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Listeriosis in Animals and Humans

S. A. Bhat M. Shahnawaz S. N. Shah B. A. Ganayi C. P. S. Solanki
Vol 2(1), 68-77

Listeriosis is an emerging zoonotic infection of humans and ruminants worldwide caused by the bacterium Listeria monocytogenes. Listeriosis is one of the most important food-borne diseases of humans. The disease manifestations include septicaemia, meningitis and encephalitis, usually preceded by influenza-like symptoms including fever. In pregnant women, intrauterine or cervical infections may result in spontaneous abortion or stillbirths. Although the morbidity of listeriosis is relatively low, the mortality of the systemic/encephalitic disease can be very high, with values in the vicinity of 30%. In case of animals, clinical listeriosis is mainly a ruminant disease, with occasional sporadic cases in other species. The main clinical manifestations of animal listeriosis are encephalitis, septicaemia and abortion. This review discusses the current knowledge about listeric infection and involved host and bacterial factors. There is an urgent need to study the molecular mechanisms of pathogenesis, which are poorly understood. Such studies will provide a basis for the development of new therapeutic strategies that aim to prevent L. monocytogenes from invading the brain and spread within the CNS.

Keywords : Listeriosis Encephalitis zoonotic


The Gram-positive bacterium Listeria monocytogenes (LM) was first isolated in a human patient with meningitis in 1921 and subsequently worldwide from a wide range of mammalian and non-mammalian species, notably farm ruminants (Dumont and Cotoni, 1921: Murray et al., 1926: Gray and Killinger, 1966).The genus Listeria, presently includes eight species. Among these L. monocytogenes and L. ivanovii are regarded as pathogenic species whereas L. innocua, L. welshimeri, L. seeligeri, L. marthii, L. rocourthiae and L. grayi are non pathogenic species (Seeliger and Jones, 1986; Mcluachlin, 1987; Graves et al., 2009; Leclercq et al., 2009). Based on serological reactions, Listeria species were divided into more than 15 distinct serotypes, L. monocytogenes serotypes having 1/2a, 1/2B, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c, 4d, 4e and 7. L. ivanovii having serotype 5 and other species of Listeria serotypes 6a and 6b and some serotypes shared with L. monocytogenes. Using genetic identification methods, L. monocytogenes was separated into three lines: Line I consists of serotypes 1/2B, 3b, 4b, 4d and 4e, line II is composed of serotypes 1/2a, 1/2c, 3a and 3c and line III of serotypes 4a and 4c. In addition, the line III can be divided into subgroups IIIA, IIIB and IIIC (Doumith et al., 2004: Gasanov et al., 2005).

It was in the year 1980s that as a result of several human epidemics that listeriosis was recognized as a serious and frequently fatal food-borne disease and research activity on the disease was substantially intensified (Schlech et al.,1983: V´azquez-Boland et al., 2001). Since then the incidence has risen steadily including large outbreaks making listeriosis a major public health issue (Denny and McLauchlin, 2008). Clinical syndromes associated with L. monocytogenes infection are similar in all susceptible hosts and include febrile gastroenteritis, septicemia, abortion and central nervous system (CNS) infections such as meningitis, meningoencephalitis and rhombencephalitis. CNS involvement is a characteristic feature and accounts for the high mortality associated with listeriosis (Siegman-Igra et al., 2002: Drevets and Bronze, 2008). Currently, the agent is one of the best-studied bacterial pathogens for various reasons. Most importantly, it serves as model system for the study of innate and cell-mediated immunity, host-pathogen interactions and intracellular survival of pathogens (Flannagan et al., 2009). More recently, the bacterium has been investigated as a vector of heterologous proteins for vaccination and immunotherapy of cancer and infectious diseases (Wallecha, 2009). Recent estimations ranked listeriosis as the second and fourth most common cause of death due to food toxi-infection in the United States, England, and Wales. This disease has been estimated to be responsible for 14-15% of all deaths due to food poisoning in both studies. A strict control of food quality and also the public education is needed to reduce the future occurrence of such episodes and sporadic cases of listeriosis (Naim et al., 2009: Nancy, 2002). The purpose of the present review is to summarize the current knowledge about L. monocytogenes.

