United States 

Department of 

Agriculture

Animal and Plant 

Health Inspection 

Service

Veterinary Services

Strategy and Policy

Center for 

Epidemiology and 

Animal Health

November 2020

Assessment of the Risk of 

Introduction of Tilapia Lake 

Virus (TiLV) by Live Tilapia 

Imported to Terminal Markets 
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Rapid Risk Assessment: Tilapia Lake Virus in live tilapia imported for terminal markets

Key Results

The Center for Epidemiology and Animal Health (CEAH) previously assessed the risk posed to 

relevant industries by the introduction of tilapia lake virus (TiLV) via tilapia frozen fillets and live 

fingerlings. This work was completed in November 2019, and the results are available in the 

document entitled Rapid Risk Assessment for Tilapia Lake Virus (TiLV). 

This current assessment supplements previous work by evaluating the additional entry pathway of 

live tilapia imported from Canada to terminal live markets in the United States for human 

consumption. 

The likelihood TiLV will enter the United States via market-sized tilapia imported from Canada 

destined for terminal live markets is estimated to be low with a moderate degree of uncertainty.

The likelihood that live market-sized tilapia carrying TiLV imported from Canada for sale in 

terminal live markets will cause infection in farm-raised tilapia is negligible, with a moderate

degree of uncertainty. 

The overall risk that live tilapia imported from Canada and destined for sale in terminal markets 

will present an exposure pathway for U.S. tilapia aquacuture is estimated to be negligible.

As described in previous assessments, the consequences of TiLV if introduced to U.S. industries 

could be high, depending upon the magnitude of spread before detection and the time needed to

respond to the outbreak. Estimated trade losses may be as high as $15,974,671.

Background

In November 2019, CEAH completed a rapid risk assessment of the potential risk of introduction of 

TiLV to U.S. aquaculture industries resulting from importation of frozen tilapia fillets and live fingerlings. 

Based on the estimated high risk of entry associated with imported live fingerlings, the U.S. Department 

of Agriculture’s Animal and Plant Health Inspection Service issued Federal Order DA-2019-01 in 

November 2019 to mitigate the entry risk. This assessment is supplementary to this previous work and 

estimates the likelihood of entry and subsequent exposure to relevant U.S. aquaculture industries of 

TiLV associated with importation of live tilapia to terminal live markets for sale and human 

consumption. 

TiLV disease was identified in Israel in 2014. Subsequently, the disease has been reported globally. In 

2019, a TiLV outbreak occurred in the United States, involving three tilapia aquaculture facilities in 

Idaho, Wyoming, and Colorado. There have been no further reports of the disease in the United States

following appropriate disease response measures and issuance of Federal Order DA-2019-01.

Because previous assessments already included information describing global TiLV distribution,

hazard identification, and a consequence assessment, as well as an assessment of trade, this report is 

solely focused on the assessment of risk and uncertainty associated with import of live tilapia to 

terminal live markets. For additional information on the distribution, pathogen characteristics, potential 
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Rapid Risk Assessment: Tilapia Lake Virus in live tilapia imported for terminal markets

consequences of TiLV in the United States, and U.S. trade in live tilapia and eggs/milt, please see the 

reports entitled Rapid Risk Assessment for Tilapia Lake Virus (TiLV) and (Analysis of U.S.

Imports/Exports of Live Tilapia and Eggs/Milt).

Entry Assessment

The likelihood TiLV will enter the United States via market-sized tilapia imported from Canada 

destined for terminal live markets is estimated to be low with a moderate degree of uncertainty. 

Currently, Canada is the only country exporting live food-sized tilapia to live terminal markets in 

the United States. This likelihood assessment is based on:

• In some cases, U.S. producers exported the fish in this entry pathway from the United 

States to Canada as fingerlings for grow-out and imported the fish back to the United 

States when they reached appropriate size. Fish exported from the United States as 

fingerlings and re-imported for marketing are unlikely to have TiLV because tilapia reared 

in the United States are unlikely to be infected with TiLV. The United States is designated 

free of TiLV, and the import of tilapia fish, fingerlings, eggs, or gametes is regulated by

Federal Order DA-2019-01.

