ECONOMIC AND DISEASE BURDEN OF SELECTED AQUATIC PATHOGENS IN THE ASIA-PACIFIC REGION

Over the past decade, Asia's aquaculture industry has increased at an av. 6.85% yr-on-yr, from 37.59 MT in 2001 to 88.90 MT in 2013. Production over 2012-13 increased at 8.64% p.a., yet despite this impressive rise, so have the losses associated with disease outbreaks.

A broad spectrum of viral, bacterial and fungal pathogens continue to impact heavily on Asia's aquaculture industries. Since its emergence in China in 2009, acute hepatopancreatic necrosis disease (AHPND) of shrimp, caused by isolates of the bacterium Vibrio parahaemolyticus with a toxin gene bearing plasmid, has had a devastating economic impact in Asia. AHPND, for example, has cost the Thai whiteleg shrimp, Litopenaeus vannamei, economy in excess of USD 5.01 billion to date (based on the value of lost tonnage since the peak of 603,227 tons in 2011). In the Vietnamese Mekong Delta, AHPND has affected 2,318 ha of Penaeus monodon ponds and 2,309 ha of ponds for L. vannamei culture, which are typically stocked at higher densities, resulting in losses estimated at USD 1.84 M and USD 8.93 M respectively. White spot syndrome virus (WSSV) has also caused notable losses within the Mekong Delta, i.e. 2,510 ha of P. monodon ponds and 1,397 ha L. vannamei ponds with resultant losses estimated at USD 2.37 M and .USD 5.65 M, respectively.

Infection of the microsporidian Enterocytozoon hepatopenaei, the causative agent of hepatopancreatic microsporidiosis (HPM) is spreading, with infections of L. vannamei and P. monodon reported from China, India, Malaysia, Thailand and Vietnam. This fungal pathogen, which infects the tubule epithelial cells of the hepatopancreas, does not, reputedly, result in the mortality of shrimp but results in the severe retardation of growth, i.e. slowed growth from 12 g onwards for L. vannamei with harvests capped at 9 tons/ha as opposed to a target of 12 tons/ha. Infections of this sort mean production costs are not covered with losses estimated at USD 32,000/ha over normal production economics.

Aquatic pathogens also result in significant losses in finfish production. Streptococcal -based infections, with significant resultant mortalities, have been reported from a large number of Asian countries. Thailand, for example, loses c. 20% of its tilapia production in the hot season; a min. 7.5% total loss across the industry (i.e. 16,270 tons valued at USD 26.57 M). Asia's tilapia productions is increasing at 10.68% yr-on-yr and production for 2015 is estimated at 4.11 MT. If a 7.5% mortality across Asia is assumed, with adjustments for Bangladesh and the Philippines, where smaller size fish are harvested, then the losses to streptococcal infections are approx. 289,440 tons valued at USD 480 M. Likewise, losses due to parasitic infections within hatcheries frequently go ignored / unreported, either because of the smaller scale of losses or because of the general acceptance that they fall within the typical, accepted margins of loss in production. Collectively, these losses can be significant and for Asia's tilapia industry, these are estimated to be USD 13.5-18.5 M annually.

In aquaculture, even occasional lapses in biosecurity can result in devastating losses. If Asia is to maximise its aquaculture potential by minimising losses due to disease agents then these must be addressed through appropriate biosecurity, health management and control programs.