SELECTIVE BREEDING TO AVOID TILAPIA LAKE VIRUS

Selective breeding to avoid Tilapia Lake Virus. Results show selective breeding increases resistance, reduces mortalities.

Nile tilapia (Oreochromis niloticus) is among the most important aquaculture species farmed worldwide, representing one of the major sources of animal protein for human consumption, particularly in developing countries in Asia, South America and Africa. However, as with other intensive production systems, infectious disease is one of the main issues threatening the success and sustainability of tilapia production.

A relatively new pathogen, the Tilapia Lake Virus (TiLV) has emerged as a major threat for Nile tilapia, and also for other farmed tilapias, including red tilapia and hybrid strains. Although the virus was discovered in 2014, it may have been responsible for mortalities since 2008-2009. To date, it has been identified in countries from different geographical regions and continents, including Peru, Ecuador, Malaysia, India, Thailand, Egypt and Uganda.

TiLV can cause high levels of mortalities, but these can vary substantially (ranging from 5 to 90 percent) in disease outbreaks, which are usually observed within the subsequent weeks post-transfer from hatcheries to grow-out ponds. After these outbreaks, it has been shown that surviving fish have a higher resistance to this infection to subsequent outbreaks suggesting some degree of resistance via acquired immunity.

Despite the serious consequences of TiLV-related infections, there are no published estimates of quantitative genetic parameters for resistance to TiLV, and its potential to be improved by selective breeding, and this is likely to be due to the current lack of a well-established and effective TiLV challenge model, although these have begun to be established.

This article – adapted and summarized from the original publication [A. Barría et al. 2020. Genetic parameters for resistance to Tilapia Lake Virus (TiLV) in Nile tilapia (Oreochromis niloticus). Aquaculture Vol. 522, 30 May 2020, 735126.] – estimated the levels of genetic variation for resistance to TiLV in a population of Nile tilapia from the Genetically Improved Farmed Tilapia (GIFT) strain, using data collected from a field outbreak of the disease.

The authors gratefully acknowledge funding from CGIAR and Institute Strategic Programme funding from BBSRC to The Roslin Institute (BB/P013759/1 and BB/P013740/1).

Study setup

The Nile tilapia population used in this study was from a major breeding program established in Malaysia and managed by WorldFish. This population originated from the GIFT strain and has been selected for improved growth rate for 15 generations. A total of 124 families were produced using 115 males and 124 females. To retain pedigree information, each individual was tagged with a Passive Integrated Transponder (PIT) tag at an average weight and age of 4.97  grams and 110.5  days, respectively. Once individuals reached typical harvest weight, their weight was recorded and the fish were then transferred to a single pond.

This population in the pond experienced a natural TiLV outbreak in February 2018; and survival or mortality data were obtained from a total of 1,821 fish from this single pond, and this formed the basis of the trait of TiLV resistance. An average of 14 fish (ranging from 2 to 21) per family were measured for TiLV resistance. Fish were collected until the mortality curve had stabilized, i.e. mortalities had returned to baseline levels. Sex was identified for all fish, with a male:female ratio of 0.74:1.00. Surviving fish were euthanized with clove oil (400  mg/liter). Necropsy assays were performed on a number of randomly selected dead fish to evaluate the cause of death and corroborate with the observed clinical signs of the disease. To confirm the presence of TiLV, spleen samples were obtained from a random sample of 39 individuals and analyzed.

For trait definitions, resistance to TiLV was defined as binary survival (BS) and time to death (TD). For the former, survivors and dead fish were assigned values of 1 and 0, respectively. In case of TD the values ranged from 1 (first day of observed moralities) to the last collection day (19). Survivor fish were assumed as censored data and each assigned the value of 18 or 19  days based on the sampling day.

For detailed information on the study population and the Tilapia Lake Virus outbreak; trait definitions; data acquisition; heritability and genetic correlations; model comparisons; and statistical analyses, please consult the original publication.

By Agustin Barría, Ph.D. Trong Quoc Trinh, Ph.D. Mahirah Mahmuddin John A.H. Benzie, Ph.D. V. Mohan Chadag, Ph.D. and Prof. Ross D. Houston, Ph.D.

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