Epidemiology of Listeriosis

Infection with L. monocytogenes is a widespread zoonosis, affecting mainly cattle, sheep, and goat herds. Bacteria are ubiquitous in the environment and are usually present in animal faeces, soil, decomposing vegetation, plants and water courses. Once an area is contaminated with microorganisms, they are difficult to eradicate, due to their high degree of survival. L. monocytogenes is able to live in the bio-film and to survive in severe environmental conditions, including a wide range of temperatures (-1.5 to 50°C) and wide limits of pH (from 4.3 to 9.6) (Rodriguez et al., 2008). In ruminants, the food-borne route of L. monocytogenes infection has been well established long before it was shown in humans (Low and Donachie, 1997). Many studies have indicated that poor-quality silage is commonly contaminated with L. monocytogenes and focused on spoiled silage as source for listeriosis outbreaks (Low and Donachie, 1997). In line with these results, fecal shedding of L. monocytogenes in cattle is associated with contamination of silage (Ho et al., 2007). The investigation of an epidemiological link between silage feeding and listeriosis in ruminants, however, gave inconsistent results. Whilst some studies could isolate matching L. monocytogenes strains in brains of affected animals and silage samples, others yielded unrelated Strains (Mohammed et al., 2009). A recent study detected a higher prevalence of the bacterium in samples collected from the immediate cattle environment (feed bunks, water through and beddings) and in cattle feces than in silage challenging the view that silage is the only source of L. monocytogenes infection (Yoshida et al., 1998). In cattle, listeriosis was first reported by Mathews in 1928 and in Romania, Rosca (1972) describes the first listerial abortion in cattle. According to the symptoms, three forms can be differentiated: sepsis, nervous and listerial abortion (Carp Cărare, 2006). Recent prevalence estimates of listeric encephalitis in cattle, sheep, and goats, based on neuropathological survey studies in Europe, range between 7.5% and 29.4% and a neuropathological survey of fallen stock in Switzerland identified listeriosis as the most important CNS disease of small ruminants (Heim et al., 1997: Oevermann et al., 2008). With reference to the small ruminant population in Switzerland the prevalence of listeric encephalitis was 216 cases/million sheep and 500 cases/million goats per year and thus exceeded significantly the number of human cases (between 1.4 and 9 cases/million inhabitants per year) (Oevermann et al., 2008). Similar data are not available for bovines. However, in neuropathological surveillance schemes for bovine spongiform encephalopathy in various countries, listeriosis scores as the most frequent neurological disease in cattle (Heim et al., 1997). The importance of these data is underlined by significant economical losses in life stock industry caused by listeriosis, the likely role of ruminants as reservoir for human pathogenic strains and therefore its impact on food safety (Nightingale et al., 2004 :Okwumabua et al., 2005).

The bacterium was also identified in the faeces of 5% of healthy human subjects, where it was considered as transient colonizers of the gastrointestinal tract. A study of patients’ families with listeriosis showed gastrointestinal porting rate of 21%, although there was no significant difference in occurrence of symptoms between carriers and non-carriers (Nancy, 2002). Rate of gastrointestinal colonization in healthy adults and the fact that 15% to 70% of food products are contaminated with L. monocytogenes suggests that people frequently ingest bacteria. However, listeriosis is a relatively uncommon infection in humans, implying that host factors related to inoculums size are important in the emergence and evolution of disease. The most numerous cases of listeriosis occur in urban areas without a clear association with animals, suggesting that food serves as a major vector of infection. Potentially contaminated foods include raw vegetables, unpasteurized milk, poultry, meat products from gourmet foods category, soft cheeses and many other foods. Although marine foods have received little attention on the potential for contamination with L. monocytogenes, the disease has been linked to consumption of shrimp, crab meat, smoked rainbow salmon, and lobster (Naim et al., 2009). In 2000, the Centers for Disease Control and Prevention (CDC) reported approximately 650 cases of listeriosis. However, because listeriosis isn’t a notifiable disease in the U.S., this number is an underestimate of disease incidence. CDC has coordinated a study of active surveillance in 1986, estimating the annual infection rate of 0.07 to 100,000 people, with approximately 1850 cases and 425 deaths per year in the US. Rate of infection is higher among adults aged over 60 years (1.4 per 100,000 people) and infants less than one month age (10 per 100,000 people). The increased incidence of listeriosis in extreme age groups is probably due to the decline or immaturity of the immune system (Aarnisalo et al., 2008). Pregnant women represents one third of cases and their risk of infection is 17 times higher than the risk of other categories. In two episodes of food infections, 50% to 87% of all affected adults that weren’t pregnant women had a poor medical condition. A study conducted by the CDC on sporadic cases of listeriosis has shown that 69% of cases of disease in patients who are not pregnant involved humans with a poor medical condition. Immunosuppressive state, which reduces cellular immunity, increases the risk of listeriosis (Takhistov et al., 2009). Especially because of prevention efforts in the food industry, the incidence of listeriosis decreased by 44% from 1989 to 1993. These strategies include a zero-tolerance policy for ready meat, poultry and dairy products.

