Abstract

The widespread usages of molecular epidemiological tools have improved the understanding of cryptosporidiosis transmission. Much attention on zoonotic cryptosporidiosis is centered on Cryptosporidium parvum. Results of genotype surveys indicate that calves are the only major reservoir for C. parvum infections in humans. The widespread presence of human-adapted C. parvum, especially in developing countries, is revealed by recent subtyping and multilocus typing studies, which have also demonstrated the anthroponotic transmission of C. parvum subtypes shared by humans and cattle. Developing and industrialized countries differ significantly in disease burdens caused by zoonotic species and in the source of these parasites, with the former having far fewer human infections caused by C. parvum and little zoonotic transmission of this species. Exclusive anthroponotic transmission of seemingly zoonotic C. parvum subtypes was seen in Mid-Eastern countries. Other zoonotic Cryptosporidium spp. are also responsible for substantial numbers of human infections in developing countries, many of which are probably transmitted by anthroponotic pathways. The lower pathogenicity of some zoonotic species in some populations supports the occurrence of different clinical spectra of Cryptosporidium spp. in humans. The use of a new generation of molecular diagnostic tools is likely to produce a more complete picture of zoonotic cryptosporidiosis.

Introduction

Cryptosporidium spp. are common parasites of humans, domestic animals and wild vertebrates. Because of the wide host range of Cryptosporidium spp., cryptosporidiosis has been considered to be a zoonotic disease for some time. The role of animals, especially farm animals and domestic pets, in the transmission of human cryptosporidiosis is nevertheless not clear. This is largely because of the oocyst morphologic similarity between human-pathogenic and non-human-pathogenic species. Recently, molecular biologic tools have been developed to detect and differentiate Cryptosporidium at the species/genotype and subtype levels (Xiao & Ryan, 2004; Caccio, 2005). The use of these tools has made significant contributions to the understanding of the zoonotic potential of various Cryptosporidium spp., the disease burden and pathogenicity of zoonotic parasites, and the role of various animals in the transmission of human cryptosporidiosis (Hunter & Thompson, 2005; Smith et al., 2007). These recent developments have enabled public health officials to better educate the public on the risk factors involved in the acquisition of cryptosporidiosis in vulnerable populations.

Epidemiologic evidence for zoonotic transmission

Cattle have been considered to be an important source of zoonotic cryptosporidiosis since the 1980s. Contact with infected calves has been implicated as the cause of many small cryptosporidiosis outbreaks in veterinary students, research technicians, and children attending agricultural camps and fairs (Preiser et al., 2003; Smith et al., 2004; Chalmers et al., 2005b; Kiang et al., 2006). Contamination of food or water by cattle manure has been identified as a cause of several foodborne and waterborne outbreaks of cryptosporidiosis (Glaberman et al., 2002; Blackburn et al., 2006). In case–control studies, contact with cattle was implicated as a risk factor for human cryptosporidiosis in the United States, United Kingdom, Ireland, and Australia (Robertson et al., 2002; Goh et al., 2004; Roy et al., 2004; Hunter et al., 2004b). In the United States, the incidence of cryptosporidiosis is the highest in mid-western states where dairy farming is most intensive (Yoder & Beach, 2007). In the United Kingdom, cryptosporidiosis case numbers are higher in areas with a high estimate of Cryptosporidium oocysts applied to land from manure (Lake et al., 2007). Indeed, massive slaughtering of farm animals and restriction of farm visits during foot-and-mouth disease outbreaks reduced sporadic human C. parvum infections in large communities in the United Kingdom (Hunter et al., 2003; Smerdon et al., 2003). In contrast, few epidemiologic studies have implicated sheep as a source of human cryptosporidiosis (Duke et al., 1996).

The role of companion animals in the transmission of human cryptosporidiosis is less important. It is has been suggested for some time that dogs can be a significant source of human cryptosporidiosis (Enriquez et al., 2001; Robinson & Pugh, 2002; Shukla et al., 2006). This, however, was largely based on the observation of direct transmission of C. parvum from calves to humans and the erroneous belief that C. parvum is responsible for cryptosporidiosis in all mammals. Only a weak association between cryptosporidiosis in HIV+ persons and contact with dogs was found in the United States (Glaser et al., 1998) or between paediatric cryptosporidiosis and contact with dogs or cats in Guinea-Bissau and Indonesia (Molbak et al., 1994; Katsumata et al., 1998). In England, contact with dogs and cats was not found to be a risk factor for cryptosporidiosis (Goh et al., 2004), and in Australia it was actually a protective factor (Robinson & Pugh, 2002).

Cryptosporidium species and genotypes in humans

Cryptosporidium parvum was once considered to be the only Cryptosporidium species to infect humans. Genotyping tools based on DNA sequences of antigen and house-keeping genes identified genotypes 1 (the human genotype) and 2 (the bovine genotype) within the umbrella of C. parvum and these eventually became Cryptosporidium hominis and C. parvum senso stricto, both infectious for immunocompetent and immunocompromised persons (Xiao et al., 2004a, b; Caccio, 2005). The PCR techniques used do not amplify DNA from some more genetically different Cryptosporidium species. At the end of the 1990s, the use of small subunit (SSU) rRNA-based genotyping tools revealed the presence of Cryptosporidium canis, Cryptosporidium felis, and Cryptosporidium meleagridis in AIDS patients in the United States, Switzerland, and Kenya, in addition to the more frequently found C. hominis and C. parvum (Pieniazek et al., 1999; Morgan et al., 1999a, 2000a). This observation has been supported by data from France, Portugal, Italy, Thailand, and Peru (Alves et al., 2001; Guyot et al., 2001; Caccio et al., 2002; Tiangtip & Jongwutiwes, 2002; Gatei et al., 2002b).

