Leptospirosis in an Ontario swine herd
Margaret Stalker
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2022;26(2):12.
Kidneys from several finisher pigs were presented for histologic examination. The kidneys had gross lesions affecting much of the renal parenchyma, varying from coalescing 0.5-1.0 cm pale/white foci to pinpoint raised well-demarcated cortical nodules with a thin red rim, to flat, 1-2 cm geographic red areas extending from the cortex into the medulla (Fig. 1).
On histologic examination, all kidneys had varying degrees of chronic and active tubulointerstitial nephritis with interstitial fibrosis, tubular atrophy, lymphoplasmacytic inflammation and formation of discrete interstitial lymphoid nodules (Fig. 2A), as well as neutrophilic infiltrates within tubules and spilling into adjacent interstitium. PCR run on renal tissues for Leptospira spp. was positive, with cycle thresholds ranging from 25.87 to 33.83. Further genotyping based on single-nucleotide polymorphism (SNP) analysis of the SecY gene was compatible with Leptospira pomona. Immunostaining for Leptospira spp. was also positive (Fig. 2B), highlighting bacteria of typical elongate curving morphology, occasionally with hooked ends, within tubular lumens.
Swine are considered the reservoir / maintenance host for Leptospira interrogans serovar pomona, in which the organism resides in the proximal convoluted tubules of the kidney. Transmission is direct, through mucosal or skin contact with urine or post-abortion discharges. Only a small proportion of infected animals develop clinical illness, usually only transient mild fever, anorexia and depression during the leptospiremic phase as the bacterium spreads to the liver, kidney, lungs and other organs. Antibody responses clear the infection from most organs, however, the renal tubules are an immunologically privileged site, and establishment of persistent renal infection is the source of ongoing herd maintenance and exposure. The more characteristic signs of Leptospira infection within the herd are abortion or premature delivery of pregnant sows, typically litters born 1-3 weeks prematurely with stillborn and weak live piglets.
Since 2008, pathologists at the AHL have identified 9 cases of swine reproductive disease/abortions putatively associated with Leptospira infection, so it appears to be uncommon. Prior to introduction of Leptospira PCR testing in 2017, this was based solely on interpretation of Leptospira serology/MAT titres on sow serum and fetal thoracic fluids. We even more rarely see/recognize the renal lesions in our caseload, although according to the literature, most chronic interstitial nephritis lesions observed in finisher swine are thought to be leptospiral in origin (1). A review of 230 Leptospira PCR tests run on porcine submissions since 2017 revealed only a single positive result in placentas from a group of aborted fetuses, prior to the PCR-positive kidney samples tested from this same herd.
Leptospirosis is an important zoonotic illness, and humans (and other incidental hosts) can be infected indirectly by exposure to environments contaminated by the infected urine of carrier animals. Such exposures can be an occupational hazard for both producers and veterinarians. AHL
Reference
1. Cianciolo R, Mohr FC. Urinary System. In: Jubb, Kennedy & Palmer’s Pathology of Domestic Animals, 6th ed. Maxie MG, ed. Elsevier, 2016; vol 2:433-438.
Figure 1. Various gross presentations of kidneys from finisher pigs with renal leptospirosis.
Figure 2. Histologic sections of renal leptospirosis. Kidney with tubulointerstitial nephritis (A). H&E stain. Immunostaining highlights filamentous leptospires within a tubular lumen (B). IHC