Lack of Evidence for Vertical Transmission of Inclusion Body Disease in Black-Necked Cranes (Grus nigricollis) at the International Crane Foundation
The causative agent of inclusion body disease of cranes (IBDC) is a herpesvirus that appears capable of persisting in a latent form with potential for intermittent shedding during periods of viral reactivation. Determination of previous exposure to this agent suggests a crane could shed the virus at any time, with or without showing signs of disease. Spread of the virus within or between flocks could also occur with vertical transmission of virus through eggs and offspring.
We reviewed the historic medical records from 22 black-necked cranes at the International Crane Foundation (ICF) for serum antibody titers to IBDC: two wild-caught, chronically seropositive adult cranes and their 20 offspring hatched between 1990 and 2004. All serologic testing was performed at the USGS National Wildlife Health Center in Madison, Wisconsin. A serum neutralization test was used to screen for the presence of IBDC-specific antibodies; any sera that inhibited the cytopathic effect in the screening test was titered using a TCID50 from diluted sera. Titers ≥1:8 were considered positive evidence for exposure to the virus.
The adult male #14-3 was seropositive in 15 of 15 (100%) serum samples tested between 1988 and 2004. The crane exhibited a four-fold rise in titer within 6 months of arrival from China, from 1:64 to 1:256. The crane’s titer subsequently declined but has remained stable since 1990 (range 1:16 to 1:64). The serologic history of this crane suggests exposure to IBDC prior to arrival in the USA followed by recrudescence of a latent infection after shipment. Despite the serologic findings, no virus has been cultured from cloacal swabs taken from this crane to date.
The adult female #14-2 was seropositive in 11 of 20 (55%) samples tested between 1985 and 2004. All titers have been low since arrival from China (<1:16). This crane has also remained seronegative for extended periods, including a 5-year period when housed with #14-3. Her initial seropositive status suggests original exposure occurred prior to arrival in the USA. The recurrence of low titers may be due to persistence of latent endogenous infection, re-exposure to the virus from #14-3, or variable detection of small amounts of antibody to IBDC in the absence of infection. All virus culture attempts from cloacal swabs have been negative, including after initial housing with #14-3 when his titer was declining.
Each egg from this pair was removed shortly after being laid and managed with a combination of artificial and natural incubation. Chicks from these eggs were either foster-reared by red-crowned cranes or hand-reared. All 20 offspring from this pair have been seronegative for IBDC (n=58 samples). Most chicks were tested a minimum of 7–14 days after hatching and again at approximately 60 days of age. Two chicks had low detectable titers (1:4) at 8 days of age possibly due to the presence of maternal antibody, though this level is below the cut-off established for a seropositive test and the results were considered equivocal. Both chicks had no detectable antibody on subsequent testing. Six chicks were tested over multiple years (mean=3.7 years); all were seronegative.
Vertical transmission of IBDC through eggs has not been documented in cranes to date, though sampling has been limited.1 The serologic results reported here support the contention that IBDC is not transferred vertically via eggs from adult black-necked cranes that have serologic histories consistent with latent infection.
Many thanks to J. Langenberg for comments on an earlier draft of this report, the USGS National Wildlife Health Center Diagnostic Virology section for laboratory services, and E. Dahlgran for technical assistance.
1. Schuh, J.C.L. and T.M. Yuill. 1985. Persistence of inclusion body disease of cranes virus. J. Wildl. Dis. 21(2):111–119.