DNA Evidence for Chagas Disease in Ancient Human Remains - Essay Example

DNA evidence for Chagas disease in ancient human remains Introduction Examination of human remains can provide us with valuable information regarding the evolution of a disease process. Much more light can be shed on the epidemiology of an infectious disease and the present host- parasite relationship if the causative organism can be identified in these remains. The sources that are examined and checked for the presence of parasites and microorganisms are mummified tissue, hair, fibres, fossilised bone fragments, and fossilised faecal matter. The term 'Archeoparasitology' has been proposed by researchers as the study of parasites in context of archaeological excavations. More commonly, it is referred to as paleoparasitology (Ferreira et al., 2000). The discovery of the presumed parasite in the human remains is then extrapolated to the present relationship between the host and the parasite Transmission and manifestations of Chagas Disease Chagas disease or American trypanosomiasis is caused by Trypanosoma cruzi. The reservoir exists in wild amongst various animal species constituting the sylvatic cycle (Afuderheide et al., 2004). The vector for its transmission is reduviid bug (family Reduviidae, subfamily Triatominae). These insects hide in the crevices, nests or human dwellings in case of domestic cycle and emerge at night to feed upon the blood of their prey (Afuderheide et al., 2004). The infection is caused by rubbing of the bitten area where the parasite deposits the faecal matter (Afuderheide et al., 2004). This leads to entry of the parasite into the blood stream from the breached skin or conjunctiva. Parasitemia may lead to acute manifestations of myocarditis or meningoencephalitis that have a mortality rate of 10% (Afuderheide et al., 2004). The disease may progress to a chronic stage characterised by flare up of febrile episodes and progressive damage to myocardium or gut leading to dilated cardiomyopathy or segmental paralysis of parts of gastrointestinal system respectively (Afuderheide et al., 2004). Material and methods for study of DNA Researchers to study the archaeological remains for the evidence of Chagas disease have used many techniques. Grossly, mega colon changes caused by the disease have been identified in mummies from Chile. These findings were supplemented by discovery of dwellings made of mud bricks, which is a common habitat of these bugs. Molecular studies included immunochemical methods and electron microscopy. Histological sections have also demonstrated the parasite even in desiccated mice tissue (Bastos et al. 1996). However, the most vital tool that has enabled the authors to draw important inferences regarding the evolution and epidemiology of Chagas disease is the extraction and amplification of DNA segments of T. cruzi from mummified tissues. Origin of samples: Atacama Desert with its dry hot winds and arid climate has been a source of mummified tissues for many researchers (Ferreira et al., 2000; Afuderheide et al., 2004; Guhl et al., 2000). This type of climate favours the preservation of body tissues in a dehydrated form and nearly arrests its decomposition. Moreover, this geographical region coincides with the distribution of the disease, along coastal region of South America in Peru and Chile. South American natives buried their dead in shallow sandy soils that led to preservation of tissues in a desiccated mummified form Laboratory techniques: This desiccated tissue is rehydrated and pulverised and DNA is extracted. The extracted DNA is then amplified using the Polymerase chain reaction. The amplified DNA participates in the hybridization process with standardised primers and probes under controlled conditions. Hybridisation of the DNA extracted from the mummified tissue samples with the probe nucleotides constitutes a positive test result. Hybridisation can be identified by gel electrophoresis and analysis of bands by radioisotope techniques. Authentication and sources of error Concern has been raised about the authenticating the presence of a parasite in a tissue sample solely based on an extracted nucleotide sequence. DNA can only help if it, indeed, has been preserved with the original tissue (Rollo & Marota, 1999). It is also important that adequate positive and negative controls be established, otherwise contamination cannot be ruled out (Ferreira et al., 2000). The PCR and other techniques should take place under standardised conditions and environment. In addition, it is important to view the evidence from a wholesome perspective. The circumstantial evidence of that of association of the parasite with the host has to be considered. Discussion DNA analysis has without doubt provided ample evidence for demonstration of the parasite in mummified human remains responsible for the causation of Chagas disease. Much of the work has been done in the excavations done in the Atacama Desert. Based on these studies authors have tried to establish the origin of the disease in humans. One hypothesis proposes that it could have been introduced in humans by the domestication of animals from the wild, which were already acting as the reservoir of the disease. These infected the reduviid bugs living in human dwellings and thus the domestic cycle was initiated. Another hypothesis proposes that inhabitation of rocky dwellings infested with triatomines and infected rodents led to human infection (Araujo et al., 1998) It appears that disease was fairly well established among wild animals (sylvatic cycle) before the contact of the human host with the infectious agent. Thus, majority of evidence suggests that before human settlement, the disease was prevalent as sylvatic cycle and humans later on became involved in this cycle, which, in due course of time, generated a domestic cycle. The evolutionary pattern and migration pattern of the disease can help the investigators in understanding the present prevalence and distribution of the disease and the parasite-vector relationship and the host-parasite relationship. The modern epidemiological concept deals with any disease in the context of the triad formed by the host, the environment and the vector/parasite. Modification of these factors can perhaps help in the epidemiologic control of disease. Genetic studies can also highlight the effect of climatic changes in the genetic makeup of the parasite. It may possibly provide clues as to the emergence of new infectious diseases. Lastly, it is fascinating, exciting and humbling to discover a disease in our ancestors that was present more than 9000 years back and is continuing to plague us. References Araujo, A., Reinhard, K. Bastos, OM., et al. (1998). Paleoparasitology: Perspectives with new techniques. Rev. Inst. Med. Trop. S. Paulo, 40, 371-376 Aufderheide,F., A.C.Salo, W., Madden, M., Streitz, J., Buikstra, J. Guhl, F. , F. Et al. (2004). A 9,000-year record of Chagas' disease. Proc Natl Acad Sci USA, 101(7), 2034-2039. Bastos, O. M., Araujo, A., Ferreira, LF., Santoro, A., Winckler, P., Morel, CA., .(1996). Experimental paleoparasitology identification of Trypanosoma Cruzi in desiccated mouse tissue. Paleopath Newsletter (Detroit), 94, 5-8. Ferreira, L.F., Britto, C., Cardoso, M.A., Fernandes, O., Reinhard, K., & Araujo, A. (2000). Paleoparasitclogy of Chagas revealed by infected tissues from Chilean mummies. Acta Tropica, 75(1), 79-84. Guhl, F., Jaramillo, C., Vallejo, G.A., A-Arroyo, F.C., & Afuderheide, A (2000). Chagas disease and human migration, Mem Inst Oswaldo Cruz, 95(4), 553-555 Rollo, F., & Marota, I. (1999). How microbial ancient DNA, found in Association with human remains, can be interpreted, Dipartimento Di Molecolare, Cellulare e Animale, Universita di Camerino, 111-119. Read More