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Hitchhiking Parasites: A Cautionary Tale

  • Blyth's Reed Warbler
    Blyth's Reed Warbler

Photo: Farah Ishtiaq / CES, IISc

Many birds undertake long journeys to warmer locations in winters. The Blyth’s Reed Warbler, a tiny brown bird that could easily fit into the palm of your hand, makes a surprisingly long migratory journey given its size. It is one of the few warblers that winter in India from the Palaearctic ecozone; a huge geographical area that expands from northern Africa, to Europe and to the Himalayan foothills.

During this long migration journey, the Blyth’s Reed Warbler stops for a few weeks in northern India, as a ‘passage migrant’ and then flies on towards southern India for the winter. It seems to be utterly innocuous. However, it isn’t.

Hitchhiking in the warblers’ blood are tiny parasites, mostly  protozoa – the same type of organisms that cause malaria. In fact, one of them causes avian malaria: Plasmodium. The other is an organism particularly virulent to pigeons called Haemoproteus but have a broad host range. If you thought Plasmodium sounded familiar, you are right – parasites of the same genus cause malaria in humans, too.

Birds of all species can suffer from ‘avian’ malaria but thankfully; it is not caused by the same species that affects humans. While this is indeed good news for people, it’s rather detrimental for birds. Both Plasmodium and Haemoproteus parasites need specific ‘vectors’ for their transmission between birds, which are usually mosquitoes for Plasmodium and biting midges for Haemoproteus transmission. When the flies bite an infected bird, the parasites get sucked up with the blood and when the flies bite another bird, they transmit the parasite, causing disease through their saliva. Disease transmission can occur be both ways – migrant to local species, or the other way around.

Traditionally, malaria parasites have been examined through blood smear for the presence of infected red blood cells. In recent times, molecular techniques have revolutionised the field of disease ecology. A “molecular based” study typically involves examining samples at the molecular level. This was the first molecular based study in the Indian subcontinent on the migrant warblers that explores the extent of blood parasite sharing between migrants and those resident in India.

In a novel study on avian disease ecology funded by Wellcome Trust/DBT India Alliance and led by Dr. Farah Ishtiaq of the Indian Institute of Science, Bangalore, blood samples from 156 warblers were screened using traditional microscopy as well as molecular techniques. 64 of these samples were positive for either parasite. Using both methods makes the analysis more robust, and also allows researchers to gauge if the parasites are in the active infection stage or not. However, none of the samples showed an infective stage under the microscope, despite testing positive in the molecular analysis. This helped confirm that there is no active transmission of parasites between migrants and resident birds, or vice versa.

A further complication is that these parasites are not the same as the local parasites to which our native birds have spent centuries getting adapted to, making infections acquired from the warblers and other migrants more devastating than can be usually expected.

“Understanding how parasites disperse between hosts populations is important to predict risk of emerging infectious diseases”, states Dr Ishtiaq. “However, we lack information on the transmission or exchange in parasites through migrant avian hosts in India”, she adds. This study revealed that although the parasites were found in the Blyth’s Reed Warblers’ blood, they were not in their infective stage and submicroscopic stage which can easily be detected via PCR but morphologically absent. This means that the parasite, while present, is not capable of attacking other birds because it is still immature or ‘spill over’ infections from migrant birds. Vector-borne parasites offer a complex system where presence of competent host and an appropriate vector is important and optimal temperature conditions are important for successful parasite transmission.

While this may seem like an anti-climax, it is an example of what should be done in the current scenario of global disease – ‘global’ as parasites are expanding their range in the face of climate change. While birds have been flying all over the world since long before the Wright brothers were born, it exactly mirrors what humans are doing nowadays.

In another study conducted by Dr Ishtiaq and a team from Sweden, this time in 394 birds of several species in both India and Sweden, screening for Plasmodium and Haemoproteus along with another parasite: Leucocytozoon which is also a vector-borne parasite and infects a wide range of bird species. This study, unlike the previous one, used 3 different molecular tools to diagnose and detect the parasites in bird samples of known parasite intensity. All the tools were variants of PCR (Polymerase Chain Reaction) technique, in which genetic material is amplified and decoded. It was found that a quantitative PCR was the best of the 3 variants of this famous tool, and this study stressed the necessity of sensitive diagnostic tests to avoid under estimation of parasite prevalence which is a crucial parameter to understand disease ecology and epidemiology.

The ‘quantitative’ PCR technique allows researchers to analyse both presence and quantity of the gene in question; the usual method only allows researchers to amplify DNA without giving information about the quantity being amplified. Using a combination of diagnostic techniques, these studies highlights the importance of accounting for methodological issues to better estimate infection in parasitological studies understanding the spread of disease from migrant to resident birds as well as confirmed that not all migrant species are involved in host and geographical shifts in transmission of vector-mediated parasites which require a definite host (e.g. insect vector) for successful transmission

The principles of spread of diseases to new populations mentioned in this study are true for human diseases as well. This has led to popularisation of the term ‘emerging infectious disease’, a term which has also been applied to diseases like bird flu and Ebola. With new diseases being constantly discovered and old foes making comebacks in new locations, it is prudent to think of the Blyth’s Reed Warbler. The ease with which they could have spread disease to new fresh hosts is a cautionary tale, well worth remembering.