4.1.2 Transmission of Communicable Pathogens

Disease transmission

• In order for a population of pathogens to survive, they must be able to successfully transfer from host to host
  • If pathogens are unable to find new hosts then they will go extinct

• Disease transmission is defined as the transfer of pathogens from an infected host to an uninfected host
• Transmission can be very risky for pathogens
  • During the infective stages, pathogens produce a large number of individuals to increase the likelihood that some will find a new host and survive

• There are two types of disease transmission:
  • Direct - from one host to another host
  • Indirect -  a second organism (vector) that is unaffected by the pathogen transfers it to a new host

Direct transmission

• The direct transmission of a pathogen can involve physical contact between individuals
  • If the leaves of plants infected with Tobacco Mosaic Virus (TMV) touch the leaves of another uninfected plant, particles of the virus are transmitted

• Sometimes individuals being within close proximity to each other is sufficient for direct transmission
  • The influenza viruses are spread in the air via tiny droplets of water. An infected individual breathes out droplets containing the virus and they are breathed in by an uninfected individual

• Spores can also be involved in the direct transmission of pathogens
  • Spores are very small reproductive structures that are released into the environment. They are dispersed via wind or water
  • Once they reach a food source (host) they begin growing
  • Depending on the organism, spores can be produced via mitosis or meiosis so they can be haploid or diploid
  • P. infestans which causes potato blight produces specialised spores called sporangia. These structures are adapted for wind dispersal

Transmission of HIV/AIDS

• Human Immunodeficiency Virus is a retrovirus
• The HIV virus is not transmitted by a vector (unlike in malaria)
• The virus is unable to survive outside of the human body
• HIV is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
• This means HIV can be transmitted in the following ways:
  • sexual intercourse
  • blood donation
  • sharing of needles used by intravenous drug users
  • from mother to child across the placenta
  • mixing of blood between mother and child during birth
  • from mother to child through breast milk

Transmission of tuberculosis (TB)

• When infected people with the active form of the disease cough or sneeze, the Mycobacterium tuberculosis bacteria enter the air in tiny droplets of liquid
• TB is transmitted when uninfected people then inhale these droplets
• TB, therefore, spreads more quickly among people living in overcrowded conditions
• The form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milk
• Very few people in developed countries now acquire TB in this way, although meat and milk can still be a source of infection in some developing countries

Indirect transmission

• Vectors are involved in the indirect transmission of pathogens
• A vector is any organism that transfers a pathogen from an infected individual to an uninfected individual
  • The vector themselves usually aren't harmed by the pathogen

• A lot of disease vectors tend to be insects
  • Insects are ideal vectors as they reproduce in large numbers which increases the likelihood of pathogen transmission

Transmission of malaria

• Malaria is caused by one of four species of the protoctist Plasmodium
• These protoctists are transmitted to humans by an insect vector:
  • Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs
  • If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal
  • When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood

• Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
• Plasmodium can also pass from mother to child across the placenta

The life cycle of Plasmodium falciparum

Factors that affect disease transmission

• The transmission of disease ultimately depends on:
  • The presence of the pathogens
    • If the pathogen is not present in the population then it cannot spread

  • The presence of susceptible individuals
    • A high number of immune or resistant individuals in a population will reduce the likelihood of transmission

• Animals and plants can be resistant to some diseases
  • Individuals who are resistant have mechanisms that prevent the infection or spread of pathogens within their body. They are not susceptible to disease.
  • These mechanisms are coded for by their genes
  • Individuals who are heterozygous for the sickle cell allele have resistance to malaria
  • Some humans are even resistant to HIV

• Resistance is different to immunity
  • When resistant individuals are exposed to the pathogen for the first time they do not develop the disease and suffer no symptoms
  • An immune individual has been previously infected with the pathogen, suffered symptoms of the disease and recovered. They are highly unlikely to develop symptoms of the disease when exposed to the pathogen again

