Babesia odocoilei infection associated with acute mortality in farmed elk in Ontario

Rebecca Egan, Felipe Reggeti, Amanda Mansz, Hugh Cai

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2023;27(1):12.

In October of 2022, a diagnostic investigation was performed in a herd of approximately 180 farmed elk in Ontario to explore the cause of acute death and illness in a subgroup of animals that had been brought in from Western Canada in late summer.  One mature female had been found dead, and clinical examination of a bull revealed lethargy, hindlimb weakness and the voiding of red urine.  A field postmortem was performed on the female, and an EDTA blood sample and formalin-fixed tissues were submitted to the AHL.

The CBC showed marked anemia (Hct: 0.11 L/L; RI:0.30 - 0.41 L/L) that was macrocytic (MCV: 50 fL; RI: 42.1–44.6 fL), along with neutropenia (0.45 x 109/L; RI: 0.86 - 5.29 x 109/L).  Examination of blood smears revealed small rounded to pear-shaped intra-erythrocytic inclusions, observed both individually and in pairs, compatible with Babesia spp. (Fig. 1).  DNA was extracted from the positive blood sample, and sequencing of PCR products from the 18S rRNA showed 100% similarity (435/435 bp) with Babesia odocoilei.  Serum Anaplasma antibody ELSIA was also performed, and the result was negative.  Other signs of regeneration were not obvious on the blood smear.  Although biochemistry profile and urinalysis were not performed, the findings of marked anemia, intra-erythrocytic inclusions, and red urine were highly suggestive of intravascular hemolysis and hemoglobinuria.

Microscopic examination of tissues from the female identified accumulation of granular red-pink pigment in many renal tubules, compatible with either hemoglobin or myoglobin (Fig. 2), and periacinar to occasionally mid-zonal hepatocellular necrosis typical of hypoxic injury (Fig. 3).  Frequent single cell death was present in mid-zonal and periportal regions (Fig. 3), prompting consideration of copper toxicosis.  Therefore, hepatic and renal copper quantification was performed; copper levels were not elevated.  Tissues from a second field postmortem case were submitted, revealing similar renal and hepatic lesions.  However, single cell death in liver was not present.  Based on the clinical presentation, the possibility of bacillary hemoglobinuria caused by Clostridium haemolyticum was a consideration; however, hepatic lesions typical of this disease were not identified.

The bull subsequently died and was submitted to the AHL for postmortem.  On examination, the bull had pale mucous membranes and was emaciated, with absence of internal fat stores and serous atrophy of fat in bone marrow.  The liver was small, and renal cortices had a slightly darkened brown-black appearance.  Examination of smears prepared from drops of blood collected from small vessels in the ears and tail revealed intracellular organisms similar to those observed in the initial blood sample.  Microscopically, the liver had extensive regions of periacinar hepatic necrosis, with remaining hepatocytes exhibiting atrophy and cytoplasmic vacuolation in periportal regions.  Pigmentary nephrosis and tubular degeneration were present in kidney, consistent with intravascular hemolysis.

Considering the combination of findings in this diagnostic investigation, together with similar reports in the literature, it was determined that these animals succumbed to an acute hemolytic crisis, presumably incited by latent Babesia infection compounded by the stress of relocation, negative energy balance, and rutting season.  Babesiosis is a tick-borne disease, and the primary vector is the blacklegged tick (Ixodes scapularis) which is present in Ontario, and also serves as the vector for Borrelia burgdorferi, the cause of Lyme disease.  In this case, it is not clear how these elk acquired the infection, and given that all four of the affected animals were from the group that were recently brought in from Western Canada, it is possible that these animals were infected prior to arrival in Ontario.   AHL

Figure 1. Blood smear demonstrating small rounded to pear-shaped intra-erythrocytic inclusions, observed individually and in pairs (arrows), compatible with Babesia spp.

Figure 2. Microscopic section of kidney (H&E, 20x) demonstrating luminal accumulation of granular red-pink pigment in renal tubules (*).

Figure 3. Microscopic section of liver (H&E, 20x) demonstrating periacinar to mid-zonal hepatocellular degeneration (*) and single cell death among periportal hepatocytes (>).  

References

1. Pattullo KM, et al. Babesia odocoilei infection in a Saskatchewan elk (Cervus elaphus canadensis) herd. J Vet Diagn Invest 2013 Jul;25(4):535-40.

2. Mathieu A, et al. Babesia odocoilei as a cause of mortality in captive cervids in Canada. Can Vet J. 2018 Jan;59(1):52-58.