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Published in the
Bulletin of Experimental Treatments for AIDS Spring 2000 issue,
by the San Francisco AIDS Foundation.
 Special
Report on HIV & AIDS
HIV is the Only Cause of
AIDS: The Potential for Journalism
to Impact the Public Health
Spring
2000 Table of Contents
Main Page
beta@sfaf.org
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HIV Causes AIDS
Proof Derived from Koch's
Postulates
Tim Teeter
The recent surge of publicity denying HIV's causal role in AIDS begs
an examination of the relationship between a specific microorganism
and a specific disease. The question is this: What scientific proof
is required to establish cause and effect?
HIV and AIDS-Related Diseases
AIDS is defined by the Centers for Disease Control and Prevention (CDC)
as the presence of a positive HIV antibody test and one or more
of the illnesses known as opportunistic infections (OIs) or a
CD4 cell count below 200 cells/mm3. AIDS wasting syndrome, which technically
is not considered an OI, and three cancers--non-Hodgkin's lymphoma,
Kaposi's sarcoma (KS), and cervical carcinoma--are AIDS-defining in
the presence of a positive HIV antibody test. HIV and AIDS have
been continuously linked in time, place, and certain population groups
(e.g., children born to HIV-infected mothers) since 1984, the year HIV
was discovered. Besides millions of cases of AIDS, some people receiving
cancer chemotherapy develop AIDS-like illnesses. And in people receiving
organ transplants, the immune system is deliberately suppressed and
AIDS-like illnesses can result. Neither of these two examples, however,
results in an AIDS diagnosis. Moreover, those illnesses resolve when
the causative medications are stopped.
Recent improvements in laboratory testing, particularly polymerase
chain reaction (PCR) testing, allow detection of HIV genetic material
in people with AIDS or a positive HIV antibody test result. Interestingly,
a 1995 study of 230,000 people with AIDS found that 168 were HIV-seronegative.
Today it is known that the immune system depletion characteristic of
advanced HIV disease can suppress the production of antibodies to HIV,
which may explain this earlier finding.
The vast majority of people with AIDS in resource-poor countries have
not had an HIV antibody test; even in developed countries, not all people
with AIDS have had viral load tests. Those who have had viral load tests
showing nondetectable virus generally have no additional tests to confirm
the presence of HIV genetic material.
However, the above facts, while strongly suggesting a relationship
between HIV and AIDS, do not by themselves establish a causal relationship.
Koch's Postulates
The German scientist Robert Koch (1843-1910) established four requirements,
or postulates, that must be demonstrated to prove that a specific microorganism
causes a particular disease:
- The microorganism must be found in all cases of the disease.
- It must be possible to isolate the microorganism from the host
and grow it in pure culture (in the laboratory).
- The microorganism must reproduce the original disease when introduced
into an experimental animal.
- The microorganism must be recoverable from that animal.
The scientific research community widely accepts that fulfillment
of these postulates demonstrates causality. Even scientists such as
Peter Duesberg, PhD, who says he believes that HIV does not cause AIDS,
accept that the postulates put forth by Robert Koch are sufficient to
establish causality. While in 1987 Dr. Duesberg noted that HIV did not
fit all of Koch's criteria, developments in HIV/AIDS research in the
13 years since that statement have shown that HIV does indeed fulfill
Koch's postulates as the cause of AIDS.
Conclusive Data from Reputable
Scientific Research
Regarding postulate one, PCR testing allows researchers to document
cell-associated proviral HIV in persons with AIDS who have been tested
(proviral DNA detection is a research test, not one of the common FDA-approved
viral load tests). Prior to this technology, HIV was often difficult
to find. In addition, combining PCR testing with the common viral load
tests has documented the presence of HIV genes as RNA freely floating
in the blood plasma, outside of cells, in persons with a positive antibody
test not taking anti-HIV medication. (Viral load testing looks for virus;
the ELISA and Western blot tests look for antibodies to HIV.)
