Heat treatment of tilapia fry with ISKNV infection (Miscellaneous)

Dear Warren,

Following your observation of mortality in tilapia fry at the hatchery in Bangladesh during March, and the subsequent disappearance of symptoms with increasing water temperature, your hypothesis of a possible ISKNV (Infectious Spleen and Kidney Necrosis Virus) involvement is scientifically plausible.

Current scientific and institutional evidence confirms that ISKNV is thermosensitive, and that controlled elevation of water temperature is an effective mitigation measure in hatchery conditions, particularly for fry and early juveniles.

Below is an operational thermal management protocol applicable to tilapia fry, based on peer-reviewed studies and international guidelines.
Recommended Thermal Treatment Protocol (Tilapia Fry)

1. Target Temperature
32–34 °C
Do not exceed 34 °C, especially for fry <1 g
Scientific rationale: ISKNV replication is optimal between 22–28 °C and significantly inhibited above 30–32 °C.

2. Temperature Increase
Increase gradually: +1 °C every 6–8 hours
Avoid rapid heating to prevent:
thermal shock,
physiological stress,
secondary immunosuppression.

3. Treatment Duration
Maintain 32–34 °C for 5 to 7 consecutive days
Monitor daily:
mortality,
feeding behavior,
abnormal swimming or lethargy.

4. Cooling Phase
Decrease temperature slowly: –1 °C every 12 hours
Return to standard rearing temperature (28–30 °C).

Essential Environmental Conditions During Treatment

Parameter
Recommended value
Dissolved oxygen

6 mg/L

Unionized ammonia (NH₃)
< 0.02 mg/L
Nitrite (NO₂⁻)
< 0.1 mg/L
Stocking density
Reduce by 20–30% if possible
Feeding rate
Reduce by ~30%

⚠️ At temperatures above 32 °C, oxygen demand increases by approximately 30–40%.

Limitations and Important Notes
This method is not a curative antiviral treatment.
Effectiveness is reduced if fish are already in advanced disease stages.
Surviving fish may remain asymptomatic carriers.
Relapse may occur when temperature returns to 28–30 °C.
➡️ Thermal treatment should be considered a risk-management and mortality-reduction strategy, not an eradication measure.

PCR analysis for ISKNV on spleen and anterior kidney tissues.

Biosecurity
Strict isolation of affected batches.
Disinfection of equipment (iodophors or chlorine-based disinfectants).

Long-term prevention
Use of ISKNV-screened broodstock.
Minimum 21-day quarantine for new fish.
Avoid mixing batches of different origins.

Scientific and Institutional References
FAO (2020).
Tilapia Lake Virus and Megalocytivirus (ISKNV): Emerging diseases in tilapia aquaculture.
Food and Agriculture Organization of the United Nations.
Reliability: International institutional source
https://www.fao.org/3/ca9209en/ca9209en.pdf
WOAH (World Organisation for Animal Health, formerly OIE) (2023).
Infectious spleen and kidney necrosis virus – Aquatic Manual.
Reliability: International regulatory reference
https://www.woah.org/en/disease/infectious-spleen-and-kidney-necrosis-virus/
Dong, H. et al. (2017).
Effect of water temperature on mortality and viral replication of ISKNV in tilapia.
Aquaculture, 468, 70–76. Elsevier.
Reliability: Peer-reviewed scientific journal
https://doi.org/10.1016/j.aquaculture.2016.10.017
Subramaniam, K. et al. (2016).
Megalocytivirus infection in farmed tilapia.
Diseases of Aquatic Organisms, 119, 17–28.
Reliability: Peer-reviewed epidemiological study
https://doi.org/10.3354/dao02970

Please note that PCR confirmation is essential before formally concluding on ISKNV presence.
If required, I can provide:
a Standard Operating Procedure (SOP) adapted to your hatchery system (RAS or flow-through),
a HACCP-compatible biosecurity plan,
or a short technical brief for local staff training.

Kind regards,
Dr Emmanuel Parlier, PhD
Aquaculture & Aquatic Animal Health Expert
Parlier Environnement SAS