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Viral Diseases of Marine Mammals


Adenovirus has been isolated from a sei whale (Balaenoptera borealis) and bowhead whales (Balaena mysticetus) and in livers from 6 stranded young California sea lions with hepatitis. No disease was noted in the cetaceans. Pinnipeds developed weakness, emaciation, photophobia, polydipsia, abdominal splinting, blood-tinged diarrhea, and eventually posterior paresis; a relative lymphopenia and monocytosis were seen. All pinnipeds developed pneumonia and died within 28 days.

The most prominent histologic lesion in all cases was hepatic necrosis. Massive coagulation necrosis without apparent zonal distribution occurred in some animals. Basophilic intranuclear inclusions in hepatocytes or granular amphophilic intranuclear inclusions in Kupffer's cells were seen. No evidence of adenovirus was detected in the lungs. Adenovirus from California sea lions is not known to cause disease in humans.

Caliciviruses have been isolated from otarid seals, walrus, Atlantic bottlenose dolphins, and opaleye fish (Girella nigricans). The marine caliciviruses appear to be serotypes of vesicular exanthema of swine virus (VESV, see Vesicular Exanthema of Swine). Several species of mysticete cetaceans have antibodies to different serotypes of VESV. By 4 mo of age, most California sea lions have neutralizing antibodies to one or more of the serotypes. Opaleye fish are probably responsible for the endemic status of caliciviruses in marine mammals that inhabit the coastal waters of California. To date, infections have not been diagnosed in marine mammals in the Atlantic Ocean.

The most consistent lesion in marine mammals is skin vesicles. In pinnipeds, the vesicles are most prevalent on the dorsal surfaces of the fore flippers. In dolphins, vesicular lesions have been seen in association with “tattoo” lesions and old scars. Vesicles are 1 mm to 3 cm in diameter. They usually erode and leave shallow, fast-healing ulcers, but occasionally vesicles regress and leave plaque-like lesions. Treatment is supportive only; skin lesions usually resolve without treatment. Infection may cause premature parturition in pinnipeds. Affected pups have interstitial pneumonitis and encephalitis and fail to thrive.

Inoculation of marine caliciviruses into pigs causes vesicular lesions identical to those seen in vesicular exanthema. In humans, heavy exposure to marine caliciviruses can result in neutralizing antibodies. Localized lesions in an accidental laboratory exposure as well as isolation of calicivirus from a clinically ill primate indicate that these viruses should be handled carefully.

Herpesviruses have been isolated from neonatal harbor seals, a California sea lion, and a gray seal. Herpesvirus-like particles have been demonstrated in skin lesions from beluga whales and dusky dolphins (Lagenorhynchus obscurus). Herpesvirus-like lesions occur in a wide variety of other pinnipeds and cetaceans. Two distinct herpesviruses have been characterized from harbor seals and gray seals. Phocid herpesvirus type-1 (PhHV-1) is an α-herpesvirus similar to canine herpesvirus. Phocid herpesvirus-2 (PhHV-2) is a putative γ-herpesvirus. Other than a postmortem diagnosis of herpesvirus encephalitis in a dead harbor porpoise, reports of herpesviral disease in cetaceans have been limited to skin and mucosal lesions with little clinical significance. A herpes-like virus was found associated with sublingual ulcers in sea otters stressed in oil spill rehabilitation processing.

Young harbor seals from Atlantic waters infected with PhHV-1 develop nasal discharge, inflammation of the oral mucosa, vomiting, diarrhea, and fever, followed by coughing, pneumonia, anorexia, and lethargy that can result in death in 1–6 days. Morbidity can approach 100% in stressed seals in crowded conditions; mortality is ∼50%. The incubation period appears to be 10–14 days. Pacific harbor seals with PhHV-1 tend to develop signs related to adrenal and hepatic dysfunction.

