Biology concepts – neural parasitology, domoic acid
toxicity
Don’t scoff at this; nature is full of examples where one
organism can cause another organism to change its behavior – just think of all
the silly things boys do trying to impress girls. But first a couple of stories
where a change in behavior has less to do with parasitism.
In August 2013, residents of Moscow began reporting that the
pigeons were acting odd. They would walk around in a funk, not get out of the
way of traffic, and not fly away from danger. One family reported that their
dinner one evening was disrupted by a pigeon on their window ledge that lost
its balance and fell into their kitchen.
These zombie pigeons (the pecking dead, as one website
called them) were freaking out the population, so the scientists went to work.
It seems that many of the dead and affected pigeons were carrying salmonella bacteria
and/or had Newcastle disease. The virus that causes this disease,
unimaginatively called the Newcastle disease virus (NDV), can be transmitted to
humans, so it's a good thing the population got freaked out.
The virus causes the birds to stagger about, stumble around
in circles, and turn their heads upside down – much like vodka does in humans.
However, when humans get NDV, they most likely will just have a flu-like
episode.
Zombie birds also led to a famous movie. Alfred Hitchcock’s
classic film, The Birds, is the story of a terrifying attack on a small fishing
village by many flocks of different birds. They attacked people, flew into car
windows and houses, and caused deaths and damage. Seems silly doesn’t it, being
killed by a shore bird? I think I would have them give some other reason in my
obituary.
The
sea lions in 1995 and 2010-11 were acting like zombies as well. They wouldn’t
get out of the way of boats or they would come up on land and just keep
scooting inland until they died.
In 1987, it was recognized that a toxin produced by certain
species of marine algae was responsible for the zombie like behaviors. Called domoic acid, the toxin is produced by
the algae and accumulates in marine organisms that feed on phytoplankton or
algae that are contaminated. Normally, levels of domoic acid are too low to
cause problems, but in years where the algae overgrows, called a bloom, the
levels will rise dramatically.
Although the acid seems to have no affect on lower life like
shellfish, bigger animals are strongly affected, including humans. When the sea
lions or birds feed on contaminated food, they begin to display the bizarre
behaviors. In the case of the 1961 birds, there happened to be a collection of
samples from the bay that had been kept all these years. Tests on the shellfish
and algae samples from 1961 showed high levels of domoic acid.
In a strange coincidence, a new paper has been published
about how infections can move through a flock of birds. It uses a mathematical
model based on many predictors and factors. The model is called the Zombie-City model, based on how a
zombie population might grow in a population of unsuspecting humans. But we
want to focus on the loss of free will in nature’s creatures.
Free will in lower animals? It does exist. Most people believe that the behaviors of
insects and such are merely responses to environmental and situational cues,
and any variation in behavior is due to misreading of cues or random errors. But studies in
fruitflies show that they can pick out their own patterns of behavior when a
blank canvas is given them.
Ampulex compressa) and the American
Cockroach (Periplaneta americana).
The wasp
lives in Africa and Asia, so this isn’t something we could use to get rid of NYC
cockroaches. P. Americana isn’t even native
to the Americas. It was introduced from Africa as early as 1625, before it was
officially named.
What the wasp steals is the roach’s ability to decide if it wants to
walk or run. Most wasps sting to kill, but the Jewel Wasp stings the cockroach
in the brain, altering its behavior with its venom. A 2010 study showed that
the wasp stings the roach continuously for up to three minutes, trying to
locate a particular part of the cockroach’s brain.
What it is searching for is called the subesophageal ganglion, the part
of the brain that allows the roach to initiate walking and running movements.
When that part of the brain is flooded with venom, the cockroach stands still,
with no will to begin leg movements. It isn’t paralyzed – it’s just a zombie.
Another study has started to investigate just how the wasp venom robs
the cockroach of its will to walk. There is an insect neurotransmitter called
octopamine that is released by some of P.
Americana’s neurons. It is this transmitter that allows the cockroach to initiate
walking.
The study hasn’t pinpointed just how the venom interrupts the octopamine
signaling, but they know if they deplete the amines in the brain, they see the
same affect. If they add back octopamine, they can rescue the cockroach’s
natural behavior. However, the study also showed that the venom doesn’t reduce
octopamine levels and it doesn’t prevent is release, so there's still more
work to be done.