Even immunocompetent persons can be infected with zoonotic species other than C. parvum. Molecular characterization of over 2000 specimens in the United Kingdom identified 22 cases of C. meleagridis, six cases of C. felis, and one case of C. canis (McLauchlin et al., 2000; Leoni et al., 2006). Ninety-nine cases of C. meleagridis, 22 cases of C. felis and two cases of C. canis infections were identified by another group among 13 112 cases in England and Wales (Nichols et al., 2006). Most of the infected persons were not immunocompromised. HIV-seronegative children in Lima, Peru (Xiao et al., 2001), and children in Kenya had these Cryptosporidium species (Gatei et al., 2006). Over 20 other cases of C. meleagridis infection have been described in immunocompetent persons in other industrialized and developing countries (Tables 1 and 2). In Peru, where a significant proportion of infections are due to zoonotic Cryptosporidium, there was no significant difference between children and HIV+ adults in the distribution of C. hominis, C. parvum, C. meleagridis, C. felis, and C. canis (Xiao et al., 2001; Cama et al., 2003).

Table 1

Distribution of common human-pathogenic Cryptosporidium species in humans in industrialized countries

Location Patient type Sample size C. hominis C. parvum C. meleagridis C. felis C. canis Mixed species Reference 
Portugal HIV+adults 29 16   Alves et al., (2003) 
Portugal HIV+adults?    Almeida et al., (2006) 
Spain Children and HIV+adults 105 69 34   Llorente et al., (2007) 
England Children and adults 2414 1005 1354 22 21 McLauchlin et al., (2000), Leoni et al., (2006) 
England Children and adults 1622 726 896     Sopwith et al., (2005) 
England Children and adults 1263 563 662     Hunter et al., (2003) 
England and Wales Children and adults 191 115 76     Hunter et al., (2004b) 
England and Wales Children and adults 2251 936 1315     Smerdon et al., (2003) 
England and Wales Children and adults 13112 6594 5981 99 22 65 Nichols et al., (2006) 
Scotland Children and adults 136 71 64    Mallon et al. (2003) 
N. Ireland Children and adults 39 34     Lowery et al., (2001) 
France HIV+adults and other immunocompromised 46 14 22   Guyot et al., (2001) 
France HIV+adults 13     Bonnin et al., (1996) 
France HIV+adults and others 64 35 16   Coupe et al., (2005) 
France Unknown 54 20 26 2? 6?   Ngouanesavanh et al., (2006) 
Switzerland HIV+adults 13   Morgan et al. (2000) 
Switzerland Children 14 11     Glaeser et al., (2004) 
Switzerland Unknown      Fretz et al., (2003) 
Denmark Various 44 25 18    Enemark et al., (2002) 
The Netherlands HIV- adults? 10 10      Caccio et al., (1999) 
The Netherlands Mostly children 41 29     Ten Hove et al., (2007) 
Czech Republic Children      Hajdusek et al., (2004) 
Slovenia Children and adults 29 26     Soba et al., (2006) 
Iran Children and HIV+adults 15 11     Meamar et al., (2007) 
Turkey Children      Tamer et al., (2007) 
Kuwait Children 62 58    Sulaiman et al., (2005) 
Taiwan HIV+adults    Hung et al., (2007) 
Japan All 19 13    Yagita et al., (2001) 
Japan Unknown     Abe et al., (2006) 
South Korea Unknown, rural area      Park et al., (2006) 
Australia All  83% 17%     Morgan et al. (2000) 
Australia Unknown 22 16     Chalmers et al., (2005b) 
New Zealand Unknown 423 198 223     Learmonth et al., (2004) 
Canada Children and adults 150 108 29    Ong et al., (2002) 
Canada Immunocompetent persons 11     Trotz-Williams et al., (2006) 
USA HIV+adults 10   Pieniazek et al., (1999) 
USA HIV+adults 29 18    Xiao et al., (2004a) 
USA All 178 119 25     Xiao et al., (2004b) 
USA All 49 44     Feltus et al., (2006) 
Location Patient type Sample size C. hominis C. parvum C. meleagridis C. felis C. canis Mixed species Reference 
Portugal HIV+adults 29 16   Alves et al., (2003) 
Portugal HIV+adults?    Almeida et al., (2006) 
Spain Children and HIV+adults 105 69 34   Llorente et al., (2007) 
England Children and adults 2414 1005 1354 22 21 McLauchlin et al., (2000), Leoni et al., (2006) 
England Children and adults 1622 726 896     Sopwith et al., (2005) 
England Children and adults 1263 563 662     Hunter et al., (2003) 
England and Wales Children and adults 191 115 76     Hunter et al., (2004b) 
England and Wales Children and adults 2251 936 1315     Smerdon et al., (2003) 
England and Wales Children and adults 13112 6594 5981 99 22 65 Nichols et al., (2006) 
Scotland Children and adults 136 71 64    Mallon et al. (2003) 
N. Ireland Children and adults 39 34     Lowery et al., (2001) 
France HIV+adults and other immunocompromised 46 14 22   Guyot et al., (2001) 
France HIV+adults 13     Bonnin et al., (1996) 
France HIV+adults and others 64 35 16   Coupe et al., (2005) 
France Unknown 54 20 26 2? 6?   Ngouanesavanh et al., (2006) 
Switzerland HIV+adults 13   Morgan et al. (2000) 
Switzerland Children 14 11     Glaeser et al., (2004) 
Switzerland Unknown      Fretz et al., (2003) 
Denmark Various 44 25 18    Enemark et al., (2002) 
The Netherlands HIV- adults? 10 10      Caccio et al., (1999) 
The Netherlands Mostly children 41 29     Ten Hove et al., (2007) 
Czech Republic Children      Hajdusek et al., (2004) 
Slovenia Children and adults 29 26     Soba et al., (2006) 
Iran Children and HIV+adults 15 11     Meamar et al., (2007) 
Turkey Children      Tamer et al., (2007) 
Kuwait Children 62 58    Sulaiman et al., (2005) 
Taiwan HIV+adults    Hung et al., (2007) 
Japan All 19 13    Yagita et al., (2001) 
Japan Unknown     Abe et al., (2006) 
South Korea Unknown, rural area      Park et al., (2006) 
Australia All  83% 17%     Morgan et al. (2000) 
Australia Unknown 22 16     Chalmers et al., (2005b) 
New Zealand Unknown 423 198 223     Learmonth et al., (2004) 
Canada Children and adults 150 108 29    Ong et al., (2002) 
Canada Immunocompetent persons 11     Trotz-Williams et al., (2006) 
USA HIV+adults 10   Pieniazek et al., (1999) 
USA HIV+adults 29 18    Xiao et al., (2004a) 
USA All 178 119 25     Xiao et al., (2004b) 
USA All 49 44     Feltus et al., (2006) 

