IN HUMANS
PREVALENCE AND RISK FACTORS • Waterborne transmission (recreational exposure or drinking water) may account for up to 75% of human cases, followed by foodborne transmission. 18 Direct human- to-human faecal-oral exposure may also occur and explains the comparatively higher probability of Giardia in young children and in adults that work with or care for them (e.g. in childcare). 19,20 • There is limited published data on zoonotic infection from dogs and cats, but the risk is considered to be low. Studies have demonstrated an association between owning a dog and Assemblage A giardiasis, and dogs have been shown to harbour zoonotic genotypes, including Assemblage A. 15,21-23 CLINICAL DISEASE • A median prepatent period of 2 weeks is reported in humans. 24 In Giardia -endemic communities, the majority of human infections are asymptomatic and do not require treatment. Clinical cases may present with bloating, flatulence and acute, intermittent or chronic diarrhoea. Infections are typically self-limiting, however prolonged infection may occur in both immunocompetent and immunocompromised individuals. Sequelae of chronic infection include malabsorption, weight loss and, in children, failure to thrive. Some studies have noted possible associations between chronic infection and irritable bowel syndrome, food allergies, arthritis and chronic fatigue syndrome. 25
Scanning electron microscopic (SEM) image of Giardia spp. protozoan on the microvillous border composed of intestinal epithelial cells. The ventral adhesive disk, which facilitates adherence to the intestinal surface, can be seen on the underside of the organism (Public Health Image Library, CDC) • Temporary disaccharide intolerance can occur post infection and may last several weeks after clearance of the organism. • Immunocompromised individuals, including those with congenital disease, hypogammaglobulinaemia, secretory IgA deficiencies and human immunodeficiency virus infection, have difficulty clearing intestinal Giardia infections. 26 • Progressive immunocompromise and low CD4+ counts also increases the risk of symptomatic Giardia infection. 27
Scanning electron microscopic (SEM) image showing group of Giardia spp. trophozoites clustered on the intestinal mucosal surface. Immediately adjacent to the organisms are a number of the characteristic circulars lesions that can be left on the surface as a result of the tight adhesion of the organism’s ventral adhesive disk (Public Health Image Library, CDC)
CONTENTS
Companion Animal Zoonoses Guidelines 47
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