The following points highlight the five major disease producing insects. The insects are: 1. Mosquitoes 2. Flies 3. Fleas 4. Bugs 5. Lice.
Disease Producing Insect # 1. Mosquitoes:
Several species of Anopheles are vectors of human malaria. The female Anopheles bites a malarial patient and takes in malarial Plasmodium along with human blood. Plasmodium multiplies and undergoes sporogony in Anopheles which transmits the parasite when it bites another person.
There are four types of malaria:
1. Plasmodium vivax causes benign tertian malaria in which the attacks of fever are every 48 hours.
2. P. malariae causes quartan malaria in which fever comes on every 72 hours.
3. P. falciparum causes malignant tertian malaria in which fever is more or less continuous.
4. P. ovale causes mild tertian malaria in which fever comes on every 48 hours. Because different species of Anopheles are widely distributed, malaria is very widespread. Malaria not only causes thousands of deaths annually in the tropics, but it also prevents the cultivation of the most fertile regions of the earth.
Culex transmits malaria in birds. Culex fatigans and C. pipiens are intermediate hosts and vectors of the larva of a nematode Wuchereria bancrofti whose adults cause elephantiasis in man. The larvae of Wuchereria come to the peripheral blood only at night when the Culex sucks blood and takes in the larvae.
The larvae develop in the mosquito and are transmitted to new human hosts where they mature in the lymphatic system. Wuchereria is also transmitted by some species of Anopheles and Aedes in warm countries. The nematode causes inflammation of arms, legs, scrotum or mammary glands of human beings.
Another mosquito of genus Mansonia transmits Wuchereria malayi in India and Southeast Asia, this nematode is confined only to legs. All cases of infection by Wuchereria do not result in elephantiasis.
Aedes is a widespread mosquito, it bites both by day and night. Aedes aegypti is the vector of the virus of yellow fever. Yellow fever is widespread in South America and Africa; it is much more deadly than malaria.
A. aegypti and other species also carry the virus of dengue or “break bone fever”, they transmit the infection from monkeys and dengue breaks out as an epidemic in warm countries; in dengue there is high temperature, rash on the skin and pain in the bones.
Disease Producing Insect # 2. Flies:
Species of Musca, the housefly transmit germs of typhoid which are picked up on the bristles of body and legs from the excreta of a patient and transferred to human food. They also transmit germs of trachoma, an eye disease of tropics, and the germs of cholera, dysentery and diarrhoea.
Glossina, the testse fly is the vector of Trypanosoma from animals to man. G palpalis and G. tachinoides transmit Trypanosoma gambiense which causes Gambian sleeping sickness. G morsitans transmits Trypanosoma rhodesiense which causes Rhodesian sleeping sicknes.
The trypanosomes are injected by the fly into the blood of man from where they reach the lymph gland and cerebrospinal fluid resulting in sleeping sickness which proves fatal. Domestic and wild animals are also susceptible to sleeping sickness, G. morsitans is also the vector of Trypanosoma brucei which causes nagana in cattle and horses, nagana is similar to sleeping sickness.
Phlebotomus, a sand-fly sucks the blood of reptiles and mammals, it spreads sand-fly fever in India and countries around the Mediterranean. Phlebotomus papatasi is the vector of sand-fly fever in which there is a pain in the eyes, stiffness in the back and neck, and a reduction of white blood corpuscles. P. papatasi and P. sergenti transmit Leishmania tropica which causes Oriental sores in India and Eastern Asiatic countries.
P. major in China and P. argentipes in India transmit Leishmania donovani which causes kala-azar fever in which the parasites enter the spleen, liver, bone marrow and endothelial cells. Kala-azar occurs in epidemic forms and proves fatal for man. P. verrucarum transmits the bacillus of Oroya fever in South American countries.
Tabanus, a gadfly and Stomoxys, a stable fly are biting and blood sucking flies, they act as vectors of Trypanosoma evansi which causes surra, a widespread tropical disease of horses and cattle, which is fatal in horses.
Tabanus and Stomoxys may also mechanically transmit Leishmania topica of Oriental sores, and bacillus of anthrax, a virulent disease of cattle. Stomoxys is the intermediate host of larvae of Hymenolepis, a tapeworm of poultry, and of larvae of Habronema, a nematode of horses.
Chrysops, the mangrove fly sucks human blood in West Africa. It is the intermediate host and vector of Filaria loa, a human nematode parasite which causes “Calabar swellings”, the nematode wanders about in the sub-cutaneous tissues, especially around the eyes.
The larvae come to the peripheral circulation by day time when Chrysops dimidiata sucks human blood, the larvae develop in the fly and are transmitted to new hosts. Chrysops discalis transmits the germs of tularaemia, a plague-like disease of rodents in America.
Hypoderma, the warble fly lays eggs on legs of cattle, in America, the larvae burrow into the skin and reach the alimentary canal. In India the larvae of Hypoderma crossii develop below the skin of the back of goats. Simulium the blackly transmits larvae of a filarial nematode Onchocera which infects man and cattle in Africa, America and Europe.
Disease Producing Insect # 3. Fleas:
Xenopsylla cheopis and X. astia are rat fleas, they suck the blood of an infected rat and transmit the bacilli of bubonic plague to man, the infection is caused by the gut getting blocked by bacilli in rat, and sucked blood is regurgitated into a wound, or infection may be caused through infected faeces of flea being rubbed into human skin by scratching.
Plague occurs in epidemic form in India. X. cheopis also transmits germs of endemic typhus from rats to man; it also transmits germs of tularaemia from rats to man. Xenopsylla is an intermediate host of larvae of two tapeworms, Dipylidium caninum of dogs and cats, and Hymenolepis of rats and man.
Disease Producing Insect # 4. Bugs:
Cimex, the bed bug has been suspected of being the cause of many human diseases, but this has not been proved. In the gut of bed bugs are anti-bacterial substances which do not permit bacteria to live for long. Cimex may carry and transmit germs of plague and relapsing fever, this is only for short periods.
Three bugs Triatoma, the “assassin bug,” Panstrongylus and Rhodnius breed in human dwellings, they are the vectors of Trypanosoma cruzi which causes Chagas’ disease in South America, this disease is similar to sleeping sickness, it is a wasting disease of the brain and body, it proves fatal.
Triatoma rubrofasciata is found in India, its nymphs are common in houses hiding in crevices and under carpets, this species is concerned with transmission of kala-azar.
Disease Producing Insect # 5. Lice:
Pediculus, the louse sucks blood and takes in Rickettsia which multiply in the louse, when they are injected into human beings they cause epidemc and endemic typhus.
Pediculus also carries Rickettsia which cause trench fever in man, and the spirochaetes of relapsing fever. Trench fever and relapsing fever are spread through the excreta of louse when it is rubbed into the human skin or the louse gets crushed on the human body.