Actinobaculum (Eubacterium) suis infections

M G COLLETT

GENERAL INTRODUCTION: IRREGULAR, NON-SPORING, GRAM-POSITIVE RODS

Introduction

Actinobaculum (Eubacterium) suis, an organism that is restricted as a pathogen to pigs, causes sporadic outbreaks of peracute to chronic cystitis, ureteritis and pyelonephritis in sows, characterized clinically by the passage of turbid and blood-stained urine.³⁰

The first in-depth study of the organism was performed in Britain less than 50 years ago, despite the fact that the condition had been recognized there for over a century.³⁰ Since then the disease has been reported in North, Central and South America,^{5, 24, 28, 33} Europe,^{7, 10, 27, 28, 29} Australia,¹¹ New Zealand,⁹ Hong Kong²³ and Taiwan.³⁹ In South Africa, the organism has been isolated on rare occasions from sows suffering from nephritis and cystitis.¹² It is likely that with improved sampling and bacteriological techniques (see below), the organism will be encountered more frequently.

Aetiology

Actinobaculum suis has undergone a number of taxonomic revisions since its first description in 1957.³⁰ Originally Corynebacterium suis, it became Eubacterium suis in 1982.³⁴ Then, comparative 16S rRNA gene sequence analysis led to it being renamed firstly Actinomyces suis²⁰ (which created confusion due to the pre-existing non-official ‘A. suis’, the cause of mammary actinomycosis in pigs — see Actinomyces hyovaginalis infections) and more recently Actinobaculum suis,¹⁸ the name used here.

Actinobaculum suis is an obligate anaerobe. The organisms are slender, nonmotile, pleomorphic Gram-positive rods, 0,5 × 1,0–3,0 μm in size, have a beaded appearance and occur singly, in pairs or small clusters.^{18, 22, 34} It is not acid-fast, neither does it produce spores or exotoxins.^{18, 29} After 48 hours of incubation, colonies on anaerobic blood agar are up to 3mm in diameter and are white and circular, and often have a slightly raised centre, rather like a fried egg.¹⁸ At 96 hours’ incubation, colonies can measure 4 mm in diameter and be extremely flat.²⁵ It is also possible to grow the organism aerobically on urea-enriched medium.³¹

Epidemiology

Outbreaks of A. suis infections occur sporadically. In an abattoir survey in the USA, A. suis was isolated from 4,7 per cent of sows examined at slaughter,³³ while 12 out of 25 sows with a urinary tract infection were positive for A. suis in an investigation in Germany.¹⁹ In one investigation on a farm in the USA on which there was an endemic A. suis infection, it was found that 26 per cent of urine samples from pregnant sows were positive but only one of them had evidence of a urinary tract infection.⁶ In the UK, infection is widespread, and A. suis is believed to be the primary pathogen in 90 per cent of pyelonephritis cases.^{3, 31} In herds where the infection is endemic, gilt piglets born from carrier sows are infected at birth, while others can acquire the infection and begin excreting A. suis within the first week of life.⁶ Gilts can also contract the infection if they are in contact with the urine of clinically ill sows.³ Although the organism is rarely isolated from healthy sows,^{6, 31} it is frequently present in the urine, semen and prepuce of healthy boars.^{1, 13, 29, 39} Indeed, most young boars over ten weeks of age are carriers of A. suis in their prepuce, probably as a result of contracting the infection from contaminated floors.^{13, 14}

Despite the above facts, there is evidence that transmission of the disease to sows is venereal since there is, in most cases, a history of service (often by a particular boar) two to four weeks prior to the development of clinical signs; affected sows are usually pregnant.^{3, 5, 11, 23, 29, 30, 31, 33} After experimental infection, however, the organism disappears within three days in 30 per cent of cases.³¹

A degree of trauma of the genital tract of sows, inflicted at breeding, is probably necessary for A. suis to gain a foothold and for the infection to establish itself.²⁵ An inadequate water supply and a high incidence of crystalluria are important predisposing factors.³⁷

The organism is very sensitive to oxygen exposure, extreme temperatures and acidic pH. Survival times at room temperature (20°C) and 4°C are 4 and 17 days respectively.⁶ It can survive on plastic sleeves used in farrowing houses even in the presence of chlorhexidine surgical scrub.⁶ The bacterium has been found on the soles of footwear and on pen floors.³

Pathogenesis

It is likely that Escherichia coli and other non-specific bacteria initially cause damage to the bladder, enabling subsequent colonization by A. suis.¹⁹ Rising oestrogen levels during oestrus in sows can cause a rise in urine pH, producing a suitable environment for A. suis.²¹ The organism has strong urease activity, and ammonia production may contribute significantly to tissue damage and the consequent inflammatory reaction.^{5, 17, 22, 31, 33}

