What are blood parasites?

A pernicious cellmate

Haemosporidian blood parasites are protists that spend a portion of their life cycle inside the red blood cells of their vertebrate hosts. Malaria—one of the most important diseases of human populations for millennia—is caused by haemosporidians of the genus Plasmodium. In domestic and wild birds, avian malaria and infections by the related parasites Haemoproteus and Leucocytozoon can cause debilitating illness and death. Consequently, haemosporidian infections can have important economic and conservation consequences.

 

Male (a) and female (b) Haemoproteus columbae gametocytes infecting mature red blood cells from a dove. Unlike those from humans and other mammals, avian red blood cells contain nuclei, which are visible here as the dark purple features. In healthy, uninfected cells, the nuclei hang out close to the center. As blood parasites grow inside the cell, they displace the nucleus, driving it against the cell membrane. (Photo by PlasmodiumLady licensed under CC BY-SA 4.0)

Like human malaria, avian haemosporidians are transmitted between hosts by blood-sucking ("haematophagous") insect vectors including mosquitoes, biting midges, and blackflies. During the early (pre-patent) stage of infection, which can last 1–3 weeks, the parasite multiplies inside host cells in the lungs, liver, spleen, and other organs. At this time, no outward signs of disease are evident, and parasites cannot be detected in the blood. Acute infection begins when the parasites cause host cells to rupture, flooding into the bloodstream. Parasites in this wave may infect additional cells in the fixed tissues (initiating another round of growth and multiplication), or may penetrate the red blood cells, where they become infective to insects taking a blood meal. When the number of parasites in the bloodstream reaches a peak, the infection is said to have reached the crisis stage. Eventually, parasitaemia (the presence of parasites circulating in the bloodstream) subsides. Typically, birds infected with blood parasites never fully recover, though disease may not be readily apparent and a certain degree of immunity against closely-related parasite strains is acquired. Instead, many hosts develop chronic, low levels of parasitaemia or latent infections in the liver or spleen. These individuals experience seasonal relapses of acute parasitaemia associated with the onset of breeding or migration.

 

Purple-backed Fairywren (Malurus assimilis) nestlings, 7 days after hatching, get a quick check-up before returning to the nest at Brookfield Conservation Park (my field site) in South Australia. Young birds in their first few months after hatching are among those most susceptible to infection by avian blood parasites, and most at-risk when infection occurs. (Photo by the author)

Symptoms

Symptoms generally occur only during the acute and chronic stages of infection, when destruction of the red blood cells causes anaemia, leading to weakness, weight loss, and a decrease in activity. Birds experiencing these symptoms may be at greater risk of predation and death due to other causes.

Although blood parasites—especially Plasmodium and Leucocytozoon—are associated with significant mortality and loss of productivity in poultry and other domestic birds, the impacts on wild bird populations are less apparent. In part, this is likely due to the infrequency with which acutely-infected birds are encountered among free-living wild populations. Critically ill birds that are inactive are less likely than healthy, active birds to be captured by traditional means, and in general the acute stage of the illness is not protracted.

The wild birds most at risk of infection by blood parasites are young individuals in their first year of life, birds experiencing other physiological stresses, and immunologically-naïve populations exposed to novel parasites. The latter scenario is exemplified by the case of honeycreepers endemic to the Hawai'ian archipelago. After the introduction of non-native birds to Hawai'i sometime around the start of the 20th Century, honeycreepers and other forest birds experienced sharp declines, particularly at low elevations where mosquitoes are abundant.

 

The vibrant 'I'iwi ( Drepanis coccinea ) is one of several native Hawai'ian forest birds at risk due to the introduction of avian malaria to the islands in the early 1900s. Its long, decurved bill allows it to extract nectar from flowers of the 'ōhi'a lehua tree and the Hawaiian lobeloids. (Photo by Bettina Arrigoni licensed under CC BY 2.0)

Experiments later showed that members of this group are particularly susceptible to infection by avian malaria, and are especially vulnerable to its effects. Many species have found refuge at high elevations, where cooler temperatures limit the abundance of mosquitoes. But as habitat and climate cause shifts in the distributions of vectors and hosts alike, species invasions can lead to the exposure of additional populations to novel threats from blood parasites.

Further reading

• Carter Atkinson, a researcher with the United States Geological Survey who has spent many years studying the impacts of avian malaria on Hawai'ian birds, prepared a short chapter on haemosporidioses of birds for the USGS' Field Manual of Wildlife Diseases. Written for wildlife managers and veterinary health practitioners, the chapter outlines the generalized parasite life cycle in addition to providing information for detecting and managing infections in wild bird populations.

•  How malaria hurts birds: a succinct but excellent piece describing the insidious and lasting effects of chronic Plasmodium infections, by Elizabeth Newbern at Audubon