die off of zebrafish babies

[Mike Kacergis]

Here is the mystery:

We have been experiencing a random die-off of juvenile zebrafish 
that began back on August 31. These fish have a wide range of 
backgrounds, from wild types to carriers of severe mutations. The 
majority of tanks die overnight but some died within three hours. 
Occasionally, one or two survivors have been found and placed into 
a fresh tank, but these typically died within a few hours.  

The fish system:

MBI zebrafish system with 2500+ 2-liter tanks. System has been 
operational for over three years. The water source comes from city 
water run through an RO filter. If it were a system problem, one 
would expect to see the die-off throughout the entire system.  

Patterns:

The majority of mortalities were confined to the right half of one Z-
mod used to raise baby zebrafish. The entire baby section consists 
of two adjacent Z-mods. A Z-mod consists of a six-shelf rack that 
houses 14, 2-liter fish tanks per rack. Babies consist of fish aged 5-
40 days post fertilization. The majority of the deaths occurred in 
tanks housing 30-day-old fish. Water exchange to these tanks was 
done twice daily for 15 minutes prior to feeding with AZ 100 and 
"baby brine". Tanks are siphoned 3 times per week.  

Factors eliminated:
All old feed has been replaced by new stock.

Just recently, die-off has been seen in the adjacent Z-mod.Thinking 
that there was a correlation to location, we moved babies to 
another location in the facility. Despite the move, five more tanks of 
fish died in the new location.  

Under the dissection scope, we saw protozoa in tremendous 
numbers inside the zebrafish carcasses. These protozoa were 
identified as Tetrahymena. We are not sure if these protozoa are 
responsible deaths or just opportunistic feeders.  

Has anyone experienced a similar sudden die-off of fish within the 
30-day-old range? Could Tetrahymena lethally effect babies without 
killing adults?  

Could the problem be bacterial? What are normal quantitative levels 
for bacteria in fish systems? We have no base line data to make a 
valid comparison.  

Mike Kacergis
Kacergis at nih.gov
Michael Kacergis
Laboratory of Molecular Genetics
NICHD, Building 6B, Room 315
National Institutes of Health
Bethesda, MD 20892
Phone (301)-435-8263
Fax (301)-496-0243
E-mail kacergis at box-k.nih.gov