Why does the wasp turn the cockroach into a zombie? I’m glad you asked. Remember,
the cockroach isn’t paralyzed, it just hasn’t the will to walk on its own. So
the wasp tugs on the cockroach’s antennae and herds the roach into its
underground nest. There the wasp lays an egg in the cockroach’s abdomen and the
emerging larva feeds on the cockroach until they are ready to emerge eight days
later.
So why not just kill the cockroach with the sting and lay the egg? The
larva need fresh meat, and a dead cockroach rots in one day. To make the meal
satisfactory for the eight days needed, the cockroach must remain alive, but in
a state where it can’t attack the wasp or the larva; hence the zombification.
It gets even creepier. The wasps have gotten so good at this strategy
that they now go to the trouble of cleaning their meal. A 2013 study shows that
the wasp larvae produces several antimicrobial chemicals that rid the cockroach
of any contaminating bacteria or parasites as the larvae munch on it. I know I’d
clean a zombie before I ate it.
There are several other examples of theft of free will, including a couple
of fungi that make ants stop their normal work and climb high in trees to allow
for the best spread of the fungal spores as they mature. There’s also a
hairworm that forces grasshoppers to commit suicide by jumping into water, just
so the worm can complete its life cycle. But I don’t want to leave this subject
without hitting the king of neural
parasitology – Toxoplasma gondii.
T. gondii is a
single-celled eukaryotic parasite that has a complex life cycle. It can reproduce asexual
in any of the hosts it infects, but can only reproduce sexually in cats, of all
things. This is important because sexual reproduction is an obligate life cycle
stage for the parasite and contributes to its evolutionary health.
The parasite has taken steps to insure that it finds its way into cats
by changing the behaviors of the mice and rats it finds itself inside. It messes with rodent brain chemistry (since it tends to form cyst organisms in
the brain) that makes rodents unafraid of cats. In fact, a recent study found
that the organism confuses the rodents into believing that cat urine smells
like a potential mate!
T. gondii
activates a certain neuronal transcription factor, which leads to increased
production of different proteins in the brain. In rodents and humans, this
leads to an increase in dopamine (similar to octopamine in the cockroach) production and a decrease in tryptophan usage.
Because the cysts target areas of the rodent brain that
control fear, the change in behaviors are involve, but are not limited to fear.
There isn’t any evidence that the cysts have a selective range in the human
brain, but considering the changes that occur in men as described below, it is a possibility.
a 2011 study showed that infected men are more attracted to cat urine, while
infected women find it less attractive.
In general, men with a long-term T. gondii infection show lower IQs,
are taller (about 3 cm on average), and are more likely to break rules, take
risks, be jealous, and exhibit anti-social behaviors. Right now – I’m not so
proud to be a guy.
On the other hand, women with long-term toxoplasmosis infections tend to
be more outgoing, friendlier, more promiscuous, and more attractive to men.
Wow, Mars and Venus to the nth degree! The question still remains – how do the
forced behavior changes in humans benefit the organism?
Next week, we return to our discussion of nucleic acid exceptions
by discussing instances where organisms can rewrite their genetic code.
For
more information or classroom activities, see:
Moscow
zombie pigeons-
Domoic
acid toxicity –
Jewel
wasp and cockroach –
Toxoplasma gondii –
Fungal
parasites and ants –
Hairworm
and grasshopper -
Zombie
apocalypse case study for class -
Infected women are also reported to be more intelligent than non-infected women, making the total alleged effects pretty much a wash. But there must be huge numbers of confounding factors, even if studies only involve Americans and not, as has happened in the past, comparisons of people from different cultures. The 3-cm height "improvement" in men is suggestive. It's not obvious how T. gondi infection might cause that. On the other hand, if you eat a lot of meat while growing up you are likely to be both larger and much more likely to acquire T. gondi. Might parents who equate a good life with heavy-duty meat-eating perhaps be less intelligent than average, or more likely to raise their kids with other lifestyle or dietary variations, e.g. bottle rather than breastfeeding to name just one of dozens, that could affect cognitive abilities? Might men who are inclined to eat a lot of meat also, for whatever reason, be more inclined to "break rules" or "take risks"? (Also note the enormous value judgement hidden in the assumption that those are bad things!)
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