CASE II: 21-031340 (JPC 4166761)



60-day-old, intact male, Katahdin, Ovis aries, ovine



Day 1: Lamb started spinning in circles, ear down, and exhibited loss of balance. Day 2: Condition worsened to the point lamb could not get up. Day 3: Lamb was found dead.


Gross Pathology:

The lamb is in thin body condition based on thin skeletal musculature and fair subcutaneous and visceral adipose tissue stores. Postmortem autolysis is mild.

Two retropharyngeal lymph nodes at the level of the atlanto-occipital junction are symmetrically and diffusely enlarged, each measuring approximately 1.0 cm in diameter, as well as red-tinged with a dark red to black medulla.


Laboratory Results:

Culture of brainstem- moderate numbers of Listeria monocytogenes.


Microscopic Description:

Brainstem: Affecting gray and white matter, bilaterally and asymmetrically, with more pronounced involvement of the left half of the brainstem, are multifocal to coalescing aggregates of viable and degenerate neutrophils with variable numbers of histocytes and gitter cells (microabscesses), and occasional aggregates of glial cells (glial nodules). Within these areas, myelin sheaths are frequently dilated with swollen and hypereosinophilic axons (spheroids), and/or with an influx of neutrophils and necrotic cellular debris that effaces the normal parenchyma. Rarely, neuronal cell bodies are shrunken, rounded and hypereosinophilic with pyknotic nuclei (necrosis). The affected neuropil is extensively rarified (malacia) with variable edema. Blood vessels are often surrounded by small to moderate numbers of lymphocytes, plasma cells, macrophages, and neutrophils (perivascular cuffing) that fill Virchow-Robbins space. The meninges are segmentally infiltrated by moderate numbers of lymphocytes and plasma cells, with fewer macrophages and neutrophils.


Contributor's Morphologic Diagnoses:

Brainstem: Severe, subacute, multifocal to coalescing, necrosuppurative and lymphoplasmacytic meningoencephalitis, with malacia, Wallerian and myelin degeneration, microabscessation and perivascular cuffing.


Contributor's Comment:

Collectively, the clinical history, lack of gross findings, and histopathologic changes are classic for Listeria monocytogenes (LM) as the cause of clinical signs and death in this lamb. LM is a gram-positive, facultative anaerobic, coccobacillus to bacillus bacterium, that is ubiquitous and durable in the environment, virtually worldwide.1,3-8,10,13 It is commonly isolated from healthy tissues and feces of ruminant animals. LM is an intracellular pathogen of leukocytes, particularly macrophages, and epithelial cells. Surface protein internalins (A and B) facilitate internalization of the bacterium, by interacting with E-cadherin to pass through the intestinal, placental and blood-brain barriers. The cholesterol-binding hemolysin, Listeriolysin-O, allows LM to escape phagosomes so they can then co-opt the host cell contractile actin to facilitate cell-to-cell transfer.1,3-8,10,14


There are 6 species of Listeria, 2 of which are considered pathogenic: LM and L. ivanovii. There are multiple serotypes, which fall under 3 lineages, the most pathogenic being 1/2a, 1/2b and 4b. Serovars 1 and 4b are most commonly isolated from cattle, while serovars 4b and 5 are most commonly identified in sheep. Serovar 5 is specific for L. ivanovii, which infrequently causes abortion, particularly in sheep.1,3-8,10


The most common syndrome of listeriosis is encephalitis. Colloquially known as "circling disease," it is almost solely observed in adult ruminants as a sporadic disease or, less commonly, in outbreak situations. Outbreaks are generally associated with feeding contaminated silage, typically during winter and spring.1,8,10 While the exact pathogenesis has not been definitively described, the most widely accepted rationale, is that the bacterium enters oral mucosal epithelium through wounds, infects the trigeminal nerve branches or other regional cranial nerves, and centripetally travels via axons to the brain. This route explains why the medulla and pons are most severely affected, with less involvement of the thalamus, hippocampus, and cervical spinal cord. Clinical signs include confusion, depression, head pressing, deviation of the head to one side or the other without rotation (causing the animal to walk in circles, hence the name "circling disease"), unilateral drooping of the ear, eyelid and lips due to paralysis of the 7th cranial nerve, and possible drooping of the masticatory muscles and pharynx. The clinical course spans from a few hours up to 2 days, and rare survivors have permanent neurological deficits. Associated lesions may include rhinitis and unilateral purulent endophthalmitis. At necropsy, there are typically no gross lesions, though in severe cases, the medullary meninges may be thickened with green-colored edema and multifocal brainstem malacia is possible. Histologically, the primary lesion is microabscessation of the brainstem with glial nodules, accompanied by perivascular cuffing, acute vasculitis, and fibrin. White matter is often edematous and rarefied. Meningitis, if present, is due to local spread of the infection.1,3-8,10,13-14


