The North Coast of NSW is not typically a sheep production area. There are less than 3000 sheep between the Queensland border and Coffs Harbour. However they are a popular hobby species. There are enthusiastic groups of Dorper and other sheep breeders and small flocks of sheep serve a useful purpose on many horticulture properties. The coastal subtropical environment of the region has regular incursions of midges and other biting insects each summer. Homebred cattle in the region routinely seroconvert to arboviruses such as Akabane and bluetongue without clinical syndromes. Less information is collected on arboviruses in sheep. The following two cases may have involved an arbovirus.
A farmer near Grafton reported lambs being born in the winter of 2011 with twisted necks. This disease syndrome had also been seen the previous year on both his property and on the neighbours property. There were no affected lambs on the neighbour's property this year. The farmer had a small flock of 20 Dorper sheep. Three lambs were affected when the farm was visited but over half of the ewes were yet to lamb.
Affected lambs were unable to stand and had the neck twisted to one side. It was not possible to bend the neck back into a normal position.
Blood samples were taken from three animals:
- An affected lamb which had not fed from its mother
- Mother of the affected lamb above
- Mother of a lamb affected last year
An affected lamb was euthanased and nerve and muscle tissues collected for histopathology.
On laboratory blood tests, none of the sheep had been exposed to pestivirus. All of the sheep had been exposed to Simbu virus. The mother of a lamb affected last year had been exposed to Akabane virus. The other two sheep were negative for Akabane virus. All sheep had normal copper levels.
There were no significant findings when tissues from the affected lamb were examined for histopathology. Histopathology was not suggestive of Akabane infection.
A definitive diagnosis was not possible in this case although an arbovirus was suspected from the history, clinical findings and laboratory results. Histopathology did not identify a myositis that could be expected with Akabane infection. However the pathologist involved in the case indicated that histopathology could not be relied upon for a diagnosis. It was significant that the lamb that had not suckled on its mother was positive for the Simbu family of viruses. This suggests exposure in utero. Its mother had also been exposed. It is hypothesised that a Simbu virus other than Akabane virus caused the defects this year. It is possible that Akabane virus has been associated with disease in the past.
A hobby farmer at Cudgera Creek near Murwillumbah had twin Dorper lambs born with severe congenital defects (see images). The ewe had been bred by a local stud and there was concern that it could be a genetic defect. All other lambs born from the ram that sired the affected lambs were normal.
The primary defect identified in the twin lambs was agnathia (absence of the lower jaw). Some of the other defects included both eyes being located ventral to their normal position and sharing the same eye socket and a brain lacking olfactory bulbs.
Blood was collected from the mother of the affected lambs. The ewe had antibody evidence of exposure to Aino and Simbu virus. She was negative for exposure to bluetongue, pestivirus and Akabane.
The aetiology of agnathia in ovines is uncertain but in bovines it has been shown to be an inherited recessive trait. The combination of defects in this case suggests a very early failure in cranial development involving the 3rd brachial arch (Windsor, 2011, personal communication). A genetic disorder is possible, although the history suggests that a toxin or viral exposure at a critical stage of embryonic development could have led to the defect.
A farm visit failed to identify any plants that could be potentially toxic. The ewe had been grazing on a clean summer dominant pasture mix.
The early summer period when the ewe in this case would have been in early pregnancy was a very active time for biting insects that are known to carry arboviruses of interest to livestock. The positive Aino and Simbu results imply that a viral cause is possible.
Sheep producers and veterinarians on the North Coast of NSW should be aware of the potential for arboviral disease. To reduce the chance of arboviral disease producers could consider moving the joining period to have ewes pregnant during winter.