A number of diseases are transmitted
through air. Such diseases are called airborne diseases. Airborne diseases are
divided into bacterial, viral and fungal diseases. Microorganisms released as
droplets, Droplet nuclei,Infectious dust and spores are easily disseminated
through air. Airborne bacterial diseases are mainly respiratory tract
infections transmitted by droplet inhalation or by contact with contaminated
inanimate objects. Upper respiratory tract consists of nostrils, pharynx and
tonsils. Lower respiratory tract consists of larynx, trachea, bronchi,
brachiolus, alveoli and lungs. Transmission of fungal diseases is more through
air. But the airborne bacterial diseases are more in number. Common airborne
bacterial diseases are as follows.
1.
Brucellosis
2.
Meningococcal meningitis
3.
Meningitis by Haemophilus
influenzae
4.
Pulmonary Anthrax
5.
Diseases by Streptococcus
6.
Atypical pneumonia
7.
Psittacosis
8.
Diphtheria
9.
Pertussis (Whooping cough)
10. Tuberculosis
11. Legionellosis
12. Tularemia
1. Brucellosis
Brucellosis is also known as
“undulant fever” because of its characteristic raising and falling nature of
temperature.Brucellosis is a zoonotic disease caused by Brucella species.
Brucella species includes B. abortus, B.
melitensis,B. suis and B. canis. They are tiny, faintly staining Gram
Negative coccobacilli. Brucellosis is a disease of humans, animals and birds,
mainly caused by B. suis, commonly occurring in infected swine. Inhalation of
aerosol during processing of infected swine causes infection in humans.
Infection is commonly found in veterinarians, butchers and slaughterhouse
workers. Hence it is considered as an occupational disease. In humans the
disease is characterized by prolonged and undulating fever, headache, chill,
generalised aches and pains of muscles and joints. Mortality rate is low, less
than 2%.
2. Meningococcal meningitis
Meningitis is the inflammation of
meninges. Meninges is the three layered membrane covering the spinal cord and
brain. The term meningitis is derived from a Greek word. “Meninx” means Brain and “itis” means inflammation. This
is considered as a potentially serious
disease as the microbial invasion is to the nervous system. Meningitis is
divided into bacterial or septic meningitis and aseptic meningitis.
Meningococcal meningitisIs a fatal septic meningitis caused by Neisseria meningitidis. Neisseria
meningitidis Is a gram-negative diplococci.
About 10% of population are healthy
carriers and harbour this bacteria in the back of the nose and throat. Healthy
carriers, Convalescent carriers and infected persons release this cocci as
respiratory droplets by activities like coughing, sneezing, kissing, talking
etc. Disease is also transmitted through sharing of utensils. No animal
reservoir is found for this disease. Inhalation of these bioaerosols will lead
to the infection
Organism first colonize in the
nasopharynx and penetrate the mucosal barrier to reach the blood stream. They
cross the blood-brain barrier and enter cerebrospinal fluid (CSF) and cause
meningitis.
3. Meningitis by H. influenzae
Mode of infection and disease are
the same as above. But the infection is commonly found in children between 6
weeks and 2 years of age. The causative agent is Haemophilus influenzae which is a Gram Negative coccobacilli.
Infection can also cause secondary pneumonia in patients.
4. Pulmonary Anthrax
This is also called ‘Wool sorters
disease”. Pulmonary anthrax is mainly a disease of herbivorous animals. It is a
highly infectious animal disease that can be transmitted to humans by direct
contact with infected animals like a cattle,goat, sheep etc or through their
products.