Other syndromes include abortion, septicemia, conjunctivitis, endocarditis, and mastitis.1,8 Infrequently, encephalitis may occur simultaneously with other syndromes, particularly abortion and septicemia, within a herd, but not within the same animal.5,10 LM has an affinity for the gravid uterus, via suspected hematogenous spread from alimentary or intravenous inoculation of the dam, who may or may not show clinical signs of septicemia before abortion.1,2,5,8-9 The incubation period is variable (days to weeks), typically causing late-term abortion. In the early part of the last trimester, LM infects the fetus, causing fetal septicemia and death. The dam typically aborts in 5-13 days.2,7-9 This means the fetus is typically and markedly autolytic, which precludes evaluation of any gross lesions. The placenta is usually retained due to mild metritis, but LM is typically not cultured. If infected in the late part of the last trimester, the dam will undergo dystocia, often accompanied by severe metritis and septicemia. In near term infections, severe, diffuse, non-suppurative cerebrospinal meningitis is possible. Gross fetal lesions may include subcutaneous edema, hydrothorax, hydroperitoneum; enlarged, pale, bronze-red, friable liver with numerous, small necrotic foci; small abomasal erosions; increased amounts of yellow-orange, mucoid meconium; and enlarged mesenteric lymph nodes.2 In bovine fetuses, there may also be severe necrotizing colitis, despite autolysis. Possible placental lesions include severe, necrosuppurative, cotyledonary placentitis; local to diffuse, yellow to red/brown exudative intercotyledonary placentitis with edema, and vasculitis.2,8-9


Septicemia syndrome is typically only observed in aborted fetuses and neonates, infected in utero. Multisystemic colonization is observed with coagulative necrosis or miliary microabscessation of the liver, as well as the heart and other viscera to a lesser degree.2,8-9


Conjunctivitis syndrome is suspected to be caused by contaminated dust in the eye.8 Acute, suppurative myocarditis with abscessation and possible bacterial emboli, is the hallmark of the endocarditis syndrome.8 Mastitis occurs most frequently in cattle, and ranges from subclinical to severely suppurative. Infection can be difficult to clear, resulting in intermittent shedding.1


Definitive diagnosis of listeriosis can be obtained via culture of fresh tissue (brain, aborted placenta, and fetus), fluids and swabs (cerebrospinal fluid, nasal discharge, urine, feces, and milk). Cytology of cerebrospinal fluid from a lumbosacral tap will have increased protein (0.6-2.0 g/L) and mild pleocytosis. Immunofluorescence can be performed on smears from necropsy specimens, milk and meat, and may be helpful for fast diagnosis in public health situations. Serology is generally unhelpful, as many animals have high titers due to frequent environmental exposure.11


Listeriosis is an uncommon, but extremely important and well-documented food-borne illness of humans.10,13 The encephalitis syndrome is suspected to be due to hematogenous spread rather than axonal migration.3,7,10


LM has been isolated from numerous animal species including birds, reptiles, and ticks.14 Clinical disease has been described in horses, pigs, dogs and cats, though incidence is rare.1,8,14


Contributing Institution:

250 McElroy Hall 

Department of Veterinary Pathobiology

Oklahoma State University
Stillwater, OK 74078 USA


Brainstem: Rhombencephalitis, necro-suppurative, multifocal to coalescing, severe, with numerous microabscesses and moderate lymphohistiocytic meningitis.


JPC Comment:

The contributor provides an excellent review of Listeria monocytogenes and its multiple associated syndromes. E.G.D Murray is credited with initially isolating this entity from the blood of laboratory animals in 1924. Unable to assign the organism to known genera at the time, he identified the new agent as Bacterium monocytogenes. The genus was later renamed Listeria by Pirie in 1940. Although commonly isolated from humans, animals, food, and the environment, L. monocytogenes wasn't fully recognized as a pathogen until an epidemic of listeriosis in newborns (granulomatous infantiseptica), occurred in Germany in 1949. Histologic examination revealed granulomas within multiple tissues, including liver, spleen, brain and lung while bacteria were cultured from the meconium, blood, and other organs. Investigators initially identified Corynebacterium infantisepticum as the etiologic agent. However, H.P.R Seeliger shortly thereafter found the bacteria to be motile, inconsistent with bacteria of the Corynebacterium genus, and identified Listeria as cause of the outbreak.6


Capable of growth within both the environment and the extracellular space within the host, L. monocytogenes is also able to penetrate and replicate within virtually every nucleated cell. Following its release from the phagosome as the result of hemolysin and phospholipases as described the contributor, the organism utilizes the cell's cytoskeleton for intracellular movement via actin polymerization, primarily at the organism's apical aspect. This movement facilitates contact with the cell membrane, from which L. monocytogenes is then extruded as a membrane bound extracellular vesicle. These vesicles shelter the organism from the host's immune system and subsequently undergo endocytosis by the next target cell, completing its cell-to-cell transmission.6


