KRM Branch 16th August 2021
For our August general meeting, a KRMB member, Daniel Heald, presented on the topic of Insects and Viruses: 300 million years of Co-evolution (and a great deal of ghastly death). The nucleic acid of a virus may be either RNA or DNA, and this determines their family. A virus contains its genome within the capsid and is housed in a lipid envelope. Viral structures are varied, bizarre and microscopic. When infected, a host cell is forced to produce thousands of copies of the virus rapidly. Viruses can be transmitted from plant to plant by insects, and in animals can be carried by blood-sucking insects.
The first example given was the Baculovirus where, after infection, caterpillars of Erinnyis ello (Sphringidae) turn into bags of pus that then burst, spreading the virus all around. The Woodlouse Iridovirus causes blue to bluish-purple iridescence in infected woodlice and was recently confirmed as present in Australia and has also been discovered in amber. Viruses can also be used as highly effective biological controls, as was the case with the European Spruce Sawfly in North America. Another virus was recently approved for use in Australia against the Fall Armyworm, Spodoptera frugiperda, a highly invasive moth that has already spread from FNQ to Tasmania and Perth and will be a major problem in corn crops. Unfortunately, insects can evolve immunity to previously used viruses, and the Coconut Rhinoceros Beetle is once again a massive threat to palms and tropical fruits across much of the Pacific and approaching Australia. There are at least 20 cryptic species and over 500 host plants of the Silverleaf Whitefly, Bemisia tabaci, all vectors for viruses. The 20 species cannot be told apart, and all the species and plants get mixed and matched. The viruses and whiteflies affect tomatoes, beans, squash, cucurbit and cassava and many other crops.
We then looked at mechanical versus biological transmission of viruses spread by insects to vertebrates (Arboviruses – arthropod borne viruses). Myxomatosis (Leporipoxvirus) was not very effective when first introduced from the Americas to Australia to control rabbits. But with the growth in the population of mosquitoes, it spread rapidly. The same rapid spread in Europe was a disaster for native rabbits there. Yellow fever virus (Flaviviridae) reproduces in Aedes aegypti and primate hosts, and it spread globally in ships’ water supplies – and later in aircraft. Symptoms were fever, headache, nausea and black vomit, with associated fatal heart, liver and kidney conditions, delirium and death in up to 60% of cases. In 1793, Yellow fever killed 10% of the population of Philadelphia. And in the construction of the first Panama Canal in 1881 to 1889, 20,000 people died from Yellow fever. In the Spanish-American War of 1898, there were 13 casualties from Yellow fever for every death in combat. Between 1896 and 1900, Major Walter Reed proved that it was a virus and not a bacterium, and that mosquitoes spread it. In 1927, the virus was isolated, and a vaccine was developed in the 1930s.
Colin Prickett