The discovery has led to calls for a new approach to identifying possible cases.
A majority of people with tuberculosis do not report having a cough and around one-quarter had no symptoms at all, according to international survey data.
And researchers suggest that given most subclinical TB is not asymptomatic, screening criteria similar to that recommended by the WHO for people with HIV could be adapted for routine use.
The so-called WHO four-symptom screen recommends people with HIV who present with any of cough, fever, weight loss or night sweats be tested for TB.
“We need to really rethink large aspects of how we identify people with TB,” said study-lead Professor Frank Cobelens of Amsterdam University Medical Center in the Netherlands.
“It’s clear that current practice, especially in the most resource-poor settings will miss large numbers of patients with TB. We should instead focus on x-ray screening and the development of new inexpensive and easy-to-use tests.”
A meta-analysis of tuberculosis screening data from countries with a high incidence of tuberculosis found that among people with lab-confirmed tuberculosis, around four in five people did not report persistent cough, and almost two-thirds reported no cough.
Yet cough is considered one of the key symptoms of the disease, raising concerns that people who are unknowingly infectious may be spreading the disease.
“Our results indicate the probable reason why, despite huge efforts to diagnose and treat the disease, the tuberculosis (TB) burden across Africa and Asia is hardly declining,” said Professor Cobelens.
“A persistent cough is often the entry point for a diagnosis, but if 80% of those with TB don’t have one, then it means that a diagnosis will happen later, possibly after the infection has already been transmitted to many others, or not at all.”
The international research collaboration analysed data from over 600,000 people collected in national TB prevalence surveys in 12 countries across Asia and Africa with high rates of tuberculosis, with findings published in The Lancet Infectious Diseases.
Screening was conducted using chest x-ray and symptom screening, with confirmed TB diagnosis defined by positive Mycobacterium tuberculosis sputum culture.
Almost 2000 people had bacteriologically confirmed TB, and while most had symptoms of some kind, the majority reported no persistent cough or indeed any cough, especially among women.
“We estimated that, overall, up to 83% of adults in the community with culture confirmed tuberculosis report no cough persisting for 2 or more weeks, up to 63% report no cough of any duration, and (based on a subset of surveys) up to 28% report no tuberculosis-suggestive symptoms (i.e., cough, fever, chest pain, night sweats, or weight loss) at all,” wrote the authors.
“Tuberculosis without persistent cough or any cough was significantly more frequent among women than among men.
“Our study also provides an estimate of the proportion of subclinical tuberculosis that is infectious based on microscopic sputum smear examination; this is 29% among people without persistent cough and 23% among people without any cough.”
While cough is the main means of transmission, TB can also be spread by sneezing, talking, laughing and singing.
According to the National Notifiable Diseases Surveillance System, there have been around 1400 cases of TB reported each year over the past few years in Australia. Most people with TB were born overseas and were likely to have acquired it while living in countries with high rates of TB.
Other people at risk are those living in overcrowded conditions where TB cases are known to occur, for example in some Australian Aboriginal and Torres Strait Islander communities, and people, especially health workers, travelling to countries where it’s endemic. Immunisation is recommended for people at higher risk of TB.