• Canada does not require TiLV disease freedom certification of tilapia prior to import; it is 

plausible that U.S. fish imported for grow-out may be comingled with tilapia sourced from 

other countries and be infected with TiLV, increasing the risk of TiLV entry via import of live 

market-sized tilapia from Canada.

Exposure Assessment

The likelihood that live market-sized tilapia carrying TiLV imported from Canada for sale in 

terminal live markets will cause infection in farm-raised tilapia is negligible, with a moderate

degree of uncertainty. The likelihood assessment is based on:

• Fish are driven across the U.S.-Canada border in trucks and delivered directly to the live 

terminal markets so exposure to U.S. aquaculture is not likely.

• The imported fish are sex-reversed males and have no value as broodstock, so will not be 

diverted from terminal live markets to aquaculture pathways.

• It is plausible that some fish may be bought for pets or placement in ornamental ponds, but 

it is unknown if these would expose aquaculture.

• Most live markets are in urban centers (i.e., cities) and are not near tilapia aquaculture 

locations, so it is unlikely that water used to transport fish or fish not sold in live markets 

will reach aquaculture pathways. 

• This is a terminal pathway. Final disposition of these fish is human consumption; it is 

unlikely that live tilapia sold via this pathway or their tissues will enter U.S. tilapia 

aquaculture. 

The moderate degree of uncertainty associated with this pathway is based on:

• Lack of information regarding the volume of tilapia imported to live terminal markets.

• Lack of information regarding the number and locations of the live terminal markets.
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Rapid Risk Assessment: Tilapia Lake Virus in live tilapia imported for terminal markets

• Knowledge gaps associated with the disposition of waste fish, trim, offal, or other tissues 

associated with food preparation.

o These materials are likely sent to landfills. Landfills used by cities are not likely located

near tilapia aquaculture facilities. Landfills are required to monitor groundwater for 

contamination; however, it is not known how the methods used correlate to the 

potential presence of aquatic animal pathogens. Scavenging gulls, other birds, or 

wildlife could serve as fomites or vectors. 

o Use of such materials as bait or food for aquatic animals is generally unknown.

• Lack of knowledge regarding State requirements for transport water treatment and disposal 

and transport truck cleaning and disinfection. 

o This reflects the lack of knowledge of terminal market locations. 

o Some producers/retailers provide oversight to ensure transport water is disinfected 

and disposed of properly and that trucks are cleaned and disinfected appropriately.

o Transport water may be disposed of via municipal water systems.

Risk Estimation

The overall risk that live tilapia imported from Canada and destined for sale in terminal markets 

will present an exposure pathway for U.S. tilapia aquacuture is estimated to be low. 

Presently, Canada is the ony country exporting live tilapia to U.S. terminal markets. Some of the

fish imported for this purpose are reared to fingerling size in the United States, exported to 

Canada for grow-out, and then imported back by U.S. producers for sale in live terminal markets. 

Post issuance of Federal Order DA-2019-01, it is unlikely that fish raised in the United States will 

be infected with TiLV virus. While Canada does not require TiLV disease freedom certification of 

tilapia prior to import, it is plausible that U.S. fish imported for grow-out may be comingled with 

tilapia sourced from other countries or share water sources with other tilapia. However, if this 

should occur, the risk posed to the U.S. tilapia industry by the tilapia raised and imported in this 

specific pathway remains low because these fish are sex-reversed males and are of no value as 

broodstock, and the terminal markets are located in urban centers (i.e., cities) and not near tilapia 

aquaculture locations. While the final disposition of the fish is generally assumed to be food, some 

fish may be bought for pets or placement in ornamental ponds. As such, these fish are unlikely to 

pose a threat to U.S. aquacuture unless they are intentionally, accidentally, or maliciously 

released into U.S. tilapia aquaculture systems. While the final disposition of waste fish and trim 

and offal associated with processing of tilpia purchased in live terminal markets is generally 

unknown, the risks of exposure to U.S. aquaculture via these pathways is low. Although the trucks 

used to transport the fish from Canada to the United States and the transport water could serve as 

potential TiLV introduction pathways, cleaning and disinfecting of the trucks and disposal of the 

transport water are often performed under observation and must comply with regulations in some 

States. Finally, municipal waste water is treated prior to release into natural water systems;

therefore, the risk of TiLV introduction via this pathway is low. 

