Aggressive Symbiosis

Virus X

Richard Preston wrote in The Hot Zone: “In a sense, the earth is mounting an immune response against the human species.”⁸³² His fanciful “revenge of the rainforest” theme makes for good reading but lacks a grounding in science. There is an emerging evolutionary theory, however, that may help provide a biological basis for the lethality of emerging infectious disease.

Viruses have long been thought of as the ultimate parasite, “feeding” off the host organism while contributing nothing of value in return. This view may be changing. Frank Ryan, a British physician-researcher, has introduced the concept of “aggressive symbiosis.” As opposed to parasitism, where one organism exclusively exploits the other, symbiotic relationships in biology can be characterized by both species benefiting in some way. Symbiosis is Greek for “companionship,” whereas parasite is from the Greek parasitos, meaning “person who eats at someone else’s table.”⁸³³ The classic symbiotic relationship is of the sea anemone and the clown fish. The clown fish is afforded protection among the stinging tentacles of the anemone. This is mutually beneficial for the sea anemone, which refrains from stinging the clown fish in return for help luring other prey into its grasp. Not all symbiotic relationships are as tender, though.

In Borneo, a species of rattan cane has developed a symbiotic relationship with a species of ants. The ants construct a nest around the cane and drink at its sugary sap. When an herbivore approaches to nibble its sweet leaves, the ants rush out and attack to defend their sugar daddy.⁸³⁴ Ryan draws an analogy between this kind of behavior—which he terms aggressive symbiosis—and that of new zoonotic agents of disease. He argues that when it comes to emerging animal viruses, animals are the cane and ants are the virus.

For example, a herpes virus, Herpesvirus saimiri, has seemingly developed a symbiosis with the squirrel monkey, passing harmlessly from mother to baby. If a rival species like a marmoset monkey invades their territory, the virus jumps species and wipes out the challenger, inducing fulminant cancers in the invaders. In this way, the virus protects the squirrel monkeys’ habitat from invading primates. It is in the squirrel monkeys’ evolutionary best interest not to try to purge the virus from their systems, and so the virus is able to replicate free of immune interference. Another similar virus, Herpesvirus ateles, protects spider monkeys in South American jungles in much the same way, killing virtually 100% of encroaching monkeys lacking immunity. The spider monkeys pass both the virus and immunity to the virus from mother to child, benefiting all. This “jungle immune system” protects the inhabitants from invading primate species, even when that invading species is us.⁸³⁵

This may explain why Ebola is so virulent—in fact too virulent. The disease evoked by the Ebola virus is so fierce that victims don’t make it very far to infect others, which would seem to make the virus an evolutionary failure. Under the aggressive symbiosis hypothesis, however, the virus may be fulfilling its evolutionary purpose after all, protecting a host species we simply haven’t identified.⁸³⁶ Interestingly, it works both ways. The herpes virus that causes nothing more serious in humans than cold sores at the corner of one’s mouth can pose a risk of lethal infection to monkeys in Central and South America.⁸³⁷

Simian (from the Latin simia for “ape”⁸³⁸ ) Immunodeficiency Virus is considered the precursor to the human AIDS virus, HIV.⁸³⁹ Chimpanzees have SIV, but never get AIDS.⁸⁴⁰ SIV is harmless in the African green monkey, but can cause acute disease in the yellow baboon.⁸⁴¹ Aggressive symbiosis is an argument for keeping viruses confined to their natural niches.