Anthrax is of three types. Human
infections through cut or abrasion of skin lead to cutaneous anthrax. If spores reach gastrointestinal tract, it
cause gastrointestinal anthrax. Inhalation of the spores leads to pulmonary
anthrax or wool sorters disease. Pulmonary anthrax is seen only in human
beings. Causative agent is Bacillus
anthracis which is large, Gram Positive, aerobic, endospore forming
bacteria. It is world wide in its distribution. Disease transmission is by
inhalation of the dust contaminated by animal products. Usually through the
sorting or combing of raw wool contaminated with spores. Spores are usually 1
to 2 to µm in diameter and can enter the
lower respiratory tract easily and lodge in the alveolar spaces. There they are
engulfed by alveolar macrophages. But can resist and survive the
phagocytosis, germinate within the endosome. Bacteria then spread to regional
lymph nodes and eventually reach the bloodstream. This will result in pulmonary
anthrax which is characterized by massive pulmonary edema, hemorrhage and
respiratory arrest.
5. Diseases by Streptococcus
A number of airborne bacterial
diseases are caused by Streptococcus species.
Streptococci are large group of Gram Positive cocci and
most significant one causing air borne infections is S.pyogenes. Infection is either due to the multiplication of
pathogen or due to the toxin produced by the
organism. Infection may range from mild diseases like most common “Strep throat” to life threatening
conditions. Organisms reside in the nose and throat and are transmitted through
respiratory droplets.
a) Streptococcal
pharyngitis
This disease is commonly called
Strep throat or tonsillitis or sore throat. It is a common mild infection
predominantly found in children of 5 to 15 years of age. Symptoms include
fever, headache, red and sore throat, enlargement of lymph nodes of neck. Often
results in bleeding throat.
b) Scarlet fever
Sometimes Streptococcal pharyngitis proceeds to Scarlet fever. This condition
is due to the production of “erythrogenic toxin” by Streptococci.
The disease characterized by strawberry tongue and red skin rashes.
c) Rheumatic fever
3% of untreated upper respiratory
tract infections by Streptococcus
pyogenes and repeated early childhood infections may lead to a condition
known as Rheumatic fever. This disease is characterized by inflammation and
degeneration of heart valves and joints.
d) Streptococcal pneumonia
About 60-80 % of all respiratory
diseases known as pneumonia are caused by Streptococcus
pneumoniae. Infection occurs in those individuals with predisposing factors
such as viral infections of respiratory tract like common cold, or physical
injury to the respiratory tract, alcoholism, or diabetics. Causative agent Streptococcus
pneumoniae is a gram-positive organism, normally found in the upper
respiratory tract. Virulence factor of this organism is the capsular
polysaccharide composed of hyaluronic acid. Capsular polysaccharide protects
the organism from ingestion and killing by phagocytes. Thus bacteria will
multiply in alveolar spaces and also produce toxins called “pneumolysin” which
may destroy the host cells. Alveoli get filled with blood cells, fluids and
become inflamed. Lung inflammation is a characteristic feature of this disease.
Sputum is often rust-colored due to the blood coughed up from lungs. Onset of
the disease is a abrupt with chills, hard laboured breathing and chest pain.
Inhalation of aerosols liberated from the infected person leads to infection.
6. Atypical pneumonia
Is an infection of lower respiratory tract and
also called “walking pneumonia”. Causative agent is Mycoplasma pneumoniae. Symptoms are gradual in onset and are milder
than typical Pneumonia. Symptoms include
persistent cough, headache, fever, sore throat, chest pain while deep
breathing etc. Cough, sneeze etc will liberate bioaerosols from the patient and
inhalation of these will lead to infection.
7. Psittacosis
This infection is mainly acquired
from birds. Infection may range between mild respiratory diseases to severe
pneumonia. Infection may sometimes lead to encephalitis, coma and convulsions.
Sometimes fatal infections may occur. Causative agent is Chlamydia psittaci. Pathogen is liberated as infectious dust from
the infected avian faeces. Inhalation of the infectious dust will cause disease
in humans. Hence this is considered as an occupational disease of poultry
workers and handlers.
