Avian malaria
Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera Plasmodium and Hemoproteus. The disease is transmitted by a dipteran vector including mosquitoes in the case of Plasmodium parasites and biting midges for Hemoproteus. The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.
Cause
Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in all parts of the world apart from Antarctica. Captive penguins in non-native environments are exposed to the protozoa without having coevolved with them and are especially sensitive to infection. The most common presentation of the disease in affected penguins is acute death. Infection of wild penguins is reported and a greater understanding of the significance of such infections is required. There are several other species of Plasmodium that infect birds, such as Plasmodium anasum and Plasmodium gallinaceum, but these are of less importance except, in occasional cases, for the poultry industry. The disease is found worldwide, with important exceptions. Usually, it does not kill birds. However, in areas where avian malaria is newly introduced, such as the islands of Hawaiʻi, it can be devastating to birds that have lost evolutionary resistance over time, like the Mohoidae family.Parasite species
Avian malaria is a vector-transmitted disease caused by protozoa in the genera Plasmodium and Haemoproteus; these parasites reproduce asexually within bird hosts and both asexually and sexually within their insect vectors, which include mosquitoes, biting midges, and louse flies. The blood-parasites of the genus Plasmodium and Haemoproteus, encompass an extremely diverse group of pathogens with global distribution. The large number of parasite lineages along with their wide range of potential host species and the pathogen's capacity for host switching makes the study of this system extremely complex. Evolutionary relationships between hosts and the parasites have only added complexity and suggested extensive sampling is needed to elucidate how global cospeciation events drive disease transmission and maintenance in various ecosystems. In addition to this, the parasite's ability to disperse can be mediated by migratory birds and thus increases variation in prevalence patterns and alters host-parasite adaptation processes. Host susceptibility is highly variable as well and numerous efforts have been made to understand the relationship between increased prevalence and host traits such as nesting and foraging height, sexual dimorphism or even incubation time length. So far, the effects of this disease in wild populations is poorly understood. A 2015 study using blood samples from Malawian bird fauna found that close to 80% of were infected with either malaria or closely related alveolates. Closed-cup nesters, such as weavers and Cisticola, were more likely to be infected with Plasmodium than with midge-borne parasites such as Haemoproteus and Leucocytozoon.There exists much controversy on what corresponds as a species in avian malaria parasites. The Latin binomials nomenclature used to describe Plasmodium and Hemoproteus parasites is based on a restricted set of morphological characteristics and the restriction to which parasites of birds they are able to infect. Therefore, considering co-speciation events or even species diversity for malaria parasites is surrounded by much disagreement. Molecular tools have directed classification towards a phylogenetic definition of lineages, based on sequence divergence and the range of hosts in which the parasite can be found. The diversity of avian malaria parasites and other haemosporidia is extremely large, and previous studies have found that the number of parasites approximates the number of hosts, with significant host switching events and parasite sharing. The current approach suggests amplification of the cytochrome b gene of the parasite and the reconstruction of genealogies based on this information. Due to the large number of lineages and different host species, a public database called MalAvi has been created to encourage sharing these sequences and aid in understanding the diversity of these parasites. Considering that no other genetic markers have been developed for this group of parasites, a ~1.2–4% sequence divergence has been determined as a cutoff value to distinguish between different parasite lineages. The molecular approach has also allowed direct comparisons between host phylogenies and parasite genealogies, and significant co-speciation has been found based on event-based-matching of phylogenetic trees.
Phylogeny of malaria parasites
There is no specific phylogeny for avian malaria parasites and related haemosporidian parasites. However, given that malaria parasites can be found in reptiles, birds and mammals, it is possible to combine the data from these groups and a well resolved large phylogeny is available. For over a century, parasitologists classified malaria parasites based on morphological and life-history traits and new molecular data shows that these have variable phylogenetic signals. The current approach suggests that Plasmodium species infecting birds and squamate reptiles belong to one clade, and mammalian lineages belonging to a separate clade. In the case of Haemoproteus, this group has traditionally been classified based on the vector host, with one clade being transmitted to columbiform birds by hippoboscid flies and a second group transmitted by biting midges to other avian families. The molecular data supports this approach and suggests reclassifying the later group as ''Parahaemoproteous.''Phylogeography of avian malaria
Although a widespread disease, the culprit most commonly associated with the disease is Plasmodium relictum and associated lineages. To better understand the parasite's epidemiology and geographical distribution, analysis of genetic variation across large geographical scales have been conducted by looking at the nuclear gene MSP1 from Plasmodium relictum ''.'' Findings have revealed that there are significant differences between lineages from the New and Old World, suggesting different introductions of the parasite to avian populations. In addition to this, considerable variation was found between Europe and African lineages, suggesting different patterns of transmission for temperate and tropical populations. Although this approach is relatively recent, detecting allelic variation in different markers is essential to unveil parasite transmission patterns and the likelihood of introduction to new susceptible host populations.Vector
Contrary to the state of knowledge on parasite-avian interactions, parasite-vector relationships are relatively less explored. MalAvi does list several known vectors however as of 2015 this is not at all complete. Generally avian malaria organisms are vectored by Culex.Its vector in Hawaiʻi is the mosquito Culex quinquefasciatus, which was introduced to the Hawaiian Islands in 1826. Since then, avian malaria and avipoxvirus together have devastated the native bird population, resulting in many extinctions. Hawaiʻi has more extinct birds than anywhere else in the world; just since the 1980s, ten unique birds have disappeared.
Virtually every individual of susceptible endemic species below in elevation has been eliminated by the disease. These mosquitoes are limited to lower elevations, below, by cold temperatures that prevent larval development. However, they appear to be slowly gaining a foothold at higher elevations and their range may be expanding upwards. If so, most remaining Hawaiian land birds may become at risk to extinction.
Most of the Hawaiian Islands have a maximum elevation of less than, so with the exception of the island of Hawaiʻi and East Maui, native birds may become extinct on every other island if the mosquito is able to occupy higher elevations.
Research on avian malaria
Ronald Ross was born in Almora, India in 1857. Although he had no predisposition to medicine, at the age of 17 he submitted to his father's wish to see him enter the Indian Medical Service. He began his medical studies at St. Bartholomew's Hospital Medical College, London in 1874 and sat the examinations for the Royal College of Surgeons of England in 1879. He took the post of ship surgeon on a transatlantic steamship while studying for, and gaining the Licentiate of the Society of Apothecaries, which allowed him to enter the Indian Medical Service in 1881, where he held temporary appointments in Madras, Burma, and the Andaman Islands. In 1892 he became interested in malaria and, having originally doubted the parasites' existence, became an enthusiastic convert to the belief that malaria parasites were in the blood stream when this was demonstrated to him by Patrick Manson during a period of home leave in 1894.In 1895, Ross embarked on a quest to prove the hypothesis of Alphonse Laveran and Manson that mosquitoes were intricately linked to the propagation of malaria. On 20 August 1897, Ross made his landmark discovery in Secunderabad. While dissecting the stomach tissue of an anopheline mosquito that had fed on a patient with malaria four days earlier, he found the malaria parasite, thus conclusively proving the role of Anopheles mosquitoes in the transmission of malaria parasites in humans. He continued his research into malaria in India, using a more convenient experimental model—malaria in birds. In 1898, he had demonstrated that mosquitoes could serve as intermediate hosts for bird malaria. After feeding mosquitoes on infected birds, he observed that malaria parasites could develop in the mosquitoes and migrate to the insects' salivary glands, enabling the mosquitoes to infect other birds during subsequent blood meals. In 1902, Ross was awarded the Nobel Prize in Medicine for his discovery of the mosquito transmission of malaria.