The Science Of Health: Welcome back to "The Science Of Health", ABP Live's weekly health column. Last week, we discussed how egg freezing is performed, what its risks are, and how much it costs in India. This week, we discuss the different kinds of diseases caused by insects and arachnids. Since it is monsoon in India in the months of July and August, the number of insect and vector-borne diseases during this time increases. 


Insects can act as primary or intermediate hosts or carriers of human diseases. Protozoa, bacteria, viruses and helminths such as tapeworms, flukes and roundworms are pathogens that can be transmitted by insects. 


According to the Smithsonian, the two modes of transmission of pathogens through insects are mechanical and biological. Mechanical vectors can pick up infectious agents on the outside of their bodies and transmit them through physical contact, while biological vectors carry pathogens that can multiply within their bodies and be delivered to new hosts, usually by biting. 


Examples of mechanical vectors are flies, and those of biological vectors are mosquitoes and ticks, according to the European Food Safety Authority. 


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Malaria


Malaria is an insect-borne disease caused through biological means. Mosquitoes of the genus Anopheles transmit malaria through a Plasmodium protozoan. Malaria affects about 250 million people in the world, and results in two million deaths annually. It is the most deadly Arthropod-borne disease in the world. 


Plague


Plague, also called black death, is mostly caused by pathogens transmitted through fleas. These insects infect humans and rodents such as rats. The three forms of plague that affect humans are bubonic, pneumonic and septicemic. Bubonic plague is the only form of plague that occurs as a result of transmission of pathogens through fleas. Yersinia pestis, a bacterium, is the pathogen that causes bubonic plague. 


Pneumonic plague is spread through the air, when a healthy person inhales the aerosols released by an infected person. Septicemic plague is a complication of pneumonic or bubonic plague, and is characterised by the multiplication of Yersinia pestis in the blood.


Fleas regurgitate plague bacteria when biting, causing the disease in the human who has been bitten. Other ways of Yersinia pestis entering the bloodstream of humans include scratching of flea faeces into the skin, and ingestion of infected fleas. 


In the 14th and 17th centuries, plague resulted in millions of deaths. The disease is still a menacing threat to society. 


Sleeping sickness


The Tsetse Fly, which belongs to the genus Glossina, is the vector for Trypanosoma brucei, which causes sleeping sickness or African sickness. Sleeping sickness has a high mortality rate among people as well as cattle. 


American Trypanosomiasis


Also known as Chagas' disease, American Trypanosomiasis is caused by a protozoan called Trypanosoma cruzi, which is transmitted by Conenose Bugs, also called Kissing Bugs. The protozoan invades the muscle cells of the digestive tract and heart, and sometimes the skeletal muscle, and after becoming an adult, the trypanosome circulates in the blood. However, adult trypanosomes do not invade blood cells the way malaria parasites do. The disease is spread through the bugs' faeces, not their bite. The bug usually feeds on their sleeping victims at night. 


Enteric diseases


Faecal contamination of food and water can spread certain diseases, either directly or indirectly, through the transmission of bacteria. This is a mechanical mode of transmission. These diseases are called enteric diseases. House flies are the most common vector of mechanically-transmitted diseases. Typhoid fever, caused by Salmonella typhi, is the most common example of enteric diseases. Vibrio cholerae, spread by house flies, causes cholera.


Escherichia coli, which causes urogenital and intestinal infections, and Shigella, which causes dysentery and diarrhoea, are other examples of enteric diseases. 


Arbovirus disease


Several diseases are spread through biological means such as the bite of mosquitoes. These diseases, which are transmitted by the bite of a variety of mosquitoes, are called arbovirus diseases. About 28 viruses of major public health importance cause horrible diseases. Mosquitoes belonging to the genus Aedes cause dengue and yellow fever. Some mosquitoes belonging to the genera Aedes and Culex transmit certain kinds of encephalitis. 


Lyme disease


Borrelia bugdorferi is a bacterium carried by an arachnid, the deer tick, and causes Lyme disease. The bacterium enters the host's bloodstream when they are bitten by the deer tick. First identified in New England and the mid-Atlantic States, the disease is now known to occur in all of the Northern Hemisphere.