Abstract

An epizootic of cutaneous nodules occurred in three species of pinnipeds at the California Marine Mammal Center during the summer of 1986. One harbor seal pup developed nodules initially, with nodules developing on all other harbor seals, one California sea lion, and at least two northern elephant seals. Initial excisional biopsies and cultures failed to reveal an etiologic agent. Subsequent biopsies of nodules from two harbor seals and one elephant seal demonstrated epidermal cytoplasmic inclusion bodies compatible with poxvirus infection. Poxvirus infection has not been previously documented in northern elephant seals. Transmission electron microscopy confirmed a poxvirus in a harbor seal nodule. The chronology of the outbreak suggests transmission of the virus from harbor seal to elephant seal and possibly to the sea lion. Transmission between pinniped species has riot been previously recorded.

Introduction

A poxvirus has been associated with benign cutaneous nodules in California sea lions, Zalophus californianus (6,10), South American sea lions, Otaria byronia (4,8,9), northern fur seals, Callorhinus ursinus (1), harbor seals, Phoca vitulina (3,9,10), grey seals, Halichoerus grypus (2), and man (2). No mortality or serious illness has been attributed to the viral infection.

Transmission of seal poxvirus to man has been recently reported (2) but transmission between host pinniped species has not been reported, thus it is not known whether the strains of this virus are host specific. Other pox viruses tend to be host specific, though the related viruses bovine papular stomatitis and pseudocowpox affect both cattle and man, and orf affects goats, sheep and man.

In the present report, we describe an epizootic of cutaneous nodules in three species of pinnipeds at a rehabilitation center. Light microscopy demonstrated eosinophilic intracytoplasmic inclusion bodies in harbor seal (Phoca vitulina richardsi) andnorthern elephant seal (Mirounga angustirostris)nodules. Electron microscopy confirmed a poxvirus in one harbor seal nodule.

Materials and Methods

Fifteen orphaned harbor seal neonates from northern California were admitted to the California Marine Mammal Center (CMMC) during spring 1986. They were housed individually or in pairs in cages for 2 to 6 weeks before placement in groups in pens. Waste material from harbor seal pens could wash into adjacent pens housing other pinniped species. During the epizootic, 3 subadult California sea lions, 29 weanling northern elephant seals, and 7 yearling northern elephant seals were housed at CMMC for a sufficient period to develop pox lesions (3 to 5 week incubation period).

Excisional biopsies of cutaneous nodules were taken from two harbor seals an one elephant seal during 1986, and from one harbor seal during 1987. Tissues were divided for processing for light microscopy and transmission electron microscopy. The interior of one harbor seal nodule was swabbed aseptically and cultured on blood agar at 37°C and potato flake agar at room temperature.

Results

On May 10, 1986, 7 weeks after a harbor seal pup was admitted to CMMC, the pup presented with a one cm firm raised cutaneous nodule on his chin. Over the next several weeks, numerous similar nodules appeared on the ventral surface of all four flippers, On June 5, similar nodules were noticed on 2 additional harbor seal pups which had been housed with the first for 3 and 6 weeks each. By mid July, all 15 harbor seals at CMMC had developed identical lesions.

Most harbor seals developed nodules 3 to 5 weeks following housing with an animal exhibiting nodules, though 3 of the 15 harbor seals did not develop lesions for several more months. Two harbor seals developed nodules even though neither was housed with affected seals. These harbor seals had been held at CMMC 4 and 6.5 weeks before nodules were evident.

The lesions first developed as small firm nodules 0.5 to 2 cm in diameter', raised 0.2 to I cm. above the skin surface, and with an intact epithelial surface. Over several weeks, the nodules enlarged, some reaching 3 cm in diameter, and often becoming confluent. Most nodules became ulcerated and exuded a small amount of purulent material. The nodules then decreased in size over several weeks, eventually leaving hyperpigmented areas of alopecia which were not elevated.

The nodules were present for at least 4 weeks, usually 6 to 8 weeks, and took another 2 to 8 weeks for the ulcers to heal. The resultant areas of alopecia were still present several months later at the time of release.

In a few of the harbor seals, the appearance of the lesions was accompanied by much flipper waving and rubbingwhich was interpreted as evidence of pruritis. In one harbor seal, superficial keratitis developed secondarily to a palpebral nodule.

Lesions were most numerous on the palmar surface of the foreflippers an d were present in all seals. Most also had fewer lesions on the dorsal surface of the foreflippers (12 of 15 harbor seals) and on the hindflippers 0 1). Fewer SE als had lesions on the abdomen (6), head (3), the perineal area(3), and the back (1).

