Introduction

Listeria and Erysipelothrix: both are gram-positive regular rods, motile and non-spore producers that are found in the intestinal tract and environment. They cause multisystemic disease and zoonoses, and are susceptible to the penicillins. However, their phylogeny and pathogenesis is very different.

Aetiology

The culture of bacteria from lesions and in foods is still the hallmark to confirm the diagnosis of listeriosis and erysipelas in animals.

Listeria

Note that whilst Listeria will grow with ease from necrotic lesions in the foetus, placenta and organs, it cultures with great difficulty from the brain and thus an enrichment method using cold-enrichment in broth is used. Alternatively, a Listeria-specific PCR can be carried out on broth cultures. Note that the histological lesions within the brainstem and spinal cord are considered pathognomonic for neurolisteriosis.

Listeria are motile at room temperature, gram-positive, non-spore forming, small, regular rods. Morphologically they can resemble Streptococcus, Corynebacterium or Erysipelothrix.  They produce beta-haemolytic, catalase- and aesculin-positive colonies.

The typical tumbling motility that they show due to the presence of polar flagella is shown in this YouTube video [:22]. They belong to the family Listeriaceae with the only other genus belonging to this family being the food spoilage bacterium Brochothrix. Both Brochothrix and Listeria will grow at 4°C. Cold-enrichment for up to 2 months in broth at 4°C has been a successful method isolating Listeria from brain samples. However, selective enrichment with an overnight incubation at 37°C is used to identify this bacterium in other samples.

The most pathogenic member of this genus is Listeria monocytogenes. It is widely distributed in nature and it and occasionally Listeria ivanovii are the cause of listeriosis.

Erysipelothrix

Erysipelothrix rhusiopathiae is a gram-positive, non-sporulating rod that is beta- or alpha-haemolytic and motile at room temperature. It is distinguished from Listeria by being catalase- and aesculin- negative and produces hydrogen sulphide in TSI (triple sugar iron) agar.

Two forms are recognised:

1) The smooth form where smooth colonies consisting of short rods predominate in samples taken from acute or subacute disease.

2) The rough form where rough colonies consisting of filamentous rods predominate in samples taken from chronic disease.

Erysipelothrix is known to infect over 50 animal species, including humans. Erysipelothrix rhusiopathiae most commonly affects domesticated pigs where it causes a multisystemic disease known as “Swine Erysipelas”.  Whilst 28 serovars are recognised, pigs are mainly affected by serovars 1a, 1b and 2. Outdoor layer hens and other poultry are also affected by a septicaemic disease caused mainly be serovars 1b, 5 and 15.

In people it causes predominantly a rare skin infection known as “erysipeloid”. People become infected when being injured after handling objects that have the bacterium present for example fish barbs are a common source.

Listeriosis

Pathogenesis of listeriosis

Intracellular survival strategy of L. monocytogenes

In the intestine Listeria attach and induce goblet cells and M-cells to engulf them. Irrespective of their portal of entry. Listeria escape from the phagolysosome by lysing the vacuole membrane with listeriolysin O, a haemolysin/ porin. They then multiply in the cytoplasm and move to the next cell by inducing the polymerisation of host actin by the ActA  transmembrane protein to form a tail that as it grows pushes the bacteria to the adjacent cells, where it is phagocytosed and the cycle begins again. Use the link provided to look at a YouTube video on the movement of Listeria from cell to cell. They are therefore able to avoid the circulating immune defence by remaining in an intracellular environment. (Burkholderia pseudomallei, a gram-negative rod, acts in a similar fashion).

(The molecular pathogenesis of Listeria is well studied. If you want additional reading for your own interest read the linked article in Nature reviews.)

Diagram illustrating intestinal invasion and intracellular invasion and cell movement by Listeria. After phagocytosis Listeria is able to lyse the phagolysome using Listeriolysin O.  ActA then causes host cell actin to polymerase forming a tail which propels listeria to the next cell. Listeria is then encased in a vesicle (phagosome) and the process repeats itself. The virulence factor production is controlled by positive regulatory factor A (prfA) which is upregulated when Listeria is in the animal host.

The exact process of how Corynebacterium pseudotuberculosis survives in macrophages is unknown. However, it is believed to use a similar mechanism as mycobacteria where phagosome-lysosome fusion is inhibited.

Prescottella equi is phagocytosed by alveolar macrophages where it is able to survive by producing VapA (virulence-associated protein A) which reduces phagosome-lysosyme fusion and disrupts endolysosomes. (More details will be provided in the relevant notes)

In ruminants, after ingestion Listeria bacteria enter via small wounds in the oral cavity and move via the trigeminal nerve to the brainstem. Due to their means of cell to cell movement (see next section), they easily cross the blood-brain barrier. (In people, neurolisteriosis occurs secondary to haematogenous spread to the brain, whereby infected monocytes accumulate and attach to cerebral endothelia, allowing cell to cell transfer of Listera – when examining the histology of lesions caused by Listeria in ruminant brains, one wonders if a similar event could also be occurring).

