"Is Bovine Leukemia Virus in Milk Infectious?"
Decades ago concern was raised
that the milk of dairy cows
frequently contained a leukemia-causing virus,
but they were talking about bovine leukemia,
the leading cancer killer among dairy cattle.
Most U.S. dairy herds are infected with the bovine leukemia virus,
thus the question of whether they’re releasing infectious virus
into milk is an important public health consideration,
so researchers at U. Penn decided to put it to the test,
and indeed infectious virus was demonstrated in the milk
of 17 of the 24 cows tested,
indicating that humans are often
exposed orally to bovine leukemia virus.
But just because we’re exposed
doesn’t mean it’s causing human disease.
How do we even know BLV can even infect human cells?
We didn’t until 1976 when it was discovered
that bovine leukemia virus can indeed infect human,
chimpanzee, and monkey cells.
OK, but that still doesn’t mean it necessarily causes
cancer in other species, though.
You can’t lock human infants in a cage and feed them infected milk
but you can infant chimpanzees.
And chimps Bois and Roger, fed the infected milk,
developed leukemia and died….
We didn’t even know chimps could get leukemia.
The fact that BLV-infected milk appeared to transmit or induce
leukemia in our closest living relatives
certainly did raise the stakes,
or at least impugn the safety of steaks...
But human beings are not chimpanzees.
Yes, our DNA may be 98% identical,
but we may share 60% of our DNA with the banana.
We need human studies.
Thanks to the pesky Nuremberg principles
we can’t do interventional trials,
but what about observational studies?
Do cattle ranchers have higher rates of cancer, for example?
Apparently so.
Leading some to suggest that milk and egg-borne viruses
may be important in the development of
human leukemias and lymphomas.
But farmers may be exposed
to all sorts of potential carcinogens such as pesticides.
It’s like, YES, large animal vets have more leukemia and lymphoma,
but some were also particularly lax
in the use of x-ray protective equipment.
So, it didn’t necessarily have anything to do with animal viruses.
What we needed were so-called serology studies,
testing people's blood for antibodies against the virus,
which would prove human exposure...
and we got ‘em.
Ten different studies looking for BLV antibodies
in cancer patients and non-cancer patients,
creamery employees vs. office employees,
veterinarians, unpasteurized milk drinkers,
the whole gamut
and not a single study found a single individual
with antibodies to BLV.
So 1981, case closed; strong evidence BLV
is not transmissible to people.
But the strength of the evidence
is only as good as the strength of the test.
Bois and Roger didn’t develop detectable antibodies either...
and they died from it.
The tests back then were just really not sensitive.
Clearly the question of whether bovine leukemia virus
poses a public health hazard deserves a more thorough investigation
using highly sensitive molecular probes.
It would take a few decades but here it is,
a landmark study that we’ll cover, next.