The pathogenesis of A. suis infection in sows resembles, to some extent, that of Corynebacterium renale in cows. Both organisms initially cause a cystitis followed by haematuria. In ascending infections, the ureters and the kidneys are involved.^{24, 29, 30} The spread of A. suis from the vagina (where initial infection occurs) to the bladder may be aided by the short and wide urethra of sows and the piliation of the organisms which enables adhesion to urothelium.^{5, 17, 31} Lesions of the ureterovesical junction may allow urinary reflux and result in an ascending infection.^{3, 19} Sows also tend to develop urinary stagnation and hydronephrosis, which predispose them to the development of cystitis, because of the anatomical structure of the neck of the bladder and the uterus, particularly when gravid, causes pressure over the pelvic brim.²⁹

Clinical signs

The earliest signs in most affected sows are a vulval discharge and blood-stained, turbid urine which may contain floccules of pus.^{3, 7, 30} However, some adult sows collapse and die of acute renal failure within 12 hours; they may even be found dead without having manifested clinical signs.^{3, 26, 31} Affected animals usually show a mild fever, anorexia, adherence of bedding to the vulval lips, twitching of the tail, general stiffness, arching of the back, lameness in the hind limbs, tense abdominal muscles, stranguria, polyuria, polydipsia, depression, constipation, weight loss, and eventually emaciation.^{3, 5, 23, 26, 30} In some sows, however, rectal temperature is normal or even slightly low.³ Clinical signs depend on the severity and duration of the disease which is frequently fatal.³¹ Mild cases may only show a vulval discharge and inappetence.³¹ Boars are seldom clinically affected, although haematuria has been reported.^{26, 30}

Pathology

Raised urea and creatinine levels provide a reliable estimation of renal function.^{3, 37} A hyperkalaemia and hyponatraemia are present, and urinary enzymes indicative of proximal tubular damage are raised.³

The specific gravity and pH of the urine of affected sows are 1,016 to 1,025 and 8,5 to 9,0, respectively, and the sediment contains white and red blood cells, mucus, bacteria, and apatite, struvite or oxalate crystals.^{3, 33} A bacteriuria, macrohaematuria and a urinary pH greater than 8,0 in a sow are probably pathognomonic of A. suis infection.^{3, 19}

Lesions are confined to the urogenital system. The affected bladder contains a thick, viscous, brown or bloodstained, foul-smelling urine. The bladder wall is markedly thickened, and its mucosa is hyperaemic and contains haemorrhages and/or a purulent to pseudomembranous inflammation, sometimes accompanied by ulceration.^{11, 19, 23, 24, 27, 30, 33, 36} The ureters may be thickened, dilated (hydroureter) or filled with pus (pyoureter) or blood.^{10, 11, 23, 29, 30, 31}

The renal lesions vary from an acute, purulent to a chronic interstitial nephritis, pyelitis or pyelonephritis.^{3, 27, 29} The surface of the kidneys is mottled greyish-red. On cut section the parenchyma reveals rays of connective tissue,^{3, 11, 24, 30} ;the mucous membrane of the renal papillae may contain haemorrhages, and there may be ulcers covered by yellow to greenish pseudomembranes.^{3, 10, 11, 23, 24, 30} The pelvis is filled with blood-stained urine containing gritty crystalline material.³ One or both kidneys may be enlarged due to hydronephrosis or distension of the pelvis with pus. Occasionally, vaginitis or metritis are encountered.^{29, 39}

Actinobaculum suis can be isolated from the prepuce of boars with or without preputial diverticulitis: the organism, on its own, does not appear to be the cause of this lesion.¹⁵

Histologically, the bladder shows submucosal oedema, congestion and thrombosis in acute cases, with degeneration and necrosis of the epithelium and other cellular components of the mucosa. These changes are often accompanied by haemorrhage and pseudomembrane formation. Gram-positive diphtheroid bacteria are usually present in the exudate or necrotic epithelium. The urethra may also be inflamed. In chronic cystitis, plasma cells and macrophages are dominant, goblet cells are hyperplastic and the bladder mucosa may be thickened with villous folds.³

The renal lesions are characterized by multiple focal infiltrates of lymphocytes, neutrophils and macrophages, in association with variable degrees of fibrosis, particularly of the medulla and pelvis, necrosis of the renal papillae, and the presence of bacterial colonies in the exudate.^{3, 11, 24}