*Including three unamplified.

Includes nine specimens with the cervine genotype.

Table 2

Distribution of common human-pathogenic Cryptosporidium species in humans in developing countries

Location Patient type Sample size C.hominis C.parvum C.meleagridis C.felis C.canis Mixed species Reference 
India HIV+adults 48 31   Muthusamy et al., (2006) 
India Children 50 47   Gatei et al., (2007) 
India Children 58 47 1 (?)   Ajjampur et al., (2007) 
Thailand 4 HIV+children, 25 HIV+adults 29 24    Tiangtip & Jongwutiwes (2002) 
Thailand HIV+adults 34 17  Gatei et al., (2002b) 
China Children     Peng et al., (2001) 
Kenya HIV+adults 24 14    Gatei et al., (2003) 
Kenya Children 175 153 15  Gatei et al., (2006) 
Malawi Children 43 41     Peng et al., (2003b) 
Malawi Children 39 25 10   Morse et al., (2007) 
Uganda Mostly HIV+children 76 56 14   Tumwine et al., (2005) 
Uganda Children 444 326 85   19 Tumwine et al., (2003) 
South Africa HIV+children 21 16     Leav et al., (2002) 
South Africa All 44 36     Samie et al., (2006) 
Haiti HIV+adults 49 31 16 1? 1?   Ngouanesavanh et al., (2006) 
Venezuela HIV+adults 10    Certad et al., (2006) 
Colombia HIV+adults    Navarro-i-Martinez et al., (2006) 
Guatemala Children 15 14     Xiao et al., (2004b) 
Peru HIV+adults 302 204 34 38 10 12  Cama et al., (2003) 
Peru Children 85 67  Xiao et al., (2001) 
Peru Children     Cordova Paz Soldan et al., (2006) 
Brazil Children 42 24 18     Bushen et al., (2007) 
Chile Children     Neira-Otero et al. (2006) 
Location Patient type Sample size C.hominis C.parvum C.meleagridis C.felis C.canis Mixed species Reference 
India HIV+adults 48 31   Muthusamy et al., (2006) 
India Children 50 47   Gatei et al., (2007) 
India Children 58 47 1 (?)   Ajjampur et al., (2007) 
Thailand 4 HIV+children, 25 HIV+adults 29 24    Tiangtip & Jongwutiwes (2002) 
Thailand HIV+adults 34 17  Gatei et al., (2002b) 
China Children     Peng et al., (2001) 
Kenya HIV+adults 24 14    Gatei et al., (2003) 
Kenya Children 175 153 15  Gatei et al., (2006) 
Malawi Children 43 41     Peng et al., (2003b) 
Malawi Children 39 25 10   Morse et al., (2007) 
Uganda Mostly HIV+children 76 56 14   Tumwine et al., (2005) 
Uganda Children 444 326 85   19 Tumwine et al., (2003) 
South Africa HIV+children 21 16     Leav et al., (2002) 
South Africa All 44 36     Samie et al., (2006) 
Haiti HIV+adults 49 31 16 1? 1?   Ngouanesavanh et al., (2006) 
Venezuela HIV+adults 10    Certad et al., (2006) 
Colombia HIV+adults    Navarro-i-Martinez et al., (2006) 
Guatemala Children 15 14     Xiao et al., (2004b) 
Peru HIV+adults 302 204 34 38 10 12  Cama et al., (2003) 
Peru Children 85 67  Xiao et al., (2001) 
Peru Children     Cordova Paz Soldan et al., (2006) 
Brazil Children 42 24 18     Bushen et al., (2007) 
Chile Children     Neira-Otero et al. (2006) 

