Fish malaria (thresher shark blood parasitism)
Fish malaria is a parasitic disease that affects the blood of certain marine fish, including the thresher shark (Alopias vulpinus). It is caused by parasites distantly related to those responsible for malaria in humans, but these parasites...
What is it?
Fish malaria is a parasitic disease that affects the blood of certain marine fish, including the thresher shark (Alopias vulpinus). It is caused by parasites distantly related to those responsible for malaria in humans, but these parasites are specific to fish and are not transmitted to humans or mammals. The parasite infects the shark's red blood cells, which can disrupt the transport of oxygen in its body. This disease remains poorly documented in pelagic sharks such as the thresher shark, with most observations coming from animals sampled at sea or found weakened.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Clinical signs in thresher sharks are difficult to observe in the natural environment and remain poorly described scientifically. \n\nWe can suspect an attack when the animal shows slowed or disorganized swimming, an apparent loss of appetite, or a general state of fatigue. \n\nWhen examining a captured or stranded animal, the mucous membranes (gums, gills) may appear paler than normal, a possible sign of anemia linked to the destruction of red blood cells by the parasite. \n\nIn many cases, the infection remains silent and causes no visible symptoms, with the animal continuing to live normally with a low to moderate parasite load.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by a unicellular blood parasite, belonging to a group of protozoa related to malaria agents but specific to the marine world. This parasite invades the shark's red blood cells and multiplies there, which can ultimately weaken or destroy these cells.
Transmission of this type of blood parasite in marine fish generally involves an intermediate host, such as certain marine invertebrates (marine leeches or copepods depending on the species of related parasites), although the exact mode of transmission in the thresher shark is not clearly established. \n\nAnimals weakened, stressed by capture, or living in areas of high density of potential intermediate hosts could present an increased risk. \n\nThe general condition of the animal and its immune system probably play a role in the intensity of the infection and whether symptoms appear.
Transmission
Exact transmission in thresher sharks is not fully understood, but is likely to involve a marine invertebrate intermediate host, as is documented for related blood parasites in other fish.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis is based on a microscopic examination of a blood smear, allowing the parasite to be visualized directly inside the red blood cells. \n\nThis examination requires a blood sample taken by a veterinarian specializing in aquatic or exotic animals, followed by a laboratory analysis. \n\nAdditional tests, such as a complete blood count, can help assess the degree of possible anemia and the general condition of the animal.
Treatment
There is no specific and validated antiparasitic treatment for this disease in thresher sharks. \n\nCare, when possible (animal in a closed structure), remains mainly supportive: improvement of environmental conditions, reduction of stress, monitoring of general condition. \n\nIn some documented cases in other fish species, broad-spectrum antiparasitic molecules have been considered by specialized veterinarians, but their effectiveness against this type of blood parasite is not clearly demonstrated.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There are no preventive measures applicable in the wild for this disease. \n\nFor structures keeping thresher sharks in tanks (public aquariums, research structures), good management of water quality, reduction of stress and regular veterinary monitoring make it possible to limit the risks of the appearance of parasitic diseases in general.
Possible complications
A significant infection can lead to marked anemia, with a reduction in the blood's ability to carry oxygen. \n\nThis can result in a general decline in the animal's vitality, a reduced ability to swim long distances, and potentially a greater vulnerability to other diseases or predators.
When to see a veterinarian
A thresher shark held in a closed structure shows abnormal swimming or persistent loss of appetite.
Abnormally pale mucous membranes are observed during routine handling or examination.
A captured or stranded animal shows signs of unexplained general weakness.
Prognosis
The prognosis strongly depends on the degree of infection and the general condition of the animal. \n\nA mild infection is often well tolerated and may go unnoticed. \n\nA significant infection with marked anemia can permanently weaken the animal, but precise data on the evolution of this disease in thresher sharks remains limited, as research on this pelagic species is difficult to carry out in the natural environment.
Frequently asked questions
Can this disease be transmitted to humans?
No. The causative parasite is specific to fish and is not transmissible to humans, despite the colloquial name "fish malaria" which refers to a resemblance to human malaria, not a transmission link.
How do you know if a thresher shark is infected?
The only reliable method is a blood test carried out by a veterinarian, with observation of the blood under a microscope. The signs visible to the naked eye, when they exist, remain not very specific.
Is this disease common in thresher sharks?
The data is too limited to give a precise frequency. As the thresher shark is a pelagic species that is difficult to study, few studies have looked specifically at this disease in this species.
Can an infected thresher shark recover on its own?
In a generally healthy animal in a natural environment, the immune system can often contain the infection without intervention. Complete and spontaneous recovery is possible in the event of a low parasitic load.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 24 September 2026
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