CHAPTER 35 LISTERIA INFECTIONS

Ezio Ferroglio

University degli Studi di Torino, Dipartimento Produzioni Animali, Epidemiologia ed Ecologia, Grugliasco, Italy

Listeriosis, also known as circling disease, is found world­wide in domestic animals, humans and, less frequently, in wildlife.

AETIOLOGY

The genus Listeria currently contains five species, although classification is under constant review. Two species, L. monocytogenes and L. ivanovii, are pathogenic, with L. monocytogenes being the more important of these. Listeria monocytogenes is a facultative intracellular Gram-positive aerobic or microaerophilic rod. Eleven serotypes have been recognized based on somatic and flagellar antigens. Cultiva­tion in the laboratory is done on common media, such as trypticase agar or brain-heart infusion agar, where the bac­teria grow in translucent colonies in 24-48 hours. When observing these colonies at a 45° angle, a characteristic blue-green sheen is appreciable on their surface. On blood agar, L. monocytogenes is haemolytic, but haemolysis occurs in a narrow area around the colonies, in contrast to L. ivanovii, which produces a wider zone of haemolysis than that seen with L. monocytogenes. Listeria monocytogenes is negative for oxidase and hydrogen sulphide and positive for catalase, and has characteristic ‘tumbling’ motility at room temperature (of animals - but only when it passes across the oral or intestinal mucosae does disease occur.

Listeria monocytogenes has been isolated from a wide range of hosts, including several mammalian families (arti- odactyls, carnivores, lagomorphs, rodents, insectivores, primates) and several bird species, and also from inverte­brate hosts (i.e. haematophagous arthropods). Listeriosis occurs particularly in domestic ruminants and has been described in more than 40 species of animal⁽¹⁾.

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

The most frequent forms of disease caused by L. mono­cytogenes are encephalitis, septicaemia and abortion or stillbirth. Genital and encephalitic forms rarely occur simultaneously in the same animal or flock. Listeria ivanovii is recorded from as wide a host range as L. mono­cytogenes, but as a pathogen it is seen only in ruminants, where it occasionally causes abortion but does not cause central nervous system (CNS) infection^(2-4).

Listeria spp. can grow in soil, water and faeces, and on vegetables, and is a facultative pathogen in vertebrate and invertebrate species. Bacteria from the environment are usually the source of infection for animals and exposure to this bacterium from environmental contamination is difficult to avoid. Listeria is able to survive and multiply in different environmental conditions and can grow at a pH range from 5 to 9 and at temperatures from 4 to 45°C. The bacteria can grow rapidly in silage that is not con­served correctly (pH higher than 5).

Listeria monocytogenes can be carried subclinically by a range of animal species and by humans, and the organism can be shed in faeces. In livestock, wildlife and human populations, the same L. monocytogenes strains may infect these different hosts, but it is also possible to find different strains of the bacterium in the environment and in water in the same area where positive animals are sampled⁽⁶⁾.

Outbreaks of infection are usually linked to ingestion of highly contaminated feed, or to heavily contaminated environments, e.g. soil, which contaminate the fodder or water with high levels of the bacterium. Silage has been frequently incriminated as a source of Listeria for domestic animals, although the organism can be readily found in fodder, grass and plants in rural or in wild environments. Populations of L. monocytogenes are probably maintained in the environment by continuous faecal-oral cycling and enrichment in ruminant hosts⁽¹⁾. Wild animals are thus continuously exposed to the bacterium, but only in rare circumstances does infection lead to disease.

PATHOGENESIS, PATHOLOGY AND IMMUNITY

Listeria monocytogenes is usually ingested; it crosses the intestinal mucosa and produces a bacteraemia. The liver is probably the first target organ. It can also cross the pla­centa and cause fetal infection. There is evidence that encephalitis is mainly due to bacteria that enter the oral mucosa, invade the trigeminal nerve and, moving along this nerve, enter the brain and thus cause inflammation. It has also been suggested that bacteria can also penetrate the tooth pulp and reach the brain via the trigeminal nerve. Less frequent routes of infection are through the conjunctiva and the nasal mucosa.

