The question of whether a shark can contract rabies involves infectious disease pathology and marine biology. Rabies, caused by the Rabies Lyssavirus, is a well-known zoonotic disease primarily affecting terrestrial animals. The virus is a neurotropic pathogen that specifically targets the host’s nervous system. This prompts a look into the biological barriers that limit this virus to specific groups within the animal kingdom.
The Biological Impossibility
The definitive answer to whether sharks can get rabies is no. The Rabies Lyssavirus is exclusively a disease of mammals, which are warm-blooded vertebrates. Sharks are cartilaginous fish (class Chondrichthyes), a separate evolutionary lineage from mammals. This biological difference prevents the virus from infecting them.
The virus requires a specific physiological environment to replicate. It has not adapted to utilize the anatomical or cellular structures found in fish. While aquatic mammals, such as seals and sea cows, can contract rabies, sharks remain unaffected due to this biological incompatibility.
Why Rabies Requires Mammals
The Rabies Lyssavirus is specialized and requires a complex biological system to complete its life cycle. As a neurotropic virus, it must gain access to the nervous system. It travels along peripheral nerves via retrograde axonal transport to reach the central nervous system (CNS), requiring specific receptor molecules found primarily on mammalian neurons.
A second factor is body temperature. Mammals are endothermic, maintaining a consistent high body temperature, typically around 37°C. Sharks are ectothermic, or cold-blooded, with temperatures that fluctuate with the surrounding water. The Rabies Lyssavirus is sensitive to temperature; its replication is significantly reduced or halted at the lower temperatures characteristic of a fish host. The virus needs the stable, warm environment of the mammalian CNS to replicate effectively.
Known Diseases and Parasites of Sharks
While sharks are immune to the Rabies Lyssavirus, they are susceptible to a range of other pathogens and afflictions. They face threats from bacterial, fungal, and parasitic diseases. Bacterial infections are common, particularly in stressed or injured sharks, with agents like Vibrio, Pseudomonas, and Mycobacterium species causing issues such as lesions, fin rot, and internal organ damage.
Fungal infections, though less common, have been documented, especially in captive individuals. Specific fungi, such as Fusarium solani, can cause fatal systemic disease, often manifesting as white, cottony patches on the skin or gills.
Parasitic infestations are the most common health issue for sharks, which host a diversity of these organisms. Parasites can be internal or external:
- Internal parasites include various species of tapeworms (cestodes) and roundworms (nematodes) that inhabit the digestive tract and other organs.
- External parasites, such as copepods, frequently attach themselves to the gills or skin of sharks.
Some parasites, like intestinal tapeworms, have been observed to accumulate high concentrations of heavy metals.