6.7 Pathogens: Routes of Entry

• In order for a pathogen to cause disease it must enter the body of the host
  • Body openings, e.g. the mouth, eyes, and urinary tract, provide easy access for pathogens to enter
  • Pathogens may enter directly into the bloodstream through breaks in the skin
• Pathogens may be transmitted in a variety of ways

Vectors

• These are living organisms that carry pathogens and transmit them between hosts
• Insects, such as flies and mosquitoes, are common vectors for diseases like malaria and yellow fever

Inhalation

• Droplets from the respiratory tract will be suspended in the air when an infected person coughs, sneezes or talks
• These droplets contain pathogens that can be inhaled by healthy people
• The airways provide an entry point into the respiratory system of a new host and another infection occurs, e.g. flu, measles, tuberculosis

Ingestion

• Pathogens can enter through the digestive system when we ingest contaminated food or drink
• This is especially probable if food is undercooked, as heat destroys most of the pathogens 
• These pathogens can make their way through the lining of the gut and cause disease (e.g. cholera, Salmonella poisoning)

Indirect contact

• Inanimate objects can contain large numbers of pathogens that may be transferred between hosts
  • An infected individual may touch or cough on an object which is later touched by a healthy individual who transfers the pathogens to their mouth or nose by touching their face
• Examples include bedding, towels, and surfaces

Direct contact

• Pathogens that spread this way will require some part of the host, e.g. skin, body fluids, to come into direct contact with a healthy individual
• Pathogens that spread by this route can then pass through the mucous membranes and enter the bloodstream, e.g.
  • When shaking hands with another person who then puts their hand to their nose or mouth
  • During sexual transmission
• Examples include HIV, ebola, syphilis

Inoculation

• This typically occurs when a pathogen enters the body through broken skin, providing it with a direct route into the bloodstream
• Transmission could be through sexual contact, sharing needles during drug use, or bites or scratches from infected animals
• Examples include hepatitis B, HIV, tetanus, and rabies

Skin

• The skin provides a physical barrier against infection
• If the skin is damaged it leaves the exposed tissue beneath vulnerable to pathogens
• The blood clotting mechanism of the body plays an important role in preventing pathogen entry in the case of damage to the skin
• Blood clotting takes time, however, so a few pathogens may still enter before a clot forms

Microorganisms of the gut and skin

• Collectively these harmless microorganisms are known as the gut or skin flora
• They compete with pathogens for resources, thereby limiting their numbers and therefore their ability to infect the body

Stomach acid

• The hydrochloric acid that makes up a large part of the gastric juices in the stomach creates an acidic environment that is unfavourable to many pathogens present on food and drink
• Sometimes a few of these pathogens may survive and make their way to the intestines where they infect the gut wall cells and cause disease

Lysozyme

• Secretions of the mucosal surfaces, e.g. tears, saliva, and mucus, contains an enzyme called lysozyme
• This enzyme will damage bacterial cell walls, causing them to burst, or lyse

The body has various barriers that prevent the entry of pathogens