Vertical Transmission of Zika: Of Epidemiologic Significance?
Amesh A. Adalja, FACP, FACEP, FIDSA, September 2, 2016
The amount of scientific knowledge on the Zika virus is accumulating at a very high rate. The large ongoing outbreak in the Western hemisphere has transformed this virus from a travel medicine trivia question to a full-fledged public health emergency. As new facts about the virus are discovered, response plans and guidance change. A new study from the University of Texas Medical Branch in Galveston provides important new confirmation of the ability of the virus to persist in the environment.
Transovarial Transmission Confirmed
In this study, Thangamani and colleagues sought to determine if Zika, like many other mosquito-borne viruses, can be vertically transmitted from a female mosquito via her eggs to her offspring, a phenomenon known as transovarial transmission. To conduct this experiment, approximately 100 female Aedes aegypti and 100 female Aedes albopictus mosquitoes were injected with the virus and then fed a blood meal. The eggs that were subsequently laid were then studied after hatching for the presence of the virus in the offspring that resulted.
Of the 69 pools tested, which contained a total of 1,738 first-generation A. aegypti mosquitoes, 6 pools were positive for the virus by immunofluorescence. The results translated to an infection rate of 1:290. None of the A. albopictus progeny were positive for the virus.
The Role of Vertical Transmission
The finding that vertical transmission occurs with Zika virus is not surprising. Many viruses are capable of being transmitted in this manner, including viruses in the same family as Zika, such as dengue, West Nile, and yellow fever. What remains to be determined is what role such transmission plays in the epidemiology of outbreaks of this virus. Though viral amplification in primate hosts is likely the major conduit for the spread of this virus, environmental persistence might occur via vertical transmission, leading to low-level endemicity in areas of the world in which A. aegypti mosquitoes flourish, even in the absence of susceptible primate hosts, who may have been rendered immune through prior infection. It will be important to confirm these laboratory findings in the field in the Western hemisphere. It would also be interesting to sample mosquitoes in areas of Africa where Zika has circulated for decades, sparking immunity in the primate population (humans included), to assess the longevity of vertical transmission without amplifying primate hosts.
Thangamani S, Huang J, Hart CE, et al. Vertical transmission of Zika virus in Aedes aegypti mosquitoes. Am J Trop Med Hyg. Published online August 29, 2016. http://www.ajtmh.org/content/journals/10.4269/ajtmh.16-0448. Accessed August 31, 2016.