SWAN IN POOR CONDITION DIED OF CHRONIC MYCOBACTERIOSIS

In September 2025, a dead mute swan ( Cygnus olor ) was submitted for necropsy via the SOS Wilde Dieren wildlife rehabilitation center and the disease surveillance program of the Agency for Nature and Forests . The animal had been brought to the center in a weakened state and died shortly thereafter. An investigation into the cause of death was therefore warranted.

Mute swan, Cygnus olor (© Leni lammens)

Necropsy: severe emaciation and striking lesions

It concerned an adult female in very poor condition. The pectoral muscles were severely wasted, a clear sign that the swan had been weakened for a long time. Around the heart there was a complete lack of fat reserves, indicating severe malnutrition. When opening the chest cavity, the liver immediately drew attention: it showed multiple round, pale, sharply demarcated foci. Such lesions are characteristic of granulomatous inflammation, a chronic disease process. Similar lesions were also found in the spleen and the glandular stomach. Because both liver and stomach were affected – while the lungs appeared normal – the infection seems to have entered via oral ingestion rather than inhalation. The granulomas occupied a considerable portion of the liver and spleen, severely disrupting the functioning of both organs.

On the liver, several granulomatous lesions can be seen as well-defined circular whitish-yellow structures. There is also no coronary fat around the heart present. (© Naomi Terriere)

A cross-section of the spleen shows the very extensive granulomatous lesions that occupy nearly the entire organ. (© Naomi Terriere)

Diagnosis: mycobacteriosis

The lesions were highly suspicious for mycobacteriosis. This was confirmed by PCR and sequencing: it involved Mycobacterium avium subsp. avium. Mycobacteria are widespread in the environment and can be found in soil, water, and dust. They are known for their high resistance to external conditions and are therefore almost impossible to eliminate from the environment.

Mycobacteriosis in wild birds

In wild birds, mycobacteriosis is only sporadically diagnosed. It usually concerns species that live in groups, such as starlings or sparrows, and often involves Mycobacterium genavense. In such cases, the impact on the population generally remains limited. True outbreaks in wild birds are rare. This contrasts with birds in captivity, such as domestic chickens or zoo animals. There, the close clustering of animals can promote transmission from bird to bird, allowing the disease to persist over long periods and necessitating specific management measures.

Did you find this blog post interesting? Then the article about mycobacteriosis in a tawny owl might also be for you!