Limitations

In this assessment, we aimed to identify the primary pathways of entry and exposure and estimate the 

chances of these events occurring, given our knowledge of current production practices and existing 

biosecurity measures. To more accurately characterize the risk, a variety of additional information is 

needed. Some of these needs include:
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Rapid Risk Assessment: Tilapia Lake Virus in live tilapia imported for terminal markets

• General knowledge associated with the

o Epidemiology of TiLV

o Final disposition of tilapia sold at live markets, and

o International and domestic tilapia trade movements and the geographical distribution 

of TiLV

• Data pertaining to the

o Export of fingerling tilapia to Canada for grow-out

o Import of live tilapia from Canada for sale in U.S. live terminal markets, and

o The final disposition of tilapia that do not get sold in the live markets

‘Rapid Risk Assessment’ Defined

A rapid risk assessment is designed to provide a quick and approximate estimate of the risk (i.e., the 

combined likelihoods of entry and exposure and potential consequences given that the risk event 

occurs) for a pathogen of interest. The objectives include 1) estimating the likelihood and potential 

impacts of emerging or evolving animal health threats; and 2) identifying data gaps.

These assessments often have the following characteristics:

• Are done when there is relatively limited information on either the pathogen or the 

transmission pathways

• Are qualitive in nature and do not provide numerical estimates of likelihood or the variability 

associated with the estimated likelihood

• Serve to guide additional data collection for in-depth risk assessments or risk analyses

• Are often performed with limited resources

• Discuss the most likely pathways of entry and do not attempt to list or evaluate all potential 

pathways

• Do not evaluate potential consequences in detail

• Do not discuss potential impact of future mitigations 

Likelihood, Uncertainty, Consequence, and Risk Categories

For this risk assessment, we have assigned qualitative likelihoods for expressing how likely it is that 

something would occur. We also use qualitative terms for discussing the uncertainty and information 

quality associated with these likelihoods. 

Table 1. Definition of likelihood categories for risk assessment

Term Definition

Negligible This event would almost certainly never occur

Low This event would be unlikely to occur

Moderate This event would be nearly as likely to occur as to not occur

High This event would be likely to occur

Very High This event is almost certain to occur
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Table 2. Definition of levels of uncertainty 

Term Definition

Low Available data are well supported, reliable, complete, and accessible 

from multiple sources or published references, and are in general 

agreement

Moderate Data are available, but have high interpretability issues, potential biases, 

reliability issues, and/or underreporting

High Some data are available but may be incomplete, unreliable, from a small 

number of published sources, and/or demonstrate conflicting evidence. 

Includes the combination of anecdotal evidence, personal 

communications, and expert opinion with available published data, if all 

sources are in general agreement

Table 3. Definition of consequence levels

Term Definition

Indiscernible The consequences of exposure are so low as to be undetectable.

Minor Minor increases in morbidity/mortality, and some decreases in 

production. Effects of exposure are controllable or reversible. 

Significant Morbidity/mortality are great enough to impose moderate production 

losses. Effects of exposure may not be reversible.

Major Morbidity and mortality are great enough threaten the economic viability 

of the sector for a lengthy period. Effects of exposure may not be 

reversible.
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Table 4. Definition of risk levels 

Term Definition

Negligible The determination of “negligible risk” suggests that the risk is low 

enough that it need not be considered, and no further mitigations are 

necessary. 

Low The determination of “low risk” suggests that although not a strict 

requirement, resources to further evaluate or mitigate this risk should 

be considered. A low risk is greater than a negligible risk due to a 

potential likelihood of occurrence, associated consequences, or a 

combination of both.

Moderate The determination of “moderate“ risk suggests that the risk is of a 

sufficient magnitude that measures to prevent or mitigate the risk 

should be considered. A moderate risk is greater than a low risk due to 

a greater likelihood of occurrence, greater consequences, or a 

combination of both.

High The determination of “high risk” suggests that the risk is of sufficient 

magnitude that measures to prevent or mitigate the risk are necessary 

and the consequences will have significant impact at the regional or 

national level. A high risk is greater than a moderate risk due to a 

greater likelihood of occurrence, greater consequences, or a 

combination of both.

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