PhHV-2 has been associated with recurring circumscribed areas of alopecia ∼0.5 cm in diameter in gray seals. Herpetic lesions in beluga whales are generally circular, up to 2 cm in diameter, and may appear slightly depressed with a target appearance or be raised and proliferative. The centers of some lesions are necrotic or may contain verrucous growths. Systemic infections have not been documented in the whales.

Diagnosis is often made at necropsy or by clinical signs and observation of characteristic intranuclear inclusion bodies in biopsies of early skin lesions. In seals, interstitial pneumonia caused by herpesvirus must be distinguished from bronchial pneumonia caused by influenza virus.

In systemic herpesvirus infection, therapy is supportive. In a documented epidemic, oral acyclovir did not eliminate the infection but appeared to significantly shorten clinical signs in primary infections. Vaccination with 1 mL of trivalent poliovirus vaccine to control recrudescence of suspected herpesvirus lesions has been used with some success; although it reduced the severity of recrudescence in seals, there is a potential public health risk because live poliovirus may be shed after vaccination. Stress and immunosuppression are associated with recrudescence of latent infections. There is no evidence that the herpesviruses of pinnipeds or cetaceans are zoonotic.

Four different influenza A viruses have been isolated from harbor seals and 2 other subtypes from a stranded pilot whale. Infection is probably common. Only nonspecific clinical signs were reported in the stranded pilot whale, which had difficulty maneuvering, was emaciated, and was sloughing skin. Disease due to influenza virus in seals is better characterized. Even well-fed captive animals become weak, incoordinated, and dyspneic. Swollen necks due to fascial trapping of air escaping through the thoracic inlet is reported. Occasionally, white or bloody nasal discharge will be evident. The incubation period during epidemics in harbor seals is ≤ 3 days. Many factors probably contribute to the explosive nature of the reported epidemics. High population densities and unseasonably warm temperatures contribute to high mortality.

In harbor seals, influenza pneumonia is characterized by necrotizing bronchitis and bronchiolitis and hemorrhagic alveolitis. In the pilot whale, the lungs were hemorrhagic and a hilar node was greatly enlarged. For differential diagnosis, seeherpesvirus, above.

The virulence of epidemics has precluded attempts at intensive supportive care. Persons whose eyes were contaminated while doing necropsies, or by being sneezed on by affected seals, have developed keratoconjunctivitis within 2–3 days, and identical virus has been recovered. All affected people have recovered completely within 7 days without developing any antibody titers, which suggests that the reaction is local, as occurs with Newcastle disease virus.

Phocid seals are susceptible to canine distemper virus (see Canine Distemper) and to a closely related but distinct morbillivirus (phocine distemper virus [PDV]). Generally, young seals are affected and show depression, anorexia, crusting conjunctivitis, nasal discharge, and dyspnea. Pneumonia develops and mortality can be high in previously unexposed animals. Outbreaks in wild harbor seals have been extensive in the North Sea. Seals vaccinated with canine distemper vaccine have been rendered immune to challenge with the virus (suspension of organ material) obtained from dead wild seals.

A delphinoid distemper virus (cetacean morbillivirus [CMV]), closely related to rinderpest (see Rinderpest) and peste des petits ruminants (see Peste des Petits Ruminants), has been implicated in the deaths of harbor porpoises and common dolphins off the coast of the UK, striped dolphins in the Mediterranean, and bottlenose dolphins in the western Atlantic and Gulf of Mexico. Pilot whale calves, white-beaked dolphins (Laegenorhynchus albirostris), harp seals, hooded seals (Cystophora cristata), and Mediterranean monk seals (Monachus monachus) have been affected by PDV and/or CMV infections. In the wild, harp seals and pilot whales have been incriminated as apparent reservoirs of PDV and CMV, respectively.

Therapy is supportive. Mortality in naive populations is high, often due to secondary infections facilitated by the immunosuppressive impact of active morbillivirus infection. Vaccination with a subunit vaccine is practiced in European rescue centers and appears to be protective, but this approach has not been applied in North America, in large part due to lack of availability of the appropriate vaccine.

