Insects in Relation to Protozoa (complete information)

Protozoa are endosymbionts in nearly all orders of insects but are best known in Orthoptera and Isoptera. Extraordinary quantities are found in termites, where flagellates constitute up to one-third of a nymph’s weight.

Protozoa enter insects via the month, and many types remain associated with the gut, but others become intracellular parasites or live in the hemocoel.

They may be pathogenic, commensalistic, or mutualistic in relation to the insect host. Pathogens causing several major diseases of vertebrates are transmitted by insects, as are a few minor protozoan pathogens of plants.

The geologic record gives no clues to the antiquity of insect-symbiont relationships. That these associations can be quite old is inferred from the distribution of flagellates among the primitive termites and Cryptocercus roaches. Cleveland and his associates (1934) found 25 species of flagellates in Cryptocercus.

Several species of the flagellate genus Trichonympha are found in both the roach and various termites. Because few opportunities for exchange of endosymbionts now seem to exist between the two orders of insects, we infer that both probably derive their Trichonympha from a common ancestor in the Mesozoic Era.

Protozoa pathogenic to insects have been reviewed by Brooks (1974). The most important are the microsporidians, which are parasites of many, if not most insects, especially of Lepidoptera and Diptera. The pebrine disease of silkworms, caused by Nosema bombycis, and nosema disease of honeybees, caused by N. apis, are examples. The coccidian Adelina is pathogenic in Coleptera and Diptera.

Relatively harmless are the eugregarines that are also known from many insects, especially Coleoptera, Orthoptera, and Diptera. Some are easily seen with the unaided eye because they may be up to 16 mm long. Mattesia grandis is. however, highly pathogenic to the boll weevil. Anthonomis grandis, and may prove useful in its control.

At one time malaria was considered the most important disease of humans. It is caused by four species of sporozoans in the genus Plasmodium and transmitted by mosquitoes of the genus Anopheles. Persistent chemical insecticides, especially DDT and antimalarial drugs have substantially reduced mortality.

The prospect was so encouraging that in 1955, the World Health Organization resolved to undertake a worldwide eradication programme. The use of residual insecticides continues to be the main strategy, but a variety of other methods are being developed to control resistant populations of the vectors. Unfortunately, malaria is now increasing in several parts of the world.

The biological transmission of plasmodia is of the cyclopropagative type. Female Anopheles ingest microgametocytes and macrogame- tocytes along with blood from infected humans. In the mosquito’s midgut, the microgametocytes produce male microgametes that seek and penetrate the female cell, now enlarged and called a macrogamete.

The resulting zygote develops into a motile ookinete. This enters the midgut epithelium and encysts as an oocyst between the epithelium and the outer tissues of the gut. Within the oocyst the cell undergoes mitotic and meiotic divisions, ultimately producing thousand of haploid sporozoites.

The oocyst bursts, releasing the sporozoites in the hemocoel. The sporozoites invade various tissues, but those reaching the salivary glands are passed into another human’s blood when the mosquito feeds. In the human, the sporozoites first enter cells.

Merozoites develop into trophozoites that destroy the blood cell and release either more merozoites or gametocytes. The release of merozoites and their toxins occurs at regular periods according to the species of Plasmodium. This is the cause of the recurring fevers and chills that characterize malarial attacks.

Other important diseases of humans in tropical regions are caused by flagellates of the genera Trypanosoma and Leishmania. Several kinds of diseases are produced by the latter, including kala azar and espundia. Phlebotomus flies (Psychodidae) are known as vectors.

Trypanosomes cause trypanosomiasis, or sleeping sickness. In Africa, both sexes of tsetse flies, Glossina (Muscidae), feed on blood and ingest flagellates. Some mechanical transmission may occur immediately by infected mouthparts, but cyclopropagative transmission follows. The flagellates multiply in the gut, and moving anteriorly to the salivary glands, they are passed to new hosts when the fly feeds.

Trypanosomiasis in America is called Chagas’ disease. The vectors are blood-sucking reduviid bugs of Subfamily Triatominae. The transmission is also cyclopropagative in the insect’s gut, but infection of new hosts is by feces. The bug defecates after feeding. Humans accidentally rub the contaminated feces into the wound or into the eyes or mouth.

Flagellates of the genus Phytomonas infest plants, especially those with latex. Vectors are various phytophagous Hemiptera-Heteroptera.