It is likely that other Cryptosporidium species can infect humans under certain circumstances. Cryptosporidium muris-like oocysts were found in two healthy Indonesian girls (Katsumata et al., 2000). A putative C. muris infection was reported in an immunocompromised patient in France based on sequence analysis of a small fragment of the SSU rRNA (Guyot et al., 2001). However, the sequence was more similar to Cryptosporidium andersoni (2-bp differences in a 242-bp region) than to C. muris (8-bp differences in the region). Several confirmed C. muris infections were documented in AIDS patients in Kenya and Peru, both by PCR-restriction fragment length polymorphism (RFLP) and sequencing of the SSU rRNA gene (Gatei et al., 2002a, 2006; Palmer et al., 2003), and a putative human C. muris infection was seen in India (Muthusamy et al., 2006). More human cases have been associated with the Cryptosporidium cervine genotype, which was reported in 10 patients in Canada, seven in the United Kingdom, three in the United States, and one in Slovenia (Ong et al., 2002; Blackburn et al., 2006; Feltus et al., 2006; Leoni et al., 2006; Nichols et al., 2006; Soba et al., 2006; Trotz-Williams et al., 2006). Other Cryptosporidium species found in humans include C. suis in an HIV+ patient in Lima, Peru, and two patients in England (Xiao et al., 2002; Leoni et al., 2006; Nichols et al., 2006), a C. suis-like parasite in two patients in Canada (Ong et al., 2002), a C. andersoni-like parasite in three patients in England (Leoni et al., 2006) and one patient in Malawi (Morse et al., 2007), the chipmunk genotype I (W17) in two patients in Wisconsin (Feltus et al., 2006), and the skunk genotype in one patient in United Kingdom (Nichols et al., 2006). The C. hominis monkey genotype has been found in two persons in the United Kingdom (Mallon et al., 2003b). Other new Cryptosporidium genotypes will likely be found in humans in future, but these parasites account for a very minor proportion of Cryptosporidium infections in humans.

Some unusual Cryptosporidium species may have a broad host range and might emerge as important pathogens in humans when socioeconomic and environmental changes favor the transmission. The avian pathogen C. meleagridis is increasingly recognized as an important human pathogen. In Lima, Peru, and Bangkok, Thailand, C. meleagridis is responsible for 10–20% of human cryptosporidiosis cases (Xiao et al., 2001; Gatei et al., 2002b; Cama et al., 2003). Likewise, the increasing number of humans infected with the cervine genotype might be related to its wide range of mammalian hosts (Feng et al., 2007a).

Cryptosporidium species and genotypes in animals

There is extensive genetic variation within the genus Cryptosporidium (Fig. 1). In addition to the 16 accepted species, nearly 50 Cryptosporidium genotypes have been described in animals and new genotypes are continually being discovered (Xiao et al., 2004a; Feng et al., 2007a). Several well-known genotypes in recent years have been elevated to species status as other biologic data have become available. Phylogenetically, Cryptosporidium species and genotypes form mostly two groups: those found primarily in the intestine and those in the stomach. Each group contains parasites of mammals, birds and reptiles (Fig. 1). Some other more primitive parasites, such as fish genotypes and the chipmunk genotype II (Ryan et al., 2004; Feng et al., 2007a), are placed outside these two groups (Fig. 1).

Figure 1

Genetic relationship among named Cryptosporidium species and genotypes inferred by a neighbor-joining analysis of the partial (~375 bp) SSU rRNA gene. Values on branches are percent bootstrapping using 1000 replicates. Numbers following species or genotypes are isolate identifications or GenBank accession numbers used in the construction of the phylogenetic tree. A few other genotypes with shorter sequences are not included.

Figure 1

Genetic relationship among named Cryptosporidium species and genotypes inferred by a neighbor-joining analysis of the partial (~375 bp) SSU rRNA gene. Values on branches are percent bootstrapping using 1000 replicates. Numbers following species or genotypes are isolate identifications or GenBank accession numbers used in the construction of the phylogenetic tree. A few other genotypes with shorter sequences are not included.

With few exceptions, most species and genotypes are host-adapted in nature, having a narrow spectrum of natural hosts. Thus, one Cryptosporidium species or genotype usually infects only a particular species or a group of related animals. Surveys conducted in cattle, sheep, pigs, kangaroos, squirrels, wild mammals, Canada geese, and reptiles have shown that most animal species are infected with only a few host-adapted Cryptosporidium species or genotypes (Guselle et al., 2003; Jellison et al., 2004; Power et al., 2004; Zhou et al., 2004a, b; Xiao et al., 2004c; Ryan et al., 2005; Langkjaer et al., 2007; Feng et al., 2007a, b). The existence of host-adapted Cryptosporidium species or genotypes indicates that cross-transmission of Cryptosporidium among different groups of animals is probably limited. Nevertheless, host- adaptation is not strict host specificity. Cross-species transmission occurs occasionally when animals share a similar habitat, such as the transmission of the Cryptosporidium skunk genotype among skunks, raccoons, squirrels, and opossums (Feng et al., 2007a).

Cryptosporidium parvum has received the most attention in zoonotic transmission of cryptosporidiosis. This was largely due to the fact that C. parvum is a major human pathogen and was traditionally considered to infect all mammals. Genetic characterizations of Cryptosporidium specimens from various animals, however, have mostly failed to detect this parasite in wild mammals (Zhou et al., 2004b; Feng et al., 2007a). It is now generally accepted that C. parvum (referred to previously as the bovine genotype) primarily infects ruminants and humans, even though natural infections have been found occasionally in other animals such as mice and raccoon dogs (Morgan et al., 1999b; Matsubayashi et al., 2004).

