Christine King BVSc, MANZCVS (equine), MVetClinStud
Hendra: why not just go ahead and vaccinate?
So, what are the risks and benefits involved here?
First, let’s talk about the risk posed by HeV itself. In horses, disease caused by HeV has a high mortality rate (reportedly about 75%) — in identified cases. However, we don't know the actual mortality rate, because some of the documented cases of HeV infection in horses were euthanised for public health reasons. Who knows how many of those horses would have survived and recovered?
Although we've been living with HeV for at least 25 years now, we still don’t have much idea about how many horses have been exposed to the virus without becoming ill or who showed only mild signs of disease (most likely attributed to some other cause) and then recovered.
In other words, we don’t know much about individual susceptibility to this disease nor about how prevalent naturally acquired immunity may be in the horse population.
Except for when HeV first made an appearance in 1994, there have been no large serologic surveys of horses in areas where HeV outbreaks have occurred (coastal Qld and NSW), so we don’t know how common it is for horses to develop natural immunity without showing signs of HeV infection or only mild signs that are attributed to some other cause.
From the limited data that are available, horses do develop natural immunity following HeV exposure which may be expected to be protective against future exposure.
The question of individual susceptibility is even harder to get a handle on. Flying foxes infected with HeV rarely show signs of infection, whereas experimental infections in ferrets (a species that is highly susceptible to HeV) cause a massive systemic inflammatory response, similar to that seen in horses and humans who have succumbed to HeV infection.
However, according to the Queensland government, "[s]everal hundred people have been exposed to Hendra virus infected horses but have not been infected." Does that mean they did not come in contact with the virus or that they encountered the virus but did not become ill? Either way, might the risk of this virus to horses and humans have been consistently overstated? Could the individual risk posed by this virus be far more nuanced than we've been led to believe?
Perhaps it's comparing apples and orangutans, but COVID-19 might provide a useful analogy. None of us had any immunity to this novel virus (SARS-CoV-2) when it came along, yet our individual responses have ranged from none (no illness, 'asyptomatic' infection) to death! Most people have experienced only a mild to moderate, cold/flu-like illness and have recovered without specific treatment. The people who have become seriously ill and needed to be hospitalised have typically had a massive systemic inflammatory response (a 'cytokine storm') which, in some cases, was overwhelming and fatal.
The limited data we have on naturally occurring HeV infection in horses suggests a similarly broad spectrum of responses. Not all horses naturally infected with HeV become ill, and not all horses who do become ill will die. Those who have died have had signs of a massive and overwhelming systemic inflammatory response.
Here are two excerpts from a report of the first Hendra outbreak to be described (occurring in the Brisbane suburb of Hendra in 1994):
In all, 20 horses in that first outbreak were identified as having HeV infection; 13 died (65%) and 7 recovered (35%). Of the two people who became ill in that outbreak, one died and the other recovered.
It is important to underscore the high mortality rate in horses who become seriously ill from naturally acquired HeV infection. Postmortem examinations of horses who died from naturally acquired HeV infection or were euthanised after experimental HeV infection have consistently revealed the widespread organ and tissue damage caused by a massive systemic inflammatory response.
But it is equally important to note that not all horses succumb to this disease. Why is that? Is it just about the number of viral particles the horse comes in contact with, or is there some component of the individual's immune response involved as well?
The factors that make one horse susceptible to illness and death while another recovers or doesn't even become ill urgently need investigation, just as they do with COVID-19 in humans. Yet all of the focus has been on vaccination. This is a very short-sighted approach, particularly in this era of personalised medicine.
In short, we don’t know how much risk HeV poses to an individual horse. It is clear that some horses become seriously ill and may die when they encounter the virus. But for what percentage of horses is that true?
Until we know who is most susceptible and why, and who is already protected through naturally acquired immunity, every horse is potentially at the same risk. This "one size fits all" approach underpins the blanket recommendation to vaccinate every horse against HeV.