1. Introduction

Listeriosis caused by Listeria monocytogenes is an infectious disease affecting a wide range of mammals, including ruminants, monogastric animals and humans. In ruminants, among which sheep are the ones more commonly affected, the main clinical features are encephalitis, septicaemia, abortion, mastitis and gastroenteritis. Listeriosis is of major importance in ruminants. At the farm level, there is a direct link between silage feeding and infection. It is also a zoonosis, occurring either after direct contact with affected animals or more frequently as food-borne disease (Brugère-Picoux, 2008). 

2. Aetiology

2.1 Bacterial characteristics

Listeria monocytogenes is a microaerophilic Gram-positive, flagellated coccobacillus which is present in a wide range of moist environments and may cause disease in man and a variety of domestic species. Listeriosis occurs sporadically in cattle, where most cases are associated with feeding poorly fermented silage during the winter months. Cattle show a similar age incidence to sheep with the majority of cases affecting two- to three-year-olds, although cattle appear much less susceptible to listeriosis than sheep. Rarely are outbreaks of listeriosis encountered in cattle (Andrews et al., 2008).

2.2 Classification

Listeria sensu strictu includes L. monocytogenes, L. seeligeri, L. ivanovii, L. welshimeri, and L. innocua, which all have been described before 1985 as well as L. marthii, which was first described in 2010. Listeria sensu strictu species form a tight monophyletic group within the genus Listeria (Fig. 1). Except for L. monocytogenes and L. innocua, all species in this group have been named after researchers that have played important roles in the study of Listeria (Orsi and Wiedmann, 2016).

3. Epidemiology

Historically, listeriosis occurs primarily in sporadic cases and is considered to be non-contagious. However, outbreaks are known to occur when sheep are exposed to a single contaminated source, such as silage. In general, the disease is more frequent in winter and early spring. This risk in winter season can be associated with silage feeding and/or because Listeria can grow at temperatures where growth of other pathogens is inhibited due to excessive cold. The case incidence rate may reach 9%, but rarely it is over 2% (Brugère-Picoux, 2008).

3.1 Susceptible hosts

Animals may be exposed to L. monocytogenes by direct contact with infective material from same or other animal species. Faeces, urine, aborted foetuses, uterine discharge and milk should be considered as infectious (Brugère-Picoux, 2008).

3.2 Transmissions

Most cases of listeriosis arise from the ingestion of contaminated food, notably silage (Fig. 2). Lambs may acquire septicaemic listeriosis from congenital infection or from contamination on the ewe’s muddy teat or contaminated milk (Brugère-Picoux, 2008).

4. Pathogenesis

Although a number of forms of listeriosis are easily recognized, such as encephalitis, abortion and septicemia, in general the pathogenesis of infection in ruminants has not been fully described (Low and Donachie, 1997).

In most animals the organism enters into the body by penetrating the epithelial barrier in the intestine (Fig. 2). Subsequently, it multiplies in hepatic and splenic macrophages aided by a haemolysin, listerolysin O which disrupts lysosomal membranes allowing the organism to grow in the cytoplasm. Bacteraemia can either be subclinical or lead to clinical septicemia. Septicaemia, with or without meningitis, occurs more commonly in neonatal ruminants and in adult sheep, particularly in pregnant ewes and when the quantity of ingested Listeria is high. Gastrointestinal listeriosis has been first described in the UK and then in New Zealand with a marked enteritis with diarrhea (sometimes with extensive haemorrhages) and ulceration of the abomasum and intestinal mucosae.

Another possibility is the ascending infection of the trigeminal (cranial nerve V) or other cranial nerves following lesion of the buccal mucosa (trauma, shedding of deciduous or permanent teeth, periodontitis) (Barlow and McGorum, 1985).

Others routes of infection have also been described, as follows: (i) ascending infection in the sensory nerves of the skin following dermatitis from prolonged wetting of fleece leading to listerial myelitis; (ii) infection of the mammary gland appears to be haematogenous, although introduction of environmental bacteria into the udder cannot be excluded; (iii) airborne transmission causing keratoconjonctivitis and iritis.

