A producer near Narrabri noticed submandibular swellings in his Dorper lambs. He had also noticed the swellings in the previous season's lambs with apparent resolution of most cases as the lambs aged. The producer was concerned that the swellings might result in the condemnation of carcasses at slaughter.
Lambs ranging in age up to four months were examined on property. Approximately 50 of 120 lambs had non-painful, firm, golf ball size, bilateral submandibular swellings. In some individuals the swelling was more diffuse and extended dorsally along the mandible. Lambs were bright and alert and in condition score 3-4. They were not vaccinated for clostridial diseases or caseous lymphadenitis.
Blood samples were taken from two lambs that appeared slightly depressed. Results were consistent with internal parasitism and hypocalcaemia. Haemonchosis was confirmed by faecal testing as a separate issue, and hypocalcaemia was suspected to be due to prolonged grazing of cereal oats.
Autopsies were conducted on two individuals slaughtered for home consumption. Histology of the submandibular swellings revealed exuberant but fundamentally normal thymus tissue.
A condition known as 'Milk Goitre' in goats is described on the internet, although scant reports exist in the peer reviewed literature. In goats, the condition is described as a 'softish palpable non-painful oedematous swelling - generally bilateral - in the thyroid region of the neck'. It can appear from approximately one week of age and resolves in most animals at about 6 to 9 months of age. Although rarely causing any clinical signs, the swellings may have adverse effects including altered vocalisation and dysphagia. A loose flap of skin may remain after resolution. Histology of the swellings revealed thymic hyperplasia. Contrary to the common name of 'Milk Goitre', the condition is not related to the thyroid gland.
In Sabi lambs in Zimbabwe, similar submandibular swellings to those seen in this case were also found to be normal extrathoracic thymic tissue. The authors suggest that the condition is probably not uncommon in lambs and kids. Radostits et al states that 'Extrathoracic thymus tissue occurs in lambs and can be mistaken for enlargement of the thyroid glands' but does not elaborate.
The main differentials for the submandibular swellings in this case were proliferation of the thyroid glands due to iodine deficiency and caseous lymphadenitis. Both conditions were excluded by histopathology of the swollen tissue.
Examination of the Dorper lamb flock at the Goondiwindi Pastoral Veterinary Centre (University of Queensland) revealed further individuals exhibiting the condition.
It is proposed that submandibular swellings due to extrathoracic thymic hyperplasia is a normal and non-pathogenic finding in Dorper lambs (and other African breeds of sheep) less than approximately 6 months of age. Some residual diffuse swelling may persist in older lambs after the thymus naturally regresses.