Other pathogenic agents: fungi, viruses and protists

In a nutshell

There are many other pathogenic agents, apart from bacteria, that are able to cause human infection. These include stem rust fungus, influenza virus and Plasmodium spp.

Stem rust fungus

Stem rust fungus is a fungal infection caused by Puccinia graminis which affects cereal crops. Its name derives from the fact it infects the plant's stem and covers it in a layer of a substance that is the colour of rust.

1.	Stem rust fungi produce dominant spores that are carried in the air.
2.	Once they land on a new plant, these spores germinate and produces hyphae which are threadlike structures that can form masses known as mycelium.
3.	The hyphae are able to secrete digestive enzymes that digest chemicals in the stem.
4.	These digested chemicals provides nutrients to the fungi.

Damage

The fungus is able to cause damage in a variety of ways. Firstly, it can weaken the stem to the plant falls over and the cereal crop cannot be mechanically harvested. It also uses nutrients that the plant needs for it's seeds, this reduces the harvest and it breaks the outer epidermis of the plant's stem. This increases the amount of water lost by the plant and makes them more vulnerable to other infections.

Controlling the spread

There are a few ways that the spread of stem rust fungus can be controlled.

Method	Description	Limitation(s)
Fungicides	Stem rust fungus can be killed using fungicides.	Fungicides are expensive and are therefore not accessible to poor farmers in low-income countries. Equally, fungicides can cause damage to local ecosystems as some fungi that are essential to plants will also be killed.
Barberry eradication programme	The lifecycle of the stem rust fungus depends on the barberry plant. This is why in North America they began a barberry eradication programme.	This reduced infection rates but did not eliminate the fungal infection as spores were still able to travel in the wind over very large distances.
Gene manipulation	Scientists could edit the genes of the cereal plants to insert fungus-resistant genes.	Currently, this has only been used in Australia.

Influenza

Influenza is the virus that causes the flu.

Biology; Microbiology and pathogens; KS5 Year 12; Other pathogenic agents: fungi, viruses and protists

1.	RNA genome	Eight short strands of RNA referred to as a segmented genome.
2.	Capsid	Protein coat.
3.	Envelope	Lipid and protein.
4.	Glycoprotein antigens	The H glycoprotein (haemagglutinin) helps the virus enter host cells and the N glycoprotein (neuraminidase) helps newly formed virus particles escape from the host cell.

Viral replication

Biology; Microbiology and pathogens; KS5 Year 12; Other pathogenic agents: fungi, viruses and protists

1.	Goblet cells secret mucus which protect the epithelium.
2.	The antigen on the influenza particle is able to hydrolyse the mucus which allows the virus to contact the membrane.
3.	The viral antigens attach to receptors on the epithelia and this triggers endocytosis.
4.	Once inside the cell, the RNA segments enter the nucleus where transcription and replication can occur.
5.	The viral proteins are replicated.
6.	New viral particles are assembled.
7.	The virus exits the host cell and takes a lipid coat with it.
8.	Virus causes new infection.

Antigens

Influenza groups are named based on their antigens - the H and N glycoproteins on their outer surface.

Example

The Spanish flu pandemic of 1918 was caused by the H1N1 strain of influenza.

Influenza has a high mutation rate meaning changes in the viral genome occur frequently. This is why there are so many strains of influenza, each with different H and N glycoproteins. Consequently, the immune system has little or no resistance to different influenza strains; existing antibodies no longer bind to the new antigens. Therefore, influenza vaccines are updated annually and administered to the most vulnerable populations.

This high antigenic variability is the reason why influenza has caused many epidemics.

Plasmodium spp.

Plasmodium spp. are the protists that cause malaria. Malaria is endemic, meaning it occurs frequently or is constantly present, in tropical and sub-tropical regions of the world. This includes sub-Suharan Africa.

There are many different species of Plasmodium that are found in different parts of the world. One species, Plasmodium falciparum, is responsible for $90\%$ of malaria infections.

Biology; Microbiology and pathogens; KS5 Year 12; Other pathogenic agents: fungi, viruses and protists

1.	Sporozoites enter via bloodstream
2.	Liver stage
3.	Red blood cell stage

1.	Transmission occurs when a female Anopheles mosquito inserts her mouthparts through the skin and into the blood vessels of a human.
2.	This injects her saliva directly into the bloodstream. The saliva contains anticoagulants to allow the mosquito to draw up blood without it clotting. It is this process that causes inflammation and the characteristic itching of a mosquito bite.
3.	If the mosquito is infected Plasmodium will be injected into the human as well. At this point, the Plasmodium spp. will be in their infective stage and they are referred to as sporozoites.
4.	The sporozoites then enter liver cells and red blood cells in their human host. They rapidly divide and form many daughter cells called merozoites.
5.	The liver and red blood cells eventually burst, which releases them into the blood.
6.	The merozoites can then keep infecting red blood cells. This is why anaemia is often seen in severe cases of malaria.
7.	The cycle then repeats when a new mosquito feeds on an infected person.