Overview of Pathogens and Disease

• Lisa Langford

A pathogen is a microorganism capable of causing infections diseases or illness to its host. Viruses thrive in various environments such as air, surfaces and soil.

Many viruses gain entry via the respiratory route. Airborne pathogens such as the common cold (rhinovirus) and influenza, passed on from human to human, become suspended in the tiny droplets from the nose during sneezing and then travel down into the alveoli of the lungs of its new host. Food borne pathogens such as bacillus cereus can cause severe vomiting and diarrhoea and even death if dehydration is extreme. This kind of bacteria multiplies quickly in room temperature. The symptoms caused by bacillus cereus are similar to those caused by staphylococcus aureus though it can also be transmitted via the skin.

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TAQ 2

Stop the spread now! Cholera is an acute diarrheal disease which can rapidly result in profound, progressive dehydration and death. Native to the Ganges in India it is widespread in parts of Africa and South America. Cholera is caused by the gram-negative, comma-shaped organism V.cholerae.

Infection and spread

Cholera is referred to as a classic water borne disease, however it is also transmitted via contaminated food, drink and by hand-mouth contact (faecal –oral route) when the organisms are ingested. The disease easily and rapidly spreads in situations of poor sanitation, over-crowding such as refugee camps and in situations when water becomes contaminated.

Cholera Cell

Image drawn from sciene-art. [online] Available at: http://www.science-art.com/image/?id=5302&pagename=The_Infection_Cycle_of_Vibrio_cholerae

Symptoms

Gastrointestinal infections such as cholera present a number of symptoms. The cardinal features which would prompt immediate medical attention include: watery diarrhoea (due to the action of the cholera peptide toxin), excessive vomiting, severe dehydration and abdominal pain.

Treatment

Commence immediate oral-rehydration therapy with sugar or starch, water and electrolytes. This will massively reduce the risk of mortality. Antibiotics can help patients recover quicker and decrease diarrhoeal purging.

Prevention

Ensure safe water supply, improve sanitation, ensuring food is safe for consumption through correct food handling procedures. Encourage cooking at high temperatures to kill bacteria and education on the disease through community outreach work.

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Image drawn from Boots WMD. [Online] Available at: http://www.webmd.boots.com/foot-care/athletes-foot-tinea-pedis

Athlete’s Foot also called Tinea Pedis is a mildly contagious, common fungal skin infection causing superficial skin damage and discomfort.

Infection and spread

The fungi thrive in moist warm environments and feed on keratin, a protein found in hair, nails and skin. Spread by skin to skin contact, sharing towels or walking barefoot through changing rooms. The fungi can spread in environments such as leisure centres because of the moist, warm climate.

Symptoms

Skin around the little toes becomes red, itchy and scaly. It can spread quickly to all of the toes on both feet. Painful blisters can also occur on the bottom of the feet. If left untreated it can spread to other areas of the body when scratched followed by other body parts being touched. In rare cases it can cause liver inflammation.

Treatment

Topical antifungal creams, powders or sprays depending on the severity.

Prevention

Self care: Wash regularly, dry in between the toes thoroughly, wear clean dry socks, made from breathable material, to ensure effective ventilation and don’t share towels with others. Whilst in environments such as swimming pools wear flip flops or socks in wet areas. Wash hands thoroughly after applying topical creams to prevent cross infection to other areas.

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Image drawn from Wikipedia. [Online] Available at: http://upload.wikimedia.org/wikipedia/commons/f/fa/Plasmodium.png

Malaria (Plasmodium) is caused by a sporozoan parasites of the genus Plasmodium which is only able to divide once inside the host cell. It is a tropical disease spread by mosquitoes. Malaria is often misdiagnosed as being influenza, viral hepatitis, epilepsy and viral encephalitis.

Infection and spread

Infection is caused by Sporozites from the saliva of a biting anophelene female mosquito. Sporozites travel to the bloodstream or the lymphatic system of the recipient. They then migrate to the liver cells where they feed, grow and asexually reproduce. A deadly infection can ensue following just one bite.

Life cycle of Malaria (Plasmodium) Image drawn from wikipedia. [Online] Available at: < http://upload.wikimedia.org/wikipedia/commons/9/9a/Life_Cycle_of_the_Malaria_Parasite.jpg>

Symptoms

Symptoms usually occur with one to two weeks of being bitten. Symptoms include high fever, muscle pains, vomiting and diarrhea. In extreme life-threatening cases patients fall into a coma caused by cerebral malaria and can also suffer renal failure.

Treatment

The WHO, advocate the use of a combination of two or more different classes of anti-malarial drugs to fight the disease. Patients should immediately be transferred to the highest level of care available. Blood glucose levels should be checked regularly especially in children, pregnant women and in the severely ill patients.

Prevention

Personal precautions aimed at reducing exposure to mosquitoes (wearing long sleeves and trousers) staying inside after dusk, mosquito nets sprayed with insect repellant and chemoprophylaxis. Travelers are advised to begin taking malaria prophylaxis one week before travel and continue for 4 weeks after return.

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Image drawn from Molecular expressions. [Online] Available at: < http://micro.magnet.fsu.edu/cells/viruses/influenzavirus.html>

Influenza is an RNA virus with a segmented genome. Influenza epidemics and endemics occur from the A-C virus groups.

