Although the investigation concluded that HIV was transmitted from the dentist to his patients, exactly how the infections were transmitted was not determined [2, 3]. This prompted myriad theories in both the mass media and scientific publications [4-7]; for instance, lack of evidence notwithstanding, transmission by contaminated instruments has been widely postulated [8]. The "murder theory," which was highlighted in the television show "20/20" last year, has also received attention [4, 9].

A different question has recently surfaced. Stephen Barr, writing in Lear's Magazine and The New York Times [10, 11], and Mike Wallace, in a "60 Minutes" segment broadcast on 19 June 1994, both posed the question "What if the dentist didn't do it?"

Given the speculation surrounding this investigation, much of which has been based on misinformation and rumors, health care professionals and the public alike may now be understandably confused about the facts of this case. Did it really happen? If so, how? As the primary investigators of the case, we would like to address some of these issues.

The magazine and newspaper articles and the "60 Minutes" broadcast relied heavily on depositions related to private litigation generated by the case and on the findings of investigators hired by the dentist's insurance companies to find information that could cast doubt on the conclusion that the patients had been infected by the dentist. These findings were used by Barr and by "60 Minutes" to suggest that each of the six patients had other risk factors for HIV infection and that the results of the HIV DNA sequence analysis are inconclusive. However, they omitted important facts that contradict their conclusions.

Our investigation found that the Florida dentist infected six patients (A, B, C, E, G, and I) under his care. This conclusion was based on an epidemiologic and laboratory investigation that failed to identify other documented sources of HIV for these six patients and found that all six patients had HIV strains with DNA sequences that closely resembled the dentist's strain. Four other infected dental patients (D, F, H, and J) acknowledged other risk factors for HIV infection and had HIV strains that differed from the dentist's strain.

The epidemiologic investigations of the patients included interviews with the patients and their families and friends; discussions with their health care providers about possible exposures to HIV; review of medical, dental, and related records; and serologic testing of persons who had been sexual partners of the patients during and after the late 1970s, when HIV first began to spread in the United States [2]. To document another source of infection for any of the six patients, we would first have to show that they had received a transfusion with blood from a donor infected with HIV or that they had had sexual contact or had shared injection equipment with a person with HIV infection. If another potential source of infection were to be identified, the virus from that person would have to be sequenced and compared with the virus infecting the dental patient. However, no other source of infection was identified for any of the six patients.

"60 Minutes" suggested that Patient A was infected by a sexual partner. We were aware of the sexual activities discussed by Patient A during a taped deposition that "60 Minutes" obtained and aired. However, the television show omitted the important fact that Patient A's two boyfriends had tested negative for HIV.

In another instance, "60 Minutes" suggested that Patient B, an elderly woman, had been infected through an extramarital affair. However, our investigation showed that since the late 1970s, her only sexual partner has been her husband, who is not infected. "60 Minutes" also implied that Patient B received a blood transfusion during surgery in the early 1980s, even though we informed them that her hospital charts showed no record of any transfusion.

"60 Minutes" stated that Patient C had had multiple sexual contacts, including some with HIV-infected prostitutes and one homosexual encounter. During our investigation, Patient C denied having had homosexual contact but reported several heterosexual partners, none of whom he paid or knew to be infected with HIV. He could name 14 sexual partners since 1978; of the 9 who could be located, all tested HIV seronegative. An alleged male sexual partner also tested negative for HIV antibodies. Furthermore, "60 Minutes" produced no information about the similarity of the HIV strains from the prostitutes with whom Patient C allegedly had sexual contact and the strain infecting Patient C.

"60 Minutes" correctly stated that Patient E, a young woman, had initially believed she had been infected by a sexual partner. Two of Patient E's 10 known sexual partners since 1978 had died from non-HIV-related causes and were not known to have been at risk for HIV infection. The remaining 8 were tested for HIV antibody during the investigation; 1 was HIV positive. Patient E tested positive for HIV antibody in October 1988; the partner tested negative in October and in December 1988 but tested positive in December 1990, after an illness compatible with an acute retroviral syndrome in September 1989 [2]. He also had a different strain of HIV than did Patient E. Thus, Patient E's partner clearly was infected after Patient E and could not have been the source of her infection.

"60 Minutes" said that Patient G was an injecting drug user. Patient G had had several admissions to drug and alcohol programs dating back to 1986. We reviewed these records, which contained comprehensive drug and alcohol histories as well as results of physical examinations; they contained no evidence to suggest that the patient had used intravenous drugs since the late 1970s. The patient also denied having had sex with prostitutes since the late 1970s. He tested negative for HIV when he donated blood in 1986, and the two female sexual partners he reported since 1986 have tested negative for HIV.