In early January 1987, proliferative ulcerative lesions were noted on the ventral surface of both foreflippers of the harbor seal which first exhibited cutaneous nodules the previous spring. No other harbor seals were housed at CMMC that winter. The lesions closely resembled the previous ones although they were lessproliferative and more quickly became confluent. The lesions gradually reduced in size over the next three months, leaving hyperpigmented areas of alopecia.

Following the occurrence of nodules in harbor seals, several northern elephant seals developed a few well demarcated raised nodules two months after being housed near the affected harbor seals. These were solitary, I to 3 cm in diameter, and tended to become ulcerated. Because these lesions were less dramatic than those of the harbor seals, they tended to be overlooked by the veterinary staff.

One California sea lion developed 3 cervical nodules identical in appearance to those of the harbor seal, except that they did not become ulcerated. These nodules, between I and 2 cm in diameter, developed several months following hovering next to harbor seals with nodules.

A mild to moderate neutrophilia (13,700 to 34,900/ul) was present in 10 of 11 harbor seals from which blood was taken, either during the time nodules were present or up to one month before. The leukograms returned to normal in 13 of 15 harbor seals after the nodules had regressed. In the remaining two, a mild neutrophila persisted at the time of release. Serum chemistry values were normal in all harbor seals sampled except for elevated total protein with normal alburnin while nodules were present (4 of 15 harbor seals sampled), and after the nodules had regressed ( 8 of 14).

No fungal organisms were isolated from the one nodule sampled. Ty is nodule, however, yielded a mixed growth of Proteus sp., Escherichia coli, Staphylococcus sp., and Streptococcus sp.

The histopathologic appearance of the 2 harbor seal and one elephant seal nodules in 1986 were similar. The epidermis showed marked acanthosis and ortho- parakeratotic hyperkeratosis of both superficial and follicular epithelin. Variably sized eosinophilic cytoplasmic inclusion bodies were present in foci of swollen cells of the stratum spinosum. Foci of ulceration, neutrophilic infiltrate and bacterial colonization were present in some areas of the epithelium. All lesions showed a prominent dermal infiltrate of macrophages, with fewer lymphocytes and plasma cells. The infiltrate was centered, around prominent dermal vessels.

The harbor seal nodule biopsied in 1987, a much older lesion than those in 1986, showed a proliferative dermitis with acanthosis. No inclusion bodies were seen.

Transmission electron microscopy confirmed the presence of poxvirus particles in the cytoplasm of stratum spinosum cells of one of the 1986 harbor seal nodules.

Discussion

Seal poxvirus infection was diagnosed by the histopathologic findings of eosinophilic cytoplasmic inclusion bodies in 2 harbor seal and one elephant seal nodules in 1986, and was confirmed by electron microscopy in one harbor seal. Seal pox has not been previously reported in northern elephant seals. Seal poxvirus infection is also strongly suggested in the other cases by the close similarity of the gross lesions of the confirmed cases of seal pox, and those of the other harbor seals, elephant seals, and the sea lion.

The gross appearance and distribution of these lesions, and the course of the disease are similar to other reported cases. As compared with harbor seals, however fewer elephant seals (2 of 36) and sea lions (I of 3) developed lesions and each individual developed only a very few nodules, suggesting a partial host specificity of the virus.

The chronology of the outbreak suggests that one harbor seal introduced the virus to the center from the wild, and that the virus subsequently spread to other pinnipeds. In most cases of seal pox in this outbreak, development of the lesions occurred after close contact with affected individuals suggesting direct spread through exposure to lesions or exudate. Two harbor seals developed lesions before being housed with affected seals and before exposure to waste material runoff. In these 2 cases, exposure to the virus probably occurred via human or arthropod vectors. The elephant seals and sea lions were not housed in the same pens as the harbor seals, nor were they handled by harbor seal keepers, thus spread to these species possibly occurred by waste material or by arthropod vectors.

Poxvirus infection may have recurred in one harbor seal. This is suggested by the similarity of the gross appearance of the two sets of nodules in 1986 and 1987. The histopathologic findings of the recurrent nodules were consistent with healing pox lesions, but failed to reveal an etiologic agent. Inclusion bodies would not be expected to be present, since the lesions were 3 months old at the time of biopsy. As previously reported, no mortality was directly associated with pox infection. Pruritis was noted in several harbor seals, and suppuration was evident in all harbor seals and one elephant seal, otherwise no clinical signs of illness were observed. A systemic inflammatory response is indicated by the neutrophilia and subsequent hyperproteinernia seen in most of the harbor seals.

Although handlers of harbor seals did not always wear gloves when handling lesions, no human cases were reported. Hick et al. demonstrated the zoonotic potential of this virus, therefore it is recommended that pinniped handlers wear gloves to prevent human infection.

Acknowledgements

The authors wish to thank the staff of the California Marine Mammal Center for providing information and assistance in the preparation of this paper. Financial support for the electron microscopy was provided by the California Marine Mammal Center.

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