In other animals and young ruminants, Listeria survives stomach passage and is phagocytosed by the intestinal goblet and M-cells cells to enter the bloodstream (haematogenous spread). Look at this excellent YouTube video [6:53] on how Listeria causes disease. It is in people, but the process is similar in animals. Note that commensal clostridia within the intestines provide some protection against listerial invasion (colonisation resistance). The majority of invading Listeria are destroyed by the immune system before they can enter cells. Virulence factors are only produced by L. monocytogenes at body temperatures of 37°C. Once they enter cells they are able to escape the phagolysome, multiply and move directly cell-to-cell avoiding humoral immunity.

The incubation period is from 2 days to 3 weeks.

Haematogenous spread after oral and intestinal penetration of Listeria, is more common in monogastric animals. It can result in sepsis in young animals or organ localisation. Infection of the uterus and placenta causes abortion in the last trimester in all mammals including people and ruminants.

The cell mediated immune response is directed by lymphokines and direct lysis of infected cells. Thus infected foci are organised as granulomas with macrophages in the centre surrounded by a zone of lymphocytes.

Pathology and clinical signs of neurolisteriosis in ruminants

Neurolisteriosis in ruminants is also known as circling disease as Listeria primarily targets the cells within the brainstem affecting the roots of the cranial nerves and balance. The presence of infected cells attracts phagocytic cells of the brain leading to microabscess formation and a meningoencephalitis.

Initially, affected animals are anorectic, depressed, and may propel themselves into corners, lean against stationary objects, or circle toward the affected side. Facial paralysis with a drooping ear, deviated muzzle, flaccid lip, and lowered eyelid often develops on the affected side, as well as lack of a menace response and profuse, almost continuous, salivation; food material often becomes impacted in the cheek due to paralysis of the masticatory muscles.

After 1-4 days, untreated animals struggle to walk, appear dull and sleepy, become paralysed and die.

A video clip of neurolisteriosis of a goat (IMG_1967) is also provided (courtesy of Elizabeth Parker). There are also excellent videos of listeriosis in ruminants on YouTube. Here is one about a doe that recovered from listeriosis: Listeriosis in Goat [3:32].

A good review on ovine listeriosis is: Brugére-Picoux J.  2008. Ovine listeriosis. Small Ruminant Research, 76:12-20 and Scott PR. The clinical diagnosis of ovine listeriosis. 2013. Small Ruminant Research, 110:138–141. The article by Scott also has some excellent video clips that show the various stages of circling disease in sheep.

Control of listeriosis

Listeria can be found in soil, water, silage, raw vegetables, raw meat and poultry, raw and pasteurized, soft cheeses, kitchen premises, and in the intestinal tract of many animals, including cattle, sheep, and humans, and in crustaceans, fish, shellfish, ticks, and insects.

Even though Listeria is destroyed by acid and heat, it can contaminate food processing plants and therefore it is one of the target bacteria in plants handling animal products for human consumption i.e. abattoirs. Listeria is unusual in that it will grow slowly at refrigeration temperatures. Thus the longer a contaminated food source such as soft cheeses and fruit is stored in the refrigerator the greater the risk of listeriosis. Listeria is not a food spoilage bacterium, so contaminated foods are not recognised as “rotten”.

Most cases in ruminants originate from the ingestion of decomposing silage or decaying vegetable material i.e. cabbages and potatoes. The disease is also more common in temperate climates in winter or early spring.

As there are no vaccines, it is best to control listeriosis by avoiding animal feeds that could have the bacterium present i.e. poorly stored silage/haylage. Refrigerated vegetables that are no longer suitable for human consumption. High doses of penicillin can be used to treat early cases before the brain damage is severe. More advanced cases usually die despite therapy. Other antibiotics that may be effective include erythromycin, ceftiofur and potentiated sulphonamides. (A novel experimental treatment is treatment with positive regulatory factor A (prfA) antagonists. prfA is a regulatory protein that Listeria uses to switch on genes that code for virulence factors such as listeriolysin A.)

Listeriosis in people (for interest only)

Listeriosis is a reportable disease in people in Australia. Humans are usually infected when they ingest the bacteria in under-cooked or processed ready-to-eat foods, especially those that have been refrigerated for a prolonged period. However, they can be infected through skin abrasions, when handling infected aborted material and transplacentally. Each year there are a number of reported cases in people, not only in Australia, but globally. Most adults develop a mild eye or skin infection, or self-limiting diarrhoea. Invasive disease is rare, but, the aged, immunosuppressed and pregnant women are highly susceptible.

Symptoms and signs of invasive disease result from sepsis or meningitis and include fever, muscle aches, headache, stiff neck, confusion and convulsions. Pregnant women can either have a miscarriage or stillbirths.

