transmission & control

Routes of transmission

~ potential entry points for microorganisms:

• mouth

• respiratory tract

• conjuctiva (thin, clear membrane that protects the eye)

• scratch or injury

• urinogenital tract

• alimentary tract (GI tract)

• anus

• skin

• arthropod (bug bite via vector)

~ the body has ways to rid ‘shed’ these microorganisms from the body as a protective mechanism i.e coughing, sneezing, flushing via urination, passing faeces

in a healthy body…

transmission:

~ dependent on 3 things

• the number of organisms shed/available

• how stable the organism is in the environment

• the infectious dose ⇒ the number of pathogens that are required to infect a host

~ also need to consider the route of entry of the organisms and whether it has the ability to cause disease

• respiratory spread ⇒ microorganims present in the respiratory tract, inhalation e.g. COVID-19, flu

• faecal-oral spread ⇒ pathogens that can be spread through GI tract e.g. norovirus, salmonella, campylobacter

• venereal spread ⇒ sexually transmitted disease

• vector/arthropod ⇒ (1) bite of mosquito e.g. malaria, sandfly fever, (2) vertebrate dogs/cattle e.g. brucellosis bacterial infection, (3) vector-vertebrate where an organism requires several different hosts to develop through several stages of its life cycle e.g. plague

transmission from respiratory tract and oropharynx:

• up to 20,000 droplets produced in a sneeze

• 100s are expelled during a cough

• size of droplets affects how far they travel

• droplets can be INHALED by another

• any means of aerosolising (making microorganisms airborne) organisms is a risk

• oropharyngeal transmission related to exchange of saliva

transmission from gastrointestinal tract

• controllable by public health measures and good hygiene (effective hand-wash strategies)

• the F-diagram ⇒ food, fluids, fields, faeces, fingers, flies

• vomiting/diarrhoea gives an additional advantage - potential aerosol leading to further spread to other individuals

• pathogens are INGESTED

transmission from the urogenital tract:

• urinary tract infections are common - usually endogenous causes (own gut flora/microorganisms from GI tract gaining entry of urogenital tract)

• genital tract infections often spread by mucosal contact between individuals (sexually transmitted disease)- ABSORPTION

• some organisms are present in semen

• perinatal transmission - during birth (mum ⇒ baby during birth)

other modes of transmission:

~ skin

• skin cells, skin flora, and dermatophytes are shed daily

• transmission is rare through shed skin - more common through direct contact

~ milk

• few organisms are shed in human milk e.g. HIV, CMV

• milk from other animals can be a source of infection ⇒ pasteurisation sterilises milk

~ blood

• some agents can be transmitted effectively in the blood e.g. HIV, hepatitis B, C

• risk factors include sharing contaminated needles

• some arthropods inject parasites directly into the blood of their host

• INJECTION

modes of transmission:

1. INHALATION

2. INGESTION

3. ABSORPTION

4. INJECTION

vertical and horizontal transmission:

• vertical transmission ⇒ when an infected individual passes that infection on to their offspring

• horizontal transmission ⇒ transmission via contact, vectors, infected air, water, food e.g. flu, salmonella

transmission from animals:

• humans and animals share a common susceptibility to SOME pathogens

• invertebrates can be passive carriers of microorganisms or form part of the life cycle

• vertebrates can also transmit disease through contact, ingestion or via an invertebrate vector

zoonosis ⇒ a disease which can be transmitted to humans from animals

Biological safety in the laboratory (control measures required for infectious diseases)

hazard groups and containment levels:

~ need to consider;

• what symptoms does a pathogen cause?

• how are the symptoms caused?

• how does the infection spread to other individuals?

• is there treatment available?

~ all of the above dictate how a potential sample/pathogen will be handled in a laboratory

• hazard group 1 organisms ⇒ lab can be containment level 1, organisms are unlikely to cause human disease, requirements for the lab are minimal e.g. standard hygiene, standard PPE lab coats, regular hand-washing and disinfectants available

~ good microbiology techniques

~ open bench work

• hazard group 2 organisms ⇒ containment level 2, these organisms can cause human disease, can be hazardous to staff, students, low risk of spreading to wider community, treatment is available, control measures include controlling who comes into the space, making sure lab surfaces are fully cleaned effectively, appropriate disinfectants, appropriate waste disposal measures (autoclave bags, biobin)

~ good microbiology practice

~ protective clothing ⇒ lab coat, gloves

~ biohazard sign

~ open bench plus bacteriology safety cabinet for potential aerosols

• hazard group 3 organisms ⇒ containment level 3, can cause severe human disease, risk of spreading to wider community TRANSMISSION, suitable treatment available e.g. mycobacterium tuberculosis, salmonella typhi

~ same as level 2

~ plus special clothing

~ controlled access

~ directional airflow (labs kept at negative pressure so air from outside sucked in to lab rather than air from lab escaping

~ bacteriology safety cabinet + other primary devices for all activities

• hazard group 4 organisms ⇒ highest containment level 4, can cause severe human disease, high risk of these agents spreading to wider community, often no suitable treatment available e.g. ebola

~ same as level 3

~ plus airlock entry

~ shower

~ class III bacteriology safety cabinet (fitted with a filter)

~ positive pressure suits

control measures:

• microbiology safety cabinet

• sharps bins

• autoclave to sterilise

• specialised lab coats

• PPE- goggles, gloves, no exposed skin, tying long hair back

• disinfectants

• SOP - standard operating procedures

risk assessment:

~ reduces the likelihood of issues;

1. identify the risk factors

2. who can be harmed and how

3. evaluate the risks (e.g. to what extent can they cause harm, understanding microorganisms and how they can be transmitted))

4. record your findings

5. monitor and review (accuracy, outdated?)

hazard vs risk:

hierarchy of controls:

MOST EFFECTIVE

elimination ⇒ physically remove the hazard

substitution ⇒ replace the hazard

engineering controls ⇒ isolate the people from the hazard (e.g. microbiology safety cabinets which direct airflow away from you)

administrative controls ⇒ change the way people work (lab script, teachers, demonstrators)

PPE ⇒ protect the worker with personal protective equipment (goggles, gloves, howie lab coat)

LEAST EFFECTIVE