Listeriosis in ruminants is classically associated the consumption of poor quality silage, a popular winter feed option utilized by producers formed under anaerobic conditions as the result of natural lactic acid fermentation. L. monocytogenes is often present, though in low numbers, in grass used for silage production. When properly performed, the silage fermentation process has an anti-listerial effect, largely as the result of organic acids. Both the initiation and maintenance of an anaerobic environment are essential for the production of suitable silage. This was demonstrated in a study that investigated the impact of both silage quality as well as a range of oxygen concentrations (0%, 0.1%, 0.5%, 1.0%, and 5%) on the inhibition of L. monocytogenes during the fermentation process.4 With the exception of poor quality late season grasses, silage exposed to 5% oxygen underwent rapid acidification to a pH of <5. This drop in pH was maintained when oxygen concentrations were maintained between 0-0.1% whereas silage exposed to higher oxygen concentrations exhibited a subsequent increase in pH that corresponded with the recovery of L. monocytogenes, though at varying levels correlating with the increase in pH. With the exception of one sample of very poor quality late season grass, L. monocytogenes survival under strict anaerobic conditions was inhibited by a pH of <4.4. Prolonged survival was noted at 0.5% oxygen and growth readily occurred at higher oxygen tensions of 1-5%. Of note, the presence of moldy silage may be a useful indicator of L. monocytogenes contamination as mold growth in microaerobic silage corresponded well to the presence of L. monocytogenes.4


As noted by the contributor, listeriosis has been reported in other species, including rare reports in companion animals. Sources of potential exposure that have become increasingly popular include raw meat-based diets. This was highlighted in 2018 study in Belgium that evaluated 35 commercial frozen raw-meat based diets from eight different brands. Listeria monocytogenes was isolated from 54% of products. Additional pathogens identified included Escherichia coli serotype O157:H7 in 23%, Salmonella species in 20%, and Sarcocystis cruzi in 11%.12



1.     Czuprynski CJ, Kathariou S, Poulsen K. Listeria. Pathogenesis of Bacterial Infections in Animals. 4th ed. Ames, IA: Wiley-Blackwell; 2010:167-187.

2.     Dennis SM. Perinatal lamb mortality in Western Australia. Australian Vet J. 1975,51:75-79.

3.     Disson O, Lecuit M. Targeting of the central nervous system by Listeria monocytogenes. Virulence. 2012;3(2):213-221.

4.     Donald AS, Fenlon DR, Seddon B. The relationship between ecophysiology, indigenous microflora and growth of Listeria monocytogenes in grass silage. J Appl Bacteriol. 1995;79(2):141-148.

5.     Dryer M, Thomann A, Bottcher S et al. Outbreak investigation identifies a single Listeria monocytogenes strain in sheep with different clinical manifestations, soil and water. Vet Microbiol. 2015;179:69-75.

6.     Hof H. History and epidemiology of listeriosis. FEMS Immunol Med Microbiol. 2003;35(3):199-202.

7.     Madarame H, Seuberlich T, Abril C et al. The distribution of E-cadherin expression in listeric rhombencephalitis of ruminants indicates its involvement in Listeria monocytogenes neuroinvasion. Neuropath Appl Neurobiol. 2011;37:753-767.

8.     Maxie MG. Jubb, Kennedy, and Palmer's Pathology of Domestic Animals. 6th ed. St. Louis, MO: Elsevier; 2016.

9.     Njaa BL. Kirkbride's Diagnosis of Abortion and Neonatal Loss in Animals. 4th ed. Ames, IA: Wiley-Blackwell; 2012:27-28, 71-72.

10.  Oevermann A, Zurbriggen A, Vandevelde M. Rhombencephalitis Caused by Listeria monocytogenes in Humans and Ruminants: A Zoonosis on the Rise?. Interdiscip Perspect Infect Dis. 2010;2010:1-22.

11.  Pritchard JC, Jacob ME, Ward TJ, Parsons CT, Kathariou S, Wood MW. Listeria monocytogenes septicemia in an immunocompromised dog. Vet Clin Pathol. 2016;45(2):254-259.

12.  Scott PR. Overview of Listeriosis. Merck Veterinary Manual. Updated March 2014. Accessed June 24, 2021.

13.  van Bree FPJ, Bokken GCAM, Mineur R, et al. Zoonotic bacteria and parasites found in raw meat-based diets for cats and dogs. Vet Rec. 2018;182(2):50.

14.  Wagner W, Melzner D, Bago Z et al. Outbreak of Clinical Listeriosis in Sheep: Evaluation from possible Contamination Routes from Feed to Raw Produce and Humans. J Vet Med B. 2005;52:278-283.

15.  Zachary JF, Huff TG, Britton R. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:186-189.


Click the slide to view.

Back | VP Home | Contact Us |