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The Discovery of Giardia1 Brian J MICROSCOPE Vol 53:4 (2005) The Discovery of Giardia1 Brian J. Ford Gonville & Caius College, University of Cambridge, UK.* KEYWORDS pared to about 5% in the developed world (3) Between 100,000 to 2.5 million Giardia infections occur annually Flagella, parasite, Leeuwenhoek, Giardia, in the United States (4). nitroimidazoles, Dobell The name of this organism has become increas- ingly familiar in recent years, yet it was first observed INTRODUCTION as long ago as 1681. Its discoverer was Antony van Leeuwenhoek, the pioneering microscopist of Delft, The pear-shaped flagellate Giardia duodenalis is a Netherlands. As Clifford Dobell (5) clearly documented, cosmopolitan protozoan parasite of humans. It has a Leeuwenhoek’s verbal description was accurate and global distribution, and indeed is ranked by Farthing unambiguous. In his watery excrement van in the top ten of human parasitic diseases (1). Infection Leeuwenhoek discovered ‘… small animalcules a- with G. duodenalis is normally self-limiting. Human gia- moving very prettily; some of ‘em a bit bigger, others a rdiasis can be divided into two disease phases: acute bit less, than a blood globule, but all of one and the and chronic. The acute phase is usually short-lived, same make, …’ which Dobell in 1932 concluded were characterised by flatulence with sometimes sulphur- the vegetative (trophozoite) stage of the infection. But ous belching and abdominal distension with cramps. a question remained: how could Leeuwenhoek have Diarrhoea is initially frequent and watery but later seen such a diminutive microorganism with the single- becomes bulky, sometimes frothy, greasy and offen- lensed microscopes at his disposal? sive. The gas may cause the stools to float. It was recently proposed to me that Leeuwenhoek’s In chronic giardiasis, malaise, weight loss and observations might be reprised, and this presentation other features of malabsorption become prominent and reveals how van Leeuwenhoek could have observed stools are usually pale or yellow, frequent and of small G. duodenalis some three and a quarter centuries ago. volume. Malabsorption of vitamins and D-xylose can occur, while disaccharidase deficiencies (most com- BACKGROUND monly lactase) are frequently detected in chronic cases. In young children, ‘failure to thrive’ is frequently due Although Leeuwenhoek discovered Giardia in 1681, to giardiasis, and all infants being investigated for its pathogenicity in humans was not formally estab- causes of malabsorption should have a diagnosis of lished and Koch’s postulates fulfilled until less than 20 giardiasis excluded (2). years ago (6). Currently, the progress of research into The parasite exists in two distinct morphological Giardia is marked by regular symposia that bring to- forms. The reproductive phase is a trophozoite which gether research on this organism with that on parasitizes the cells lining the upper small intestine. Cryptosporidium (an organism which I discussed at Inter There is also a resistant, resting phase, an environ- Micro seven years ago) (7). As keynote speaker at the mentally-resistant cyst which is voided in the faeces. International Conference on Giardia and Cryptosporidium This is the infective and disseminating stage. Giardia is in September 2004, I discussed Leeuwenhoek’s revela- the most commonly detected intestinal protozoan tion of Giardia (8). Although the literature on his dis- parasite in the world, and the prevalence of giardiasis covery is well established, nothing had been done to in developing countries is approximately 20% com- investigate whether the organism could be satisfacto- 1 Presented at Inter/Micro 2005, Chicago, IL 147 * Rothay House, Mayfield Road, Eastrea, Cambridge PE7 2AY, UK. rily observed through primitive microscopes, and this The shape resembles that of a pear cut in half along remained a crucial question. Professor Huw Smith of it long axis (a pyriform shape). On the anterior half of the Scottish Parasite Diagnostic Laboratory, Glasgow, the flattened ventral surface is located a distinctive Scotland, proposed that I should attempt to repeat the concave disc with a raised ridge at its anterior end. experiments from 1681, to confirm the observations This ventral disc is used to attach the trophozoite to reported by Leeuwenhoek. He offered to send living the epithelium of host enterocytes, so that the parasite trophozoites for micrography. As the holder of a is not swept away with digested food. Once the tro- NESTA Fellowship (see acknowledgements) I was able phozoite loses hold and leaves this preferred environ- to set aside time for this research project. ment to pass out of the body of the host, it is unable to Antony van Leeuwenhoek (1632-1723) wrote his survive. Attachment onto the surface of the enterocyte description of Giardia in 1681 as he was examining his is therefore crucial. This tiny ‘sucker’ holds the cell own diarrhoeal stools under the microscope. The next fast to its host, and the outline of the ventral disc can description of this parasite was not until 1859 when