Listeria monocytogenes and L. ivanovii are intracellular pathogens that survive in macrophages, epithelial cells, hepatocytes and endothelial cells. After cellular uptake, probably by M cells of the intestinal mucosa, but also by crypt cells, Listeria escapes from the phagosome, multiplies in the intracytosol and spreads to adjacent cells. When the bacteria leave infected cells they are phagocytosed by mac­rophages and neutrophils; however, this can lead to sys­temic spread. Listerolysin, a cholesterol-binding cytolysin, is the factor that, in phagocytosis and cell invasion, regu­lates intracellular survival by facilitating the escape from the phagocytic vesicle.

In septicaemic forms, miliary foci of necrosis are usually visible in liver, spleen, lung, kidney and lymphoid tissues. Septicaemia may also result in uterine infection with abor­tion. Foci of necrosis are also visible in abortion material, in the cotyledons with a suppurative placentitis. The fetus has necrotic foci resembling those of septicaemic infection. The brain of infected ruminants may appear normal at gross inspection. Typical histopathologic lesions are menin- goencephalomyelitis characteristically affecting the white and grey matter of the brainstem, which can extend ros- trally to the diencephalon and caudally to the cervical spinal cord. Histologically there is early necrosis, with bacteria in the necrotic foci. Later there is a predominance of neutrophils and necrosis (microabscesses) and gitter (phagocytic) cells. These changes are accompanied by perivascular cuffing of mononuclear cells. There may also be leptomeningitis, perineuritis, and ganglioneuritis (trigeminal ganglion).

Infection is more widespread than disease, and the onset of a strong cellular response renders the host resistant to the organism. The cellular response seems to decrease after some months in experimentally infected mice, but in nature animals are probably frequently challenged by the bacteria. The humoral response has a lesser role in protec­tion.

CLINICAL SIGNS

Listeriosis can manifest as a CNS infection, as abortion, as a generalized septicaemia and as mastitis in dairy cattle. When the CNS is involved, adult ruminants show menin­goencephalitis with depression and confusion, protrusion of the tongue, and salivation, blindness, head pressing, and paralysis of facial and throat muscles. The affected animal can be unstable on its legs and seeks support from objects such as fences; when it walks, the tendency is to move in a circle — hence the common name of circling disease. At the terminal stage animals remain immobile on the ground and tremors are frequently seen. Death occurs in 2 to 3 days after the onset of clinical signs. Listeria abortion usually takes place in late gestation. Septicaemic listeriosis is usually associated with very few clinical signs, as affected animals die without these being observed. In young rumi­nants and monogastric animals, signs of meningitis without brain involvement may be seen, whereas clinical listeriosis is relatively rare in non-ruminant species; however, listerio­sis has been noted in pregnant female European brown hares.

DIAGNOSIS

For cultivation of Listeria, to increase the recovery from visceral organs, placenta and fetuses, the tissues can be mixed with a liquid nutrient medium and incubated at 4°C for 1 week or longer. Subcultures should be done at 1, 3, 4 and 12 weeks in blood agar from the original broth. Selective broths (e.g. Listeria Enrichment Broth or Fraser Broth) are more sensitive and specific than direct non- selective plating. There are also chromogenic agars avail­able for presumptive identification of L. monocytogenes. Antibodies against L. monocytogenes can be tested by sero­logical tests such as the agglutination test, but the sensitiv­ity and specificity are poor, so serology is of limited diagnostic value⁽¹⁾.

