The identification of the role seabirds play as possible transmission vectors for the major shrimp viruses, (WSSV, IHHNV, TSV, YHV), was the focus of this study. From anecdotal observations of seabirds feeding on dead and dying shrimp, our hypothesis was that seagulls and other seabirds may serve as paratenic or mechanical vectors of the major shrimp viruses.

An early association between seagulls and human disease was noted in 1925, when researchers observed gulls foraging at dumps, and then roosting at nearby reservoirs. The gulls were blamed for depositing pathogens (such as bacteria and parasites) into the water supply.
Seagulls and other seabirds are very mobile and freely move between open feeding sites, where they may consume diseased farmed shrimp and fish, and then defecate in the surrounding waters. For example the herring gull, Larus argentatus and the black-headed gull, Larus ridi-bundus are often seen scavenging on shrimp trawler discards, and may be capable of transmitting viruses from wild shrimp to nearby shrimp farms.


Literature on bird feces as a possible source of viral transmission in aquatic environments is relatively scarce, however several reports demonstrate bird-associated viral transmission in fish aquaculture. These reports implicate the heron, Ardea cinerea as a mechanical vector of fish pathogenic VHS, SVC, and IPN to trout fry. The study determined the viral shedding duration of IPN, and a water-borne infection of trout fry was diagnosed. This study resulted in a conclusion that herons serve as mechanical vectors for VHS, SVC, and IPN, and may be a potential source of infection and spread of the diseases.

Birds pose an obvious predation threat to farmed aquatic animals, and for many years aquaculturists have suggested that as a possible link between predatory birds and the spread of aquatic diseases. Early studies to support this theory illustrated that wild herons and mallards that have eaten infected fish, excrete infectious IPN virus in their feces at similar titers found in afflicted hatchery fish. Furthermore, a reduced viral disease prevalence was observed at fish hatcheries that used electric fence barriers and netting as bird deterrence devices.

Inconclusive results were obtained when the role of loons, Gavia immer, as possible transmitters of IPN in trout waters was investigated in 1983. However, the study implied that regurgitation of infected material may serve as a transmission mechanism.

Seabirds have been shown to carry and transmit bacteria in their feces as well. For example, transmission of Yersinia rukeri, the causative agent of enteric redmouth disease (ERM), and Myxosoma cerebralis, the causative agent of salmonid whirling disease, have been demonstrated.

Experimental Set-up
The US Marine Shrimp Farming Consortium funded this study as part of a Master’s Thesis for the author at the University of Arizona, Aquaculture Pathology Lab. State and federal permits for wildlife collection, holding and transportation were acquired for this study to comply with
state and federal regulations.

The hypothesis that shrimp eating birds may carry infectious viral particles in their feces from diseased ponds to nearby unaffected ponds, was tested in a two part study. The initial part of the study consisted of testing the feces collected at regular intervals of seagulls fed shrimp infected with either WSSV, TSV, YHV, or IHHNV by PCR/RT-PCR. A Day 0 sample of the seagull feces, prior to ingestion of infected material, was also tested to ensure negative status of the feces prior to the bird consuming
infected tissue.

Bioassay challenge studies were performed as the second part of this study to determine if any of the viruses detected in seagull feces remained infectious. Four separate bioassay challenge studies were run to determine if WSSV, TSV, YHV, or IHHNV were infectious following passage through the seagull digestive tract. SPF Litopenaeus vannamei were challenged by injection with homogenized and diluted seagull feces that were collected during the first part of the study. Histological examination and molecular methods such as PCR/RT-PCR, and in-situ DNA hybridization were used to identify and confirm viral infect-
ion in the challenged SPF shrimp.

Results
Detection
In the feces of gulls fed the four viruses, standard PCR detected WSSV and IHHNV DNA in seagull feces, and RT-PCR detected TSV RNA in seagull feces. However, YHV RNA was not detectable by RT-PCR.

Bioassay Challenges
In shrimp challenged with PCR/RT-PCR positive feces, pathognomic lesions for IHHNV and TSV were observed in histological sections and confirmed by PCR/RT-PCR in the challenged shrimp. Histology and PCR/RT-PCR were negative for the presence of WSSV or YHV in the challenged shrimp. Although WSSV was detectable in the seagull feces and inoculum, the inoculum containing the virus was not infectious when injected into susceptible SPF shrimp. YHV was not detectable in gull feces in this study.

Discussion & Conclusion
In this study, seagulls were shown to be mechanical vectors of certain shrimp viruses. Seabirds are mechanical vectors because they do not become infected with the viruses that they carry in their feces. Seabirds naturally scavenge food whenever possible, thus gulls feeding on dead shrimp in open ponds can remain at the feeding site until they have become engorged with shrimp. This study demonstrated that seagulls which consume shrimp infected with the viruses IHHNV and TSV are capable of shedding those infectious viral particles in their feces. The shed viruses remain infectious and can, therefore, pose a risk to native shrimp in nearby ponds or other bodies of water if the seagulls defecate virus contaminated feces, or if they regurgitate ingested shrimp soon after feeding.

DNA is typically more stable than RNA, this fact may help to explain why the DNA viruses, (WSSV and IHHNV) were detectable in seagull feces fed infected shrimp, whereas only one of the RNA viruses, (TSV), was detectable by RT-PCR.

Prevention of shrimp viral diseases are dependent on increased knowledge of the pathogenic pathways of the shrimp viruses as well as technologically advanced shrimp farms. Control of scavenging seabirds is a direct implication of this study. Future construction and planning of shrimp farm sites may need to include design features to deter predation of shrimp by seabirds.

In summary, TSV and IHHNV, non-enveloped icosahedrons, remained infectious following passage through the seagull digestive system. The enveloped viruses, YHV and WSSV, were apparently degraded during passage through the seagull gut, and were unable to remain infectious. The injection bioassay confirmed the infectivity, or lack of infectivity, of these viruses by both PCR/RT-PCR and routine histology.