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Letters to the Editor  |   October 2012
Giardiasis Revisited: An Underappreciated Reemerging Zoonotic Disease
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Gastroenterology
Letters to the Editor   |   October 2012
Giardiasis Revisited: An Underappreciated Reemerging Zoonotic Disease
The Journal of the American Osteopathic Association, October 2012, Vol. 112, 649-651. doi:10.7556/jaoa.2012.112.10.649
The Journal of the American Osteopathic Association, October 2012, Vol. 112, 649-651. doi:10.7556/jaoa.2012.112.10.649
To the Editor: 
Presently, the world population exceeds 7 billion humans.1 Less than 50 years ago, the population barely exceeded 3 billion.1 People are now living in areas that were previously uninhabited by humans, especially in developing countries. Globally, municipalities are more crowded than ever and basic human needs, such as nutritious food and clean drinking water, are becoming less attainable for many people. This increase in population, along with the increase in globalization and the breakdown of international barriers through travel and trade, has led to an increase in prevalence of several common parasitic diseases, with many of them reemerging in zoonotic contexts. 
Management of these diseases has become more difficult because parasites have developed resistance to many of the commonly used treatments such as metronidazole. In the United States, parasitic infections are frequently underdiagnosed or misdiagnosed because health care providers either are unfamiliar with the parasitic infection or because the providers do not have sufficient resources to identify parasitic infection in the laboratory setting. Examples of these undiagnosed cases have been seen in the Department of Microbiology, Infectious and Emerging Diseases laboratory at the Edward Via College of Osteopathic Medicine–Virginia Campus; during the past 18 months, we have identified Giardia duodenalis in the stool of 8 osteopathic medical students, all of whom had chronic gastrointestinal infections lasting from 2 to 5 years. 
Giardiasis knows no geographic borders or political boundaries. According to a review,2 the prevalence of giardiasis ranges from 2% to 7% in developed countries to more than 30% in developing countries.2 Certainly, this is an underestimation—the same review cites recent studies that have reported infection rates surpassing 40% in countries such as Italy and Uganda.2 In the United States during the past 15 years, giardiasis has been recognized as one of the most common waterborne and food-borne parasitic diseases in humans and in companion animals such as dogs, cats, rabbits, and horses.3 During 2011, the number of cases of giardiasis reported to the Centers for Disease Control and Prevention exceeded 20,000.3 We believe that the actual number of cases is undoubtedly higher, with more cases being misdiagnosed or undiagnosed. 
Cases of giardiasis are most likely undiagnosed or misdiagnosed because of a lack of access to proper health care in rural or underserved areas or because of inadequately trained health care providers.4 It is imperative that health care providers are properly trained to diagnose and manage a disease that is as common as giardiasis. Often, health care providers only associate giardiasis with diarrhea. Yet, other gastrointestinal manifestations of the disease, including nausea, vomiting, dehydration, abdominal cramping, bloating, weight loss, and malabsorption, are often seen in patients with chronic giardiasis.5,6 Other manifestations unrelated to gastrointestinal symptoms are also seen in patients with chronic giardiasis. These manifestations include itchy skin, hives, periorbital puffiness, and joint effusion. In addition, some studies5,6 revealed stunting of cognition, stunting of intelligence, and the impeding of psychosocial development in language-cognitive and fine-motor development–affected individuals—disturbing outcomes for patients with chronic giardiasis. 
No country or population is immune to endemic giardiasis, although certain groups within a population may be at greater risk for acquiring an infection. These groups include international travelers, back-packers, hikers, and campers who drink unfiltered or untreated water and people who drink from shallow wells.7 The groups also include children who attend day care centers, child care workers, and parents of infected children, especially those who care for children in diapers.8 People who have contact with animals that have the disease, men who have sex with men, and those who have oral sex are also at a higher risk for the disease.9,10 All these risk factors must be taken into account by health care workers when developing a differential diagnosis of any patient with related symptoms of giardiasis. 
