The
HeV outbreaks have several features in common. All the index cases were
mares; in the Mackay and Hendra outbreaks, the mares were pregnant, and in
the Cairns outbreak, the mare had been pregnant in November but was without
foal upon necropsy in January. Two index cases had seronegative non-pregnant
companions; the companion of the third acquired infection by licking the
index case’s face. Two of the outbreaks occurred during the peak birthing
period of Pteropus alecto, P. poliocephalus, and P. conspicillatus. The
pathology of HeV in guinea pigs and P. poliocephalus suggests that the virus
has evolved to target the female reproductive tract. This evidence is
further corraborated by profiles of the index cases, the discovery of
vertical transmission in P. poliocephalus in the wild, and the pathology of
other paramyxoviruses (MenV, NiV, LPMV). Urine and saliva are unlikely to
play a large role: though NiV has been isolated in bat saliva and HeV in cat
and guinea pig urine, anti-HeV antibodies have not been found in any bat
carers, who would be expected to come into contact with these fluids. Urine
from infected bats did not infect horses in experiments, nor did in-contact
male P. poliocephalus infect one another in the lab. Non-vertical
transmission by bats may thus depend on consumption of leftover placental
tissue or contact with the female during parturition. A virus capable of
being transmitted vertically might be able to maximize horizontal
transmission by inducing abortions in some percentage of pregnant hosts.
Photo: K. Halpin.
Author: S. Cobey.