8. Diphtheria
Causative agent is Corynebacterium diphtheriae which is a
gram positive bacilli. Is mainly found in children. Toxins produced by these
microbes cause generalized toxemia in patients. Infection occurs in the upper
part of the respiratory tract and nose, throat and tonsils get inflamed. Also lymph glands in the neck region swell
and result in “bull neck appearance”. Powerful exotoxin produced by the
pathogen destroys the cells cof the epithelial lining. Thus the dead cells, mucus and scavenger cells
pile up and form a pseudo membrane. This membrane is leathery in consistency
and cause respiratory blockage and death by suffocation, particularly in young
children. Tracheotomy (Cutting hole in throat) are performed to save children.
Sometimes, toxin diffuses to blood stream, causing widespread damage
particularly to the heart.
9. Pertussis (Whooping cough)
Is highly infectious and potentially
lethal disease caused by Bordetella
pertussis which is a Gram negative coccobacilli. This disease mainly
affects infants and young children below 4 years of age. No non human reservoir
for this disease and infected children act as source of infection. From
infected person the pathogen is liberated as aerosols through activities like
talking, coughing, sneezing, laughing etc. Bacilli
binds to ciliated epithelium of upper respiratory tract and secrete toxins. The
function of toxin is to clear mucus from air passages and hence they damage
cells. Net effect is the build up of stick glue like mucus in airways. This
will result in common cold like symptoms followed by spasms of violent,
hacking, persistent, recurrent cough with at least 15 to 20 coughs at a time.
This is actually an attempt to remove the accumulated mucus from the
respiratory tract. But these episodes result in oxygen deficiency, triggering
deep and rapid inspirations through the partially obstructed air passages,
resulting in characteristic ‘whoop’. Coughing may be so violent that it may
cause vomiting, hemorrhage and even
brain damage. It may last for several weeks or months.
10. Tuberculosis
Tuberculosis or TB is a severe lower
respiratory tract disease. Causative agent is Mycobacterium tuberculosis which is an acid fast bacteria. These
bacteria have high mycolic acid content in the cell wall which makes a waxy
coating on its surface and organisms are seen as clumps. These are highly
virulent bacteria and even a single organism is
sufficient to initiate an infection. In a susceptible host, the
inhalation of a single particle carrying a viable tubercle bacillus and small enough to reach the lungs is capable of
producing infection. Disease is characterized by loss of appetite, fatigue,
weight loss, night sweats and persistent cough. Inhalation of aerosols
liberated from the patient is a mode of infection.
11. Legionellosis
This is also a lower respiratory
tract infection. This disease is considered as a type of bronchopneumonia or
atypical pneumonia. Causative agents are coming under Legionella species. Legionella species Are Gram Negative
bacteria found naturally in fresh water bodies. They may contaminate Air
conditioners, cooling towers or showers and cause infection. No person to
person transmission of disease occurs.
Legionellosis is of two types. Pontiac fever and Legionnaires
disease.
Pontiac fever is a self limiting, non-fatal disease. It is caused mainly
by Legionella pneumophila.
Symptoms include fever, chills, dry cough and headache with mild upper respiratory tract infections
resembling acute influenza. This
resolves spontaneously and often goes undiagnosed. Inhalation of mist from
water sources like air conditioners, cooling towers, showers etc., contaminated
with the pathogen is mode of infection.
Legionnaires disease, unlike Pontiac fever, is severe bronchopneumonia and can be
fatal. Here also the causative agent is Legionella
pneumophila.
Symptoms include fever, chills, dry cough, vomiting and diarrhoea. Pathogen occurs in natural air and in
freshwater. At times they enter and proliferate in cooling towers, air coolers
and showers. Praying and splashing of water containing pathogens may produce
aerosols which are disseminated in air and inhalation causes infection.
12. Tularemia
Causative agent of the disease
is Francisella
tularensis, which is a Gram negative bacilli. This is one of the most
virulent bacteria, inhalation of as few as 10 organisms is sufficient to
establish the disease. It is also known as rabbit fever as it is a zoonotic
disease transmitted from natural reservoirs of small mammals like rabbits, mice and squirrels. Here also,
there is no transmission of disease from person to person. Inhalation of dust
particles liberated from the infected animals result in severe respiratory
illness leading to pneumonia.
