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Bright blindness in a goat flock associated with fern (Pteridium aquilinum) intake
Hernandorena, M.R.; Albite, A...
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Objectives: In the month of August 2021, a clinical case of blindness in a goat flock was referred to the Ruminant Clinical Service of the University of Zaragoza by a practitioner working in the Basque Country (Spain). Affected animals belong to a pet flock of 12 Valais breed goats reared in an extensive production system.The goats were grazing in a 4-hectare field along with some animals of other species. According to the veterinarian, there was little food in the pasture, and ferns predominated, which the farmer reported that the goats ate greedily.
The practitioner referred that in June 2021, six goats were presented with nervous clinical signs. After neurological examination, he noticed almost complete blindness in all six animals.
Material and methods: A clinical examination of the affected animals was performed. They presented a low body condition (2/5), heart and breath rates were normal, as well as body temperature. In addition, the examination revealed mild blindness as the only neurological clinical sign. The farmer referred that goats presented a bright greenish light in the eyes at night.
One of the goats died and was referred to the Pathological Service of the Veterinary Science Faculty for the post-mortem exam. Cerebellum, encephalon and the eyes were taken for histopathology, while sterile swabs were collected for microbiology and Rt-PCR.
Results: A differential diagnosis was carried out based on the clinical signs and the history. Some disorders such as heavy metal or fern intoxication were taken into consideration. Likewise, listeriosis, vitamin A or B deficiency and individual causes such as retinal detachment, cataracts or an eye ulcer were included in the differential diagnosis.
The main suspicion was intoxication based on the chronic ingestion of fern (Pteridium aquilinum) referred to by the farmer. These plants contain high concentrations of ptaquiloside, a toxic component that causes stenosis in the retinal vessels damaging and vanishing the layer of rods and cones completely. This narrowness caused an increase in tapetum lucidum reflectance,which is called "bright blindness".
Microbiological study and Rt-PCR showed negative results for Listeria monocytogenes, and thus the necropsy revealed it was not an inflammatory process but degenerative.
At the post mortem macroscopic examination, there were no major findings in the brain, and no signs of cerebrocortical necrosis were found under ultraviolet light. The following histo-pathological study only revealed representative damage in the eye: the layer of rods and cones had almost completely disappeared, and the consequent retinal degeneration reached the macula.
Conclusions: The ingestion of Pteridium aquilinum is associated with intoxication that can have an acute or chronic evolution with different clinical pictures. Acute poisoning, also called haemorrhagic fever, is more common in cattle than sheep and presents the characteristic clinical picture of a haemorrhagic syndrome. The continued intake of low doses of toxins (chronic intoxication) is associated with the appearance of bovine enzootic haematuria, gastrointestinal carcinomas, polioencephalomalacia (action of thiamine) and a particular form of the disease described in sheep known as "bright blindness". This clinical picture has not been described so far in goats.
Progressive bilateral blindness characterises this disorder due to a degeneration of the retinal neuroepithelium. The eyes glow abnormally in the gloom. There is no inflammation or opacity of the cornea. After the eye exam, the veins and arteries are narrower, and the main vessels are more separate than normal.
Keywords: Brigth Blindness, Fern, Goat.
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Affiliation of the authors at the time of publication
Universidad de Zaragoza, Zaragoza, Spain
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