2. Historical Perspective and Taxonomy
2.1. Cryptosporidium
Cryptosporidium was first discovered by
Ernest Edward Tyzzer in 1907. He identified life cycle stages of a parasitic protist in the gastric glands of laboratory mice and proposed the name
Cryptosporidium muris for this new species (
Tyzzer, 1910). Subsequently, another new species, called
Cryptosporidium parvum, was described by
Tyzzer in 1912.
C. parvum infected the small intestine, and the oocysts were smaller than those of
C. muris. After its first discovery by Tyzzer,
Cryptosporidium was not recognized as an economically or medically important parasite for the next 50 years. In 1955, it was first identified as a potentially disease-causing agent, when it was isolated from diarrhoeic turkeys (
Slavin, 1955). In 1971,
Cryptosporidium was reported to be associated with diarrhoea in young calves, for the first time (
Panciera et al., 1971). Five years later, the first two human cases of cryptosporidiosis were reported (
Meisel et al., 1976;
Nime et al., 1976). Then, in 1982, the Centers for Disease Control (CDC) reported cryptosporidiosis in 21 HIV/AIDS patients in the USA (
MMWR, 1982), after which
Cryptosporidium received major attention around the world.
Traditionally,
Cryptosporidium taxa have been identified based on oocyst morphology, host specificity and/or predilection site within the host (
Ryan and Xiao, 2014). Subsequently, using immunological or molecular methods, many distinct species and genotypes have been identified within
Cryptosporidium. Over the years, the taxonomy of
Cryptosporidium has undergone substantial change, leading to some controversies. The development of new molecular tools has allowed the identification and characterisation of species and/or genetic variants (genotypes) (reviewed by
Jex et al., 2011a;
Ryan and Xiao, 2014).
The genus
Cryptosporidium belongs to the Phylum Apicomplexa, Order Eucoccidiorida and Family Cryptosporidiidae (
Fayer and Xiao, 2008). Presently, more than 26 species of
Cryptosporidium have been recognised (
Chalmers and Katzer, 2013;
Ryan and Xiao, 2014). In addition, there are more than 40 genotypes that have not yet been formally recognised as species, because of a lack of sufficient morphological, biological and molecular data to comply with the International Code for Zoological Nomenclature (ICZN) rules of describing new species (
Table 1) (cf.
Jex et al., 2011a;
Ryan and Xiao, 2014).
2.2. Giardia
Although
Giardia was first observed in 1681 by Antonie van Leeuwenhoek, the first detailed description of this protist was not published until 1859. Subsequently, species of
Giardia were described based on the host occurrence, because of a lack of characteristic or differentiating morphological features. Later, in 1952, an increasing number of
Giardia species and uncertainty regarding host specificity led to a taxonomic rationalisation. Based on this classification, most members infecting vertebrates were named as one group,
Giardia duodenalis (
Filice, 1952). Although this ‘species’ was isolated from humans and many animal hosts, the zoonotic significance of
Giardia was controversial until the World Health Organization (WHO) recognised it as a zoonotic agent in 1979 (
WHO, 1979).
Giardia is a binucleate, flagellate protist belonging to Phylum Metamonada, Order Giardiida and Family Giardiidae (
Plutzer et al., 2010). Other recognised species of
Giardia include
Giardia agilis,
Giardia ardae,
Giardia microti,
Giardia muris and
Giardia psittaci (
Table 2) (
Thompson, 2011). At the trophozoite stage, they can be distinguished based on morphological characteristics using light and electron microscopy.
Giardia duodenalis (syn.
Giardia intestinalis,
Giardia lamblia) is known to infect at least 40 host species, including humans (
Thompson, 2011). The other species are known to be host specific or have a limited host range:
G. agilis in amphibians,
G. ardae and
G. psittaci in birds and
G. microti and
G. muris in rodents (
Adam, 2001). Currently,
G. duodenalis is recognised as a complex of at least eight different assemblages (A–H) (
Koehler et al., 2014a;
Monis et al., 2003). Although there are little or no morphological differences, there is substantial genetic diversity among these assemblages (
Nash et al., 1985;
Nash, 1992). Assemblages A and B are described to have a relatively broad host range, including humans and various other mammals. The other assemblages are either host specific or have narrow host ranges. Assemblages C and D are commonly found in dogs, whereas assemblage F mainly infects cats. Assemblage E is found in cloven-hoofed animals, and G is found in rodents (
Feng and Xiao, 2011;
Xiao and Fayer, 2008). Recently, assemblage H was identified in marine vertebrates (
Lasek-Nesselquist et al., 2010).
Table 2
Currently recognised species of Giardia and genetic groupings (assemblages) within Giardia duodenalis
| Assemblage A | Humans, primates, dogs, cats, livestock, rodents, wild mammals |
| Assemblage B | Humans, primates, dogs, cattle, some species of wild mammals |
| Assemblage C | Dogs, other... |
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