Our understanding of the epidemiology of Giardia has changed dramatically during the past decade. This change is because of the presence of different Giardia species, strains, and genotypes and their wide and variable host ranges. These changes have made understanding the dynamics of disease transmission difficult for health care workers and research scientists. Understanding Giardia epidemiology is particularly important in determining the zoonotic potential of infected domestic animals and in determining the human disease burden attributed to Giardia of animal origin. The taxonomy of Giardia is complicated: there are 7 known species, including G duodenalis. Presently, G duodenalis is the only taxonomically recognized species that causes giardiasis in humans and in some mammals.2 There are many synonyms for G duodenalis in the literature. These include G intestinalis, G lamblia, Cercomonas intestinalis, Lamblia intestinalis, and Megastoma enterica. G duodenalis of nonhuman host origin can become infectious to humans. Many common farm animals, such as cattle, pigs, goats, and sheep, can be infected with G duodenalis. In addition, many wild animals, including gorillas, reindeer, dolphins, coyotes, harbor seals, and herring gulls, can be infected, making giardiasis one of the most important zoonotic diseases. G duodenalis is divided into 8 assemblages, suggesting that G duodenalis may represent a multispecies complex.2 The assemblages are based on phylogenetic analyses of nucleotide sequence of the small-subunit rRNA. The 8 assemblages include the following: assemblage A (humans, nonhuman primates, and many mammals); assemblage B (humans, nonhuman primates, canines, and cattle); assemblages C and D (canines); assemblage E (domestic ruminants and pigs); assemblage F (cats); assemblage G (rodents); and assemblage H (seals).2 
Giardiasis has classically been identified from stool samples using standard light microscopy, which remains an important method for diagnosing an infection. Even under ideal conditions, both the trophozoite and cyst stages of G duodenalis are difficult to find and to identify and are often missed when examining fresh stool smears or prepared stained slides, especially when the samples contain a low number of parasites. 
According to the Centers for Disease Control and Prevention, the use of concentration methods and trichrome staining techniques for processing fecal samples might not be sufficient to identify G duodenalis. This may be because of the variability in the concentration of organisms recovered from the stool sample, making infection difficult to diagnose. For this reason, fecal immunoassays, which are more sensitive and more specific, should be used in conjunction with classical concentration and staining techniques when possible. Rapid immune-chromatographic cartridge assays are also available but should not take the place of routine microscopic ova and parasite examination. Only molecular testing (eg, polymerase chain reaction) can be used to accurately identify the subtypes of Giardia, although there are currently many new methods being developed. 
Our understanding of giardiasis transmission can be improved through the systematic use of molecular diagnostic tools in well-designed epidemiologic studies conducted in various socioeconomic and geographic settings. The use of molecular diagnostic tools has substantially changed our present understanding of the epidemiology of giardiasis, including its zoonotic impact and global distribution. 
References
US Census Bureau, International Programs Data Base. World population: total midyear population for the world: 1950-2050. US Census Bureau Web site. http://www.census.gov/population/international/data/worldpop/table_population.php. Accessed August 30, 2012.
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Centers for Disease Control and Prevention. Introduction. MMWR Morb Mortal Wkly Rep. 2011;60(suppl):2-6.
Palmieri JR, Elswaifi SF, Fried KK. Emerging need for parasitology education: training to identify and diagnose parasitic infections. Am J Trop Med Hyg. 2011;84(6):845-846. [CrossRef] [PubMed]
Berkman DS, Lescano AG, Gilman RH, Lopez SL, Black MM. Effects of stunting, diarrhoeal disease, and parasitic infection during infancy on cognition in late childhood: a follow-up study. Lancet. 2002;359(9306):564-571. [CrossRef] [PubMed]
Simsek Z, Zeyrek FY, Kurcer MA. Effect of Giardia infection on growth and psychomotor development of children aged 0-5 years. J Trop Pediatr. 2004;50(2):90-93. [CrossRef] [PubMed]
Xiao L, Fayer R. Molecular characterisation of species and genotypes of Cryptosporidium and Giardia and assessment of zoonotic transmission. Int J Parasitol. 2008;38(11):1239-1255. [CrossRef] [PubMed]
Ang LH. Outbreak of giardiasis in a daycare nursery. Commun Dis Public Health. 2000;3(3):212-213. [PubMed]
Peters CS, Sable R, Janda WM, Chittom AL, Kocka FE. Prevalence of enteric parasites in homosexual patients attending an outpatient clinic. J Clin Microbiol. 1986;24(4):684-685. [PubMed]
Ballini A, Cantore S, Fatone Let al. Transmission of nonviral sexually transmitted infections and oral sex. J Sex Med. 2012;9(2):372-384. [CrossRef] [PubMed]