A number of observations suggest that there may be differences in pathogenic tropism between strains of L. monocytogenes. In sheep, the two major clinical forms of listeriosis, meningoencephalitis and abortion, usually do not tend to occur simultaneously in the same flock. However, other observations do not confirm this opinion: in an outbreak linked with the feeding of the same batch of silage, abortive (nine ewes), encephalitic (one ewe) and septicaemic (four ewes) form of listeriosis were observed with an incubation period of 18 and 26 days for the abortive and the encephalitic forms respectively (Wagner et al., 2005)

It is likely that during silage feeding, subclinical infections commonly occur and that animals become protected against the septicaemic form of listeriosis. Nevertheless, these responses may not be wholly effective in protecting against encephalitis. This can be an explanation for the rarity with which encephalitis and abortion are recorded in the same flock and why encephalitis occurs more frequently in many silage-fed animals (Brugère-Picoux, 2008)

5. Clinical signs and pathology

5.1 Clinical signs

Encephalitis is the most readily recognized form of listeriosis in ruminants. It affects all ages and both sexes, sometimes as an epidemic in feedlot cattle or sheep. In cattle, the disease course is less acute, and the recovery rate approaches 50%. Lesions are localized to the brain stem, and the clinical signs indicate dysfunction of nerve nuclei, including those of the third to seventh cranial nerves.

Initially, affected animals are anorectic, depressed, and disoriented. They may propel themselves into corners, lean against stationary objects, or circle toward the affected side. Facial paralysis with a drooping ear, deviated muzzle, flaccid lip, and lowered eyelid often develops on the affected side, as well as lack of a menace response and profuse, almost continuous, salivation; food material often becomes impacted in the cheek because of paralysis of the masticatory muscles. Terminally affected animals fall and, unable to rise, lie on the same side; involuntary running movements are common.

Listeric encephalitis may recur on the same premises in successive years. The number of animals clinically affected in an outbreak usually is < 2%. Listeric abortion usually occurs in the last trimester without prior clinical signs. Fetuses usually die in utero, but stillbirths and neonatal deaths also occur. Fatal septicemia of the dam secondary to metritis is rare. Encephalitis and abortion usually do not occur simultaneously in the same herd or flock (Constable, 2022). 

5.2 Pathology

Grossly, lesions affected mainly the leptomeninges of the Central nervous system - CNS and consisted of opacity, hyperemia and mild to moderate fibrin deposition over these surfaces (Fig.3A). Additionally, malacic areas were observed in one bovine, which were characterized by multifocal irregular greyish cavitations that extended from striated body to diencephalon (Fig.3B). Histologically, the leptomeninges presented a marked inflammatory infiltrate composed by healthy and degenerated neutrophils, besides mild to moderate infiltrate of macrophages, lymphocytes and plasma cells, associated to fibrin deposition and intralesional basophilic coccobacilli bacteria myriads (Fig.3C). Distinct histological lesions were observed in three cattle, which consisted of severe blood vessel degeneration and fibrinoid necrosis, multifocal thrombosis and marked Gitter cells infiltrate, associated to perivascula cuffs composed by neutrophils, macrophages, lymphocytes and plasma cells (Fig.3D). E. coli was cultured from six cattle and one goat samples, while T. pyogenes was cultured from one cattle sample. On immunohistochemistry for E. coli, 12/17 cases (nine cattle, two goats and one sheep) presented immunostai ning in the cytoplasm of macrophages and neutrophils at the leptomeninges and interspersed at the fibrin deposition (Fig.3E). Also, the three cases that presented meningoencephalitis showed a marked immunostaining around blood vessels (Fig.3F) (Konradt et al., 2017).

6. Diagnosis

6.1 Laboratory diagnosis

Listeriosis is confirmed only by isolation and identification of L. monocytogenes. Specimens of choice are brain tissue from animals with CNS involvement and aborted placenta and fetus. If primary isolation attempts fail, ground brain tissue should be held at 4°C (39°F) for several weeks and recultured weekly. Occasionally, L. monocytogenes has been isolated from spinal fluid, nasal discharge, urine, feces, and milk of clinically ill ruminants. Serology analysis is not used routinely for diagnosis, because many healthy animals have high Listeria titers. Immunofluorescence assay testing is effective to rapidly identify L. monocytogenes in smears of samples obtained from dead animals or aborted fetuses and from samples of milk, meat, and other sources (Constable, 2022).