• The proportion of resistant or immune individuals in a population influences the potential for transmission (as they are not susceptible to disease)
  • The higher the proportion, the lower the probability of transmission

• Different types of disease transmission are affected by different factors
• Human pathogens are affected by specific factors dictated by human behaviour and population size

Factors affecting direct transmission

• Pathogens that spread through direct contact or by droplet infection need potential hosts to be within close proximity to each other
• Places or areas with high population densities are more likely to have high infection rates
  • E.g. cities and schools

• Tuberculosis (TB) transmission is very high in places where many people have to sleep in confined quarters
  • E.g. poor housing and homeless shelters

• Farmers who use monocultures to maximise yield and profit can experience large disease outbreaks
  • Farmers grow a large number of crop plants in a small area
  • As the crops grow the leaves of different plants touch each other, making the transmission of pathogens such as tobacco mosaic virus (TMV) very easy

Factors affecting indirect transmission

• The indirect transmission of a pathogen can be affected by the biology of the vectors involved
  • Common disease vectors include mosquitoes and aphids

• The population of vectors (usually insects) is influenced by weather and climate

Factors affecting the distribution of malaria

• Malaria is caused by one of four species of the protoctist Plasmodium but these protoctists are transmitted to humans by an insect vector (female Anopheles mosquitoes)
• The Anopheles mosquitoes favour habitats that have high rainfall, high temperatures and high humidity
• This means malaria can occur where these mosquitoes are present and, as a result, is found throughout the tropics and sub-tropics (about 80% of cases are in Africa)
• The Anopheles mosquitoes found in Africa also have longer lifespans and prefer biting humans than animals
• In the 1950s, the World Health Organisation (WHO) coordinated a worldwide eradication programme. Whilst malaria was eradicated from some countries, the programme was mainly unsuccessful because:
  • Plasmodium became resistant to the drugs being used to try and control it
  • Anopheles mosquitoes became resistant to DDT and other insecticides being used against them

• There is evidence that there are an increasing number of malaria epidemics due to climatic and environmental changes that favour the spread of the Anopheles mosquitoes
• A social factor that has caused the number of cases of malaria in Africa, in particular, to increase in recent years is the increased migration of people due to war (when migration happens due to war the parasite can be transferred from areas that have the infection to new regions, and, if the Anopheles mosquito is breeding in the new region, then the mosquito vector will transfer the disease from one human to the next)

Factors affecting the transmission of human diseases

• The degree or level of poverty in an area often correlates with the transmission of human diseases in that area
• Water-borne disease like typhoid, cholera and polio spread when human faecal matter enters and contaminates drinking water
• Those below the poverty line usually live in areas with crowded housing with no sewage systems, sanitation facilities or water treatment facilities. In addition, many people in these areas have limited access to hygiene products
• In the last 200 years, humans have spread across the globe, bringing their diseases and pathogens with them
• The level of human movement and migration that currently exists means that populations are more connected than ever
  • In the past, the ocean and bodies of water would have acted as natural geographic barriers to prevent the spread of pathogens
  • The first flu pandemic in 1918 took one year to spread around the globe. The flu pandemic in 2009 only took 3 months to reach West Africa from North America
  • An individual can become infected in one country (where the disease exists) and get a flight to another country thousands of miles away. They may not show any symptoms until they have already arrived in the new country

• The historical danger of human migration is well known
  • When colonisers arrived in the Americas they brought many European diseases with them, such as smallpox
  • The Native Americans had no immunity or resistance as they had never been exposed to these pathogens before (they were a fully susceptible population)
  • The invasive pathogens rapidly spread through the population causing a large number of deaths

• The behaviour or cultural practices of humans can also affect the transmission rate of diseases
  • For example, in parts of Africa, it is a religious and cultural tradition to touch and kiss the dead. This was a major problem during Ebola outbreaks there and scientists had to work with the public to try and inform them that this tradition was increasing the spread of the virus