Earthly viruses can alter behavior

Special to The Times

“The Invasion,” Warner Bros., premiered Aug. 17

The premise: In this fourth attempt at putting Jack Finney’s classic “The Body Snatchers” to the screen, there is a new twist. This time, instead of plant-like pods, it’s an alien virus-like particle attached to the wreckage of the NASA Shuttle Patriot, and it begins to spread rapidly through the human population. The virus (in the jargon of the movie) interferes with sweat, causes a “cellular condensation,” a “metabolic reaction” and alters the body’s “genetic expression” by the “integration of alien DNA” -- while turning everyone into emotionless robots.

The medical questions: Gobbledygook aside, are there real-life examples of infections that infect brain tissue and alter behavior and emotion, or is this all science fiction?

The reality: In animals, there are plenty of examples of viruses and parasites that change a creature’s behavior, sometimes in a way that appears to benefit the invader. The rabies virus, for example, causes increased saliva flow and aggression. Both, it’s been pointed out, benefit the virus since it is transmitted in spittle through bites. Rodents infected with a parasite called Toxoplasma gondii lose their fear of the scent of cats, which is useful for the parasite since it replicates in the guts of cats.


In humans beings, small infectious protein particles known as prions are responsible for an array of rare, related, degenerative brain disorders that cause changes in behavior as early symptoms. Prions mostly affect the brain’s gray matter, causing loss of nerve function and spongy holes to form in the brain. In Creutzfeldt-Jakob disease, one of these rare prion disorders, patients suffer progressive dementia, volatile emotions, difficulty walking and muscular jerks. (Kuru was another historical example of CJD in Papua, New Guinea, in the early 20th century: The disorder spread because of the tribal practice of eating brains at funerals.) Bovine spongiform encephalopathy, also known famously as mad cow disease, is a similar disorder that has been epidemic in cattle but has rarely affected humans because of a species barrier.

Fatal familial insomnia is a human prion disease characterized by wild shifts in the body’s vital signs as well as by loss of sleep.

Viral encephalitis (inflammation of the brain -- mostly the gray matter) is commonly caused by arboviruses including West Nile, herpes viruses and enteroviruses. Common symptoms include irritability, lethargy and seizures. And rabies, a deadly form of encephalitis, causes irritation and agitation and later, hallucinations. Luckily, rabies is rare in the U.S., with one or two human cases reported each year.

Borna disease virus was first described in horses in 1766, but since has been associated with a variety of animals, including humans. In animals, it has been known to cause hyperactivity, somnolence, depression and agitation. Although a causal link hasn’t been established, traces of the virus in man have been found in significant numbers of depressed and schizophrenic patients. Borna appears to spread in animals by direct contact through saliva and nasal secretions. Once in the body, it spreads from peripheral nerves back toward the central nervous system.

Subacute sclerosing panencephalitis is caused by direct invasion of the brain by mutant measles viruses. There are fewer than 10 cases a year in the U.S. The disease tends to occur years after the patient appears to have recovered from measles. Symptoms include gradual behavioral change leading to bizarre behavior, muscle jerks, unsteady walking, seizures and frequently death within one to two years.


Dr. Marc Siegel is an internist and an associate professor of medicine at New York University’s School of Medicine. In the Unreal World, he explains the medical facts behind the media fiction. He can be reached at