Broad-ranging bacteria

Broad-ranging

bacteria

From the benign to the deadly. Clinical pathologist Lisa Hulme-Moir, from Gribbles Veterinary Auckland, looks at the many forms and effects of listeriae.

LISTERIAE ARE A highly adaptable group of saprophytic bacteria that grow in a wide range of environmental locations. They primarily proliferate in decaying plant matter, but can also reside temporarily in animals’ gastrointestinal tracts, usually with few consequences. However two species, Listeria monocytogenes and Listeria ivanovii, have evolved the ability to invade and replicate in animals’ cells, with potentially catastrophic effects. The following is a brief overview of the bacterial genus and the diseases the bacteria cause in New Zealand.

THE ORGANISM

Listeria monocytogenes is one of the most intensively studied models of bacteria capable of living intracellularly and evading the body’s immune response (Freitag et al., 2009). The bacterium binds to the surface of cells such as enterocytes and uses their endocytic pathways for uptake.

Within the cell, Listeria escapes into the cytosol by lysing the membranes surrounding it. There it uses the host cell’s nutrients to replicate and hijacks the host’s cytoskeleton to create actin tails for motility. This actin machinery is also used to push the bacteria into adjacent cells, enabling Listeria to travel long distances through and between cells without exiting to the extracellular environment.

Listeria is also taken up by macrophages and neutrophils, again escaping from the phagosome and evading these cells’ usual killing mechanisms. Inside the macrophage’s cytosol, the bacteria may be disseminated around the body as the macrophage traffics to various tissues.

As Listeria is an intracellular pathogen, innate immune responses and in particular cytotoxic T-cells are extremely important in the body’s response to listerial infection. Listeria is a potent stimulator of cytotoxic T-cells, which target and lyse infected cells, and much of what we know about cytotoxic T-cells has been derived from studies of L. monocytogenes in mice.

This ability to stimulate cytotoxic T-cells and act as a vehicle for intracellular processing of antigen has been harnessed to create anti-cancer vaccines. Currently there are a number of vaccines under development that use attenuated L. monocytogenes as a vector, including one for cervical cancer in humans and one for osteosarcoma in dogs.

SHEEP, CATTLE AND GOATS

Ruminants are the most common species affected with listeriosis (Table 1), with encephalitis and abortion being the most frequent presentations. Enteritis also occurs and, less commonly, ocular keratoconjunctivitis (or ‘silage eye’) and neonatal septicaemia.

Listeriosis is generally a sporadic disease but outbreaks may occur, particularly in sheep. It is rare for different disease syndromes to present together, but cases of encephalitis in sheep have been observed several weeks after outbreaks of enteritis (Fairley, 2018). Deaths due to metritis or gastroenteritis may occur in ewes during abortion outbreaks (Gill, 1999).

Encephalitic listeriosis presents with the classic signs of ‘circling disease’ and cranial nerve deficits, due to lesions primarily centred on the brainstem

(rhombencephalitis) (Cantile and Youssef, 2016). The bacteria gain access to the brainstem by travelling up the cranial nerve fibres from breaks in the oral mucosa or dental lesions, with disease resulting about three weeks later. Rhombencephalitis is uncommon in non-ruminant species; in these animals listerial infections of the central nervous system are typically due to bloodborne spread and result in meningitis and meningoencephalitis (Schlech, 2019). It is difficult to culture Listeria from cerebrospinal fluid or central nervous system tissue, but diagnosis can usually be obtained on histopathology as long as the brainstem and cranial spinal cord are available for examination.

Pregnant animals are particularly susceptible to listerial infection as their innate immune function is lowered. Abortions due to Listeria typically occur during the last trimester (Schlafer and Foster, 2016). If close to term, the aborted foetus may survive but can succumb to septicaemia. Septicaemia may also occur one or two weeks after birth as a result of infection contracted during the birthing process or in the immediate perinatal period.

Enteritis due to Listeria occurs in both sheep and cattle in New Zealand and clinically resembles salmonellosis (Clark et al., 2004; Fairley and Colson, 2013). Unlike the experience of encephalitic listeriosis, the incubation period is short – only a few days after exposure to a high number of organisms, and most often in spoiled silage. Care is needed when diagnosing listerial enteritis based on faecal culture.

