Air borne bacterial diseases

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.