What Ebola virus means for primate populations
For most, the mere mention of Ebola virus provokes abject fear - the archetypal blockbuster contagion - but just how dangerous is it to our ape cousins?
A recent report highlighting possible Ebola virus infection in Indonesian orangutan populations provides a timely reminder of the threat to animal and human health that emerging virus infections pose.
Ebola virus was first described in the 1970s as the cause of deadly outbreaks of haemorrhagic fever in humans in the Democratic Republic of Congo and Sudan, and outbreaks continue to this day.
Early studies of human epidemics showed they had been ignited by contact between hunters and animals - particularly gorillas and chimpanzees. Most significantly, these animals were dead and Ebola virus was the prime candidate for their deaths.
Great apes in trouble
Population surveys of these great apes revealed a shocking truth. Massive declines in ape numbers had consistently accorded with Ebola virus outbreaks. In one study population in the Lossi Sanctuary in DR Congo, more than 90% of gorillas - an alarming 5,000 animals - were lost.
However, proving a definite link between virus infection and the great ape demise was not easy.
In jungle environments dead animals are quickly dispersed by scavengers, or decay rapidly and become usurped by the thick forest undergrowth. But the repeated discovery of ape carcasses coincidental with human outbreaks was compelling evidence; that these corpses harboured Ebola virus was even more so.
Maybe not the proverbial smoking gun, but incredibly persuasive evidence just the same.
But if the infection was so harmful to apes and caused so many deaths, it was unlikely that these animals were its natural reservoir.
Viruses require large numbers of susceptible hosts to ensure continued spread and rapid wholesale killing is not conducive to this. Great apes - humans, gorillas and chimpanzees - are essentially dead-end hosts for these Ebola viruses.
Ebola virus infections
• Ebola virus takes its name from the river in DR Congo near to the area where the first cases were described.
• There are five recognised species of Ebola virus: Sudan, Zaire, Ivory Coast, Bundibugyo located in Africa and Reston in the Philippines.
• All Ebola virus species cause fatal infections in non-human primates.
• The Zaire strain of virus is the most likely cause of the majority of gorilla and chimpanzee deaths.
• Clinical symptoms observed in wild animals dying during Ebola virus outbreaks include vomiting, diarrhoea, weight loss and bleeding from the nostrils.
Virus-hunters soon discovered that the most likely natural hosts for Ebola viruses were fruit bat species living in regions where outbreaks had occurred.
Great apes and bats, as well as Ebola virus, have co-existed for millennia, yet gorilla and chimpanzee population collapses seem a recent event.
Infection through interaction
As habitats are destroyed, great apes and a whole host of animals retreat into precious sanctuaries and protected parks. An inescapable irony is that these remaining bastions, supporting large populations of apes living cheek-to-cheek with bats, have probably aided the passage of this deadly virus.
And it's not just the infections carried by other animals that are a problem.
Great apes and humans share around 98% of their genetic material. This remarkable similarity means that these creatures are also susceptible to many human viruses: measles and a whole host of respiratory viruses, such as respiratory syncytial virus, human metapneumovirus and influenza, amongst many others.
Great ape encounters, through eco-tourism - a once-in-a-lifetime opportunity to get up-close to these mighty creatures, to see the "whites of their eyes" - is putting animals at risk.
But this activity can also be a force for good. The great wealth generated provides incentive to secure the safety of these animals for generations to come.
So, conservationists find themselves stuck between an ecological rock and a hard place: damned if they try to support the remnants of these once widespread animals and equally damned if they don't.
Increased human interaction, through habitat encroachment or eco-tourism, is inevitable. So how can we mitigate the risks that this brings?
Understandably, great ape conservation is a highly emotive subject. If we are to secure the long-term survival of our closest relatives can we cling to the vestiges of non-interventionist strategies or do we need to be more proactive?
To one group of scientists the answer is simple: direct action, through a pioneering programme of vaccination, will be necessary to protect these primates from "natural" and human-wrought infection.
