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Keywords:

  • biomedicine;
  • birds;
  • disease biology;
  • ecological genetics;
  • insects;
  • life history evolution

Abstract

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References

Many species of malaria and related haemosporidian parasites (Haemosporida) are responsible for diseases in wild and domestic animals. These pathogens are exclusively transmitted by blood-sucking dipteran insects (Diptera). Traditional vector studies, which are based mainly on experimental infection and subsequent dissection of insects, are time-consuming, so progress in the identification of the vectors has been slow. Since the discovery of haemosporidians in wildlife by V. Danilewsky in 1884, it took over 70 years to determine the main vector groups of these parasites. However, precise vector–parasite relationships remain insufficiently investigated in wildlife, particularly at the species level of haemosporidians and their vectors. Molecular tools have provided innovative opportunities to speed such research. In this issue of Molecular Ecology, Martínez-de la Puente et al. (2011) collected, for the first time, a significant PCR-based set of data on the presence of lineages of the pigment-forming haemosporidians (species of Haemoproteus and Plasmodium) in biting midges (Culicoides). They identified numerous associations between Culicoides spp. and Haemoproteus spp., indicating directions for future targeting vector studies of haemoproteids.

Pigment-forming haemosporidian parasites (Haemosporida) of the genera Haemoproteus and Plasmodium are widespread in birds on all continents except Antarctica. These pathogens are transmitted exclusively by blood-sucking dipteran insects and are responsible for diseases, sometimes deadly, both in domestic and wild birds (Garnham 1966; Valkiūnas 2005). Recent findings indicate that some common species and lineages of Haemoproteus spp. are probably relatively benign in naturally adapted hosts, but might cause lethal disease in non-adapted birds. This has been demonstrated in zoos and private collections in America (Ferrell et al. 2007) and Europe (Olias et al. 2011) using both histopathology and PCR-based methods. However, the true extent of pathology and mortality caused by Haemoproteus parasites remains unclear because severe haemoproteosis and death of infected birds have been reported mainly during the tissue stage of parasite development, before the development of gametocytes and appearance of parasitaemia. Such abortive infections are difficult to diagnose using blood samples either by microscopy or PCR-based tools because parasites are absent in the blood. Morphologically, the tissue stages of Haemoproteus spp. in dead hosts resemble the megalomeronts of Leucocytozoon spp. That is why mortality in birds caused by unusual pathology in internal organs and muscles was formerly described as an ‘aberrant Leucocytozoon infection’ (Walker & Garnham 1972). Recent PCR-based findings (Ferrell et al. 2007; Olias et al. 2011) supplement former histopathology studies (Atkinson et al. 1988; Earléet al. 1993) and indicate that species of Haemoproteus are responsible for some instances of mortality in birds. Therefore, traditional opinion about the harmlessness and insignificant veterinary importance of avian haemoproteids requires partial reconsideration. Certainly, more attention should be given to vector studies and transmission of haemosporidian parasites in wildlife.

The majority of avian Haemoproteus species are transmitted by biting midges of the genus Culicoides; exceptionally, hippoboscid flies (Hippoboscidae) act as vectors of these parasites (Garnham 1966; Valkiūnas 2005). However, precisely which Culicoides species transmit which haemoproteids remains unknown for the majority of avian Haemoproteus species and their lineages. For the first time, Martínez-de la Puente et al. (2011) used molecular methods and determined numerous associations between Culicoides spp. and avian Haemoproteus spp., which indicate possible parasite–vector specificity. These findings suggest a possibly exciting field of research in co-evolution, particularly because of new data on possible vector capacity of avian Haemoproteus spp. (usually specialist parasites) and Plasmodium spp. (often generalist parasites). Their original hypothesis that Plasmodium spp. are more likely to establish associations with novel blood-sucking dipterans than Haemoproteus spp. has evolutionary consequences for the transmission of diseases, so warrants experimental investigation. That might contribute to the better understanding of evolution of parasite–vector associations in malaria parasites and related haemosporidians. This information also is important for future epidemiological studies and better understanding of avian disease transmission, particularly in the light of recent discoveries on the pathogenicity of Haemoproteus spp. in avian hosts (Ferrell et al. 2007; Olias et al. 2011). However, one point about this vector study deserves additional discussion.

