Bacteria are nowhere near the only agents of disease of biological origin. We’re talking parasites (protozoa), viruses and fungi for starters. Fungi are very different from bacteria, they’re eukaryotic! They’re also more closely related to animals than plants. Viruses, as previously covered, aren’t cells nor alive. Protozoa are (also eukaryotic) single-celled organisms. So without further ado, let’s see what the transmission, mode of infection and pathogenic effect are in stem rust fungus on cereal crops, common flu and malaria.
This infection of cereal crops (including wheat) by fungi, specifically Puccinia graminis, has been an issue in agriculture since time immemorial, although fungus-resistant wheat strains became available since the 1950s. Can you see why it’s called stem rust? The fungus colonises the parts of the crop above ground level, and compromises the harvest very rapidly by disrupting seed development, as well as other symptoms of a diseased plant such as dead stems by breakdown of the surface barrier used for transpiration. This leads to uncontrollable water loss by evaporation.
The infection starts when the fungal spores carried by air land on the crop and establish themselves at the site through uredinia which are those reddish pustules. The infection affects the outer layers of the stalk and decreases the number of tillers produced by the plant. Fungicides can be used to prevent this infection.
Influenza, what a joy! I think by this point we know that flu is caused by a virus that has an infinite mutation potential that makes vaccines very fleetingly efficient (taken on a yearly basis). But! Who is influenza and what does it do?
Influenza is a virus part of the mind-blowingly named Orthomixovirus family, and is transmitted between hosts via droplets carried by sneezing or coughing. It presents symptoms between 2 and 10 days post-infection, such as fever, cough and nasal congestion. Unlike the rhinoviruses that cause the common cold, influenza actually damages tissue. However, the symptoms are mostly the result of the body’s own immune response caused by a release of proinflammatory cytokines from infected cells.
The mechanism of infection of influenza consists of its outer hemagglutinin protein being cleaved by proteases present in human cells. These are found in the throat and lungs, so other tissues don’t get infected as a result. More virulent strains like the bird flu strain H5N1 on the other hand, can have their hemagglutinin cleaved by other types of proteases, hence enable their spread to other parts of the body.
Malaria is transmitted via a vector which carries it without being affected, before passing it onto the final host of the parasite. In this case it is carried by the mosquito which transfers it via its bite in saliva.
Symptoms upon infection include fever, headache, vomiting and fatigue.
The reproductive cells of the parasite (gametocytes) fuse inside the mosquito which then delivers the sporozoites into the host via the bite. The Plasmodium falciparum sporozoites get taken up by the lymphatic system in a human, and later they pass through the liver where they asexually reproduce, and then travel to red blood cells before releasing their gametocytes again.
Their reproduction in the liver doesn’t bring any symptoms, but once they invade red blood cells and destroy them to invade more red blood cells, fever occurs in waves as new parasites move from the liver cells into red blood cells.
Red blood cells can become sticky due to so-called adhesion knobs on their surface. The parasite can easily hide inside the red blood cells to evade the host’s immune system, as well as in the liver.