A horrendous disease, at one point called
"swine mystery disease," "blue abortion,"
and "swine infertility," and now referred to as
"Porcine Reproductive and Respiratory Syndrome,"
or PRRS, has infected
some pigs in about 75 percent of American pig herds,
according to experts. Vaccines have only partially been
effective.
The disease also has been creating a
nightmare for many other nations since at least the mid-1980s.
Pigs
Are Dying
The reproductive and respiratory syndrome,
which often kills baby pigs, is characterized by a variety
of conditions and is causing economic hardship for pig farmers.
Affected mothers lose up to 10 percent of their pregnancies.
Their babies are spontaneously aborted or are stillborn.
As many as 20 percent to 30 percent of survivors may suffer
and die from respiratory disease, such as pneumonia.
The PRRS virus is said to primarily
attack a pig's immune system, leaving the body
open to a host of other infections, particularly in the
lungs. Some pigs develop a chronic infection and become
carriers but show no symptoms.
Research reveals that the virus is transmitted
via semen, saliva and blood. Those pigs herded closely together
and transported at close quarters by trucks may be more
susceptible to infection.
To date, there is no
evidence that the virus can infect humans from
any source, including via food. Researchers looking for
signs of the virus in pig meat haven't found any.
Scott Dee with the University of Minnesota
College of Veterinary medicine, a PRRS researcher, says
the disease is the "most
economically devastating swine disease there is," and
that the "problem is getting bigger," but he bristles
a little when I introduce the term "AIDS" into
the conversation.
The PRRS virus, while not a member of
the family of viruses to which HIV belongs, is also said
to strike the immune system and then cause some changes
in the body that are similar to AIDS.
No
Virus Trace in People or Meat
Beth Lautner, vice president of science
and technology at the National Pork Producers Council, says
PRRS is a complex disease, acknowledges that some
of the symptoms are AIDS-like and notes that
no trace of the disease has been found in people or meat.
She worries that "some people will jump to the wrong
conclusion, that pigs gave AIDS to humans."
Monte McCaw, a PRRS researcher with
North Carolina State University's College of Veterinary
Medicine, believes that while the differences between PRRS
and AIDS are obvious to researchers, it is also important
to study the similarities. McCaw
has so far been low-key about his AIDS-related findings.
Some
Similarities to AIDS
McCaw has concluded that the following
key conditions in PRRS-infected pigs are similar to what
is found in AIDS:
-
Secondary
infections, mainly in the lungs, are common due to the
immune-suppressive abilities of the PRRS virus.
-
PRRS
reproduces in cells called macrophages, which are front-line
cells in the body's immune system.
-
PRRS
primarily reproduces in cells called alveolar macrophages,
which are immune cells in the lung. Damaging these lung
immune cells makes the animal susceptible to opportunistic
infections.
-
Key
white blood cells of the immune system (lymphocytes) go
through some of the same changes that occur in AIDS.
-
Lymphocytes
produce higher levels of a variety of biochemical substances,
as in AIDS.
McCaw adds, however, that some baby
pigs that manage to survive the onslaught of infections
in the lungs that the PRRS virus triggers end up thriving.
"This is an obvious difference
in the way [AIDS and PRRS] generally develop," he says.
But McCaw thinks enough of the similarities
between AIDS and PRRS that he indicated in a report that
he would like to study novel immune therapies in pigs. He
also said, "We would hopefully learn more about PRRS
in pigs and maybe AIDS in humans," and to possibly
"find ways to help treat both diseases."
A recent finding in Dee's camp should
further encourage McCaw's foray into the AIDS-like dimension
of this disease. "We have just learned that PRRS can
be transmitted from one pig to another via the repetitive
use of [vaccination] needles," Dee says.
Dee also says that PRRS hides out in
the lymph nodes. The same is claimed for HIV.
More Research
Necessary
McCaw, Dee and Lautner all agree that
much more research will be necessary in order to get a proper
handle on PRRS, in the hopes of better understanding and
controlling it. They point to the manner in which the PRRS
virus is capable of changing and the difficulty this creates
for vaccine strategies against the virus.
Much more will need to be learned about
how the pig's immune system behaves in PRRS. Will PRRS,
for example, be capable of unleashing some previously undetected
microbe in pigs that could potentially be transmitted to
humans?
Or, for that matter, is this pig disease
being oversimplified by focusing so much of the attention
on the PRRS virus? There may well be a number of factors
that initially combine to trigger PRRS. The disease has
often been referred to as a "mystery," and in
many ways it remains so.
While progress has been made, no one
should be overconfident about any aspect of this terrible
disease in pigs