RESUMO
Shrimp viruses can remain infectious in frozen shrimp tissue and have been found in frozen commodity shrimp. Therefore, the threat of viral outbreaks in wild and cultured shrimp via frozen commodity shrimp exists. Because frozen shrimp are imported with and without the cephalothorax, more knowledge is needed concerning the infectivity of a cephalothorax relative to that of an abdomen. We compared the mortality rates from shrimp exposed to a WSSV-infected cephalothorax, abdomen, or whole shrimp cadaver. Estimates of transmission coefficients from the exposures to the infected cephalothorax, abdomen, or whole shrimp were also calculated because the transmission coefficients account for differences in the initial doses. In addition, we compared the variability in infectivity of pieces of shrimp by feeding 24 equal-sized pieces of cephalothorax and abdomen to 24 individually isolated shrimp. In Expt 1, susceptible shrimp did not completely consume the infected abdomen, and a significant difference was detected among shrimp exposed to the abdomen (mortality rate = 0.40), cephalothorax (mortality rate = 0.75), and whole shrimp cadaver (mortality rate = 0.67). The calculated transmission coefficients were 0.95 from an infected cephalothorax, 0.59 from an infected abdomen, and 0.69 from an infected whole shrimp cadaver. In Expt 2, susceptible shrimp were starved to ensure complete ingestion of each dose. No significant difference was observed in the estimated mortality rates from an infected cephalothorax (0.58), abdomen (0.63), or whole shrimp (0.67). The calculated transmission coefficients were 0.84 from an infected cephalothorax, 0.83 from an infected abdomen, and 0.60 from an infected whole shrimp cadaver. In Expt 3, no difference was observed in the mortality rates resulting from exposures to pieces of infected cephalothorax (0.57) or abdomen (0.58). Our results suggested that there was no difference in the viral loads of a WSSV-infected cephalothorax or abdomen, but that the cephalothorax was more infectious, probably because it was more palatable. In addition, our results are inconsistent with some assumptions of pathogen transmission used in epidemiological models. Some shrimp may be less aggressive feeders; therefore, susceptible shrimp are differentially contacting the dead infected shrimp in the exposure tanks, violating the random mixing assumption. Moreover, virus is probably not homogeneously distributed throughout an infected shrimp, suggesting that contacts between susceptible and infected shrimp are not equally likely to result in transmission.