Rabies is a severe viral disease that attacks the central nervous system of mammals and is almost always fatal once symptoms begin. Historically, the risk of a seal carrying rabies has been considered extremely low because the disease is overwhelmingly a terrestrial phenomenon. Marine mammals are usually isolated from the continuous transmission cycle required for the virus to persist. Despite this, recent events have shown that seals are not immune to spillover from land-based wildlife, introducing a new health consideration.
Rabies Virus Ecology in the Marine Environment
The rabies virus, a Lyssavirus, depends on a dense population of land-based mammals to maintain its life cycle. Transmission typically occurs through the saliva of an infected animal entering a bite wound. This sustained cycle requires terrestrial reservoir species, such as raccoons, foxes, skunks, and bats, which are found in close proximity to one another.
The open ocean acts as a significant barrier, naturally isolating seals from these primary carriers. However, this barrier is breached when terrestrial animals come into direct contact with seals on beaches or in coastal areas. A rare but confirmed outbreak of rabies in Cape fur seals along the South African coast illustrates this spillover risk.
Genetic analysis suggests the virus originated in a terrestrial animal, likely a black-backed jackal, which then bit a seal. The virus established itself within the seal population, leading to seal-to-seal transmission within their highly social and densely populated colonies. Prior to this outbreak, the only other documented case in a marine mammal was a single ringed seal in the 1980s, believed to have been infected by a rabid Arctic fox.
Phocine Distemper and Other Zoonotic Risks
The behavior of a sick or stranded seal, such as disorientation, aggression, or neurological symptoms, is often what prompts public concern about rabies. While rabies is one possibility, these signs are more frequently caused by other serious diseases that mimic the effect of a neurological infection. Phocine Distemper Virus (PDV) is one of the most common and devastating diseases in seal populations.
PDV is a Morbillivirus, related to the canine distemper and measles viruses. It causes symptoms including labored breathing, ocular and nasal discharge, and fever. The virus can progress to cause neurological signs such as tremors, seizures, and erratic behavior, which may be mistaken for rabies. Large outbreaks of PDV have caused mass mortality events in North Atlantic seal populations, significantly reducing their numbers.
PDV is not considered a zoonotic disease, meaning it is not transmissible to humans. However, it can potentially infect domestic dogs, which is why keeping pets away from seals is a necessary precaution. Other zoonotic threats that seals can carry include bacteria such as Leptospira and Brucella, which can be transmitted to humans.
Leptospirosis can cause lethargy and acute renal failure in humans, typically after contact with contaminated urine or water. Brucella species found in marine mammals can cause an influenza-like illness. A localized skin infection known as “seal finger” can also result from wounds contaminated with bacteria like Erysipelothrix rhusiopathiae after handling seals. Aggressive behavior may also be caused by domoic acid poisoning from harmful algal blooms, which produces neurotoxins that cause neurological dysfunction.
Safe Encounters with Sick or Stranded Seals
Encountering a seal exhibiting unusual behavior, whether due to rabies, PDV, or another illness, requires a clear response to protect both the public and the animal. The rule is to maintain a significant distance from all seals. Federal guidelines recommend staying at least 50 yards away from seals and sea lions on the water or on shore.
Seals are protected under the Marine Mammal Protection Act, making it illegal to harass, feed, or attempt to interact with them. Approaching a seal, especially one that appears sick or injured, can cause the animal significant stress, which may prompt a defensive attack. Never attempt to push a stranded seal back into the water, as the animal hauled out for a reason, which is often to rest or because it is ill.
If a seal appears sick, injured, or is in an unusual location, the proper action is to immediately contact local marine mammal stranding networks or authorities. These specialized teams are trained to assess the animal’s condition safely and have the necessary equipment to test for diseases, including rabies. They can also determine if the animal is genuinely stranded or simply resting, which seals often do for extended periods.
Keeping pets, particularly dogs, away from seals is necessary. Dogs can disturb the resting animal, transmit or contract diseases like PDV, or be injured by a defensive bite. By maintaining distance and contacting the proper authorities, members of the public ensure their own safety while providing the best chance for the seal to receive appropriate care.