Recently, media coverage of the Ebola virus epidemic has taken new twists.
A Spanish nurse, Teresa Romero Ramos, who caught Ebola from the patient she nursed, suffered additional distress when her dog, Excalibur, was euthanized. The dog was killed despite a vigorous social media campaign to save his life. “They want to kill him just like that, without following any procedure,” said her husband, Javier Romero.
Authorities maintain that the dog posed a theoretical risk of transmitting Ebola to people, while the protesters say they acted prematurely.
Now the United States is faced with a similar dilemma over the fate of a dog belonging to an Ebola-positive healthcare worker in Dallas. However, in this case the mayor says the dog will be “cared for” and not euthanized.
So who, if anyone, is right? What is known about the role of dogs in the transmission of the Ebola virus? This article aims to present the known facts about Ebola and dogs so that you can make up your own mind.
Ebola Virus: The Basics
Ebola (Ebola hemorrhagic fever) is a zoonotic disease — which, according to the World Health Organization (WHO), is “a disease naturally transmitted from animals to people.”
The way a zoonotic infection works is that an animal acts as the reservoir host — a source of infection for people. The infectious agent (in this case Ebola virus) does not usually make the reservoir host ill but reproduces inside the body to pass infectious particles in the host’s feces. When other animal species (other than the reservoir host) come into contact with these infectious feces, they are at risk of catching infection and may become sick.
Which raises the important question: Can dogs catch Ebola?
The short answer is: Yes, scientists report that dogs can and do catch Ebola virus.
Less straightforward is to know the risk that infected dogs pose to people.
The implication of a dog being capable of passing infection onto people is immense and something researchers in Africa are keen to understand. Scientists are still piecing the puzzle together.
To this end, researchers have studied village dogs in the regions of confirmed Ebola outbreaks.
This work came about because clusters of Ebola outbreaks were traced to villagers coming into contact with single infected animal carcass (usually a primate that died in the jungle)¹ that had been infected by the reservoir host (a bat). People in contact with that carcass became ill, and then direct contact between humans transferred infection between them.
However, in 2001-2002 there was an Ebola outbreak in Gabon that had no obvious originating source. Investigators learned that the village dogs were not fed but were expected to scavenge for their food. This raised the possibility of dogs hunting in the forest and eating monkeys or other primates who had died from Ebola. Could these dogs have transferred Ebola back to the villagers?
To test this theory, researchers took blood from dogs in the village where the Ebola outbreak was confirmed, and for comparison took blood from dogs living in villages where there had been no human cases. This blood was later tested for the presence of antibodies to the Ebola virus.
The results were startling:
- In the first group (dogs from villages with human cases), 27.2 percent of dogs had seroprevalence, or evidence of an immune response to the Ebola virus.
- However, in the second group (dogs from villages with no human cases), 22.4 percent of dogs also had mounted an immune response.
- Both groups of dogs had been in contact with Ebola, but only the population of 1 village was ill.
So what does this mean?
Exposure vs. Active Infection
To understand the implication of these results means knowing a little of how the body’s immune system works.
When a virus infects an animal, it becomes an antigen. An antigen is something that stimulates the body’s immune system to produce an antibody. It is the job of these antibodies to fight the infection and get rid of it from the body.
Think of antigens as the “baddies” and antibodies as the “goodies” in a battle between infection and recovery. When you pick up a common cold the cold bug is the antigen, and your antibodies fight off the cold.
Antibodies are a good thing. The higher the level of antibodies, the more vigorous the body’s fight against infection.
This is where things get complicated. The presence of antibodies does not mean the animal is infectious to others — in fact, his immune system may be so strong that he kills the virus and poses no risk to anybody. The only conclusion that can be drawn from finding antibodies is that at some point the animal was exposed to the virus and mounted an immune response. It does not tell you if the patient is infectious to others.
Unfortunately, what those research workers in the African jungle were unable to do was to test for the presence of the virus in those village dogs. The remote location and the limits of technology made this impossible.
So the crucial question of whether humans can get Ebola from dogs remains unanswered.
Next, let’s look at things the other way round: Does infection with Ebola make dogs ill?
Do Dogs Get Sick From Ebola?
The short answer is: No. Dogs seem to be asymptomatic — when they come into contact with the Ebola virus, it doesn’t make them ill.
Now, some of the more astute among you may reason that this makes dogs sound an awful lot like a reservoir host.
Indeed, a highly respected scientific paper² concludes among the comments:
“Asymptomatically infected dogs could be a potential source of human Ebola outbreaks, or of virus spread during human outbreaks….”
Are Dogs a Reservoir Host?
Before we get carried away and condemn dogs as being a reservoir host, it is worth looking back to the results from that village where no human Ebola cases were found.
With nearly a quarter of those dogs showing seroprevalence for contact with Ebola, you might expect at least a handful of human cases among those living in close proximity — but no.
The reason for this goes back to the difference between mounting an immune response and killing the virus and becoming a “carrier” who poses a risk to others. At the present time, thinking with regards to dogs lies somewhere in between these 2 scenarios.
Researchers suspect that dogs may shed virus for a limited time before ridding themselves of infection.
However, this does little to help us decide if it is appropriate to euthanize dogs that have been in contact with Ebola:
- On the one hand, it appears the dogs are unlikely to become ill.
- But on the other, they may shed the Ebola virus and pose a risk to people for an unknown period of time.
If all this leaves you wondering what species is the true reservoir host, the answer is: fruit bats.
This evidence came to light in 1976 when an outbreak of Ebola centered on a cotton warehouse in Nzara, Sudan. A colony of bats roosting in the warehouse eaves tested positive, both on blood and fecal tests, for the Ebola virus. The bats themselves were not affected by the disease, but human workers who came into contact with infected bat feces became seriously ill and died³.
Ebola and Dogs: The Facts
So what do we know?
The facts as they stand are these:
- Dogs may be infected with Ebola virus but not show symptoms.
- The mechanism of viral excretion by dogs needs further investigation.
- Asymptomatic dogs are a potential source of infection for people.
Unfortunately, there is a lot of work to be done before we have a definitive answer about the risk dogs pose to people, and so the final word must go to workers in the field of Ebola research:
“Although dogs can be asymptomatically infected, they may excrete infectious viral particles in urine, feces and saliva for a short period before virus clearance, as observed experimentally in other animals.”
—Lois Allela, veterinary inspector with the Ministry of Environment of Gabon, whose specialist field is Ebola in the dog
- Multiple Ebola virus transmission events and rapid decline in central African wildlife. Leroy, Formenty, Souqueires et al. Science. 2004 Jan 16; 303 (5656): 387-90.
- Ebola Virus Antibody Prevalence in Dogs and Human Risk. Allela, Bourry, Pouillot et al. Emerg Infect Dis. March 2005; 11(3): 385-390.
- Ebola hemorrhagic fever in Sudan, 1976. Report of a WHO/ International Study Team, Bull World Health Organization. 1978; 56 (2):247-70.
This pet health content was written by a veterinarian.