Poxvirus has been identified morphologically in skin lesions of both captive and free-ranging pinnipeds and cetaceans. Lesions in California sea lions, harbor seals, and gray seals are probably due to parapoxviruses; lesions in South American sea lions (Otaria byronia) and Northern fur seals are probably not. An orthopox virus has been isolated from pox-like lesions on a gray seal. An unclassified poxvirus has also been associated with skin lesions in Atlantic bottlenose dolphins and in a stranded Atlantic white-sided dolphin. Cases in killer whales, dusky dolphins, long-beaked common dolphins (Delphis capensis), Hector's dolphins (Cephalorhynchus hectori), and Burmeister's porpoises (Phocoena spinipinnis) have also been reported.

Outbreaks typically occur in postweanling pinnipeds recently introduced into captivity. The incubation period is 3–5 wk. A break in the epithelial surface may be required to start an infection. Lesions can recur. Small, cutaneous, raised nodules (0.5–1 cm in diameter) occur on the head, neck, and flippers of affected pinnipeds. These may increase to 1.5–3 cm in diameter during the first week and may ulcerate or develop satellite lesions during the second week. After the fourth week, lesions begin to regress, although nodules are reported to persist as long as 15–18 wk. Areas of alopecia and scar tissue may remain after resolution.

Cutaneous poxvirus infections in cetaceans can occur on any part of the body but are most common on the head, pectoral flippers, dorsal fin, and tail fluke. They range from ring or pinhole lesions to black, punctiform, stippled patterns (“tattoo” lesions). Ring or pinhole lesions appear as solitary, 0.5–3 cm, round or elliptical blemishes, which sometimes coalesce. They are usually light gray and may have a dark gray border, although the reverse color pattern is also seen. Lesions may persist for months or years without apparent ill effects.

Major differential diagnoses include cutaneous streptothricosis and calicivirus. Diagnosis is based on the presence of eosinophilic, intracytoplasmic inclusion bodies in lesion biopsies and is confirmed by identification of typical poxvirus particles by electron microscopy.

Poxviruses of marine mammals do not appear to cause systemic infections. Although animals with cutaneous poxvirus lesions have died, other factors were responsible. Therapy to control secondary bacterial infections is indicated only when skin lesions suppurate. The parapoxviruses of pinnipeds can cause isolated lesions on the hands of persons not wearing gloves during contact with infected animals.

A ringed seal (Phoca hispida) in Norway was diagnosed with rabies, which was confirmed by immunofluorescent examination of the brain. At the time, there was an epidemic of rabies in foxes in the area. Other rhabdoviruses isolated from cetaceans, which are not recognized by antisera to representatives of the Lyssavirus, Ephemerovirus, or Vesiculovirus genera, may be related to rhabdoviruses of fish.

Papillomavirus infections have been reported in a wide range of cetaceans including narwals (Monodon monoceros), and several species of mysticete whale. Lesions are typical of those found in terrestrial species. No therapy is available. Lesions are usually self-limiting.

Hepadnavirus infection with a hepatitis B-like agent has been documented in a Pacific white-sided dolphin with a long history of recurrent illness in captivity. No evidence of zoonotic transmission was identified.

The only retrovirus identified to date in a marine mammal was a spumavirus isolated from recurring skin lesions in a California sea lion that subsequently died of Pasteurella pneumonia complicated with herpesvirus.

Immunohistochemical staining consistent with coronavirus infections was found in 2 adult harbor seals that died without clinical signs and a third that died acutely after a brief period of anorexia and behavioral aberration.

An enterovirus of unknown pathogenicity isolated from a rectal swab of a California gray whale has now been reclassified as a calicivirus. Antibodies, unassociated with disease, against human influenza virus (after challenge) and poliomyelitis virus were found in bottlenose dolphins.

Severe enteritis and vomiting that rapidly led to death in a captive beluga whale were suggestive of parvovirus enteritis, but no virus was isolated.

Last full review/revision April 2012 by Michael K. Stoskopf, DVM, PhD, DACZM

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