A cloudburst threatens to ruin your summer hike. You dart
under a tree for protection from the rain and break out a granola bar. You
decide to wait it out, but after a few minutes, your skin starts to itch and your eyes sting. After a few more minutes, you notice a rash on your arms
and your throat feels like it's closing. Is it bad granola? Is it acid rain?
Are you going to die?
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The
manchineel tree is toxic enough that just touching it can
do
you serious harm. The sign should say, “Don’t even get
near
me!” because dripping sap can be just as bad. On the
right
you see the apples of the manchineel. They look good,
they
are sweet, and the price is right – and then you die.
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The latex that oozes from this tree contains the toxins
hippomanin A and B. Both toxins are present in the latex, leaves, bark, wood,
roots, fruit, flowers, and nectar of the manchineel. Eat it or rub against it,
and you get sick. Cut it up and the sawdust makes you sick; get it in your eyes
(even the smoke from burning it) and you can go blind!
Most deaths have occurred from eating the apple of the
manchineel; hence the common name for the tree – the Death Apple. Your mucosal surfaces
blister, your larynx swells shut, your GI system rebels loudly and explosively.
Massive hemorrhage can follow the closing of your throat, so you drown in your
own blood.
In the 1500’s, South American Indians threw death apples
down their own wells to poison the invading Spanish conquistadors. It worked because this
toxic plant is an exception; it is sweet. Most often, plant toxins taste bitter
and that's how we know to avoid them. The death apple’s taste prevents
us from making a judgment that could save our life. Definitely, this is a plant
to be respected and feared.
Now that I have your attention, let’s talk about a seemingly
more common plant toxin. Urushiol is the name for the offending group of
molecules in poison ivy, poison oak, poison sumac, even more exotic plants like
mango, lacquer trees and cashew nuts.
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Mangoes
(left) and cashews (right) are in the same family
of
plants as poison ivy and poison oak. They contain
urushiol
and can cause significant damage to those who have
a
heightened allergic response. Urushiol is present in the
skin
of the mango fruit and in the shell of the cashew nut.
This
is why you can’t buy cashews in the shell, the shell
must
be removed to make them safe. Cutting a mango will
give
you a small exposure, but most people tolerate this well.
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Urushiol toxicity comes from the immune reaction it
generates in about 60-80% of the population. However, urushiol doesn’t spark an
immune response on its own. It turns your body against itself. Immune responses
are aimed at antigens (not born of, so not self), but urushiol
breakdown products are haptens (to fasten to); think of them as half
antigens. Haptens must combine with something else to become full antigens. In
the case of urushiol, they combine with proteins from our own cell membranes.
When portion of the urushiol combines with the integral protein,
now the protein is seen as foreign and your immune system might start to
attack, in a process called type IV delayed hypersensitivity. This produces
inflammation and tissue damage in a reaction termed allergic contact dermatitis.
 |
Allergic
contact dermatitis is different from irritant contact
dermatitis
in that irritants damage the skin directly, while
allergens
invoke an immune response that causes the
damage.
The hapten, in the case of urushiol, penetrates and
is
modified by Langerhan cells. The lymphocytes are
exposed
to the modified urushiol + membrane protein and
initiate
a response. The activated T cells then circulate and
react the next time the urushiol is touched. |
Not everyone’s immune system recognizes or overreacts to the
hapten + membrane protein, so not everyone gets a rash from poison ivy – lucky
devils! Other permutations are possible as well. You can be resistant and then
develop an allergy late in life, or you can have contact dermatitis when young
and later on become resistant. We know a lot about allergic hypersensitivity,
but there's also a lot we don’t know. Much research is underway on plant
toxins and allergens.
And herein lies the rub - pun intended - with many toxic
plants. They cause pain, damage and irritation, yet Paracelsus said, “only the dose makes the poison.” Does that mean that a lower dose has no effect? Well,
most of our medicines – antibiotics, anti-cancer, anti-depressive - come from
fungi and plants. It isn’t just that less may not be harmful; less might
actually be helpful!
Take urushiol for instance. A 2011 study shows that urushiol
can kill H. pylori, the bacterium
that causes many stomach ulcers. Within 10 minutes, urushiol can strip the
membrane off of the bacterium and cause it to lyse. Traditional treatments were
found to eradicate the disease in 75% of cases, but adding urushiol brought a
100% cure rate. It even worked in a mouse model, but no one asked the mice if
their stomachs itched. Even hippomanin A is an inhibitor of herpes simplex virus 2 replication. It seems that every toxic plant we talk about here has
some medicinal use – nothing and nobody are completely evil.
