THE GENUS ACTINOBACULUM

Actinobaculum suis is the current stopover on a taxonomic odyssey beginning with the genus Corynebacterium and moving to Eubacterium and Actinomyces. However, 16S rDNA sequence analysis demonstrated that Corynebacterium-Eubacterium-Actinomyces strains from cystitis in sows are a different subline within the Actinomyces-Arcanobacterium spp. complex and, in light of a 6% sequence divergence, became Actinobaculum suis.

Actinobaculum suis is a normal component of the preputial flora in boars and the vaginal flora in sows. The bacterium produces cystitis, nephritis, and metritis in sows. Boars may transmit the bacterium to sows at breeding. Piglets may be infected when they come in contact with the organism during parturition. The organism has been detected on the pen floor of pig barns and is transmitted via urine. Distribution is worldwide. In several large surveys of swine disease, cystitis/pyelonephritis caused by A. suis ranked as a major cause of death in the U.S. sow population.

Actinobaculum suis requires alkaline conditions for growth and replication. The healthy vaginal tract of the sow, being slightly acidic, is an unsuitable environment. During estrus, the urine pH rises in response to the rise in estrogen, enhancing the growth of A. suis and leading to migration into the upper urinary tract. Inadequate water supply, with resulting crystalluria, is an important predisposing factor. Sows housed in gestation stalls have a higher prevalence of infection because there is more fecal contamination of the perineum and a reduction of daily activity leading to urine stagnation.

The only clinical sign indicating initiation of cystitis is bacteriuria. In mild cases, lesions include epithelial cell hyperplasia, desquamation of superficial epithelial cells, and goblet cell metaplasia with intraepithelial cyst formation. Severe cases are characterized by necrotizing ureteritis and pyelitis (Figure 34-1), with accumulation of bacterial colonies, ascending renal infection, and uremia. Infection is frequently accompanied by hematuria and urinary pH greater than 8. The most severe infections are characterized by purulent ureteritis and pyelitis/pyelonephritis, and are likely to be caused by A. suis in concert with other bacteria, especially Escherichia coli. Recent studies suggest that a toxin produced by E. coli may damage the ureteric valve, allowing retrograde flow of urine into the kidneys, resulting in hydronephrosis.

FIGURE 34-1 Actinobaculum suis infection in a sow.

(Courtesy Peter Moisan.)