Ziehl-Neelsen stain of M. tuberculosis in sputum
In 1962 Wells and Riley demonstrated the “droplet nuclei” theory for transmitting TB by positioning guinea pigs in areas above the wards where patients, who were in the coughing and sneezing stages of disease, were isolated. Small particles of viable bacteria were carried (in drops of moisture) through the air circulating to the animals housed above the patients, leading to infection of the guinea pigs with the same TB strains. A causal relationship was established when the antimicrobial susceptibility patterns of patients and guinea pigs exactly matched.
The test used to assess the transmission status of TB patients is the acid-fast bacillus (AFB) smear prepared from the patient’s sputum. When positive, as shown in the image, the patient can transmit droplet nuclei with the contagious bacteria during episodes of coughing, talking, or sneezing. The same diagnostic criteria are used to determine the length of isolation or hospitalization required before patients are considered non-infectious. From the 1960s to the mid-1980s, the antimicrobial drug combinations were effective against TB; the disease appeared to be appropriately managed and unlikely to exhibit further out-of-control conditions, particularly in the US. The closing of sanatoria and the success of antibiotics caused a dramatic decrease in incidence and transmission, promoting a relaxed public health attitude toward TB.