MANAGEMENT, CONTROL AND REGULATIONS

Control of the bacteria in the environment is not feasible apart from good standards of hygiene that aim to reduce food and water contamination by organisms shed in the faeces. Outbreaks in wildlife are usually reported in captive or managed herds (e.g. foraging herds). Listeria monocy­togenes sporadic infections with low mortality, less than 3%, are reported in the European brown hare⁽³⁾. Avoiding wild animals congregating at supplementary feeding sites, and preventing the provision of supplementary food on the ground, as well as avoiding access to foodstuffs left on pasture for livestock, are management actions that can be adopted to control this infection in wildlife. Considering that many livestock infections are due to poor-quality silage, use of such silage to feed wildlife should be avoided.

PUBLIC HEALTH CONCERN

Listeria monocytogenes is one of the most important food- borne zoonoses. Direct transmission to humans, even through contact with infected animal tissues, rarely occurs. The main risk for human health is through consumption of infected animal food, and infection from milk, pates, cheese or other food items contaminated by the bacterium. There are many reports of the occurrence of Listeria, mainly L. monocytogenes, in game carcasses, where preva­lence of L. monocytogenes infection ranges from 0 to 12.5% of the wild game meats sampled. Wild boar appear to be the species with the highest prevalence, because up to 23.5% of meat from this species is contaminated¹-⁷). This apparently high prevalence in wild boar meat could be the result of different hunting techniques (animals driven to guns instead of stalking or stand hunting), which tend to lead to more abdomen shots (resulting in meat contamina­tion with Listeria spp. tracking from the digestive tract), or to the dressing of carcasses in the field. However, this prevalence is not directly linked to primary carcass infec­tion (i.e. systemic infection of the animal prior to slaugh­ter), because meat infection is usually due to carcass dressing in the field and in the later processing stages (cold storage rooms or processing facilities), where environmen­tal contamination with the bacteria can occur⁽⁷⁾.

SIGNIFICANCE AND IMPLICATIONS FOR ANIMAL HEALTH

Although there are occasional reports of outbreaks of lis­teriosis in captive or managed herds, cases in free-ranging wild animals are sporadic. Therefore it seems that Listeria generally has a minor influence on wildlife populations. A report on white stork chicks indicates that there was no effect of Listeria spp. infection on the subsequent survival of the infected chicks⁽⁵⁾.

The main implication of L. monocytogenes infection in domestic animals is in dairy cows, which develop sub- clinical mastitis, with shedding of L. monocytogenes in the milk. If this milk is not then pasteurized there is a potential for causing human infection from the consumption of the unpasteurized dairy products.

REFERENCES

1. Songer, J.G. & Post, K.W The Genera Listeria and Erysipelothrix. Veteri­nary Microbiology: Bacterial and Fungal Agents of Animal Disease. St. Louis, MO: WB Saunders; 2005.

2. Tham, W, Bannerman, E., Bille, J. et al. Listeria monocytogenes subtypes associated with mortality among fallow deer (Dama dama). Journal of Zoo and Wildlife Medicine. 1999;30:545-9.

3. Wuthe, H.H. & Schonbergerg, A. Listeriosis in the European brown hare in northern Germany. Berliner undMunchener Tierarztliche Wochen- schrift. 1999;112:98-9.

4. Morner, T Listeriosis. In: Williams, E.S. & Barker, I.K. (eds). Infectious Diseases of Wild Mammals, 3rd edn. Ames, Iowa: Iowa State University press; 2001; pp. 502-5.

5. Andrzejewska, I., Dolata, PT, Jerzak, L., Ptaszyk, J., Tryjanowski, P. & Zduniak, P. Prevalence of agglutinating antibodies against Listeria mono­cytogenes in chicks of the white stork (Ciconia ciconia Linnaeus, 1758) in Poland. European Journal of Wildlife Research. 2004;50:218-20.

6. Lyautey, E., Hartmann, A., Pagotto, F. et al. Characteristics and fre­quency of detection of fecal Listeria monocytogenes shed by livestock, wildlife, and humans. Canadian Journal of Microbiology. 2007;53: 1158-67.

7. Atanassova, V., Apelt, J., Reich, F. & Klein. G. Microbiological quality of freshly shot game in Germany Meat Science. 2008;78:414-9.