There are several plants that could wrestle for the title of
most toxic, but anyone’s top five contenders would have to include oleander (Nerium oleander). You can die just from
eating honey collected from bees that landed on the plant and partook of the
pollen or nectar.
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Nerium oleander is a bush like plant that
can have red or
white
flowers. It is deadly, but is repairing its reputation
by
being used as a medicine. Oleander is the official flower
of
Hiroshima, Japan, as it was the first flower that grew
after
the atomic bomb was dropped. In Texas, oleander is
used
as a decorative plant in road medians – don’t mess
with
Texas medians!
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A 2012 study has shown that oleander distillate is
therapeutic in diabetes. Rats with induced diabetes were treated with oleander
extracts for 12 weeks. Those treated rats had better blood sugar levels, reduced
insulin resistance and cholesterol, and improved insulin levels. Not only was
the diabetes positively affected, but fat levels were also positively affected
– all through treatment with a lethal poison.
But wait - there’s more! Oleandrin has been show to be
effective in inhibiting cancer. In at least five different kinds of cancer,
oleandrin can stop cancer cells from increasing in number (proliferating), and
can even induce the cancer cells to kill themselves (apoptosis). That’s a good
start, but it gets better.
A 2005 review discusses the idea of resistance to treatment
that develops in many cancers over time. Wouldn’t it be great if we had
something that could make the cancer cells sensitive to the drug treatments
again? Well, this review discusses studies that show oleandrin can do just
that. Oleandrin acts not only as a chemosensitizer, but makes cancer cells more
sensitive to radiation therapy. Therefore, oleander is synergistic with other cancer
therapies and makes them work better.
Can you stand any more wondrous uses for this poison? A more
recent study indicates that oleandrin reduces infectivity of HIV. AZT, a
traditional HIV drug, reduces replication but not infectivity, while oleandrin
reduces infectivity but not replication, so they could work together. Oleander can save you from infectious
diseases, cancers, and metabolic diseases – but eat the berries on your next
hike and you’ll die a horrible death.
So humans are animals that can’t just willy-nilly start
munching on toxic plants. But other animals can. We have talked about animals
that use the toxins they eat (2˚ toxin sequestering), usually from either
insects or plants. But is there an exception – does any animal sequester a
toxin that its prey sequestered from a plant? I looked for one. |
Both
the cinnabar caterpillar (left) and the moth (right) are
brightly
colored. This is called aposematism, warning predators
that
they are toxic. It usually works, but the common cuckoo is
apparently
opposed to aposematism; it has learned to avoid the
most
toxic portions of the larvae and adult.
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There is a cuckoo that eats the cinnabar moth caterpillar that
eats toxic ragwort. The plant has alkaloids. In the stomachs of most animals,
they are quickly converted to toxins. But a 2012 study shows that the cinnabar
moth caterpillar’s enzymes can convert the metabolic products back to their
non-toxic alkaloid forms. Then they are ready to poison the unwitting animal that
eats the caterpillar and hasn’t had the forethought to evolve a detoxification
process!
However, the common cuckoo (Cuculus canorus) avoids the toxins in
the cinnabar caterpillar by biting off the head of the larvae and discarding
it, then shaking the carcass to expel the liquid toxin. This is like how some
birds can eat monarch butterflies. Monarchs are toxic, having sequestered
milkweed toxins they ate as caterpillars. Shining cuckoos in New Zealand and
some North American birds know to get rid of the most toxic portions and just
eat the rest. Therefore, these birds are not 3˚ toxin sequesterers. I couldn’t
find an example – can you?
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The
monarch caterpillar eats only milkweed – ONLY milkweed.
This
is where is picks up its toxins. The adult will drink nectar
of
many flowers, but the toxin is maintained as the larvae
metamorphed to the adult.
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Most birds just stay away from monarchs most of the time, but even this has weirdness associated with it. Monarchs lose toxicity as they age, and males usually have less toxin than females, yet somehow the birds can sense it. Research has shown that monarchs with higher levels of toxin are less likely to be attacked by a predator. How do the birds know?
We have just touched the surface of toxic plants; there are
more than we can mention. In Australia alone there are said to be over 1000
toxic plants! Let’s next look to our exceptions; plants that aren’t just toxic,
they’re venomous.
For
more information, see:
Plant
toxins –
