Fish malaria (nurse shark blood parasitism)
“Fish malaria” is a term sometimes used to designate an infection of the blood by unicellular parasites close in their life cycle to the agents of malaria in mammals. In the nurse shark (Ginglymostoma cirratum), this type of blood parasitis...
What is it?
“Fish malaria” is a term sometimes used to designate an infection of the blood by unicellular parasites close in their life cycle to the agents of malaria in mammals. In the nurse shark (Ginglymostoma cirratum), this type of blood parasitism remains rare and poorly documented compared to bony fish. It affects the animal's red blood cells or other blood cells. This sheet remains general because scientific data specific to this species of shark is limited.
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:
The signs are often discreet and not very specific in sharks.\nWe may observe general fatigue, an animal less active than usual, slowed or abnormal swimming.\nAppetite may decrease.\nThe mucous membranes (internal areas of the mouth, gills) may appear paler than normal, a sign of possible anemia linked to the destruction of red blood cells by the parasite.\nIn many cases, the infection remains silent and is only discovered during an examination. routine blood, especially in aquariums.
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 protozoan parasite (single-celled organism) that infects the shark's blood cells. These parasites belong to groups similar to those responsible for malaria in mammals and birds, hence the colloquial name “fish malaria”. The parasite multiplies in the host's blood and may, depending on its cycle, require an intermediate host (such as a marine leech or other invertebrate) to transmit from one fish to another.
Nurse sharks living in natural environments where vectors (marine leeches, small parasitic crustaceans) are present seem more exposed.\nA state of stress, weakened immunity or cohabitation in a tank with carrier animals can promote the appearance of clinical signs.\nYoung individuals or otherwise weakened animals could be more sensitive to the consequences of anemia.
Transmission
Transmission would occur via a marine invertebrate vector, probably a leech or a parasitic copepod, which bites the animal and transmits the parasite by feeding on its blood. Direct contact between sharks is generally not enough to transmit this type of parasite.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The aquatic animal veterinarian takes a blood test to examine the blood cells under a microscope and look for the presence of the parasite inside the red blood cells or other cells.\nAdditional analyzes may be necessary to assess the degree of anemia and the general condition of the animal.\nPrecise identification of the species of parasite in question often requires the advice of a laboratory specializing in fish pathology.
Treatment
There is no well-established specific treatment for this type of blood parasitism in sharks.\nThe treatment aims above all to support the animal: improvement of breeding conditions, reduction of stress, supportive care.\nIn certain cases, families of antiparasitics used in fish medicine can be considered by the veterinarian, depending on the parasite identified and its cycle.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is mainly based on limiting potential vectors in ponds and aquariums (control of leeches and other external parasites).\nForty new animals before their introduction into a common pond makes it possible to identify a possible infection before it spreads.\nGood regular veterinary monitoring, with control blood tests, helps to detect the problem early.
Possible complications
A significant infection can lead to marked anemia, with a general decline in the animal's vitality.\nIn severe cases, anemia can weaken the shark in the face of other secondary diseases or infections.\nMortality directly linked to this parasite remains considered uncommon, but serious cases are not excluded, particularly in already weakened animals.
When to see a veterinarian
Abnormally slow swimming or unusual behavior of the animal.\nPersistent loss of appetite.\nPale color visible on the mucous membranes or gills.\nGeneral decrease in activity without obvious cause.\nAny unusual sign in a shark cohabiting in a tank with an animal already diagnosed.
Prognosis
The prognosis depends on the degree of anemia and the general condition of the animal at the time of diagnosis.\nIn the majority of mild cases, the infection remains discreet and compatible with a normal life of the animal.\nMore severe forms, with marked anemia, require close monitoring and can affect long-term vitality.
Frequently asked questions
Can “fish malaria” be transmitted to humans?
No, this parasite is specific to fish and their marine vectors. It does not infect humans.
How do you know if a nurse shark is infected?
The diagnosis is made only by a blood test and a microscopic examination carried out by a specialized veterinarian, because the visible signs are often absent or very discreet.
Is this disease common in nurse sharks?
It remains rare and poorly documented in this specific species; most data come from broader observations in other marine fishes.
Should an infected shark be isolated from others?
This depends on the suspected mode of transmission and veterinary advice, as direct transmission between sharks seems unlikely without an intermediary vector.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 25 September 2026
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