Viral Zombies
Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. ~ American biologist Laura Ingwell
Numerous viruses practice mind control: altering host behavior to improve the probability of infecting others.
The Wasp & Its Virus
Many parasites of insect hosts have evolved associations with bacteria and viruses that help them perform their often-deadly deeds. ~ American entomologist Nancy Beckage
Caterpillars are eating machines. The parasitoid wasp Cotesia congregata chucks a spanner in the machine by infecting tobacco hornworms. A compromised caterpillar slows down and loses much of its appetite. This is a profound change.
Cotesia wasps finesse their parasitism via a polydnavirus: a virus genetically integrated with its wasp in a mutualistic relationship. This symbiotic system independently evolved with different viruses at least 3 times.
The wasp provides a home base for the polydnavirus, which does not reproduce in the caterpillar. The virus is replicated in special cells – calyx – within a female wasp’s ovary. Male wasps carry the viral sequence but cannot produce it.
Infection begins with the wasp injecting her eggs into a caterpillar. The virus paves the way for a happy incubation for wasp larvae hatching within the hornworm.
The polydnavirus provides the smarts for the wasp’s parasitism: suppressing the caterpillar’s immune system and controlling the cytokines that alter the hornworm’s behavior.
Beside genomic integration, the polydnavirus has other oddities. It has double-stranded DNA packaged much like chromosomes in eukaryotes. Despite having few viral genes, polydnavirus’ genome is one of the largest, and largely composed of introns, which is rare for a virus. 70% of the DNA is noncoding; far from the genic efficiency typical of viruses; but then, human understanding of genetics is rudimentary.
The origin of polydnaviruses remains a mystery. Many of its protein products are inscrutable and have no known homologs (similar structures).
The polydnavirus defies placement in an evolutionary niche, lending support to the theory that it was assembled rather than evolved.
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Healthy gypsy moth caterpillars climb out onto leaves to feed at night. At night, they crawl back onto branches or bark to hide from predators.
Contrastingly, caterpillars infected with a baculovirus disregard normal safety procedures and are readily found on leaves in broad daylight. For the virus, the caterpillar being picked off by an aerial predator is a free flight ticket.
Eventually an infected caterpillar climbs to the top of the tree it is on. There the caterpillar is converted into a sac of virus that liquefies and rains down on foliage below. The viral particles sprinkle leaves and await being eaten by the next caterpillar victim. Fast flight or slow fall: both tactics spread the virus.
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Some plant viruses get insects to do the heavy lifting for them. By altering feeding preferences, targeted herbivorous insects act as viral agents to spread infection through a plant population. Different viruses independently evolved this trickery.
Aphids not infected with Barley yellow dwarf virus (bydv) prefer grazing on barley plants that are infected, while infected aphids prefer uninfected plants. Hence, bydv promotes both its acquisition and transmission by its insect porter.
Aphids are similarly manipulated by the potato leafroll virus for potato plants. The tomato spotted wilt virus pulls the same trick with thrips that suck on tomato leaves.
The manipulative wiles of viruses are widespread. To engender transmission, an active herpes virus often makes its host lusty.
Cytomegalovirus – a cousin to herpes – can drive the offspring of its carriers crazy. Though this common virus invokes at most mild symptoms in adults, women infected with cytomegalovirus run an increased risk of bearing a child who will develop schizophrenia.
People vaccinated with flu viruses become more socially active shortly after being inoculated.
Viruses were early adopters in the parade of parasites that make their carriers do their bidding. Many other pathogens know how to manipulate their hosts minds, often rendering a host so insensible as to put its own life in peril for the parasite’s sake.
That viruses can influence the minds of other organisms in specific ways which advantage the pathogen is convincing evidence that Nature is an exhibition that cannot be explained by merely biomechanical means.
