Silent Ebola?

Embedded in this article on the pro and cons of plasma transfusions from Ebola survivors as a treatment for the disease is word that not all cases of Ebola may result in illness; some may go unremarked, and result in immunity: What if Lots of People Have Ebola-Proof Blood?

“There is limited evidence from past outbreaks that suggests there probably are quite a few people who get exposed, who get infected, without ever developing symptoms and without ever developing illness but they develop immunity,” said Lauren Ancel Meyers, a professor in the Department of Integrative Biology at The University of Texas at Austin.

In other words, Ebola might be acting like its own vaccine.

This, of course, is par for the course for most viral infections. Once the immune system has been ginned up to produce specific antibodies, and assuming the patient survives, they often get more or less long lasting immunity to the disease, and this has been the basis of vaccination since Jenner used Cowpox (Vaccinia) to prevent Smallpox (Variola).

Meyers hasn’t tested any blood herself, but she and her team went through published reports about studies done on Ebola patients. In some outbreaks, up to half of people at high risk of infection who were watched never got sick, but they had antibodies against Ebola in their blood.

“If this turns out to be true, there is widespread silent Ebola infection that is immunizing.”

The best evidence of this happening with Ebola was a study done during a 1996 outbreak in Gabon, when researchers monitored 24 contacts of known Ebola patients who never became sick. Eleven of them had antibodies to Ebola.

So, 11 out of 24 more people may have become infected but never became ill, and developed antibodies against Ebola. This implies that Ebola may be even more contagious than than thought, and less reliably dangerous. Mixed news at best.

“If this turns out to be true, there is widespread silent Ebola infection that is immunizing,” Meyers said. “We should really investigate it further.”

It’s not unheard of. Immunologists know that people can get exposed to a virus and never become ill. One study showed that people in Peruappeared to have survived rabies — a virus believed to be virtually 100 percent fatal. One in 10 people tested had antibodies to rabies but many couldn’t recall having gotten sick.

I have several questions about this.  First, if these people are contracting Ebola, but not getting obviously sick (a few stray fevers in malaria country might not be considered that big a deal), are these people capable off passing the virus to someone else? Do they become like Typhoid Mary at least for a while, before they clear the virus?

Second, could this possibly be an evolutionary response among the people native to the Ebola ridden regions of Africa? Would similar rates of immunity be found say, in Europe or North America? We know that European descendants of plague survivors have higher rates off natural immunity to  HIV. If true, even if the African populations have a high rate of natural protection, others may not. That may not be good news.

Third, could there be non-lethal Ebola-like viruses running around in Africa that are causing the "anti-Ebola" antibodies?  We know that Ebola is a variable virus; at least four strains are known, of varying potency, and the virus has poor fidelity in its replication and mutates easily. Could strains be circulating that only cause mild non-descript illness that result in immunity to Ebola?  A similar situation occurred with Smallpox, where the much milder Cowpox resulted in long lasting immunity to Smallpox. Edward Jenner noted this, and developed the procedure of infecting people with live Cowpox to immunize them from Smallpox.  The process is called vaccination from the Latin name of the Cowpox virus, Vaccinia.

If we could find "safe" strains of Ebola that provide long lasting immunity, it would help the vaccine production process enormously. Nobody wants to work with a potentially lethal virus like Ebola.