Tracing Encephalitozoon cuniculi in a research rabbit colony - From clinical outbreak to seronegative status.

Encephalitozoon cuniculi is a unicellular, gram positive, microsporidial fungus detected in rabbit populations worldwide. Acute infection targets the lungs, kidneys and liver, while chronic infection establishes in the kidneys and central nervous system (CNS). Infection is a confounding variable for histological studies and has adverse health and breeding effects including reduced growth, altered immunity, illness and death. Following clinical and subclinical signs of infection in a New Zealand White (NZW) rabbit breeding colony, ongoing infection surveillance was supplemented by a retrospective investigation of a bank of 61 serum samples collected between 2019 and 2023 from 18 rabbits (4 males, 14 females). This investigation identified 13 seropositive rabbits which may include two cases of adult horizontal transmission and seven cases of transplacental and/or doe to kitten horizontal transmission during close contact after birth. Four other rabbits were seropositive when they entered the colony. Ten infections were intergenerational and commenced from an introduced seropositive buck. E. cuniculi was not detected in 7/7 urine samples collected from 5/7 rabbits which tested seropositive and 2/7 which tested seronegative, before and/or after PCR assay. Sequencing of DNA extracted from the kidney of one seropositive rabbit and amplified by nested PCR showed significant alignment (E-value 1e-36 and Bit Score 167) with Encephalitozoon cuniculi strain American Type Culture Collection (ATCC) 50602/strain designation CDC:V282. The housing, biosecurity and disinfection practices used reinstated a completely seronegative population status while maintaining an active breeding colony.