Toxic lung condition in African ruminants
A cluster of cases of severe breathing difficulties and sudden death were experienced in Grey Duiker and Nyala at the National Zoological Gardens, Pretoria. Necropsies in all animals were similar: the lungs were congested, wet, uncollapsed and heavy. In the Grey Duiker, minor emphysema (clear air-filled spaces) affected the soft tissues of the thorax. In the Nyala, large (< 10cm dia) similar bubbles, some of which were filled with clotted blood, distorted the surfaces of the lungs. White froth drained from the cut surface of the lungs and filled the distal airways. Histological lesions were similar in all cases and were characteristic of acute toxic lung damage.
In cattle, acute pulmonary emphysema and oedema is caused by a variety of toxic factors, all of which are activated by the enzymes in the cells that line the airways of the lungs. The toxins thus created cause damage to the lungs, possibly as a result of the release of tissue damaging free radicals. In Europe “fog fever” results from ingestion of high levels of fresh green herbage (including lucerne). Other plants associated with similar lung lesions include: mouldy sweet potatoes, Brassica spp (turnip, rape, kale), Mexican poppy (Argemone), Australian stinkwood (Zieria arborescens), Crotalaria spp, Pyrollizadine alkaloids; Gnidia burchelli and Perilla mint. Hypersensitivity to Bovine Respiratory Syncytical virus infection can also cause similar lesions.
Careful examination by Professor Naude of the camp and lucerne hay fed to the Nyala and Grey Duiker did not identify any of the above plants, although one batch of lucerne was contaminated by many weed species. However, after a task team investigated the outbreak at the Zoo, and new sources of food were obtained, no further deaths occurred. Cubes and mineral blocks are being fed to sheep to see if they contain any toxic material. It is possible that all the offending material was fed to the animals concerned, or that a previously unknown toxic plant was responsible. Interestingly, two similar Nyala cases were recorded from distant game farm, so a common source of contaminated lucerne hay may have been responsible. No viruses have so far been isolated from the tissues of affected animals. This report highlights the importance of obtaining high quality food for captive African ruminants, and suggests that Grey Duiker and Nyala may be particularly susceptible to toxic products that are activated in the lung.