Vector-borne Transmission Zika virus was maintained in Africa in sylvatic cycle between non-human primate and mosquito Aedes africanus with a cyclic epizootic in monkeys. In places where there is no non-human primates are present, the virus is maintained through man–mosquito–man cycle. In Asia, sylvatic cycle does not exist and virus maintenance occurs trough transmission between man–mosquito–man.
The virus was first detected in Aedes africanus mosquito in Africa. Subsequently it has been isolated from various other species, such as Aedes polynesiensis, Aedes luteocephalus, Aedes vittatus, Aedes apicoargenteus, Aedes furcifer, Aedes albopictus and Aedes aegypti. Vector competency has been experimentally proved in A. aegypti. In Yap state, A. henselii was the primary vector.
Non-Vector-borne Transmission
Mother to Fetus Transmission
Transmission of Zika virus from infected mother to fetus can be transplacental, perinatal or through breast milk.
Perinatal transmission has been reported in other flaviviruses such as dengue virus and West Nile virus. In two cases, perinatal transmission of Zika virus has been reported where Zika virus RNA was detected in serum of mother and infants and mother’s milk.
Transplacental transmission was first noted during the outbreak in Brazil when babies of infected pregnant women were born with severe malformation. This was confirmed by the detection of Zika virus RNA in amniotic fluid, blood and brain tissues of the neonates born with severe microcephaly and died soon after birth.
Zika virus particle has been demonstrated in the breast milk but transmission through breast milk has not yet been documented.
Sexual Transmission
Zika virus RNA has been detected in semen of patients during acute and convalescent phase of illness. Several cases of sexual transmission have been reported in various outbreaks.
Blood Transfusion
During the French Polynesia outbreak, 3% of the donor’s blood was found to be positive for Zika virus RNA. Transmission through blood transfusion is not yet documented. However, considering the epidemiology of other flaviviruses and asymptomatic infection in a large number of individuals, the possibility of transmission through blood transfusion remains for Zika virus. Figure 1 depicts the various modes of transmission of Zika virus.
Fig1. Transmission of Zika virus