6.2 Differential diagnosis

Listeriosis can be differentiated from pregnancy toxemia in ewes or ketosis in cattle via careful clinical examination, CSF testing and beta-hydroxybutyrate concentrations well below 3 mmol/L. Furthermore, facial and ear paralysis are absent in pregnancy toxemia or ketosis. In cattle, the unilateral signs of trigeminal and facial nerve paralysis (often subtle) help differentiate listeriosis from bovine spongiform encephalopathy, thrombotic meningoencephalitis, polioencephalomalacia, sporadic bovine encephalomyelitis, and lead poisoning. Rabies must always be considered in the list of differential diagnoses for listeriosis. Animals with brain abscesses and coenurosis present with circling, contralateral blindness, and proprioceptive deficits; however, they show no cranial nerve deficits. Vestibular disease is common in growing ruminants; these animals typically show ipsilateral spontaneous nystagmus or strabismus, and they remain bright and alert without trigeminal nerve dysfunction (Constable, 2022).

7. Treatment

Penicillin or trimethoprim-sulpha remain the antibiotics of choice for listeriosis. Oxytetracycline is not considered an appropriate antibiotic for listeriosis because of its large molecular size, although good results have been claimed. A minimum dose rate of 44000iu/kg procaine penicillin injected intramuscularly b.i.d. must be considered for at least 10 days in addition to 44000 iu/kg penicillin G injected intravenously b.i.d. on the first day. Penicillin dose rates as high as 300000iu/kg have been recommended for the first day of antibiotic therapy because it is essential to exceed MICs by 10 to 30 times to achieve a successful outcome.

Loss of saliva may lead to dehydration and metabolic acidosis. Care must be taken when replacing fluids by orogastric tube because contraction of the rumen caused by anorexia of some days’ duration may result in passive regurgitation of these fluids around the orogastric tube. The amount of fluids administered in this way should be restricted to 15 to 25 litres four to six times daily. Transfaunation with rumen liquor from a healthy cow may promote rumen function and aid recovery (Andrews et al., 2008).

8. Control and prevention

There are no published studies which have reported the efficacy of prophylactic antibiotic administration in the face of an outbreak of listeriosis, but such epidemiology is unusual in cattle.

Listeriosis occurs sporadically in cattle and is prevented by management practices which ensure the making and storage of high-quality silage. Soil contamination is limited by rolling grass fields at the beginning of the growing season. Good fermentation is guaranteed by cutting grass at an early growth stage (digestibility value >72) when it contains a high fermentable sugar content, and the use of various silage additives, whether sugars, organic acids or bacterial cultures. Compaction of the silage clamp is important to expel all air followed by air-tight sealing to prevent aerobic bacterial multiplication. Poor quality or spoiled silage should be discarded. Prevention of listerial encephalitis by vaccination is not an established procedure in ruminant species (Andrews et al., 2008).

9. References

Andrews, A.H., Blowey, R.W., Boyd, H., Eddy, R.G., 2008. Bovine medicine: diseases and husbandry of cattle. John Wiley & Sons.

Barlow, R., McGorum, B., 1985. Ovine listerial encephalitis: analysis, hypothesis and synthesis. The Veterinary Record 116, 233-236.

Brugère-Picoux, J., 2008. Ovine listeriosis. Small Ruminant Research 76, 12-20.

Constable, P.D., 2022. Listeriosis in Animals. MSD Manual. Veterinary Manual.

Howard, J.L., 1993. Current veterinary therapy 3: food animal practice. WB Saunders Company.

Konradt, G., Bassuino, D.M., Prates, K.S., Bianchi, M.V., Snel, G.G., Sonne, L., Driemeier, D., Pavarini, S.P., 2017. Suppurative infectious diseases of the central nervous system in domestic ruminants. Pesquisa Veterinária Brasileira 37, 820-828.

Low, J., Donachie, W., 1997. A review of Listeria monocytogenes and listeriosis. The Veterinary Journal 153, 9-29.

Orsi, R.H., Wiedmann, M., 2016. Characteristics and distribution of Listeria spp., including Listeria species newly described since 2009. Applied microbiology biotechnology 100, 5273-5287.

Wagner, M., Melzner, D., Bago, Z., Winter, P., Egerbacher, M., Schilcher, F., Zangana, A., Schoder, D., 2005. Outbreak of clinical listeriosis in sheep: evaluation from possible contamination routes from feed to raw produce and humans. Journal of Veterinary Medicine, Series B 52, 278-283.