Zoonotic infections, which transmit from animals to humans, form the
majority of new human pathogens. Following zoonotic transmission, the
pathogen may already have, or may acquire, the ability to transmit from
human to human. With infections such as Lassa fever (LF), an often
fatal, rodent-borne, hemorrhagic fever common in areas of West Africa,
rodent-to-rodent, rodent-to-human, human-to-human and even
human-to-rodent transmission patterns are possible. Indeed, large
hospital-related outbreaks have been reported. Estimating the proportion
of transmission due to human-to-human routes and related patterns (e.g.
existence of super-spreaders), in these scenarios is challenging, but
essential for planned interventions.
### Methodology/Principal Findings
Here, we make use of an innovative modeling approach to analyze data
from published outbreaks and the number of LF hospitalized patients to
Kenema Government Hospital in Sierra Leone to estimate the likely
contribution of human-to-human transmission. The analyses show that
almost 20% of the cases at KGH are secondary cases arising from
human-to-human transmission. However, we found much of this transmission
is associated with a disproportionally large impact of a few individuals
('super-spreaders'), as we found only 5% of human cases result in an
effective reproduction number (i.e. the average number of secondary
cases per infectious case) >1, with a maximum value up to 12.
This work explains the discrepancy between the sizes of reported LF
outbreaks and a clinical perception that human-to-human transmission is
low. Future assessment of risks of LF and infection control guidelines
should take into account the potentially large impact of super-spreaders
in human-to-human transmission. Our work highlights several neglected
topics in LF research, the occurrence and nature of super-spreading
events and aspects of social behavior in transmission and detection.
Iacono, L.; Cunningham, A.A.; Fichet-Calvet, E.; Garry, R.F.; Grant, D.S.; Khan, S.H.; Leach, M.; Moses, L.M.; Schieffelin, J.S.; Shaffer, J.G.; Webb, C.T.; Wood, J.L.N. Using Modelling to Disentangle the Relative Contributions of Zoonotic and Anthroponotic Transmission: The Case of Lassa Fever. PLoS Neglected Tropical Diseases (2015) 9 (1) e3398. [DOI: 10.1371/journal.pntd.0003398]
Using Modelling to Disentangle the Relative Contributions of Zoonotic and Anthroponotic Transmission: The Case of Lassa Fever