NEWS IN THE ANDLAB VIEWS
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.
52 – VetScript November 2020
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
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