Even though cattle have been considered to be a major host for C. parvum, only preweaned calves are frequently infected with this species. Most Cryptosporidium infections in postweaned calves are due to C. bovis and the deer-like genotype, which can be frequently found in yearlings and adult cattle. Cryptosporidium andersoni is first found in juveniles but more frequently in yearlings and adults (Santin et al., 2004; Fayer et al., 2006b; Langkjaer et al., 2007; Feng et al., 2007b). Cryptosporidium bovis and the deer-like genotype are genetically related and the age pattern of the host is very similar for both parasites, even though the deer-like genotype is usually less common. Thus, only preweaned calves are major contributors of zoonotic C. parvum.

Cryptosporidium parvum has only been detected in small numbers in other farm animals. Studies conducted in Australia and the United States suggest that C. parvum infection is not common in sheep, which are more often infected with the Cryptosporidium cervine genotype and other genotypes (Ryan et al., 2005; Santin et al., 2007). However, lambs are sometimes naturally infected with C. parvum and direct transmission of C. parvum from lambs to children was confirmed for at least one small outbreak of cryptosporidiosis by subtyping (Chalmers et al., 2005b). Possible transmission of C. parvum among humans, calves, and zoo ruminants in Lisbon, Portugal, was supported by subtyping (Alves et al., 2003). Although C. parvum was detected in a few horses, its prevalence is not known (Grinberg et al., 2003; Hajdusek et al., 2004; Chalmers et al., 2005a) and horses are known to be infected with a Cryptosporidium horse genotype (Ryan et al., 2003).

Cryptosporidium parvum has been detected in a few dogs in Italy (Giangaspero et al., 2006), but most studies indicated that dogs are almost exclusively infected with C. canis (Morgan et al., 2000c; Satoh et al., 2006; Huber et al., 2007; Rimhanen-Finne et al., 2007). Likewise, most cats are infected with C. felis, although C. muris were also found in two cats (Pavlasek & Ryan, 2006; Santin et al., 2006; Fayer et al., 2006a; Rimhanen-Finne et al., 2007). Because C. canis and C. felis are minor pathogens of humans, these genotyping data suggest that the role of dogs and cats in the transmission of human cryptosporidiosis is probably limited. Direct transmission of C. canis to humans, however, has been speculated in a recent report, in which two children and one dog in the same household were shown to be infected with C. canis during the same period (Xiao et al., 2007b).

One Cryptosporidium with a noticeable broad host range is the cervine genotype. Because its initial finding in storm runoff in a feral area, it has been found in domestic and wild ruminants (sheep, mouflon sheep, blesbok, nyala, and deer), rodents (squirrels, chipmunks, woodchucks, beavers, and deer mice), carnivores (raccoons), and primates (lemurs and humans) (Xiao et al., 2000; Perz & Le Blancq, 2001; Ong et al., 2002; da Silva et al., 2003; Ryan et al., 2003, 2005; Blackburn et al., 2006; Feltus et al., 2006; Leoni et al., 2006; Nichols et al., 2006; Soba et al., 2006; Trotz-Williams et al., 2006; Feng et al., 2007a). Because it is the most common Cryptosporidium found in pristine water, it is likely the some other wild mammals are also hosts (Jiang et al., 2005).

A few human-pathogenic Cryptosporidium spp. have been found in unusual hosts. For example, C. hominis was detected in several calves and sheep in the United Kingdom, United States, Australia, and India (Giles et al., 2001; Ryan et al., 2005; Smith et al., 2005; Feng et al., 2007b), C. suis was found in a calf in the United States (Fayer et al., 2006b) and a few lambs in Australia (Ryan et al., 2005), C. meleagridis was seen in one dog in Czech Republic and one deer mouse in the United States (Hajdusek et al., 2004; Feng et al., 2007a), and the C. canis dog genotype was seen in a fox (Zhou et al., 2004b). The role of these animals in the transmission of these species to humans is probably minimal. Mechanical carriage of C. hominis and C. parvum oocysts has been reported in a few Canada geese. However, Canada geese are normally infected with two unique Cryptosporidium genotypes: goose genotypes I and II (Zhou et al., 2004a).

Zoonotic cryptosporidiosis in industrialized countries

Over the last decade, extensive studies have been conducted to examine the transmission of human cryptosporidiosis in industrialized nations using both genotyping and subtyping tools. Among the five common Cryptosporidium species in humans, C. parvum and C. hominis are responsible for >90% of human cases of cryptosporidiosis in most areas (Xiao & Ryan, 2004). Geographic differences exist in the disease burdens attributable to these two species. Results of a series of large scale studies showed that C. parvum and C. hominis are speonsible for 96–98% of sporadic cases in England and Wales, with C. parvum responsible for slightly more infections than C. hominis (McLauchlin et al., 2000; Chalmers et al., 2002; Leoni et al., 2006). This may also be the case in some other parts of the Europe (such as Northern Ireland, France, Switzerland, Portugal, Slovenia, and the Czech Republic) and New Zealand (Table 1). In contrast, C. hominis in general is responsible for more infections than C. parvum in the United States, Canada, Australia, Japan (Peng et al., 1997; Morgan et al., 1998; Sulaiman et al., 1998; Ong et al., 1999, 2002; Xiao et al., 2004b). Interestingly, in the highly urbanized Kuwait City, almost all cryptosporidiosis cases in children are caused by C. parvum (Sulaiman et al., 2005), which may also be the case in Iran and Turkey (Meamar et al., 2007; Tamer et al., 2007). This geographic difference in the distribution of C. parvum and C. hominis in humans is true in both immunocompetent and immunocompromised individuals. Immunocompromised persons in these countries seemingly have slightly more infections caused by C. meleagridis, C. canis, and C. felis than immunocompetent persons (Table 1).