Regarding postulate two, improvements in laboratory culture techniques
have allowed the growth of HIV in vitro (in laboratory models)
from blood samples obtained from persons with AIDS who have undergone
such testing and from almost all persons with a positive antibody test
without AIDS who have undergone such testing.
The last two postulates stipulate that inoculating the organism into
an animal model (i.e., exposing or infecting the animal) leads to the
same disease and that the organism is recoverable from that animal.
The evidence satisfying these postulates was established in 1997, when
Francis J. Novembre, PhD, and colleagues from Emory University in Atlanta,
GA, published in the Journal of Virology that a chimpanzee inoculated
with HIV ten years earlier had developed an AIDS-defining OI. Prior
to the OI, the HIV RNA viral load had increased (partially documenting
recovery of the organism from the animal model) and the CD4 cell count
had decreased in the chimpanzee. Cultures of blood from the animal also
were positive for HIV, establishing recoverability of the organism.
Subsequently, blood from that chimp was transfused into a second, healthy
chimpanzee. This second chimpanzee later had an increase in the HIV
viral load and a decrease in the CD4 cell count.
Prior to this 1997 report, fulfillment of Koch's
third and fourth postulates was lacking. Interestingly, the incubation
period for clinical AIDS in this chimpanzee, with whom humans share
98% gene homology (structural similarity), was essentially equivalent
to the average incubation period in humans--ten years. This finding
and publication were reported in the September
1997 issue of BETA. While evidence from one chimpanzee may
not seem compelling to the lay person, in the scientific arena and in
conjunction with other, cumulative data, it is considered persuasive.
A good source of information on this topic is www.niaid.nih.gov.
Ancillary and Epidemiologic
Supporting Evidence
Several reports document the transmission of the organism to a human
host that reproduces the original disease, and the subsequent recovery
of the microorganism in that second person. For example, at least
three laboratory workers developed AIDS after accidental exposure
to concentrated HIV in the laboratory. All three developed immunosuppression
and related opportunistic diseases, including Pneumocystis carinii
pneumonia (PCP), following infection. In all three cases, HIV was
isolated, sequenced, and shown to be the infecting strain of the virus.
In addition, the development of AIDS following known HIV seroconversion
has been repeatedly observed in widely diverse populations, including
each of the following:
- pediatric and adult blood transfusion cases
- hemophiliacs who received infected blood clotting factor protein
infusions
- monogamous sexual partners of those transfusion recipients
- health-care workers with accidental needlestick or other occupational
exposure, similar to the lab technicians described above
- mother-to-child transmission
- male-to-male and male-to-female sexual transmission
- injection drug users with secondary sexual transmission
- extremely rare outbreaks including transmission to dental patients
from an infected dentist (e.g., the Kimberly Bergalis case in Florida)
In conclusion, although the specific molecular mechanisms of
HIV's causative role in AIDS are not yet completely understood,
Koch's postulates have been fulfilled, thus establishing causality.
Tim Teeter is Associate Director of
Treatment Support and Publications at the San Francisco AIDS Foundation.
Related BETA Articles: "Special
Report on HIV & AIDS"; "HIV
is the Only Cause of AIDS: The Potential
for Journalism to Impact the Public Health"
Selected Sources
Burnett, M. and White, D.O. Natural History of Infectious Disease.
Cambridge University Press, Cambridge. 1972.
Cohen, J. Fulfilling Koch's postulates. Science 2666(5191):
1647. December 1994.
National Institute of Allergy and Infectious Diseases (NIAID) Fact
Sheet. The evidence that HIV causes AIDS. July 1995.
Novembre, F.J. and others. Development of AIDS in a chimpanzee infected
with human immunodeficiency virus type 1. Journal of Virology
71(5): 4086-4102. May 1997.
Weiss, R.A. and Jaffee, H.W. Duesberg, HIV, and AIDS. Nature
345(6277): 659. June 1990.
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last updated 2 June 2000
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