Listeria monocytogenes is a gram positive facultative anaerobic organism that is ubiquitous in the environment of farm animals (Baird et al. 2012). L. monocytogenes is an intracellular pathogen with a unique potential to spread from cell to cell, thereby crossing the blood-brain barrier, intestinal and placental barriers (Dhama et al. 2015). Many animals can be healthy carriers that shed the organism in their faeces, tears, urine, nasal secretions and milk. The organism may be present in soil, water, manure, feed and can survive for greater than five years (Sherman et al. 2009). There are 16 known serotypes and multiple subtypes of L. monocytogenes (Sherman et al. 2009). Each strain of L. monocytogenes has varying pathogenicity causing a range of disease syndromes in a variety of hosts (Thompson et al. 2011). On this property we had septicaemia in the sheep and abortion and encephalitis in the goats. Birds and mammals (including humans) can be infected with the organism L. monocytogenes and it is found throughout the world. L. monocytogenes is capable of growth over a wide pH range of 5.5-9.6 (optimal growth 7-7.2) and temperature range of 3-45C (optimal growth 20-40C).
Listeria is a common organism in the environment, but certain environmental conditions are more ideal for growth of the organism. The predisposing factors to clinical listeriosis in sheep and goats include; sudden change in weather; change in feeding regime; introduction of carrier animals; confinement during winter; overcrowding; poor sanitation; increased stress (poor nutrition or concurrent diseases); advanced pregnancy; feeding of silage, mouldy hay, sewerage, or manure piles; contact with rotting vegetation; provision of dirty water; feeding on the ground, contact with products of abortion; venereal transmission; and contact with birds (including chickens). Listeriosis is characterised by sudden death, encephalitis, septicaemia, abortion, keratoconjunctivitis and metritis/vaginal discharge (Matthews 2009). Sheep and goats are highly susceptible to Listeriosis.
In May 2020, a sheep and goat producer in the Lower Hunter introduced two new sheep onto their property from Glenlyon, Queensland. These two sheep had respiratory symptoms when they took delivery of the sheep in Tenterfield. They were given antibiotic medication but no history was provided by the previous owner. These sheep were kept in a separate pen (in close proximity to the resident flock) and drenched on arrival with monepantel. The health of the new sheep deteriorated, and initially private vets were called to the property. The vet treated the new sheep for shipping fever with oxytetracycline, but unfortunately one became extremely ill with respiratory signs and was euthanised. No postmortem was performed.
The following week, three of the resident sheep died. The clinical signs noticed were anorexia, scours and death over 2-3 days with some sheep also having respiratory signs. The private vet was called to the property again and the sheep and goats were drenched, all treated with oxytetracycline, a faecal egg count was done which had 0 eggs per gram of faeces and all sheep and goats were vaccinated with a 5-in-1 clostridial vaccination. During this period the feeding had stayed the same except they had noticed some of the hay they were feeding had become mouldy and it had been thrown out.
In mid-June the local District Veterinarian was contacted by the EAD Hotline and the producer to perform a post-mortem on a sheep that had died suddenly. The ewe was three years old and vaccinated six weeks prior with a 5-in-1. On post-mortem the lungs were clear with no sign of pneumonia, there were petechial haemorrhages over the surface of the duodenum and scours noticed, and the fluid content of the rumen appeared to be twice the normal amount. Samples were sent to EMAI for histopathology. The laboratory conclusion was a moderate small intestinal multifocal, haemorrhagic enteritis that may have been due to viral, bacterial or parasitic agents. A Mycoplasma ovis PCR came back as negative and the faecal egg counts of 10 sheep from the flock came back as 0 eggs/gram of faeces.
The following week, one doe aborted twins (no notification or testing was done on this animal or the aborted kids at the time of abortion) and a 12-month-old goat from the property became dysphagic with excessive drooling, circling in one direction, followed by an inability to stand and torticollis. When the goat initially showed dysphagia and excessive drooling, bloods were sent to EMAI for bluetongue exclusion. The bluetongue PCR was negative. As the clinical signs progressed to circling, recumbency and torticollis the private veterinarian attempted treatment (IV fluids, meloxicam, oxytetracycline and vitamin B1 injection) but was not successful. The goat was euthanised and cerebrospinal fluid was sent for Listeria culture and PCR and fixed brain was sent for histopathology.
The histopathology of the goat's brain revealed a meningoencephalitis that was necrosuppurative, subacute, and moderate with micro abscesses and lymphohistiocytic perivascular cuffs in the brainstem. These findings were consistent with Listeria infection. The goat also had a suppurative, multifocal bronchopneumonia that was most likely related to the Listeria. Culture of the cerebrospinal fluid confirmed the presence of Listeria and PCR identified Listeria monocytogenes.
Control of listeriosis is achieved by excluding animals from the highly infectious source. In this case the source was most likely the introduced sick animals or the mouldy hay. The producers had a small flock of 15 sheep and goats in total, with most of the sheep and goats on the property showing respiratory signs, nasal discharge and/or scours over a three-week period and five deaths occurred. This property would be highly contaminated with Listeria.
Listeria is difficult to eliminate from the environment as the organism quickly becomes resistant to disinfectants (Muller et al. 2013). Scientists looking at Listeria DNA gene sequences noticed that a section of DNA was very different from the remainder of the genome (Tn6188) and concluded that this bacterium had acquired an alteration reasonably recently (Muller et al. 2013). Researchers found that Listeria that harboured the Tn6188 sequence were much less sensitive to benzalkonium chloride disinfectant. Scientists were able to show that deleting the Tn6188 gene made Listeria once again sensitive to benzalkonium chloride.
Control measures that were recommended in this case included:
All in-contact animals were treated with a dose of long-acting oxytetracycline as they were potential carrier animals and preventative treatment of clinically unaffected animals in an outbreak has been suggested.
Listeria bacteria can cause severe illness in people, with pregnant women, newborns, the elderly and those people with a weakened immune system being most susceptible to disease. Clinical signs in people include fever, headache, vomiting, diarrhoea, miscarriage and stillbirths in pregnant women, convulsions, coma and death. When handling these animals, it is essential that the producers use personal protection and hand washing and ensure people most susceptible to disease are kept away from the sheep and goats (NSW Health).
Morpeth Veterinary Hospital (initial private veterinarians to the property).
Cessnock Veterinary Centre and Hospital for their involvement (2nd private veterinarian involved in this case).
EMAI for laboratory services.