"60 Minutes" mentioned that Patient I, a teenager, had had sex with several men and implied that she had been infected by one of these partners. Although Patient I did report having had sexual contact with six men, five of these six were seronegative for HIV. The sixth partner, with whom the patient reported having a single sexual contact using a condom, could not be located. As was the case for many of the patients in the practice, dental records documenting dates and types of treatment for Patient I were not available. During several interviews, the patient and her family members stated that she had visited the dentist on at least three occasions during the summers of 1987-1989. "60 Minutes" reported that records indicated that she made only one visit; however, their statement was based on insurance billing records, which do not document every patient visit.

"60 Minutes" implied that CDC either ignored or overlooked important information about the six dental patients. This is incorrect. We have previously released all information directly relevant to the investigation. We describe additional details in this paper only to indicate the depth of our inquiry. Maintaining patient confidentiality has always been an important priority for CDC and others involved in public health investigations. During sensitive investigations, patients are asked to reveal many personal aspects of their lives. Only information that is well documented and pertinent to the investigation is disclosed. For example, it would not be relevant to this investigation to disclose a sexual contact that had occurred years before HIV began to spread in the United States.

The CDC also based its conclusions on laboratory studies that showed that the DNA of the HIV strains infecting these six patients were closely related to the strain infecting the dentist. In an editorial in this issue, Dr. Gerald Myers discusses the usefulness of DNA sequencing data in epidemiologic investigations of HIV transmission [12].

The "60 Minutes" broadcast implied that CDC's DNA sequencing data were inconclusive. In saying this, they relied heavily on HIV DNA sequencing results obtained by Dr. Lionel Resnick [13], who served as a consultant to the dentist's insurance companies. In the "60 Minutes" broadcast, Dr. Resnick stated that he found other persons living in the community where the dentist practiced who were infected with HIV strains similar to the strains infecting the dentist and the six patients.

In the spring of 1994, all available sequence data from Dr. Resnick and CDC were independently reanalyzed by Drs. David Hillis and John Huelsenbeck from the University of Texas. In a recent letter to Nature, these authors note that the Resnick sequences support dental transmission to three of the patients (A, B, and E) even more strongly than suggested in the original CDC analysis [14]. Hillis and Huelsenbeck state that, at best, the Resnick sequences "cast doubt only on patient G." "60 Minutes" was given this letter but did not mention it in their report. Furthermore, none of the local controls were infected with HIV strains that were closely related to the strain infecting the dentist (Hillis DM. Personal communication).

At the request of the Human Resources and Intergovernmental Relations Subcommittee of the House Committee on Governmental Operations, the U.S. General Accounting Office reviewed the entire Florida dental investigation. Their report, published in September 1992, supported the conclusions of the CDC/HRS investigation [15].

In the "60 Minutes" broadcast, Mike Wallace asked us if there was "absolute certainty" that the dentist infected his patients. Although the nature of epidemiologic investigations precludes "absolute certainty," we feel that the scientific evidence overwhelmingly indicates that HIV transmission from the dentist to his patients did occur. However, we still do not know how this happened. There are three possibilities. First, the dentist may somehow have contaminated his dental instruments with his blood, failed to properly sterilize the instruments, and exposed the patients to the blood by reusing the contaminated instruments. We could not identify any breaches of infection-control practices that could indicate that transmissions by contaminated instruments occurred. According to his office staff, the dentist received treatment with his dental equipment on only one occasion, and they were not aware that he treated himself in his office [2]. Some believe that the likely culprit is a contaminated dental handpiece or prophylaxis angle. These instruments, if not sterilized properly, can theoretically transfer blood from one patient to the next. Thus, if the dentist had infected one of his patients with his strain of HIV, this strain could have been transmitted to other patients. However, an analysis of the patients' visits and procedures failed to show the pattern of visits and instrument use that would have been necessary for patient-to-patient transmission of HIV [16].

The second theory is that the dentist somehow injured himself and exposed the patients to his blood. Recent self-reported percutaneous injury rates among dentists indicate these injuries may occur four to six times per year [17]. However, most of the injuries appear to occur outside the patient's oral cavity. During our one interview with the dentist, he denied having had any needlesticks after he was diagnosed with HIV infection, and none of the staff or the six patients recall any injuries to the dentist.

The third theory is that the dentist intentionally transmitted these infections. Interviews with family, staff, health care providers, patients, and others have not provided any support for this hypothesis. The dentist readily agreed to provide a blood specimen for genetic sequencing. Most of the procedures done by the dentist were routinely observed by staff, all patients were awake during the procedures, and no unusual behavior was noted or suspected by either patients or staff [2]. On "20/20," an acquaintance of the dentist told Barbara Walters that the dentist had implied that he had intentionally infected his patients. However, this same acquaintance said during a sworn deposition that the dentist specifically did not tell him that he had, or was going to, intentionally infect patients.

Thus far, these six patients are the only ones known to have been infected with HIV by an infected health care worker. As time goes on, it becomes clearer that the risk for this type of HIV transmission is extremely small and that whatever happened in this dental practice was unusual. However, our inability to determine how these transmissions occurred does not mean that they did not occur.