Listeria are ubiquitous in the environment and are frequent temporary inhabitants of the gastrointestinal tract. This means a positive faecal culture result alone is not diagnostic; ideally it should be supported by appropriate lesions on histopathology or (if premortem) a close consideration of all the clinical data.

There have been occasional reports of keratoconjunctivitis in New Zealand. The absence of corneal ulceration and its occurrence in the winter months at a time of feeding supplement help to distinguish this disease from other causes of pink-eye, which typically occurs in the summer months when flies are active.

TABLE 1:

Number of Listeria cases submitted to Gribbles Veterinary laboratories between 2010 and 2020 for large animals and exotic species.

Sheep

Cattle#

Goats

Alpacas

Horses

Other Neurological

132

57

15 Primary disease presentation

Abortion

86

26

2

1* Enteritis

47

10

2 Ocular

3

1

# One case of Listeria monocytogenes isolated from mastitic milk * Cotton-top tamarin ** Bearded dragon and a marmoset Septicaemia

2** Mastitis has been reported overseas, but Listeria is a rare isolate in mastitic milk in New Zealand (Table 1).

Listerial disease in sheep and cattle is strongly associated with the feeding of silage and baleage and therefore is often seen during late autumn, winter and early spring. A pH of 4–4.5 is most often recommended to inhibit the growth of Listeria in silage (Avila and Carvalho, 2019). However, recent research has shown that Listeria can grow at lower pH levels, particularly if oxygen is available or if the decline in pH has been slow. When investigating outbreaks of listeriosis, it is therefore important to consider not only the pH of the feed sample but also the degree of packing and the exclusion of air in the stacks or bales. Other factors to consider include whether the covering was broken during storage, the rate of feeding out once opened and the degree of soil and faecal contamination during the initial harvesting process.

Not all listeriosis cases are associated with silage feeding; the disease may also be seen in animals grazing pasture only. High stocking density or hard grazing during and after droughts are particular risk factors, as animals graze into the lower sward where Listeria may be proliferating in dead plant material. Listeria’s ability to grow in low temperatures may also be a factor in the winter, along with pasture flooding, inclement weather and concurrent disease placing stress on animals.

ALPACAS

Only a few cases of listeriosis have been recorded in alpacas in New Zealand (Staples, 1997). These included abortions in imported hembra and two recent cases of enteritis, which displayed similar histopathologic changes to those seen in sheep and cattle. Overseas, encephalitic listeriosis and endocarditis have been reported in adult alpacas, and septicaemia and meningoencephalitis may be seen in neonatal crias.

HORSES

As with other non-ruminant species, listerial infections in horses are uncommon. Occasional instances of abortion have been recorded in New Zealand, and recently Listeria monocytogenes was isolated from the eye of a horse with a chronic non-healing ulcer. Overseas, several papers have documented cases of listerial keratitis in horses, which were often poorly responsive to therapy and required prolonged/repeated treatment (Revold et al., 2015). Risk factors such as feeding haylage or silage were identified. Encephalitis, neonatal septicaemia and one case of enteric listeriosis with an enlarged mass-like caecal lymph node have also been reported overseas (Lee and Mogg, 2016).

CATS AND DOGS

Infections due to Listeria are generally very uncommon in dogs and cats. However, in recent years a small number of cases of mesenteric lymphadenitis have been seen in New Zealand cats (Fluen et al., 2019). These cases have involved young to middle-aged cats presenting with anorexia, weight loss, mesenteric lymphadenopathy or abdominal masses. One case report of cellulitis due to L. monocytogenes on the foot of a cat following a tree weta bite has also been reported in New Zealand (Jones et al., 1984). Overseas, rare cases of encephalitis, septicaemia and pyoderma have been recorded in cats and dogs. The feeding of raw meat diets has been proposed as a risk factor for listerial disease.

PUBLIC HEALTH

Approximately 0.6 cases of listeriosis per 100,000 people are seen each year in New Zealand, and it is one of the leading causes of death due to foodborne illness in first-world countries.