The VaccinApe project aims to take human vaccines and adapt them for use in wild apes: initially those at most risk from human disease, but ultimately protecting as many gorillas and chimpanzees as possible from a range of infections, including the deadly Ebola virus.
Identifying Ebola virus
But what of the Indonesian orangutans: are we on the verge of a catastrophic Ebola virus-induced population collapse? To answer this key question we need to look at what the researchers did and understand what their data is telling us.
Dr Chairul Nidom and colleagues from Airlangga University, Surabaya, Indonesia, tested over 300 orangutan blood samples for the presence of antibodies that reacted with Ebola virus proteins.
The bat connection
• Ebola or Ebola-like virus infection is evident in bats in Africa, the Philippines, Spain and China.
• Ebola virus has been detected directly in three African species of fruit bat: the hammer-headed or big-lipped fruit bat (Hypsignathus monstrosus), Franquet's epauletted fruit bat (Epomops franqueti), and the little collared bat (Myonycteris torquata).
• Ebola virus-specific antibodies - a signature of past infection - have been detected in the Gambian epauletted fruit bat (Epomophorus gambianus), Veldkamp's dwarf epauletted fruit bat (Nanonycteris veldkampii) and the straw-coloured fruit bat (Eidolon helvum)
• The proportion of bats with evidence of current or past infection is very low - usually less than 5%.
• Geoffroy's rousette bats (Rousettus amplexicaudatus) in the Philippines were shown to harbour antibodies to Reston Ebola virus. This bat is widely distributed throughout south-east Asia including Indonesia.
• Ebola virus infection of bats is usually symptomless - this 'silent infection' is often a sign of extensive co-evolution between the virus and its host. However, the recent discovery of a novel Ebola virus in bats in Spain coincided with major die-offs of that particular bat in the region where the virus was found.
• Bats represent a unique mammalian virus reservoir - they can live in very large colonies, have a range of diets and are capable of flight - all of which aid virus transmission and spread.
Antibodies help defend the body from marauding viruses. They are produced by specialised white blood cells and each cell produces a unique type of antibody.
The virus and antibody interaction is specific: just like a lock and key. So the presence of antibodies to a particular virus is a tell-tale sign that we've seen it before, either through infection or by vaccination.
Around one-fifth of the orangutan samples tested by Dr Nidom contained antibodies that recognised Ebola virus proteins and 1% had antibodies to a related virus called Marburg virus. These animals had possibly been infected.
Antibodies are usually specific for their target, but non-specific or "sticky" antibodies do occur. Many of the study's orangutan samples reacted with proteins from different Ebola virus species. Could the data be explained by the presence of non-specific antibodies rather than past virus infection?
Possibly, but antibodies present in humans who recover from Ebola virus infection recognise many different strains just as the orangutan samples did.
These findings are tantalising, but only isolation of the virus or its genetic material will conclusively prove that this infection is happening. This will require large-scale screening of orangutans and other animals, such as bats, for the presence of virus. According to a recent review, finding Ebola virus in animals is a very rare event.
Sampling animals in the field is not easy, but extensive analysis of faeces - the approach that helped our understanding of a malaria parasite and HIV-like virus infection of African apes - may hold the key.
It would be no great surprise to discover another Ebola-like virus outside of Africa. The Reston species of Ebola virus originated from the Philippines, and Ebola-like viruses have been documented in bats in Spain and China. They appear ubiquitous.
If confirmed, the discovery of Ebola virus infection in orangutans would not necessarily be their death knell nor automatically pose a serious risk to human health.
The presence of antibodies in these animals shows that they successfully fought the virus infection and became immune. Perhaps other orangutans perished, but there is no evidence for this whilst Reston Ebola virus infection in humans often passes without symptoms.
Nor would Ebola virus spread easily through orangutan populations. Unlike their gregarious African counterparts, these are solitary animals that rarely meet.
Great ape populations are under unprecedented threat from illegal hunting and massive habitat loss.
Ebola virus infections are turning the screw further on African gorillas and chimpanzees.
But so far, the evidence suggests that orangutans have no worries on this score.
Let's hope it stays that way.