Listeria are ubiquitous, facultatively intracellular, short, gram-positive rod bacteria (Hodgson et al., 2011, Morin, 2004, Oevermann et al., 2010). There are six species of Listeria, although the only pathogenic species of veterinary and human significance are Listeria monocytogenes and Listeria ivanovii (Hodgson et al., 2011, Radostits et al., 2007, Oevermann et al., 2010). L. monocytogenes is a major cause of meningoencephalitis and abortion in sheep, cattle and goats, and has also caused cases of spinal myelitis, septicaemia, gastroenteritis, ophthalmitis and mastitis in ruminants (Radostits et al., 2007). L. ivanovii is responsible for abortion in cattle, sheep and goats and for septicaemia especially in lambs (Hodgson et al., 2011, Windsor, 2011, Radostits et al., 2007, CFSPH and IICAB, 2005, Santagada et al., 2004).
In July 2012, goats from two properties in the central tablelands showed neurological dysfunction.
On Property A, 30 Boer goats (of 80 in total) grazed in a paddock that had previously been slashed in the summer. The herbage was not baled and was left on the pasture. In early July, a buck showing signs of hypersalivation, stupor and recumbency before death was autopsied. As the rumen was impacted with grain, grain poisoning was suspected. Two weeks later, the owner found a 3-year-old doe that was separated from the group, showing signs of disorientation, circling, followed by recumbency, leg paddling and death. On 17 July 2012, the owner found a 4-year-old doe separated from the mob. The doe was depressed and circling to the left then became recumbent with leg paddling, an asymmetrically droopy face, head tilt, strabismus and horizontal nystagmus.
Despite treatment with trimethoprim/sulfadiazine (Trisoprim 480 Troy Ilium), thiamine and electrolytes the doe died over night. The goat was autopsied and the brain submitted for laboratory investigation.
On property B, running 105 adult Boer crossbreed goats, the owner slashed large tracks to allow vehicle access, with the grass left on the pasture. On 25 July 2012, the owner found a recumbent 1-year-old buck. The buck showed horizontal nystagmus, depressed mentation and ataxia when attempting to walk. In view of the prognosis, the owner euthanized the buck, which was subsequently autopsied.
On both properties, some does appear to have aborted but the extent and cause of these was not determined.
Histopathology from case 1 revealed a mild to severe non-suppurative meningoencephalitis and severe multifocal necrotizing and suppurative encephalitis. Case 2 revealed meningioencephalomyelitis with glial nodules, microabscesses and some malacic foci. The lesions were considered suggestive of, if not pathognomic for listeriosis.
In both cases, the rapid onset of disease is consistent with the more acute and typically fatal form of listeriosis commonly seen in sheep and goats (Windsor, 2011).
Both cases occurred in winter, following prolonged cold and wet weather. Both cases occurred on properties that had slashed and left the vegetation on the pasture to rot. There was no history of recent transport, handling or a stressful incident. However, it is possible that some degree of immune compromise was involved, as there was evidence of moderate parasitism by H. contortus affecting both properties and the majority of animals in both flocks were suspected pregnant does.
Goats on Property A were moved off the affected pasture. Slashed vegetation was removed from the paddock of Property B. No further cases of encephalitis have been seen on either property, although it is uncertain if Listeria was involved in subsequent abortions.
Listeria monocytogenes is a significant cause of disease in humans. Food-borne outbreaks have been caused by human consumption of unprocessed foods including uncooked vegetables, unpasteurized milk or dairy products and raw meat; however may occur from contaminated processed foods including cold cut meat, seafood and dairy products (e.g. butter, ice cream, soft-ripened or grated cheese) (Hodgson et al., 2011, Radostits et al., 2007, Nightingale et al., 2005, CFSPH and IICAB, 2005, Katre et al., 2009, Sherman, 2011). Faecal contamination of milk and consumption of unpasteurized dairy products has been responsible for outbreaks of human listeriosis.
Healthy people appear to be able to consume most listeria-contaminated foods without clinical signs (CFSPH and IICAB, 2005). Approximately 1-10% of humans are asymptomatic carriers of L. monocytogenes in their gastrointestinal tract and clinical disease is rare and usually limited to a self-resolving gastroenteritis following consumption of heavily contaminated food (CFSPH and IICAB, 2005). Direct contact with infected animals is unlikely to cause disease in humans, however handling of abortus material has been shown to occasionally cause papular dermatitis and conjunctivitis in veterinarians and farmers (Hodgson et al., 2011, CFSPH and IICAB, 2005).
Disease is most frequently seen in immunosuppressed, elderly, very young and pregnant women (Hodgson et al., 2011), and may result in septicaemia, meningoencephalitis and abortion (CFSPH and IICAB, 2005), with mortality in approximately 20-30% of cases (CFSPH and IICAB, 2005)
Prevention requires stringent food safety practices on a production, processing and household level. At risk individuals should cook all animal sourced foodstuffs, washing vegetables well and avoid high risk foods (e.g. unpasteurized dairy products, soft cheeses and cold-cut meats) (CFSPH and IICAB, 2005).
We thank the owners for their willing assistance in these cases.