Infection and spread

Influenza is transmitted via the respiratory tract spread from person to person through coughs and sneezes. It can also be spread by touching an infected surface and then touching the eyes or mouth. Someone can be infectious even before they have begun to show symptoms.

Image drawn from Quick flu diagnostic tests. [Online] Available at: < http://www.geldin.com/wp-content/uploads/2013/12/Flu-sumptoms.jpg>

Symptoms

Symptoms are similar to the common cold though the respiratory symptoms are more serious. Fever, lethargy and myalgia are also common features.

Treatment

Patients are advised to stay at home and keep hydrated and warm and to contact their GP if complications occur. New treatments such as the neuraminidase inhibitor oseltamivir may reduce the duration of symptoms.

Prevention

A vaccine is available but it is only effective against previously isolated strains. The vaccine is therefore only offered to those in high risk groups, i.e asthmatics, the elderly and those with cardiac disease. Vaccines can cause the patient to suffer from symptoms of the flu though they are not going to be as severe as a full blown flu virus infection.

“How does the body defend itself against pathogens which cause infectious diseases?”

Our bodies are constantly under attack, every second of everyday, our immune systems do not rest. In order for pathogens to penetrate the body they must first encounter a number of physical and biochemical barriers through active or passive means.

Our first lines of defence from invasive microbes include the epidermis and keratinocytes of the skin. Other physical barriers include mucous membranes which line all of the body’s cavities which come in to contact with the open environment, such as the respiratory, gastro-intestinal and urogenital tracts. Goblet cells within the mucous membrane epithelium secrete an estimated 4 litres of mucous a day. In the respiratory tract the mucous traps inhaled bacteria, fungi and other particles. Also within the respiratory tract are millions of hairs (cilla) lining the airways, which help to keep the passageways clear of debris. Mucosal surfaces of the moist epithelium allow the exchange of molecules within the environment whilst also resisting microbial invasion. The urinary tract itself is a physical barrier to pathogens in a similar way the respiratory tract is, it expels in an outward movement. Urination works to inhibit the movement of microbes from the environment up into the kidneys and bladder. The washing action of tears and saliva prevent attachment of microbes to the epithelial surfaces.

Our skin also has a role as a chemical barrier against pathogens as sweat glands secrete oils, some of which are slightly acidic which inhibit the growth of some bacteria. The stomach, urinary tract and tear ducts also play a roll in the chemical barrier as they all have antimicrobial properties, enzymes which break down bacteria to help keep these areas free from infection.

Once pathogens have penetrated our first physical lines of defence they are met by our innate immune system, our second line of defence, which we are born with. An example of your innate immune system at work is when you cut yourself and notice in a few hours that the site of the cut is red and swollen. This is a sign that white blood cells or leukocytes have rushed to the site, to help fight any infections and begin the healing process of the damaged tissue. Leukocytes patrol the tissues and organs of the body via the lymphatic and blood circulatory systems like the soldiers of the immune system. The most virulent of these cells is called a macrophage. They’re sole purpose is to search and destroy bacteria. They don’t just happen upon bacteria in the body the have sensors or receptors which are tuned to recognise “danger molecules”.

Lymphocytes are the cells involved in the adaptive immune response, specifically called T cells and B cells. The adaptive immune system is a defence system that can adapt to protect us against almost any invader. There are three main types of T cells, killer T cells, known as CTLs, helper cells and regulatory T cells. A large number of T cells are produced in the primary lymphoid organs/tissues, derived from stems cells in the bone marrow. Each T cell has receptors specific for only one antigen. T-cells which survive the selection process migrate to the peripheral lymphoid tissues where they complete their functional maturation and provide protection against invading microbes. B cells develop from hemopoietic stem cells primarily in the micro- environment of the foetal liver and, after birth in the bone marrow. There are two kinds of B cells B1 and B2. B & T cells are selected by specific antigens to go on to take part in an immune response. This process of selection is called the clonal selection. When B cells are activated by an antigen, sometimes with T cells help (helper cells), they multiply and mature into memory cells or plasma cells. Cytoxic T-cells or CTL’s play an integral role in killing virus infected cells. They release perforin and granzymes on to the surface of the target cells to induce direct cell death (apoptosis). T cells are not adept at recognising and so responding to free antigen. They can only see an antigen when it is presented to them. This process is called antigen presentation and is facilitated by carrier molecules, such as MHC’s (Major histocompatability complex proteins).

An antigen that can cause an allergic reaction is called an allergen. During an allergic reaction, histamines and other chemicals are released from mast cell granules, which increase capillary permeability so that fluid escapes from the capillaries in the tissue. This is the reason you get a runny nose and watery eyes when you have an allergic reaction. Histamine binds to two types of receptors on target cells H1 and H2. After binding to H receptors, histamine induces contraction of smooth muscle, increases vascular permeability and mucous secretion by goblet cells. Via H2 receptors, histamine increases gastric secretion, decreases mediator release by basophils and mast cells.

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References

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Inglis, TJ 2003. Microbiology and Infection. London, UK, Churchill Livingstone P91-2

Image. Illustrator unknown. Life cycle of the malaria parasite. [online] Available at:http://en.wikipedia.org/wiki/File:Life_Cycle_of_the_Malaria_Parasite.jpg [Accessed 4th Feb 2015]

World health organisation, 2002. Prevention of hospital-acquired infections. [online] Available at: http://www.who.int/csr/resources/publications/whocdscsreph200212.pdf [Accessed 9th Feb 2015]

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