The report on Haemoproteus and Plasmodium lineages in biting midges is based exclusively on molecular analysis. Vector interpretations of this and similar studies need validation because PCR will amplify haemosporidian DNA regardless of the life stages present in these biting insects; for example, it cannot distinguish between gamete (Fig. 1a), ookinete (Fig. 1b), oocyst (Fig. 1c) and sporozoite (Fig. 1d) stages. The sporogonic development of haemosporidian parasites is frequently abortive in resistant or partly resistant dipteran insects, with, for example, no development of sporozoites. Additionally, in resistant insects, sporogonic stages of haemosporidians might persist at the ookinete and/or oocyst stages for several days before degenerating; asynchronous haemosporidian sporogony contributes to that (Garnham 1966; Valkiūnas 2005). Hence, PCR detection of Haemoproteus and Plasmodium haplotypes in biting midges does not necessarily mean that these haplotypes originated from parasites that are capable of completing sporogony and producing viable sporozoites, the essential stage for haemosporidian transmission; Martínez-de la Puente et al. (2011) briefly acknowledged that. Abortive development of haemosporidian parasites in vectors is well documented and might happen at each stage of sporogony (Fig. 1) in resistant insects (Garnham 1966). It is worth mentioning that even records of haemosporidian DNA in the thorax of a dipteran insect, the site of location of salivary glands and infective sporozoites, cannot be a guarantee of completion of sporogony, as was experimentally documented in Plasmodium gallinaceum in partly resistant mosquitoes (Kim et al. 2009).

image

Figure 1.  Vector stages of avian (a, b, d) Haemoproteus sp. and (c) Plasmodium sp.: (a) microgamete (left) and macrogamete (right) before fertilization, (b) mature ookinete, (c) developing oocyst, (d) sporozoite. Arrowheads indicate parasites. (a, b, d) Giemsa- and (c) mercurochrome-stained preparations. All bars = 10 μm. All images are by G. Valkiūnas.

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During abortive sporogonic development, dipterans with parasites would be scored as infected by PCR when, in fact, they are not competent vectors of the parasites. It is likely that the report of lineages of Plasmodium spp. in biting midges, which are not known to be competent vectors of avian malaria, are such cases (Martínez-de la Puente et al. 2011). Detection of sporozoites in biting midges that fed on infected hosts is essential for the definitive demonstration that these blood-sucking insects could act as vectors.

Martínez-de la Puente et al. (2011) determined numerous significant links between biting midge haplotypes and Haemoproteus spp. haplotypes; these links could indicate true vector–parasite relationships. If proven by demonstration of sporozoites in the biting midges involved in these significant links, the methodology used by Martínez-de la Puente et al. (2011) for determining such links might be applied in PCR-based vector studies of haemosporidian parasites in the future. That would accelerate haemosporidian vector studies and is particularly important owing to the urgent need to determine the vectors of emerging lethal avian haemoproteosis (Ferrell et al. 2007; Olias et al. 2011).

It is difficult to imagine future epidemiological and ecological research of haemosporidian parasites without application of innovative molecular techniques. Study by Martínez-de la Puente et al. (2011) is an important first step in vector molecular ecology of Haemoproteus spp. However, because of the complicated life cycles of haemosporidians, it should be pointed out that a combination of advanced molecular and microscopical approaches is an essential urgent task in this field of biology. Such a marriage of molecular and microscopic approaches is already an established practice in studies of haemosporidians in their vertebrate hosts (Bensch et al. 2009; Perkins & Austin 2009; Križanauskienėet al. 2010), but has still rarely been applied in the vector research of these parasites in wildlife. An important issue of haemosporidian vector research is the need for increased collaboration between researchers from different fields with a common interest.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References

The author acknowledges the support of the Global Grant (VPI-3.1.-ŠMM-07-K-01-047).

References

  1. Top of page
  2. Abstract
  3. Acknowledgements
  4. References
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G.V. has been studying ecology, taxonomy, evolutionary biology and pathology of malaria and related haemosporidian parasites in wildlife for over 25 years; the author and coauthor of 3 books and over 80 articles. Main results of his research are generalized in the monograph ‘Avian Malaria Parasites and Other Haemosporidia’ (2005). His recent studies aim to bring traditional parasitology knowledge and molecular biology innovations in research of wildlife malaria parasites and related haemosporidians into a profitable wedding.