Ebola Transmission Studies

[MHSC]

The reviewed studies show a low risk of transmission in the early phase of symptomatic patients, even if high risk exposure occurred. However, risk of transmission may increase in later stages of the disease with increasing viral titres [19] and increased viral shedding.

In a household study, secondary transmission only took place if direct physical contact occurred [20]; In an outbreak in 2000 in Uganda, the most important risk factor was direct and repeated contact with a sick person’s body fluids, as occurs during the provision of care. The risk was higher when the exposure took place during the late stage of the disease. However, one case was probably infected by contact with heavily contaminated fomites, and many persons who had had a simple physical contact with a sick person did not become infected. Therefore transmission through heavily contaminated fomites is apparently possible [21]. In summary, physical contact with body fluids seems necessary for transmission, especially in the early stages of disease (as is likely in passengers still able to travel on a plane), while in the later stages contact with heavily contaminated fomites might also be a risk for transmission.

http://www.biomedcentral.com/1471-2458/12/1014/

[MHSC]

Re: Ebola Transmission Studies

Regarding indirect transmission, sleeping on the same mat (PPR = 2.78, 95% CI 1.15 to 6.70), participating in the ritual handwashing during the funeral ceremony (PPR = 2.25, 95% CI 1.08 to 4.72), and sharing a communal meal during the funeral ceremony (PPR = 2.84, 95% CI 1.35 to 5.98) were significantly associated with disease. Although the differences were not statistically significant, sharing meals, washing clothes, and sleeping in the same hut were associated with a higher risk of acquiring the disease.

...

By contrast, simple physical contact with a sick person appears to be neither necessary nor sufficient for contracting EHF. In fact, one person in whom the disease developed was probably infected by contact with heavily contaminated fomites (patient 7), and many persons who had had a simple physical contact with a sick person did not become infected.

Transmission through contaminated fomites is apparently possible. In fact, the association found for having slept on the same mat or having shared meals with a sick person or with funeral participants remained after controlling for direct contact. However, having washed the clothes of a sick person and having participated in the ritual handwashing during the funeral ceremony were not significant risk factors.

Finally, although we cannot exclude the possibility of airborne transmission, this mode probably plays a minor role, if any. In fact, the association between having slept in the same hut and acquiring the disease was weak and could have been produced by some unidentified confounding variables. Furthermore, the reported Ebola virus aerosol transmission among nonhuman primates (17,18) has been demonstrated in laboratory experiments, which may be irrelevant in the natural context.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3035551/

[MHSC]

Re: Ebola Transmission Studies

Of particular concern is the frequent presence of EBOV in saliva early during the course of disease, where it could be transmitted to others through intimate contact and from sharing food, especially given the custom, in many parts of Africa, of eating with the hands from a common plate. However, the isolation of EBOV from only 1 saliva specimen, in contrast to the 8 that were RT-PCR positive, could suggest that the virus is rapidly inactivated by salivary enzymes or other factors in the oral cavity that are unfavorable to virus persistence and replication. EBOV has been previously documented in saliva by RT-PCR, but no attempt was made to culture virus or to explore the temporal dynamics of virus shedding in that study [12]. Marburg virus, the other member of the Filoviridae family, has been isolated as well as detected by RT-PCR in saliva from a patient with a fatal case of Marburg hemorrhagic fever in the Democratic Republic of the Congo (authors' unpublished data). The higher mortality among patients with RT-PCR-positive saliva likely reflects increased virus shedding in patients with high viremia, which has been previously noted to be an indicator of a poor prognosis [9, 11].

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As indicated by RT-PCR and ELISA antigen results from blood (data not shown), the shedding of EBOV in saliva corresponded almost exactly to the period of viremia, with the last positive saliva specimen noted at day 8 after disease onset. In contrast, specimens of breast milk and semen were found to be culture positive and RT-PCR positive at days 15 and 40 after disease onset, respectively, when EBOV was already cleared from the blood.

http://jid.oxfordjournals.org/content/196/Supplement_2/S142.full

[JJackson]

Re: Ebola Transmission Studies

MHSC I had read the Oxford Journals paper previously and was more than l little surprised by the data in table 1. On reading the rest of the paper I found the authors were also skeptical.

There was a significant discrepancy between the results of virus culture and RT-PCR testing in our study, with many more frequent positive results from RT-PCR. Possible explanations for this finding include virus degradation from breaks in the cold chain during sample collection, storage, and shipping; the greater sensitivity of RT-PCR relative to culture; and, in the case of the saliva specimens, possible virus inactivation by salivary enzymes. The less-than-ideal storage conditions of the specimens in the isolation ward immediately after acquisition and the fact that even the nasal blood from 1 patient was culture negative suggest that some virus degradation indeed occurred. Nevertheless, we cannot exclude the possibility of a true absence of viable virus in the original samples. We hope to be able to repeat this study in the future with better maintenance of the cold chain to resolve this question.