Major differences in the transmission routes may be responsible for the differences in the Cryptosporidium species distribution. This is supported by results of studies in the United Kingdom, which reported that C. hominis infection was more common in patients with a history of foreign travel (McLauchlin et al., 2000; Goh et al., 2004; Hunter et al., 2004b; Hunter et al., 2007). Indeed, restriction of farm visits and culling of farm animals during a foot and mouth disease outbreak in England have greatly reduced the occurrence of cryptosporidiosis due to C. parvum (Hunter et al., 2003; Smerdon et al., 2003). It is possible this and other factors such as drinking water treatment improvement may have permanently changed the transmission of cryptosporidiosis in North West England (Sopwith et al., 2005). In recent years, C. hominis has been more prevalent than C. parvum in humans in England and Wales (Nichols et al., 2006).

Not surprisingly, geographic differences in the distribution of Cryptosporidium species can occur within a country (McLauchlin et al., 1999, 2000; Learmonth et al., 2004). Thus, C. hominis infection is generally more common in urban areas and C. parvum is more common in rural areas. It seems likely, but remains unproven, that the high prevalence of C. parvum in humans in these areas may be due in part to the intensive husbandry practiced for ruminants and the associated high concentrations of young animals at these feeding operations. In the United States, even though C. hominis is usually more common than C. parvum in humans (Zhou et al., 2003; Xiao et al., 2004b), most human cryptosporidiosis cases in the dairy state Wisconsin are attributable to C. parvum (Feltus et al., 2006).

Seasonal differences in the distribution of C. parvum and C. hominis have been reported. In the United Kingdom and New Zealand, the spring increase in the cryptosporidiosis cases reported was mostly due to C. parvum whereas the autumn increase was largely due to C. hominis (McLauchlin et al., 2000; Learmonth et al., 2003, 2004; Hunter et al., 2004b), suggesting that seasonal differences in the relative importance of specific transmission routes might exist. It was speculated that the increase in C. parvum in spring was due to lambing, calving, and farm runoff from spring rains, and the autumn C. hominis peak in these countries was likely the result of increased recreational water activities and international travel during late summer and early autumn (Goh et al., 2004; Hunter et al., 2004b).

What proportion of C. parvum infections in humans are attributable to zoonotic transmission remains unclear, as the source of C. parvum in humans can be of bovine or of human origin. Results of subtyping studies at the 60 kDa glycoprotein (GP60) locus support the occurrence of zoonotic transmission in industrialized nations. One major GP60 C. parvum subtype family, IIa, is common in humans in rural areas in the United States and in Europe (Glaberman et al., 2002; Alves et al., 2003, 2006; Stantic-Pavlinic et al., 2003; Chalmers et al., 2005b; Feltus et al., 2006). Many of the IIa subtypes found in humans have also been found in calves in the same area. For example, in Portugal, one C. parvum subtype in humans, IIaA15G2R1, is the predominant C. parvum subtype in calves and zoon ruminants (Alves et al., 2003, 2006). In Wisconsin, where patients were almost exclusively infected with C. parvum (Feltus et al., 2006), many of the subtypes found in humans were found previously in calves in neighboring Michigan and Ontario (Peng et al., 2003a; Trotz-Williams et al., 2006). Likewise, in Northern Ireland, even though calves are infected with many subtypes in the C. parvum subtype family IIa, most of the common subtypes have been found in human outbreak or sporadic cases (Glaberman et al., 2002; Thompson et al., 2007). In an apple-cider-associated outbreak of cryptosporidiosis in 2003 in Ohio, all patients had C. parvum, with either subtype IIaA15G2R1 or IIaA17G2R1 (Blackburn et al., 2006). One of the two C. parvum subtypes detected in patient specimens, IIaA17G2R1, was found in the implicated apple cider (Blackburn et al., 2006). This subtype is rare in eastern United States, having only been reported in some calves in Ohio and Vermont (Xiao et al., 2007a). Thus, cattle were attributed as the likely source of apple contamination with Cryptosporidium oocysts.

Another less common bovine C. parvum subtype family, IId, may also be responsible for some zoonotic infections. In southern Europe (Portugal, Italy, Serbia and Hungary), although IIa subtypes were the dominant C. parvum in calves, IId subtypes were found occasionally (Alves et al., 2003, 2006; Wu et al., 2003; Misic & Abe, 2007; Plutzer & Karanis, 2007). Four of the IId subtypes, have been found in HIV+ persons in Portugal (Alves et al., 2003, 2006). About half of the C. parvum infections in children in Kuwait City are caused by IId subtypes, although the transmission appears to be anthroponotic in origin (Sulaiman et al., 2005). IId subtypes of C. parvum, nevertheless, have never been found in calves or humans in the United States, Canada, Australia, and the United Kingdom (Glaberman et al., 2002; Peng et al., 2003a; Chalmers et al., 2005b; Trotz-Williams et al., 2006; Thompson et al., 2007; Xiao et al., 2007a). All these are further indicators of differences in the role of zoonotic parasites in the transmission of C. parvum among geographic areas.

