Bats
Opportunistic sampling by Dr. Field and colleagues found that 25-30% of flying foxes (including representatives from all four species in mainland Australia) had antibodies to HeV (Field pers. comm.). Positive samples were obtained from bats along the whole of the eastern coast of Queensland, from Cairns to Brisbane. Titers ranged from 1:5 to 1:640 (Young et al. 1996). Retrospective surveys have identified HeV antibodies in sera collected in 1982, the earliest sample tested (Field et al. 2001). Virus has been isolated from each species (Halpin et al. 1996). Seroprevalences appear to differ consistently among species, even when taken from individuals sharing the same camps: seroprevalence is around 10% in P. scapulatus but closer to 50% in P. alecto and P. poliocephalus (Field pers. comm.). It is interesting to note that P. scapulatus tends to remain clumped in shared camps, while P. alecto and P. poliocephalus intermingle. P. alecto and P. poliocephalus also have overlapping reproductive schedules, while that of P. scapulatus is offset by half a year (Field pers. comm.).
Horses
No HeV antibodies were found in a survey of 4000 horses (Field et al. 2001). Another seroprevalence survey on the Mackay index farm yielded negative results. This study tested the 98 horses present on the property at the time the mare and colt died, 35 horses on 13 surrounding properties, 136 horses on traceback properties, and 124 horses from traceforward properties (Ward et al. 1996). A survey of 2024 non-racing horses from Queensland also did not yield evidence of HeV infection (Ward et al. 1996). Retrospective investigation of diagnostic laboratory records by Ketterer and Hooper failed to supply evidence of past HeV infection of horses (Field et al. 1999).
Other animals
In May and November 1995, 5264 sera from 46 species, including rodents, marsupials, birds, amphibians, and insects were serosurveyed near Brisbane and Mackay. Some of these individuals were sick and injured wildlife supplied by wildlife carers and veterinarians. There was no evidence of HeV infection in any of the animals (Barclay and Paton 2000). Another serosurvey of over 5000 domestic animals, including 4000 horses and cattle, dogs, cats, and poultry found no HeV antibodies (Field et al. 2001). The Queensland Department of Primary Industries and University of Queensland supplied 500 cat sera from metropolitan Brisbane, which were all negative (Westbury et al. 1995).
In May 1999, five samples from each of 100 “finisher” herds (pigs’ mean age 6 months) around Queensland all tested negative for HeV antibodies (Black et al. 2001).
Between February 17 and 22, 1999, sera from 52 insectivorous bats (Mormopterus loriae), four rodents (Rattus rattus), and 4000 mosquitoes were trapped around the Cairns outbreak site were negative for HeV infection using a direct ELISA (Field et al. 1999).
Pooled samples of Cyclopodia albertisii (family Nycteribiidae), an obligate blood-sucking ectoparasite of pteropids, have been negative for HeV under PCR (Field et al. 2001).
Humans
Selvey et al. (1996) conducted a serological survey of 128 bat carers and found no evidence of infection with HeV. Serology surveys in Mackay and Brisbane, each conducted within a month of the local outbreak, of 298 individuals with close contact to horses and/or the three infected humans found no HeV antibodies (McCormack et al. 1999).
From bats
Transmission between four infected and four uninfected, in-contact male P. poliocephalus was not demonstrated (Williamson et al. 1998). Halpin et al. (2000) posit that the high seroprevalence of HeV in bats indicates efficient transmission, e.g. direct contact with aborted fetuses and birthing fluids, and that virus is excreted almost entirely through this route. More spillovers and seropositive bat carers would be expected if the virus could be efficiently transmitted through urine or feces. They also note that the HeV outbreaks occurred during the birthing season of P. poliocephalus, P. alecto, and P. conspicillatus. Williamson et al. (1999) suggest that horizontal [sic] transmission may occur from licking of female genitalia during parturition, which has been observed. Vertical transmission was demonstrated experimentally (Williamson et al. 1999) and naturally in P. alecto (Halpin et al. 1996).
Transmission from bats to horses has not been demonstrated, though Williamson et al. (1998) added the urine and feces of infected bats to the horses’ water supply.
From cats
Westbury et al. (1996) showed that HeV could be transmitted from infected to non-infected cats through undetermined route(s), but transmission was unlikely. Transmission occurred in only one of two pairs of cats kept in the same cage and not between cats in adjacent cages. Cats have infected horses in the lab; 12 infected cats housed with three uninfected horses caused one to contract disease (Williamson et al. 1998).
From other animals
Experimental transmission between horses or from horses to bats has not been accomplished. Williamson et al. (1998) found high HeV titers in equine bladder urine post-mortem but were unable to isolate virus from fresh urine swabs from the stall floors.
Vertical transmission appears possible in many animals. Williamson et al. (1999) observe that guinea pig, human, and fruit bat placentas are all haemochorial, having one layer of trophoblasts in direct contact with maternal blood.
Author: S. Cobey.