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

Abstract

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.

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

“…However, this idea has been contested by Tuyen et al [101], who found that indirect water-borne transmission was more important than direct transmission through cannibalism in L. vannamei. A more recent study attempted to explain these contradictive findings, postulating that cannibalism can facilitate direct peroral WSSV transmission by ingestion of infected tissues, or it can promote indirect water-borne WSSV transmission because the act of chewing potentially releases multiple virus particles in the water, immersing the cannibal and other shrimp in its vicinity [102]. This suggests, however, that an in vivo infection model that mimics or leaves room for the occurrence of cannibalism might provide better insights into the natural infection dynamics [93].…”

“…If animals are housed in a group, for instance, it can often not be guaranteed that every animal receives the same amount of infectious virus during a challenge with WSSV-infected tissues [76,88,113]. Individual housing of the experimental animals offers a potential solution to this issue [76,102,113]. Indeed, a WSSV challenge model in which shrimp are individually housed has certain advantages over a WSSV group challenge model.…”

“…This is especially useful during per os feeding of infected tissues, because in this case, it is unlikely that all individually housed animals eat the same amount of tissue at the same time [113]. In addition, the clinical outcomes on the level of the individual shrimp can be evaluated [102,123]. On the other hand, a challenge in a group simulates more closely the on-farm reality of a WSSV outbreak, as it allows for disease transmission between shrimp [102].…”

“…In addition, the clinical outcomes on the level of the individual shrimp can be evaluated [102,123]. On the other hand, a challenge in a group simulates more closely the on-farm reality of a WSSV outbreak, as it allows for disease transmission between shrimp [102]. This might indeed generate results that can potentially be easier to extrapolate to the field, but due to the less controllable nature of this experimental setting, results may be less reproducible and accurate data collection becomes more challenging.…”

“…This might indeed generate results that can potentially be easier to extrapolate to the field, but due to the less controllable nature of this experimental setting, results may be less reproducible and accurate data collection becomes more challenging. Nevertheless, both model types can be very useful and complementary tools to use in WSSV research [102,123]. The choice for individual or group housing may also be largely determined by the specific research question that one desires to be answered [54].…”