Australia has had numerous human infections from soft cheeses and in February 2018, 22 cases were contracted from rock melons provided by a single grower in NSW. Seven of the 22 died and a pregnant woman aborted. Antibiotics such as ampicillin, ciprofloxacin, linezolid and azithromycin, are effective in treating listeriosis and can prevent foetal infection. Although the invasive disease is rare, the case fatality rate can be as high as 30%.  Ensure that hands are washed after the preparation of food and handling raw meats, meats are properly cooked, that dairy products are pasteurised and that uncooked meat is not mixed with uncooked meat and fresh salads and fruit in the refrigerator.

It is also important that trace-backs are done in disease outbreaks and that strain fingerprinting is done, usually by whole genome sequencing (WGS), to better understand the origin of the causative agent. This enabled Public Health to trace the 2018 outbreak of listeriosis to a single grower.

Erysipelas – Erysipelothrix rhusiopathiae infections

pathogenesis of erysipelas

In pigs

Bacteria gain entry to the body after ingestion via the tonsils or through skin damage. Virulent strains produce both a capsule and neuraminidase. The capsule allows the bacteria to resist phagocytosis by macrophages and when phagocytosed to resist killing, most probably by decreasing the oxidative burst response. The neuraminidase is thought to assist in adherence to and the invasion of host cells, especially endothelial cells. Neuraminidase cleaves the sialic acid present in the cell membrane which not only stimulates the coagulation cascade but leads to inflammation.

Immunity is both humoral by an IgG response allowing opsonisation of the bacteria and cellular. Antibodies to the SpaA surface protein on E. rhusiopathiae is protective.

Once in the bloodstream, Erysipelothrix disseminates throughout the body causing endothelial damage and inflammation leading to sepsis in non-immune suckling pigs. In older pigs and those with partial immunity the bacteria will lodge in end-artery systems causing a localised thrombosis. In growing pigs the arterioles of the skin are affected, leading to ischaemic necrosis of the skin supplied by that arteriole. The classical rhomboid areas of necrosis in the skin give the disease its common name of “Diamond Skin Disease”.

In older animals the heart valves and joints are usually affected. Adult animals with endocarditis tend to be poor doers and may die suddenly due to heart failure. Heavy animals may show lameness (stiffness and reluctance to move)  and boars may find it difficult to mate. Reproductive failure may occur as a result of fever or foetal infections with embryo absorption, abortions and stillbirths.

In birds

Erysipelas is increasingly being reported in commercial poultry, mainly on farms where birds are reared outdoors. The disease is more common in turkeys and chickens both broiler and layer operations. Outbreaks occur sporadically. Infection is usually through damaged skin or red poultry mites with occasional oral transmission. The fatal septicaemic form is the most common where there is a dramatic drop in egg lay and depressed birds followed by death within 24 hours. The chronic form can occur where chickens have skin lesions and swollen hocks. Turkeys can develop a valvular endocarditis.

In other animals

Sheep plunge-dipped post-shearing may have their wounds infected with E. rhusiopathiae. This disease is known as “post-dipping lameness” and is easily prevented by the use of bacteriostatics in dips or by delaying dipping until wounds heal. Plunge dips are rarely used nowadays.  However, erysipelas arthritis is still common and is associated with the contamination of wounds in procedures carried out during marking, such as mulesing and tail docking and when shearing if contaminated equipment is used.

Aquatic mammals, such as dolphins and reptiles, such as crocodiles and fish (ornamental and eels) are also prone to infections with Erysipelothrix species.

In people (for interest only)

The bacterium is found in the faeces of animals and often enters slurry and then water bodies. Erysipelothrix will lodge in the slime layer covering fish. Fishermen who scale the fish they caught can be locally infected especially if jabbed by fish barbs. People can also be infected via infected animals and contaminated water bodies. Infected people develop, localised pruritic lesions, rashes and arthritis. This disease in people is known as “Erysipeloid”. Note this should not be confused with erysipelas in humans which is a skin disease caused by Group A streptococci.

Control of erysipelas in piggeries

Erysipelothrix rhusiopathiae is found in the tonsils and lymphoid tissue of the intestines of up to 50% of pigs. It is also an intestinal inhabitant of mammals, birds and fish that can be found in sewage, the intestines and mucous skin layer of fish, and soil contaminated with animal faeces.

Control is generally very effective, as evidenced by the decline in the number of cases in pigs in recent years. Control is aimed at the treatment of clinical cases, environmental sanitation and immunisation. Penicillin is the drug of choice and prevention is by the use of bacterin vaccines. In pigs these are usually combined with Leptospira vaccines. The vaccines are usually administered to all unvaccinated stock from 16 weeks of age, with a booster 4 weeks later and then twice a year in pigs. It should be administered to sows 4 to 6 weeks prior to farrowing to provide maximum colostral immunity to the piglets. (You only need to learn that there are bacterin vaccines available, to whom and when, but not the commercial names of the vaccines)

Suggested reading:

Wang Q, Chang BJ, Riley TV (2010) ‘Erysipelothrix rhusiopathiae‘, Veterinary Microbiology, 140:405-417, doi:10.1016/j.vetmic.2009.08.012.

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