it be seen impressed on the surface of affected enterocytes. was observed by Vilem Dusan Lambl and described Trophozoites possess four pairs of flagella ar- by him as Cercomonas intestinalis. The term Lamblia ranged in bilateral symmetry. These are the organs of intestinalis was coined in 1888 by Raphael Anatole Émile locomotion and trophozoites can attach and detach Blanchard. In 1915, Charles Wardell Stiles (1867-1941) from the microvillous surface of enterocytes. Two cu- et al introduced the name Giardia lamblia to commemo- riously “claw-hammer” shaped median bodies, com- rate the work done by Professor A. Giard in Paris and posed of microtubules, lie transversely in the mid-por- Dr. Lambl in Prague. Today, the organisms that infect tion of the organism, though the function of these re- humans are variously referred to as G. lamblia, G. mains unknown. intestinalis or G. duodenalis, indicating that intense de- Motile trophozoites exhibit forward movement bate continues to surround this intriguing protozoan during which the organism tends to rotate around its parasite. longitudinal axis displaying both a tumbling move- ment resembling that of a falling leaf and an up and THE ORGANISM down movement referred to as ‘skipping’. Trophozoi- tes can become detached and can subsequently re-at- G. duodenalis is a binucleate flagellated protozoan tach themselves to the surface of another enterocyte. with a life cycle that alternates between an actively The cycle of attachment, detachment and subsequent swimming trophozoite and an infective, resistant cyst. re-attachment may be necessary to compensate for Trophozoites measure 12 - 18 µm in length, from their rapid enterocyte turnover and the ensuing sloughing broad (anterior) end to their narrow (caudal or poste- of host cells into the intestinal lumen. rior) end, up to 10 µm in breadth and 2 -4 µm in thick- Giardia was once though to be a primitive cell, lack- ness. Cysts are smaller, measuring about 10 µm x 8 ing identifiable glycosomes, peroxisomes or mitochon- µm. Cysts form as trophozoites pass through the in- dria. That’s not the case, and the discovery of the Giar- creasingly alkaline intestinal tract. The process of en- dia mitosome has changed this view. It now seems that cystment can be observed under the light microscope; the amitochondrial state is not primitive, but is the the trophozoites cease active motility, become rounded result of reductive evolution. Under electron micros- and increasingly refractile as encystment begins. copy we can observe numerous cytoplasmic cisternae Nuclear, but not cytoplasmic, division occurs to pro- and tubular elements that appear to be a transitional duce the quadrinucleate, mature and infectious cyst. condition between a formal endoplasmic reticulum As the cysts pass through the acidic pH regime of and the true Golgi as seen in more specialized cells the stomach, the low pH and elevated CO2 followed by (10). Giardia is itself parasitized by other organisms, slightly alkaline environment of the proximal small and has been found to contain bacteria or Mycoplasma. intestine induce excystation. One trophozoite emerges There is also a 32 nm double-stranded ribonucleic acid from each quadrinucleate cyst, which now undergoes virus which is characteristic of the genus. This, known rapid cytoplasmic, but not nuclear division, to form as the Giardia lamblia virus (GLV), has been identified in two binucleate trophozoites. Trophozoites attach many isolates of the host. themselves to the luminal surface of the epithelial cells Transmission of Giardia to humans can occur (enterocytes) that line the duodenum and jejunum, through any mechanism by which material contami- then undergo further division by asexual binary fis- nated with faeces containing infectious cysts from in- sion. fected human beings or animals is ingested by a sus- 148 MICROSCOPE(2005)53 BRIAN J. FORD ceptible host. Transmission routes include person to “I have sometimes also seen tiny creatures mov- person, waterborne, foodborne and zoonotic transmis- ing very prettily; some of them a bit bigger, others a sion. Waterborne transmission of Giardia associated bit less, than a blood-globule but all of one and the with community water systems, drinking untreated same make. Their bodies were somewhat longer than water whilst backpacking and immersion watersports broad, and their belly, which was flattish, furnished is well documented, and epidemic giardiasis, associ- with sundry little paws, wherewith they made such a ated with contaminated potable water, has been fre- stir in the clear medium and among the globules, that quently reported. Giardia is the most commonly identi- you might even fancy you saw a woodlouse running fied agent of waterborne disease in the USA with over up against a wall; and albeit they made a quick motion 120 waterborne outbreaks affecting more than 25,000 with their paws, yet for all that they made but slow persons, since 1965 (11). Until purified water became progress.” generally available in the United States since World This description is remarkable.
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