Avian malaria is a disease caused by protozoan parasites of the genus Plasmodium that infect birds. Just like human malaria, avian malaria is transmitted by mosquitoes. When an infected mosquito bites a bird, it injects Plasmodium parasites into the bird’s bloodstream where they infect red blood cells and undergo asexual replication. Avian malaria causes pathogenic effects like anemia, weight loss, enlarged spleen, and even death in severe cases. Understanding the pathology or the functional changes associated with avian malaria is key to diagnosing and treating this disease.
What causes avian malaria?
Avian malaria is caused by protozoan parasites of the genus Plasmodium. There are over 50 species of Plasmodium that can infect birds. The most common ones are:
- Plasmodium relictum
- Plasmodium elongatum
- Plasmodium juxtanucleare
- Plasmodium ashfordi
- Plasmodium durae
- Plasmodium gallinaceum
These protozoan parasites have a complex life cycle involving a vertebrate host (birds) and an invertebrate vector (mosquitoes). In the bird host, the parasites infect red blood cells, multiply asexually and cause disease symptoms. In the mosquito vector, sexual reproduction of parasites occurs resulting in development of infective sporozoites that are injected into a new bird host during a blood meal, thereby continuing the transmission cycle.
Life cycle of avian Plasmodium parasites
The life cycle of avian malaria parasites involves stages in the bird host and mosquito vector. It includes the following steps:
- Infective sporozoites are injected into the bird’s bloodstream by a mosquito bite.
- The sporozoites enter liver cells and undergo asexual multiplication into thousands of merozoites.
- Merozoites are released from the liver into the bloodstream where they invade red blood cells.
- Within red blood cells, the parasites undergo asexual replication into ring trophozoites, schizonts and then merozoites which burst out to infect new red blood cells.
- Some merozoites develop into male and female gametocytes which are taken up by a mosquito during a blood meal.
- Within the mosquito gut, the gametocytes undergo sexual reproduction forming zygotes.
- The zygotes develop into motile ookinetes that invade the mosquito’s gut wall and form oocysts.
- Thousands of sporozoites form within each oocyst.
- The oocysts rupture releasing sporozoites that migrate to the mosquito’s salivary glands.
- The cycle repeats when the infected mosquito bites another bird and transmits the sporozoites.
This complex life cycle allows the parasite to multiply in both bird and mosquito hosts leading to transmission of avian malaria.
How is avian malaria transmitted?
Avian malaria is transmitted from infected birds to healthy birds only by mosquitoes. When a female Anopheles mosquito (usual vector) bites an infected bird, it ingests gametocytes along with the blood meal. These undergo development in the mosquito into sporozoites which accumulate in the salivary glands.
During subsequent blood meals, the infected mosquito introduces sporozoites into the healthy bird completing the transmission cycle. The sporozoites travel to the liver to start the next asexual multiplication phase. Birds cannot directly transmit malaria to other birds without a mosquito vector.
Pathogenesis of avian malaria
The pathogenesis or mechanism by which avian malaria causes disease involves:
Tissue damage due to parasite replication
The cyclical asexual replication of Plasmodium parasites within red blood cells results in their rupture and release of parasites. This destroys the red blood cells leading to anemia. Replication in other tissues like liver, spleen, lungs, kidneys and brain can damage these organs.
Host immune response
The bird’s immune system is activated in response to the parasite. This triggers inflammatory reactions and release of cytokines. An overactive immune response can sometimes cause pathology through exacerbated inflammation and tissue damage.
Sequestration of infected red blood cells
Infected red blood cells tend to adhere to blood vessel endothelium. This sequestration impedes blood flow in organs like the brain leading to hypoxia and organ dysfunction.
Metabolic disturbances
Parasite replication requires glucose and amino acids which are derived from the host. This increases the metabolic demand on the bird leading to increased protein catabolism and loss of weight.
Blockage of blood vessels
The swollen infected red blood cells have reduced deformability. They can block narrow blood capillaries causing ischemia and infarction in organs like the brain and kidney.
Pathology in acute avian malaria
The pathological changes seen in acute stages of avian malaria infection include:
Enlarged spleen and liver
Hyperplasia and swelling of the spleen and liver occurs due to increased demand to remove damaged red blood cells and parasite debris.
Pale organs
Anemia due to red blood cell destruction causes pale discoloration of organs like the spleen, liver, kidneys and brain.
Mottling of the liver and kidney
Ischemic necrosis of hepatocytes and renal tubules occurs in a spotty distribution causing dark red mottling against a pale organ background.
Brain capillary blockage
The rigid infected red blood cells block brain capillaries leading to neuronal ischemia, micro-infarcts and hemorrhages.
Pulmonary edema and heart enlargement
Impaired cardiac function and increased pulmonary pressure due to severe anemia results in lung edema and right ventricular dilation.
Pathology in chronic avian malaria
Some birds can survive the acute infection and develop a chronic form of malaria showing:
Persistent anemia
The ongoing destruction of red blood cells leads to a non-regenerative anemia.
Atrophy of organs
Long-standing parasite infection causes wasting of body fat stores and muscular atrophy. Organs like the spleen and liver show atrophic changes.
Fibrosis of organs
Recurrent inflammation and cell necrosis leads to collagen deposition and fibrotic scarring in organs like the liver, spleen, kidneys and lungs.
Secondary infections
The immunosuppressive effects of chronic malaria predisposes the birds to secondary bacterial, viral and fungal infections. These further compromise organ function.
Stunted growth
Chronic malaria in fledglings and juvenile birds can impair their growth leading to low body weight.
Diagnosis of avian malaria
Avian malaria is diagnosed by:
Microscopic examination of blood smears
Thin blood smears stained with Giemsa or Wright stain are examined under the microscope for presence of parasites within red blood cells. The parasitic forms like trophozoites, schizonts and gametocytes can be visualized.
Polymerase chain reaction (PCR)
PCR to detect Plasmodium DNA provides sensitive and specific diagnosis. The primers target conserved sequences like the cytochrome B gene of the parasite mitochondrial genome.
Immunological tests
Antibody detection tests like ELISA or indirect fluorescent antibody test (IFAT) indicate exposure to the parasite. However, they cannot distinguish between past and current infections.
Complete blood count (CBC)
CBC reveals anemia, thrombocytopenia and leucocytosis providing supportive evidence for malaria. But confirmation requires parasite visualization.
How is avian malaria treated?
Avian malaria is treated using anti-parasitic drugs like:
- Chloroquine
- Mefloquine
- Primaquine
- Pyrimethamine
- Quinine
- Artemisinin
Along with specific anti-malarial therapy, supportive treatment for anemia and organ dysfunction is provided. Preventive measures involve mosquito control and keeping birds in insect-proof aviaries. A partially effective vaccine (Serum sporozoite vaccine) is also available for high value captive birds.
Conclusions
In summary, avian malaria pathology involves:
- Asexual replication of Plasmodium parasites in tissues like liver, spleen and red blood cells
- Destruction of red blood cells leading to anemia
- Inflammatory damage mediated by the host immune response
- Ischemic necrosis due to sequestration of infected red blood cells
- Metabolic disturbances causing wasting and weight loss
Diagnosis is by microscopic demonstration of parasites and molecular PCR. Treatment involves anti-malarial drugs and supportive therapy. Preventive measures aim at interrupting the parasite transmission cycle involving the mosquito vector.