The use of a multilocus typing tool also identified the occurrence of zoonotic transmission in a case-control study in Wales and northwest England (Hunter et al., 2007). At the ML1 locus, significantly more persons with C. parvum subtype ML1–242 had touched or handled farm animals than those with ML1–227. Similarly, at ML2, significantly more isolates with alleles between 223 and 237 were from patients who had touched or handled farm animals than were strains with alleles 193 and 197. At the GP60 locus, patients who had contact with farm animals yielded significantly greater product sizes than those who reported no animal contact before onset of illness (Hunter et al., 2007).

Not all C. parvum subtypes are zoonotic. One study in Portugal showed that the genetic diversity of C. parvum was much higher in HIV+ persons than in calves or zoo ruminants, and of the three C. parvum subtype families, one (IIc) was not found in animals. The anthroponotic nature of the IIc subtype family has been demonstrated subsequently in comparative subtyping studies of human and bovine cryptosporidiosis in Portugal, United States, Canada, United Kingdom, and Australia, where IIc subtypes have only been found in humans. In urban areas in the United States, IIa subtypes are rarely seen in humans. Instead, the anthroponotic IIc subtype family is responsible for most human C. parvum infections in these areas (Xiao et al., 2004a). In European countries such as Portugal and the United Kingdom, both IIa and IIc are fairly common in humans (Alves et al., 2003, 2006).

Results of multilocus typing studies support the occurrence of anthroponotic C. parvum. PCR product length polymorphism analysis of three minisatellite and four microsatellite markers has identified two large groups of C. parvum in human and bovine specimens from Scotland, with one group exclusively found in humans and the other groups found in both humans and in calves (Mallon et al., 2003a, b). A Similar finding was obtained more recently in England and Wales using three of the same microsatellite markers (Leoni et al., 2007). Like the previous observation of greater C. parvum genetic diversity in humans than bovines in the GP60 gene in Portugal, humans in Scotland were infected with significantly wider spectra of C. parvum multilocus types than cattle. Thus, a significant fraction of human C. parvum infections may not have originated from bovine reservoirs (Grinberg et al., 2007). In contrast, lower genetic diversity of C. parvum was observed in France in humans than in animals (Ngouanesavanh et al., 2006). Although two populations of C. parvum were also seen in France, they were not restricted to a particular host (Ngouanesavanh et al., 2006). It is not clear whether the omission of some more polymorphic markers such as GP60 in the French study has contributed to this different observation.

Zoonotic cryptosporidiosis in developing countries

The distribution of Cryptosporidium spp. in humans in developing countries is very different from that in most industrialized nations. All studies conducted so far have shown a dominance of C. hominis in humans in developing countries, responsible for 70–90% of infections (Peng et al., 2001, 2003b; Xiao et al., 2001; Leav et al., 2002; Tiangtip & Jongwutiwes, 2002; Cama et al., 2003; Gatei et al., 2003, 2006, 2007; Tumwine et al., 2003, 2005; Das et al., 2006; Muthusamy et al., 2006; Bushen et al., 2007). In contrast, the disease burden attributable to C. parvum is much lower. This strongly suggests that zoonotic infection is much less common in developing countries than in industrialized countries. Children and HIV+ persons in developing countries, however, usually have a higher prevalence of C. meleagridis, C. canis, C. felis, and C. muris, with the cervine genotype seen rarely. In fact, most human C. canis infections have been reported in persons in developing countries. In Peru and Thailand, C. meleagridis, C. canis, and C. felis are responsible for 15–20% of Cryptosporidium infections in AIDS patients and children (Table 2).

Another unique feature of cryptosporidiosis in developing countries is the anthroponotic origin of C. parvum. Unlike European countries, Australia, and the United States, the zoonotic IIa subtypes are rarely seen in humans in developing countries. Instead, the anthroponotic IIc subtype family is responsible for most human C. parvum infections in these areas (Leav et al., 2002; Peng et al., 2003b; Xiao & Ryan, 2004; Xiao et al., 2004b; Akiyoshi et al., 2006). In some regions such as Lima, Peru, the IIc subtype family is the only C. parvum in humans, whereas in other developing countries such as Malawi and Kenya, another anthroponotic C. parvum subtype family, IIe, is also seen in humans (Peng et al., 2003b; Xiao & Ryan, 2004; Xiao et al., 2004b; Cama et al., 2007). In Uganda, even though IIc subtypes are the dominant C. parvum in children, several new subtype families are present (Akiyoshi et al., 2006). Most of the genotyping and subtyping studies were carried out in urban areas. A recent study in Malawi has shown a higher C. parvum infection rate in rural areas than in urban areas (Morse et al., 2007). Unfortunately, subtyping was not carried out to determine the source of C. parvum in rural areas, even though an earlier study clearly demonstrated an almost exclusive anthroponotic transmission of cryptosporidiosis in the country (Peng et al., 2003b).

Whether C. meleagridis, C. canis, C. felis, and C. muris are transmitted in developing countries by the zoonotic pathway remains to be decided. Using C. hominis and C. parvum-specific genotyping tools, the analysis of C. canis- and C. felis-infected specimens from HIV+ persons in Lima, Peru, revealed the concurrent presence of the C. hominis and C. parvum IIc subtype family in 6 of 21 patients, indicating that infection with mixed Cryptosporidium spp. is more prevalent than believed previously. The concurrent presence of the human-specific C. hominis and C. parvum also suggests that many of the C. canis and C. felis infections in humans were transmitted through the anthroponotic rather than the zoonotic pathway (Cama et al., 2006). There are no multilocus subtyping studies to determine whether there is any host segregation in C. canis or C. felis, although an earlier study of a small number of human and bird specimens failed to show this in C. meleagridis (Glaberman et al., 2001).

