A RARE FIND: MYCOBACTERIOSIS DETECTED IN TAWNY OWL

Tawny owl (c) Unsplash

In April 2025, a dead tawny owl (Strix aluco) was brought in for necropsy via the Nature Inspection of the Agency for Nature and Forests. The animal was found by cyclists along a roadside, with a striking posture: eyes wide open, tail feathers straight up and one claw tensely clenched. Initially, poisoning was suspected, a cause of death that unfortunately occurs frequently in birds of prey in the wild.

However, during the necropsy it quickly became clear that it was an adult male who had died as a result of a chronic disease process, specifically granulomatous hepatitis. The liver showed multiple round, pale, well-defined foci, suggestive of granulomatous inflammation.

Macroscopic image of a tawny owl liver with multiple pale, well-circumscribed nodular lesions (arrow), typical for granulomatous inflammation associated with mycobacteriosis (c) Naomi Terriere

To further investigate the nature of these lesions, histological examination was performed. Hematoxylin-eosin (HE) staining revealed multinucleated giant cells arranged around central zones of necrosis—characteristic of granuloma formation.

Circular granuloma in the liver, with central necrosis (A) surrounded by inflammatory cells (B). The surrounding liver tissue appears histologically normal aside from freezing artifacts (C) (HE, 25x).

Ziehl-Neelsen staining, specific for acid-fast bacteria, revealed multiple grouped, pink-staining rods, consistent with infection by Mycobacterium spp.

Clusters of pink-stained, acid-fast rod-shaped bacteria (arrow) visible within granulomatous liver tissue, consistent with Mycobacterium spp. (Ziehl-Neelsen, 100x oil immersion).

The presence of Mycobacterium was subsequently confirmed by PCR testing, and the bacterium was further identified as Mycobacterium avium subsp. avium , a known causative agent of avian mycobacteriosis.

All bird species are susceptible to infection with the Mycobacterium avium complex (MAC), although certain bird orders, such as Galliformes, Columbiformes and Gruiformes, may be more susceptible to the disease (Friend, 1999). Avian mycobacteriosis (AM) is a relatively common disease in domestic poultry (González et al., 2002; Bougiouklis et al., 2005) and captive wild birds (Singbeil et al., 1993; Marco et al., 2000; Dvorska et al., 2007). This is mainly due to the ability of the bacterium to persist in the body for long periods, its high degree of infectivity and its ability to survive in the environment (Thorel et al., 1997). However, sporadic cases of AM are also reported in free-living wild birds (Smit et al., 1987; Millán et al., 2004; Gerhold & Fischer, 2005).

In Europe, avian mycobacteriosis has been reported in, among others, two kestrels ( Falco tinnunculus ) (Wilson, 1960; Bucke & Mawdesley-Thomas, 1974) and a barn owl ( Tyto alba ) (Wilson, 1960). Smit et al. (1987) isolated Mycobacterium avium from several dead birds of prey, both diurnal and nocturnal species, found in the Netherlands, while Millan et al. (2010) investigated the disease in birds of prey in Mallorca (Spain). The prevalences in those countries range from 0.96% to 3.8% in the Netherlands and 0.9% to 4% in Spain (depending on the bird species tested), which is very low.

Unlike Galliformes and Columbiformes, where mycobacteriosis is often linked to high bird densities and therefore easy transmission and maintenance of infection, birds of prey are solitary. In their case, infection typically results from consumption of infected prey, secondary infections of wounds or exposure to the bacteria in the environment.

The rarity of avian tuberculosis in humans, even in areas with high levels of infected poultry, suggests strong human resistance. The low prevalence in wild birds means that the disease poses only a limited risk to public health.