Listeriosis can present as straightforward food-poisoning cases in healthy individuals, or as invasive diseases in at-risk groups such as the very young, the elderly, pregnant women and those

APPROXIMATELY 0.6 CASES OF LISTERIOSIS PER 100,000 PEOPLE ARE SEEN EACH YEAR IN NEW ZEALAND, WHILE IN WESTERN COUNTRIES IT IS ONE OF THE LEADING CAUSES OF DEATH DUE TO FOODBORNE ILLNESSES.

immunosuppressed due to disease or drug treatment. Invasive diseases include abortion, meningoencephalitis and septicaemia in adults, neonatal septicaemia and neonatal meningitis (Schlech, 2019).

Listeria’s ability to proliferate at low temperatures makes it a particular problem in ready-to-eat products such as salads, delicatessen products, raw milk products, soft cheeses and fresh and smoked fish. It also readily forms bio-films on food preparation surfaces that are resistant to low and high pH, chlorine, iodine and quaternary ammonium compounds. This makes it particularly challenging for the food production industry to control. Although studies of bulk milk and meat carcasses suggest contamination rates of these products are low in New Zealand, faecal shedding by ruminants are likely to act as a source of contamination of raw products and the factory environment.

Strategies to minimise the carriage and shedding of listeria on farms therefore benefit not only farmers by reducing disease in their stock but also public health by reducing a source of Listeria in the food-production and processing environments.

Finally, cutaneous listeriosis has occasionally been reported in veterinarians handling aborted material. This causes cutaneous papules and pustules with mild fever and is another example of the importance of taking extra hygiene precautions and wearing personal protection when dealing with abortion cases.

REFERENCES:

Avila CLS, Carvalho BF. Silage fermentation – updates focusing on the performance of microorganisms. Journal of Applied Microbiology 128, 966–84, 2019

Cantile C, Youssef S. Nervous system. In: Maxie GM (ed). Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. 6th Edtn. Pp 362–4. Elsevier, Missouri, USA, 2016

Clark RG, Gill JM, Swanney S. Listeria monocytogenes gastroenteritis in sheep. New Zealand Veterinary Journal 52, 46–7, 2004

Fairley R. Enteric listeriosis of sheep and cattle. VetScript 31, 46, 2018

Fairley RA, Colson M. Enteric listeriosis in a 10-month-old calf. New Zealand Veterinary Journal 61, 376–8, 2013

Fluen TW, Hardcastle M, Kiupel M, Baral RM.

Listerial mesenteric lymphadenitis in 3 cats. Journal of Veterinary Internal Medicine 33, 1753–8, 2019

Freitag NE, Port GC, Miner MD. Listeria monocytogenes – from saprophyte to intracellular pathogen. Nature Reviews in Microbiology 7, 62337, 2009

Gill JM. Abortions in sheep caused by Listeria species. Proceedings of 29th Annual Seminar, Society of Sheep and Beef Cattle Veterinarians 41–2, 1999

Jones BR, Cullinane LC, Cary PR. Isolation of Listeria monocytogenes from a bite in a cat from the common tree weta (Hemideina crassidens). New Zealand Veterinary Journal 32, 79–80, 1984

Lee C, Mogg TD. Enteric listeriosis in a horse. The Australian Equine Veterinarian 35, 48, 2016

Revold T, Abayneh T, Brun-Hansen H, Kleppe

SL, Ropstad EO, Hellings RA, Sorum J. Listeria monocytogenes associated kerato-conjunctivitis in four horses in Norway. Acta Veterinaria Scandinavica 57, 76–86, 2015

Schlafer DH and Foster RA. Female genital system. In: Maxie GM (ed). Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. 6th Edtn. Pp 408–9. Elsevier, Missouri, USA, 2016

Schlech WF. Epidemiology and clinical manifestations of Listeria monocytogenes infection. In: Fischetti VA, Novick RP, Ferretti JJ, Portnoy DA, Braunstein M, Rood JO (eds). Gram Positive Pathogens. 3rd Edtn. Pp 793–802. ASM Press, Washington DC, USA, 2019

Staples P. Listerial infection of animals and birds in New Zealand. Surveillance 24, 12–13, 1997

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