Pathogenicity of zoonotic Cryptosporidium spp. in humans

The clinical significance of various Cryptosporidium species and genotypes in humans is not yet clear. Biologically, C. parvum and C. hominis differ from each other in host specificity. Cryptosporidium parvum infects humans and calves in natural situations and mice in cross-transmission experiments, whereas C. hominis does not infect calves or mice readily (Peng et al., 1997). In antibiotic pigs, C. parvum and C. hominis differ from each other in the prepatent period, infection site, and disease severity (Pereira et al., 2002). In immunosuppressed Mongolian gerbils, no difference in oocyst shedding was observed between the two Cryptosporidium species (Baishanbo et al., 2005). Human volunteers inoculated with a C. hominis isolate also had an infection course and symptoms similar to those infected with C. parvum isolates (Chappell et al., 2006). The number of isolates tested for both species in these models is very small. It remains to be determined whether any observed differences were due to intrinsic biologic differences between the two species or variations in biologic characteristics at the isolate level.

Results of recent genotyping studies nevertheless support the theory that C. hominis and C. parvum behave differently in humans. In sporadic cases of cryptosporidiosis in the United Kingdom, samples with C. hominis were more likely scored microscopically for 2+ or 3+ than those with C. parvum (McLauchlin et al., 1999). Similarly, in a longitudinally followed cohort of children in Peru, stools with C. hominis had a significantly higher mean oocyst score than those with the zoonotic genotypes (1.7 vs. 1.3 out of 3, P=0.02). In addition, children with C. hominis had a significantly longer duration of oocyst shedding than those with the zoonotic genotypes (13.9 vs. 6.4 days, P=0.004) (Xiao et al., 2001). Likewise, oocyst shedding intensity was higher in Brazilian children infected with C. hominis than C. parvum (Bushen et al., 2007).

In addition to the differences in host specificity and oocyst shedding, C. parvum and C. hominis differ from each other in pathogenicity and clinical presentations. In sporadic cryptosporidiosis in England, C. hominis but not C. parvum was associated with an increased risk of nonintestinal sequelae such as joint pain, eye pains, recurrent headache, dizzy spells, and fatigue, even though there were no significant differences between the two species in the presence and duration of diarrhea (Hunter et al., 2004a). In AIDS patients in Lima, Peru, infections with C. canis and C. felis were more likely associated with diarrhea, and infections with C. parvum, C. canis and C. felis were associated with chronic diarrhea, and C. parvum was more likely associated with vomiting. In contrast, infections with C. meleagridis and C. hominis, especially its Ia and Ie subtype families, were more likely asymptomatic (Cama et al., 2007). These results demonstrate that C. hominis and zoonotic Cryptosporidium spp. are linked to different clinical manifestations in different populations of humans.

In a Brazilian study, although there were no differences between C. hominis and C. parvum in the occurrence of diarrhea, these two species seemingly had different nutritional effects on infected children. Height-for-age (HAZ) Z-scores showed significant declines within three months of infection for children infected with either C. hominis or C. parvum. However, in the 3–6-month period following infection, only C. hominis-infected children continued to demonstrate declining HAZ scores and those with asymptomatic infection showed an even greater decline (P=0.009). Thus, C. hominis was associated with greater growth shortfalls, even in the absence of symptoms (Bushen et al., 2007).

Conclusions

Molecular diagnostic tools are being used increasingly in studies of zoonotic cryptosporidiosis. Significant progresses has already been made in the understanding of the zoonotic potential of Cryptosporidium spp. from animals, the human disease burden attributable to several zoonotic species, the contribution of various groups of animals to cryptosporidiosis transmission in humans, and the infection sources for the so-called zoonotic species. Data are emerging to show different spectra of clinical illness between C. hominis and zoonotic Cryptosporidium spp. in humans, and differences in cryptosporidiosis transmission between developing and industrialized countries, or between rural and urban areas in industrialized nations, especially the relevance of zoonotic transmission. The use of GP60-based subtyping and more recently multilocus subtyping and multilocus sequence typing tools has increased our appreciation of anthroponotic transmission of C. parvum, which is very important in developing countries and has becomes increasingly important in industrialized countries. The development of new subtyping tools for other more divergent species will no doubt lead to better assessment of their infection sources in humans.

The use of genotyping and subtyping tools in well-designed case–control studies and longitudinal cohort studies has played a very important role in achieving this progress (Xiao et al., 2001; Hunter et al., 2004b, 2007; Bushen et al., 2007; Cama et al., 2007). Continued efforts in this area, especially more studies in developing countries and the utilization of new-generation typing tools, would provide the scientific data needed to better advice public health professionals, governmental officials, and the general public on the importance of zoonotic transmission of cryptosporidiosis, as a recent flurry of outbreaks of cryptosporidiosis, including a major one in Botswana, has attracted wide attention in the general news media. This would require close collaborations among epidemiologists, clinicians, molecular biologists, and parasitologists. Such an integrated approach will undoubtedly lead to better utilization of available molecular diagnostic tools and a better understanding of zoonotic cryptosporidiosis.

Statement

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.

Acknowledgements

Y.F. was supported by the National Natural Science Foundation of China (No. 30771881).

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Author notes

Present address: Yaoyu Feng, School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China.
Editor: Willem van Leeuwen