Diagnosis and differential diagnosis

A diagnosis of A. suis infection should be suspected in a sow when blood-stained, turbid urine is detected clinically or at necropsy.^{5, 11, 15, 32} The diagnosis should be confirmed by anaerobic bacterial isolation from urine or affected tissues.^{5, 10, 11, 30} Apart from the urinary tract, the organism can be cultured from renal lymph nodes and from the peritoneal cavity but not from the blood.³

A selective medium for the isolation of A. suis is commercially available as Columbia CNA agar consisting of Columbia agar with colistin sulphate (10 mg/l) and nalidixic acid (15 mg/l).^{4, 5} Where specimens are severely contaminated, such as those from the prepuce, metronidazole (50 mg/l) can be added to the medium.²⁵

Pure infections with A. suis are rare and a mixed flora is present in most cases.¹⁹ Various other bacterial species may also be isolated from sows with cystitis if both aerobic and anaerobic culture methods are employed. These include E. coli, Proteus and Pseudomonas spp., and a number of Gram-positive bacterial species including Trueperella pyogenes, haemolytic and non-haemolytic streptococci, staphylococci, Peptostreptococcus spp. and Enterococcus faecalis^{2, 3, 19, 23, 24, 30, 31, 33}

The isolation of A. suis from the urinary tract of a sow is highly significant, but, in a boar, isolation of A. suis from a normal bladder can occur due to retrograde invasion of the organism from the prepuce after death.⁸

An immunofluorescence technique applied to smears of urine sediment of sows or swabs collected from the preputial diverticulum of boars is an inexpensive method of identifying A. suis.^{16, 27} Some cross-reaction with streptococci can occur.⁸

Serum antibody (measured by indirect immunofluorescence) is detectable three weeks after infection and lasts four to nine months, but the magnitude of the titre does not correlate with the extent of the infection and false negatives can occur.^{31, 35}

Acute illness and death due to A. suis infection in sows should be differentiated from other diseases such as heart failure, septicaemia, the porcine proliferative enteropathy syndromes, and many others. In the more subacute to chronic disease, other causes of ‘thin sow syndrome’, such as pneumonia, infestation with parasites such as the kidney worm (Stephanurus dentatus), poor nutrition, swine dysentery, gastric ulceration, toxic nephropathy, leptospirosis, and other causes of cystitis should be eliminated.^{5, 25, 26}

Control

It is important to isolate infected sows from the rest of the herd. In the early stages, infection due to A. suis can be treated by the intramuscular injection of penicillin G (15 000 units/kg for five days), lincomycin (10 mg/kg) or ampicillin. The latter is the drug of choice if one attempts to save a pregnancy, and has been used at 2,5 to 11 mg/kg twice daily for five days.^{3, 5, 6, 26, 32} Such treatment can be monitored by a reduction in urine pH but is probably not curative.²⁶ Provided infection is limited to the bladder, treatment with enrofloxacin (Baytril-Premix 2,5 per cent, 10 mg/kg body weight per day for ten days) and, if necessary, with ampicillin (20 mg/kg per day for 20 days) is usually curative.³⁷ Sows with renal failure, advanced or chronic lesions or those that have failed to respond to treatment should be culled.³¹

Treatment of non-specific urinary tract infections in sows may also be important as damage to the bladder mucosa by other bacteria may support colonization by A. suis.³⁸

Local therapy (installation of penicillin, penicillin/streptomycin or enrofloxacin into the preputial cavity) and medicated feed (enrofloxacin) does not eliminate A. suis from the preputial diverticulum. Washing of boars and disinfection of pens is also ineffective.³⁸

The organism is sensitive to phenol, formaldehyde, 3 per cent iodine and 5 per cent hypochlorite, but resistant to chlorhexidine, quaternary ammonium compounds and 70 per cent alcohol.⁶

Stalls in which pregnant sows are kept should be properly designed to prevent urine cross-contamination.⁵ Because of the sensitivity of A. suis to oxygen exposure, manure should be removed regularly and the pens cleaned with an effective disinfectant and/or hot water (> 76 °C).⁶

In order to reduce the likelihood of trauma at breeding, smaller boars should be used on gilts and overaggressive mating should be avoided.²⁵ Non-slip floors should be provided in mating pens and sows injured during mating should be given prophylactic antibiotics.²⁶ In affected herds, artificial insemination may be used to prevent transmission of A. suis to sows.^{3, 38}

Other management aspects that may need attention include the provision of an adequate, functional and accessible water supply and a twice-daily feeding regimen, the latter to ensure that the sow stands, drinks and urinates more frequently than if fed once daily.^{3, 31}

References

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