasites. We need to change our approach.
If you kill the migrating M worms just once per month on a day or two,
then none of that "30 day migration cycle" can get into the spinal cord
and cause damage as they are dead. Giving multiple doses of wormer
every day, all month long doesn't do "more" as the M worm when dead is
dead. Giving a dose of doramectin is like "giving ivermectin every
day" as doramectin lingers and has a long residence in the body. But
doramectin needs to be given every thirty days to replenish the supply
in the blood stream.
So . . . . once per month ivermectin kills the migrating M worms and it
does not expose the gut worms to prolonged doses of wormer. Once again,
we are back to Dr. Steve Purdy's (director University of Massachusetts
Camelid Program) recommendation that ivermectin once per month (every
30 days) all year round is the best thing.
Giving a monthly shot is a pain - literally a pain for your alpaca! But
it is critical to do this. Miss the killing vulnerability "migration"
30 day window means that the M worm can now enter the safe haven of the
spinal cord where ivermectin or doramectin at preventing doses cannot
kill it. Now it chews on neural tissues and nerves slowly killing the
alpaca. Very expensive and time consuming treatment is now necessary to
save the alpaca. Some will not recover.
This is a terribly painful disease with the alpaca often not being able
to get up and then often dragging around a non-functioning limb. The
momentary shot is well worth protecting from the disease.
Giving the prevention shot is the proverbial "stitch in time saving
nine". I suggest picking the first Saturday or Sunday of every month
and using that as your herd health day. Give shots, record body
scores, trim toenails and go over every single alpaca. This approach
is easier than calculating the exact 30 day time every month and having
this rotate when some months have more than 28 days.
In summary, we have an increased appreciation for handling M worm. Next
month, we will consider rational gut worm prevention.