[pct-l] Why Rockwell is largely wrong on Giardia

[Bruce 'Buck' Nelson]

Years ago I got Giardia at least twice, diagnosed by physicians, once
by a lab test. I carefully treated all water after that. Before my
2010 PCT hike I read the Rockwell paper. I trust science over
anecdotal evidence and the paper made sense. I usually didn’t treat my
water. Like many PCT hikers I contracted Giardiasis in the Sierras.

I took a closer look at the Rockwell paper and found numerous problems
with it. Firstly it is dated. What matters is what is in that water
bottle you just filled up, not what was found in a tiny percentage of
water sources 25 years ago.

Rockwell says it takes 10 or more cysts to become infected with
Giardiasis. That figure is the basis of many of his calculations. He’s
wrong. It has been estimated that one cyst has a 2% probability of
causing Giardiasis. The odds get worse from there. An infected person
can shed 1-10 billion cysts a day. Many animals, including marmots,
carry the disease. Even way back when the Rockwell water surveys were
done, Giardia was detected in 1/3 of the Sierra water sources tested.

Rockwell has a quote comparing the threat of Giardia to backpackers to
that of shark attack to beachgoers. Two studies I found that did tests
before and after backpackers and campers headed afield found that from
5.7-24% had picked up Giardiasis! (Many people are asymptomatic.)

One study of 691 individuals and a separate survey of nearly 300
outdoors enthusiasts each showed that the infection rate for
Giardiasis is TRIPLE for those that don’t treat their water vs. those
that do. Those studies of what IS happening trump speculation on what
SHOULD be happening.

Rockwell says “Almost always, Giardiasis goes away without treatment”
and “If you get a Giardia infection, you are unlikely to have
symptoms.” A Colorado survey of 256 infected people showed they were
sick an average of 3.8 weeks and lost an average of about 12 pounds.
No problem. You’ll feel better in a month!

I have read, over and over, that the odds of getting Giardiasis are
low and people hardly ever get tested for it. Real world experience
shows that many hikers get Giardiasis on the PCT and they are commonly
tested for it. Almost every year there is a spate of Giardiasis
somewhere north of Kennedy Meadows.

Quoting this thread on Backpackinglight: “What Buck said: ‘As I
related before, my last physician (Mammoth Lakes) said he commonly
treats backpackers with Giardia. He doesn't report it to the health
department. And he said something to the effect that as far as he
knows none of his colleagues do either.’
Exactly what the doctor told me at Stateline Medical center in South
Lake Tahoe.
He had treated many cases of Giardiasis and said it was not an unusual
event at the clinic.
One of my thru hiker friends in 2009 also tested positive for
Giardiasis at Mammoth Lakes and was told the same thing by the doctor
there. He had to wait 5 days for the test results!” Rare as shark
attack?

You will find Rockwell, Welch and others arguing that the risk of
Giardiasis from backcountry water is minimal, but that flies in the
face of the advice of nearly every health agency, such as the CDC.

Rockwell makes a big point of comparing the concentrations of Giardia
to those of municipal water sources, implying that since his (decades
old) findings show that Sierra water is often naturally cleaner that
it is safer. What he fails to say is that Sierra water is much more
likely to have CONCENTRATIONS of Giardia and that these cities TREAT
their water. For Giardia untreated Sierra water is MUCH more risky to
drink than treated city water.

The NOLS policy of good hygiene and treating all water has been very
successful and seems sensible to me.

My Giardia manifesto has more information and source links:
http://bucktrack.blogspot.com/2011/03/waterborne-giardia-for-backpackers-no.html