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...