Ruminants – Zoonotic Reservoir for Human Infection

The link between ruminant and human listeriosis is not completely understood. Listeriosis is defined a zoonosis, but direct transmission between ruminants and humans rarely occurs and is in most cases associated with non life threatening cutaneous infections through contact with infected cattle or after handling of abortive material (Regan et al., 2005).. However, it appears reasonable to implicate ruminants as an important natural reservoir for strains causing human infections given that one epidemic clone responsible for a significant proportion of human epidemics has been frequently isolated from cases of ruminant listeriosis (Okwumabua et al., 2005: Wiedmann et al., 1997). Furthermore, dairy farms are frequently contaminated with L. monocytogenes, particularly as compared to other environments and its subtype populations in the farm environment encompass commonly strains that have been associated with human illness, whether sporadic or epidemic (Roberts and Wiedmann, 2003: Ueno et al., 1996: Nightingale et al., 2004). Ruminants, particularly cattle, contribute to amplification and dispersal of L. monocytogenes into the farm environment. The bacteria can be shed in the feces of clinically affected animals, but also healthy carriers (Ueno et al., 1996: Mohammed et al., 2009). Raw milk might contain L. monocytogenes either as a consequence of bacterial shedding in the milk or due to exogenous contamination from the dairy farm environment (Pintado et al., 2009). Human listeriosis is principally a food-borne infection and most reported outbreaks of listeriosis in men are attributed to the consumption of contaminated products of animal origin (Danielsson-Tham et al., 2004). Transmission may occur indirectly through food products from infected animals or healthy carriers that are not processed before consumption as well as raw vegetables that are contaminated by L. monocytogenes containing manure (MacDonald et al., 2005). Most foods of animal origin are treated by procedures that effectively kill L. monocytogenes in raw foods. Therefore, a possible means of transmission of L. monocytogenes strains from ruminants to humans is their introduction and establishment in food processing facilities and their ability to produce biofilms and to adhere to inert surfaces may significantly contribute to the latter. Supporting this hypothesis, one study identified several L. monocytogenes genotypes that contaminated both dairy-processing and farm environments (Arimi et al., 1997). Although all these results strongly implicate ruminants as a natural reservoir for L. monocytogenes and a source of human infections, at present, there are no data with regard to the extent of strain population overlap between human and ruminants.. The identical neuropathology of listeric encephalitis in humans and ruminants, however, indicates that neurotropic strains common to both hosts are responsible for the disease.

Identification and Serological Test for L. monocytogenes: A variety of conventional and rapid methods are available for the detection and identification of L. monocytogenes in food samples and specimens from animal listeriosis. Conventional methods remain the ‘gold standard’ with which other methods are compared. They are usually very sensitive. These methods use selective agents and enrichment procedures to reduce the number of contaminating microorganisms and allow multiplication of L. monocytogenes. Although not required for regulatory purposes, different levels of subtyping of L. monocytogenes strains are available, including serotyping, phage typing, multilocus enzyme electrophoresis, DNA restriction enzyme digestion patterns (conventional and pulse-field gel electrophoresis), nucleic acid sequence-based typing and random amplification of polymorphic DNA.

Serological tests for the detection of antibodies have not been traditionally used for the diagnosis of listeriosis. A number of formats have been tried and they have all been found to be largely unreliable, lacking sensitivity and specificity. Experimental serological assays based on the detection of anti-listeriolysin O have been used in some epidemiological investigations and as support for the diagnosis of culture-negative central nervous system infections. Immunohistochemical detection of L. monocytogenes antigens is a useful tool for the diagnosis of the encephalitic form of the disease.


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