The possibility is discussed in a just-released report by Dr Nigel Perkins, a veterinary epidemiologist, into Hendra cases in Queensland in July, which occurred in Brisbane and Proserpine.
The worst of the outbreaks was at Brisbane's Redlands veterinary Clinic and resulted in two human cases - one of which proved fatal for 33-year-old veterinarian Ben Cunneen. In total, eight horses either died from the infection or were euthanised.
"Information collected during the response indicates that infected horses may shed Hendra virus while in the incubation stages of disease, before they develop clinical signs of illness that could be attributed to Hendra infection," Dr Perkins said.
"The possibility of apparently healthy, incubating horses posing exposure risk to people is important information in the development of effective recommendations for risk management to minimise the risk of exposure for all people dealing with horses," he said.
Before the Redlands cases it was suggested or assumed that human exposure was most likely as a result of very close contact with an obviously sick or dying horse or through performing invasive procedures such as a post mortem on a Hendra case, he said.
It was possible that one of the human infections could possibly have occurred while performing procedures on a horse that was not at that time displaying signs of illness that could be attributed to Hendra.
"This information is considered to be very important for all people involved with horses - horse owners, carers, riders, farriers, dentists, veterinarians and others - since it indicates that there may be risks involved in interacting with horses that are not displaying obvious signs of disease," Dr Perkins said.
"It is also very important to place this information in context to avoid alarming people unduly and to provide information on risks and risk management that allow people, including veterinarians, to be informed and to take suitable precautions.
"Hendra virus in horses is a rare disease. The risks of human infection appear to be very low for people who are engaged in routine horse-handling and care."
That said, there may be certain procedures associated with elevated risk, most likely involving exposure to fluids or tissues that may contain virus such as blood, nasal secretions, oral secretions and urine.
This may have implications for the protective equipment worn by veterinarians when exposed to equine body fluids, "and reiterates the need for continued information and education of the general horse community and veterinarians".
It was important, he said, that all individuals and industry groups that have an interest in horses participate in the development and implementation of education and awareness programmes.
Dr Perkins, whose report was tabled in Queensland's parliament on Thursday, found that the response of the state's Department of Primary Industries was both rapid and effective after veterinarians at both outbreak locations had expressed their concerns.
Dr Perkins recommendations include:
Native fruit bats are understood to be the natural reservoir for the Hendra virus and can occasionally infect horses, probably through bodily discharges on to pasture or into water troughs.
While humans are able to catch the virus from horses, there has been no reported human-to-human transmission.