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Parasite Influence on Host Behavior - Part 2

My idea in writing about evolution is to arouse the curiosity of the readers and inform them that truth is often stranger than fiction. The premises of Darwin's theory are that the innate nature (genes, not acquired characteristics) of an organism are faithfully passed on to the progeny via chromosomes; there are random mutations that occur in genes during the process of inheritance and the environment (natural selection) leads to differential survival and more importantly differential reproduction (survival of the fittest) of those organisms whose genes are better adapted to the current environment. In part one examples were given of parasites that modify the behavior of intermediate hosts to make them more susceptible to predation by the final hosts of the parasite to enhance the survival and reproduction of the parasite. 

Other interesting examples are certain flukes which infect snail shells and secrete substances that make the snail grow a thicker shell than normal, thus protecting the snail from easier predation and prolonging the parasite's survival. Mice infected with the tapeworm Spirometra mansonides grow faster and more than uninfected mice and thus provide a more robust home for the parasite. Trilobium beetle larvae when infected by the sporozoan Nosema don't mature by molting into adult beetles and repeatedly molt into bigger larvae up to six times instead of the normal just once. They then become a super-sized larvae. The Nosema micro-parasites who are all sibs, achieve this by collectively secreting the beetle juvenile hormone to keep the larva repeating its molt without progress.

Some parasites like the barnacle Sacculina castrate the host, a crab. When it comes to sexual behavior there are other fascinating examples. For this it is important to understand that most plants and animals we know are Eukaryotes formed by the merger of two separate creatures nearly three billion years ago. Before that the earth had very little oxygen and bacteria the only life forms, lived anaerobically without oxygen. Then Cyanobacteria evolved and with their chlorophyll they could use sunlight, carbon dioxide and water to survive and breathed out oxygen. This is the opposite of what we do which is breathe in oxygen and breathe out carbon dioxide. Slowly the atmospheric oxygen level increased. A new bacterium evolved that could live on oxygen. The merger of an old bacterium with a new mitochondrion (oxygen consuming) became a eukaryotic cell from which we evolved. A merger of an old bacterium cell and a cyanobacterium (sunlight using by chlorophyll) gave rise to a eukaryotic cell from which plants evolved.

Bacteria reproduce asexually by simple division. They don't need a date on Saturday night. Now and then they do have sex, but it is purely recreational. They don't need it for reproduction. The donor bacterium donates a copy of some or all of its genes and the recipient incorporates the genes in its gene pool and both go their merry way with no further contact just like a one night stand. Each of them divides into two later on to reproduce. The recipient has an enhanced gene pool that it passes on to its progeny by bifurcation. Whenever a bacterium is infected by a plasmid (smaller extrachromosomal ring of gene bits, sometimes a virus) it really gets an itch to have sex, an unnatural behavior for it but very necessary for the plasmid or viral bacteriophage to spread. This is why indiscriminate misuse of antibiotics leads to the development of resistant bacterial strains which cause an itch for sex in the resistant bacterium with a plasmid and thus spread of the antibiotic resistance both by sexual and normal propagation.

A bacterium called Agrobacterium often gets infected by a plasmid named Ti. The infected bacterium then infects a plant and causes cancer in the plant. The indiscriminately growing cancer is called a crown gall and is of no benefit to the plant. The Ti plasmid has a gene which is turned on in the plant and induces it to make a substance called opine for which the plant has no use. Amongst the genes of the Ti plasmid is another gene that allows infected bacteria to use the opine as food. This leads to a profuse growth of the Ti infected bacteria which ensure survival of the Ti gene by simple division of the infected bacteria and increase in their penchant to have sex with uninfected bacteria to spread the Ti plasmid.

Another fascinating case is of a tiny parasitic wasps. Such wasps lay their egg in the tail end of the larvae or pupae of other insects after paralyzing them. The wasp larva slowly eats its way through the body of the paralyzed caterpillar and emerges as an adult wasp after devouring the caterpillar's head after a few days. This is what made Darwin lose faith in a god. William Hamilton discovered that the parasitic wasp Trichogramma lays its eggs within the egg of a butterfly or moth. The wasp larvae eat the host egg, then pupate and emerge as adult wasps. Hamilton found that all the progeny were female. This means that the wasps were all females with no males and gave birth to only females without having sex. This is called parthenogenesis, literally virgin birth as in the case of Jesus Christ (except he was a male).

On further study it was found that the cytoplasm of the wasp egg had bacteria in it. When Hamilton fed the adult female wasps with a course of honey laced with the antibiotic tetracycline, the wasps resumed having sex with other males and produced a mixed litter of males and females. It is important to understand that most animal sperms have hardly any cytoplasm and few mitochondria (our ancestral merger partner) and they dump them before penetrating the ovum which is loaded with mitochondria. All children male or female get their mitochondria from the egg and hence the mother. For mitochondria, being in a male body is a dead end and being in a female body is the passport to immortality. Mitochondria would love it if all progeny are female. Thus it was a conspiracy between the cytoplasm based (not in the cell nucleus) mitochondria and bacteria that subverted the development of wasp eggs in this particular case to all females. Some of the eggs were male but the bacteria directly or at the prompting of egg mitochondria, both being cytoplasm based and not wanting to end up dead in the future in a male body sabotaged the male egg to develop as a female.

Host behavior modification is seen in rabies. The rabies virus infected dog has a noticeable change in temper because of brain infection. For a day or two after infection, it is more affectionate, salivates more and has increased licking of human or canine contacts. Then it wanders restlessly and bites aggressively. Even fruit eating bats when infected with rabies will bite humans. All these help the rabies virus to spread, infect and propagate.

Human beings infected with cold, influenza and respiratory viruses have increased sneezing and coughing which help the virus spread. An experiment done by channeling mosquitoes in a controlled environment has shown that the female Anopheles mosquito is preferentially attracted by the body odor of a malaria infected person. The periodic rigors and breaking of fever by sweats tends to make the malaria infected person throw off body covers and increase exposure to mosquito bites.

Small roundworms like pin-worms that parasitize the human intestine are passed on by consuming food contaminated with their eggs. They utilize a fiendish strategy. The pin-worms crawl out to just outside the anus and lay their eggs. These eggs secrete a highly irritant chemical that makes the infected person feel a strong peri-anal itch. The natural tendency is to scratch. The eggs stick to the fingers or nails and lead to self-infection or infection of others when food is handled without proper hand washing.

Self-destructive behavior induced by parasites reminds me of old Sanskrit and English adages surprisingly independently and identically coined. In Sanskrit it is 'Vinaash kaale viparit buddhi'. In English it is 'Those whom the gods wish to destroy they first make mad'. Apparently history also repeats itself, first as a tragedy and then as a farce. Mad King George caused the loss of American colonies to the British Empire and now Mad Boy King George is going to lose the Middle East for the American Empire. Unfortunately we cannot do a brain MRI on the current George to diagnose the illness and cure it because he is so afraid that the brain MRI may show absence of a brain and then some child may cry out ' Look, The King has no brain'.                   


More by :  Gaurang Bhatt, MD

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