A mob of 21 rams were born April 2010 from bought-in pregnant Dorper ewes. In January 2011, the producer near Balranald, telephoned to advise that some rams had swollen scrotums. It was unclear for how long they had swollen scrotums but the affected rams appeared to have recently lost condition.
A property visit was undertaken. Three rams had grossly swollen scrotums and appeared listless. Blood samples were taken from the three. The remainder of the mob appeared normal. All 3 had temperatures of approx. 41. One was sacrificed for necropsy.
On completion of the necropsy, the remaining two affected sheep were euthanased.
A month later, another ram in this mob developed similar symptoms of epididymitis. On necropsy, similar pathology to the first ram was evident. The owner decided to send all the remainder to slaughter.
Approx 300-400 mls of white pus was released under pressure from an incision of the scrotum on one side. A fistula was present from the tail of the epididymus to the ventral scrotum. The pus was odourless.
The affected epididymitis was approx 3-4 times normal size. The other epididymus was normal. A spermatic granuloma was evident.
Blood and tissue samples were negative for Actinobacillus seminis and Brucella ovis. Tissue and pus samples were positive for Histophilus somni.
Diseases associated with H. somni are associated with differing pyogenic conditions including septicaemia, polyarthritis, meningoencephalitis, metritis, epididymitis, mastitis, abortion and general pyaemia1. Whilst they occur worldwide, diseases caused by H. somni are not common. The most common H. somni disease in lambs is lameness and septicaemia. Although diseases caused by H. somni are usually fatal, the organism can also be a normal inhabitant of the prepuce and vagina. The interesting feature about H. somni infection is that the mode of transmission is not known. Attempts at transmission via nasal, oral and conjunctival routes have failed. Environmental or other stress factors are thought to be a predisposing feature to clinical disease.
The practice of buying in replacement sheep and having cattle on the same property are considered to be two significant risk factors in the manifestation of clinical disease. Work in Canada2 identified that flocks containing replacements which were bought-in the previous year were 8.5 times more likely to be infected than those which did not. On properties which bought-in replacements and had cattle, the risk factor increases to 13.2 times. This work identified that 32% of studied rams were positive for H. somni. Other work has suggested that neonatal rams are infected as they pass through the contaminated birth canal of infected dams.3 Other workers could not identify the organism in any rams under 12 weeks of age but found 87% of rams were infected at 20 weeks4.
In the case described, cattle were run on the property until April, the month in which these rams were born. Their dams had been bought-in some months earlier as pregnant ewes. So this case had both risk factors2 of bought in stock and grazing cattle. The property had no history of previous disease due to H. somni.
It is postulated that the bought-in ewes were infected, and this infection was passed to their offspring. Clinically affected rams (4/22) may have been the tip of the disease iceberg. It would have been interesting to have sampled the remainder of the mob and observe over time if other animals within the mob eventually developed the disease.