Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp.

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

The Way of Water: Unravelling White Spot Syndrome Virus (WSSV) Transmission Dynamics in Litopenaeus vannamei Shrimp.

White spot disease (WSD) is a severe viral threat to the global shrimp aquaculture industry. However, little is known about white spot syndrome virus (WSSV) transmission dynamics. Our aim was to elucidate this in Litopenaeus vannamei using peroral in vivo WSSV challenge experiments. We demonstrated that WSD progression was rapid and irreversible, leading to death within 78 h. Viral DNA shedding was detected within 6 h of disease onset. This shedding intensified over time, reaching a peak within 12 h of the time of death. Isolating shrimp (clinically healthy and diseased) from infected populations at different time points post-inoculation showed that host-to-host WSSV transmission was occurring around the time of death. Exposing sentinels to environmental components (i.e., water, feces, molts) collected from tanks housing WSSV-infected shrimp resulted in a significantly (p-value < 0.05) increased infection risk after exposure to water (1.0) compared to the risk of infection after exposure to feces (0.2) or molts (0.0). Furthermore, ingestion of WSSV-infected tissues (cannibalism) did not cause a significantly higher number of WSD cases compared to immersion in water in which the same degree of cannibalism had taken place.

Susceptibility of juvenile Macrobrachium rosenbergii to different doses of high and low virulence strains of white spot syndrome virus (WSSV).

As some literature on the susceptibility of different life stages of Macrobrachium rosenbergii to white spot syndrome virus (WSSV) is conflicting, the pathogenesis, infectivity and pathogenicity of 2 WSSV strains (Thai-1 and Viet) were investigated here in juveniles using conditions standardized for Penaeus vannamei. As with P. vannamei, juvenile M. rosenbergii (2 to 5 g) injected with a low dose of WSSV-Thai-1 or a high dose of WSSV-Viet developed comparable clinical pathology and numbers of infected cells within 1 to 2 d post-infection. In contrast, a low dose of WSSV-Viet capable of causing mortality in P. vannamei resulted in no detectable infection in M. rosenbergii. Mean prawn infectious dose 50% endpoints (PID50 ml⁻¹) determined in M. rosenbergii were in the order of 100-fold higher for WSSV-Thai-1 (105.3 ± 0.4 PID50 ml⁻¹) than for WSSV-Viet (103.2 ± 0.2 PID50 ml⁻¹), with each of these being about 20-fold and 400-fold lower, respectively, than found previously in P. vannamei. The median lethal dose (LD50 ml⁻¹) determined in M. rosenbergii was also far higher (~1000-fold) for WSSV-Thai-1 (105.4 ± 0.4 LD50 ml⁻¹) than for WSSV-Viet (102.3 ± 0.3 LD50 ml⁻¹). Based on these data, it is clear that juvenile M. rosenbergii are susceptible to WSSV infection, disease and mortality. In comparison to P. vannamei, however, juvenile M. rosenbergii appear more capable of resisting infection and disease, particularly in the case of a WSSV strain with lower apparent virulence.

Molt stage and cuticle damage influence white spot syndrome virus immersion infection in penaeid shrimp.

Transmission of white spot syndrome virus (WSSV) in shrimp has been reported to occur by feeding and immersion. In the present study, the impact of the molt process and artificial lesions in the cuticle on shrimp susceptibility to WSSV was examined using intramuscular and immersion routes. For the intramuscular route, Penaeus (Litopenaeus) vannamei shrimp (n=450) were injected with 10(-2.3) up to 10(2.7) shrimp infectious dose 50% end point (SID(50)) of WSSV in early and late post-molt, inter-molt, early and late pre-molt; resp. A-, B-, C-, D1- and D2-stage. The resulting infection titers demonstrated that no difference (p>0.05) in susceptibility existed between different molt stages when virus was injected. For the waterborne route, shrimp in different molt stages were immersed in seawater containing 10(4)SID(50)ml(-1) of WSSV. In a first study, P. vannamei (n=125) incubated in cell culture flasks, became infected with WSSV mostly in post-molt stages. In a second study, 2 groups of P. vannamei (n=100) and P. monodon (n=100) were transferred into plastic bags to prevent damage to the cuticle; and in 1 group a pleopod was cut off prior to incubation. Induction of damage increased infection significantly (p<0.05) in A-stage from 0-40% to 60-100%, in B-stage from 0-20% to 40-60%, in C-stage from 0-20 to 20-60%, while infection was 0% in D-stages with both immersion methods. This study proved that shrimp are more susceptible to WSSV infection via immersion after molting than in the period before molting and wounding facilitates infection.