Listeriosis in farmed ruminants
Emily Rätsep,
Animal Health Laboratory, University of Guelph, Kemptville, ON.
AHL Newsletter 2024;28(2):12.
Listeriosis, caused by the bacterium Listeria monocytogenes, is an infectious disease that affects a wide range of mammalian species, including humans. This environmental bacterium is present in soil, vegetable matter, and feces of healthy animals. Although ubiquitous, it can cause serious disease under certain conditions. This is most often seen in farmed ruminants due to feeding poorly fermented silage above a pH of 5.0-5.5. The bacteria optimally grow in temperature ranges between 30 and 37 °C, although they are also capable of reproducing in temperatures as low as -0.4 °C and as high as 45 °C. They also can reproduce in a wide pH range, from 4.5-9.6. While present in the natural environment, Listeria usually only results in sporadic disease and is considered non-contagious. However, outbreaks are known to occur when animals are exposed to a single contaminated source such as silage. Disease igenerally occurs most often in winter and early spring, although outbreaks have also been reported in animals fed poor quality pastures or rotting vegetable matter. Moist preserved feed such as rotting hay or feed with significant soil contamination may also represent a potential risk.
Clinical disease can vary greatly, and may include: neurological disease, sepsis, abortion, mastitis, or gastroenteritis. The most common disease form is generally considered to be listerial encephalitis which is often a chronic disease with approximately 4-6 weeks between exposure and presentation. The clinical course itself is generally rapid in sheep, with death occurring within 24-48 hours after onset of neurological signs. In cattle, the disease course is more chronic, lasting about 1-2 weeks. In these affected animals, neurological signs can vary, but are generally unilateral due to an ascending infection along the trigeminal nerve following penetration of the oral mucosa. Infection results in ataxia, nystagmus, dropped jaws, chewing problems, head tilt and cranial nerve deficits. Other less specific clinical signs such as fever, anorexia, depression, proprioception deficits, circling or head pressing can also be observed. Grossly, changes are minimal and often subjective. These can include meningeal congestion and occasionally reddening of the parenchyma of the brainstem. Histologic lesions are notable, consisting of microabscesses and perivascular cuffing primarily affect the brainstem (Figs. 1, 2).
Abortion or early neonatal mortality is the second most common manifestation of Listeria infection, and is most often reported in small ruminants. In a pregnant ewe, bacteremia with subsequent placental and fetal infection can result in abortion, usually 5-10 days post exposure. If infection occurs later in pregnancy, it can result in stillbirths or early neonatal mortality due to sepsis. There are few gross lesions; however, areas of necrosis can be observed in the placenta, liver, spleen and heart. Histological changes include intercotyledonary placentitis characterized by multifocal areas of necrosis with neutrophils, and frequent, dense colonies of coccobacilli (Fig. 3).
Septicemia is most commonly recognized in neonates or pregnant ewes exposed to high numbers of bacteria. Rare presentations include dermatitis and gastrointestinal listeriosis typified by enteritis with diarrhea, possible hemorrhaging, and ulceration of the gastrointestinal mucosa. Listerial mastitis is rarely clinically apparent.
Diagnosis is dependent on characteristic histological lesions combined with results of bacterial culture on enriched media. At the AHL, an immunohistochemical stain is also available for L. monocytogenes, allowing for diagnosis in histological sections if no fresh tissue is available for culture.
It is important to recognize that exposed animals may become latent carriers and could potentially shed the bacteria in milk, thereby representing a potential zoonotic risk, particularly for immunocompromised people. Feces, urine, aborted fetuses and uterine discharge may also be potentially infectious; therefore, practicing good hygiene when handling known or suspected cases of listeriosis is of critical importance. AHL
Figure 1. Goat brainstem. Necrosis of the parenchyma with microabscesses composed of neutrophils and macrophages. H&E stain, 40x.
Figure 2. Bovine brain. Perivascular cuffing of cerebral blood vessels with macrophages, lymphocytes and fewer neutrophils. H&E stain, 40x.
Figure 3. Goat fetal liver. Focus of necrosis with neutrophils and dense colonies of coccobacilli (*) amongst the necrotic debris. H&E stain, 40x.
References
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4. Scott PR. Listeriosis. In: Diseases of Sheep, 4th ed. Aitken ID, ed. Wiley-Blackwell, 2007:255-258.