Airborne Pathogens: Tuberculosis and Chalara Ash Dieback

Two diseases, one route through the air

A pathogen causes disease by reaching a suitable host. Tuberculosis (TB) is caused by bacteria and usually affects the lungs. Chalara ash dieback is caused by a fungus that infects ash trees. Both can spread through the air, but they travel in different ways: TB bacteria are carried in droplets released by a person, whereas infectious ash dieback spores are blown by the wind.

How tuberculosis spreads

When a person has active TB affecting their lungs, coughing releases small droplets containing the bacteria. Another person may breathe them in. Infection is more likely after regular, prolonged close contact with someone who has active TB, such as living in the same home; it is not usually the result of briefly passing someone outdoors. A person with latent TB has the bacteria in their body but no symptoms and cannot spread TB to others, although the infection may later become active.

Finding and treating active TB reduces the opportunity for bacteria to pass between people. Treatment involves a course of antibiotics, which must be completed even if the person feels better. Avoiding prolonged close exposure to someone with untreated active TB also reduces risk. The BCG vaccine helps protect some people at higher risk; in the UK it is recommended for particular risk groups rather than for everyone.

How Chalara ash dieback spreads

The fungus responsible for Chalara ash dieback produces infectious spores on fallen ash leaf stalks. Wind can carry these spores to other ash trees, where they can land on leaves and begin an infection. The disease can cause leaf loss and bark lesions. Unlike TB, this is not spread by a person coughing: airborne here means wind-blown spores.

Once spores are being produced outdoors, wind dispersal is difficult to stop completely. In an affected area, covering collected ash leaves and leaf mould while they break down can help prevent spores dispersing from that material. Avoiding the movement of diseased ash plants or infected wood can also reduce spread by a route other than air. Neither measure can guarantee that an ash tree will remain uninfected, especially where the disease is already widespread.