Hendra virus (HeV) is a zoonotic paramyxovirus harboured by Australian mainland flying foxes from which it is believed to be transmitted directly to horses. HeV first emerged in 1994 in Hendra, a suburb of Brisbane, Australia, leading to an outbreak of acute respiratory disease in 21 thoroughbred horses, of which 14 died [1–3]. During this outbreak, two horse trainers also became infected with the virus, one fatally. In horses, HeV causes a severe, often fatal, febrile illness associated with respiratory and neurological signs . Since its emergence in Queensland, Australia in 1994, HeV spillover from flying foxes to horses has regularly recurred: with an increase in disease events occurring over the past two years . Thirty-nine disease events have occurred resulting in the death or euthanasia of seventy-six horses and one dog, with a case fatality of 75% in the horses.
HeV is the prototype species of the genus Henipavirus, within the subfamily Paramyxovirinae[6, 7]. Nipah virus (NiV) is the only other virus officially classified within the Henipavirus genus. NiV was first identified during a major outbreak of acute respiratory disease in pigs in peninsular Malaysia in 1998–99. Over one million pigs were culled in Malaysia to prevent the continued spread of the virus. Over 265 farm and abattoir workers exposed to infected pigs were infected by NiV, resulting in a total of 105 deaths in Malaysia and Singapore [8–10]. Since the outbreak of disease in Malaysia and Singapore, NiV re-emerged in Bangladesh in 2001, with continued re-emergence and human cases almost annually since in Bangladesh and sporadically in India [11, 12]. Differences in transmission have also been observed between the Bangladesh and Malaysian strains of NiV. NiV Bangladesh has been shown to cause direct bat-to-human transmission without the involvement of an intermediary or amplifying host. Human-to-human spread of NiV in Bangladesh has also been documented [12–15]. Because of the broad host range and the high mortality rates associated with these viruses, both HeV and NiV have been classified as a biosafety level 4 (BSL-4) agents.
Recently, we described a novel paramyxovirus, Cedar virus (CedPV), which displayed many of the distinguishing characteristics of the henipaviruses, including similar genome length and organisation, it displayed antigenic cross-reactivity with henipaviruses and used the same host cell molecule (ephrin B2) as a receptor for entry during infection . Interestingly in preliminary animal challenge studies, CedPV did not cause disease in ferrets and guinea pigs, both of which are susceptible to fatal disease by the henipaviruses. In addition, a near full length genome sequence has been described for a bat-borne virus from Ghana, Africa , that shows around 50% sequence identity with the henipaviruses, including CedPV. Henipa-like viruses have also been detected serologically in bats in Thailand , China , Madagascar  and West Africa , with successful virus isolation obtained from Lyle’s flying foxes in Cambodia .
Reverse genetics of negative strand RNA viruses allow for the creation of recombinant infectious and replication-competent viruses with specific mutations or insertions. Often, researchers have inserted the green fluorescent protein (GFP) gene into such viruses allowing for the real-time monitoring of virus replication and spread, either within cell culture or in vivo within an infected host. The expression of GFP allows for the detection of virus infection in tissues without the need for antibody-specific detection methods. The generation of recombinant henipaviruses will be an extremely powerful tool to monitor viral infections both in real-time for imaging and in a high-throughput approach for screening activities. They will also play a pivotal role in our understanding of pathogenesis of henipaviruses at the molecular level through the generation, rescue and testing of specific mutation variants. Rescue systems have previously been reported for NiV [23–25], here we report the generation of recombinant HeV in which the GFP (HeV-GFP) or firefly luciferase gene (HeV-Luc) has been inserted as an additional transcriptional unit between the P and M genes, and we assess their biological characteristics both in vitro and in vivo.