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Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera Plasmodium and Hemoproteus (phylum Apicomplexa, class Haemosporidia, family Plasmoiidae). 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.

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 (Culicidae), biting midges (Ceratopogonidae), and louse flies (Hippoboscidae) .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.

There exists a lot of 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 a lot of 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 amount 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 even-based-matching of phylogenetic trees.

Phylogeny of malaria parasites
To date, 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 most commonly associated culprit to the disease itself 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 (merozoite surface protein) 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.