**4. Companion animals**

#### **4.1. Dogs and cats**

Three assemblages of *G. duodenalis* have been recognised in sheep, livestock assemblage E, and the two zoonotic assemblages A and B [13, 56, 59]. The non-zoonotic assemblage E is the most frequently reported compared to the zoonotic ones [59, 66, 68, 69]. However, assemblage E appears to occur most frequently in cattle compared to other livestock; this was demonstrated by an extensive, longitudinal study of dairy herds in Australia over several months

In small ruminants, there are considerably more surveys from sheep populations than goat populations and therefore fewer publications on *Giardia* in goats. Furthermore, only a few molecular studies regarding *Giardia* have been performed worldwide [13, 58, 71–74] compared to other ruminant hosts (see [4]). In the reported studies, *Giardia* prevalence was reported to range from <10 to >40% depending on the age, geographical location and diagnostic technique used [75]. Infections are normally significantly higher in pre-weaned goat kids compared to that in older goat kids [74]. Most infections are asymptomatic, however, foul-smelling diarrhoea which is lightly coloured, greasy and mixed with mucous; reduced weight gain are clinical signs that may be observed, mostly in young animals that are symptomatic [71]. A study in Spain reported a high infection rate in young animals, agreeing with the hypothesis that to a great extent, young animals contribute to the environmental contamination with *Giardia* cysts [71]. A study in Nigeria also reported a high prevalence (46.9%) in goats with pre-weaned (≤3 months) goats having a much higher prevalence (58.1%) compared

Even though a large number of *G. duodenalis* genotyping studies in ruminants report a higher occurrence of genotype E, with genotypes A and B being less frequent [13, 58, 76, 77], other studies, [13, 72] have reported zoonotic genotype A infections in goats in Belgium and Côte d'Ivoire, respectively. In Malaysia, one study [73] reported genotypes A and B in goats. These

There is limited information on the *Giardia* infections in pigs. From the limited studies, *Giardia* infections have been reported in all age groups from nursing piglets to boars and sows worldwide, from Australia, Asia, Europe and North America, Africa with varying prevalence ranging between 0.1 and 20% [62, 78–86]. Natural infections are typically asymptom-

Both assemblages E and A have been identified in pigs with assemblage E being most common [4]. In one study in Australia, assemblage E was the most common genotype detected in positive specimens of both pre-weaned (64%) and post-weaned (67%) pigs [87]. In Denmark, assemblage E was also the most common genotype, being identified in 62% of samples from post-weaned pigs, while assemblage A was detected in only 12% of specimens [85].

findings suggest that goats could be a potential source of zoonotic infection.

Interestingly, the canine assemblage D has also been reported in pigs [85, 88].

and another study in Canada [12, 56, 70].

to those that were over 3 months (38.2%) [74].

**3.3. Goats**

44 Current Topics in Giardiasis

**3.4. Pigs**

atic with no evidence of illness.

*Giardia* is commonly recovered from the faeces of both symptomatic and asymptomatic dogs worldwide [90, 91]. Several studies have reported high prevalence of *Giardia* in stool samples of companion animals (i.e. cats and dogs) (reviewed by [92]). *Giardia* infection rates in dogs differ considerably based on many variables, including the composition of dog populations (owned/stray/kennel), the test used for diagnosis and its sensitivity. Similar to other animal species, severity of disease depends on host age and ability of the immunity to eliminate the infection. Reports of giardiasis range from 0.1% in owned dogs to as high as 100% in kenneled dogs, the risk factor being overcrowding and intensive contact between large numbers of dogs sharing the same shelter in kenneled dogs. This favours transmission of infections [6, 15, 93–96]. Some studies have indicated *Giardia* to be the most common enteric parasite of dogs and cats. For example, studies in Australia found that *G. duodenalis* was the most common enteric parasite of domestic dogs and cats [97, 98] while [99] also reported the parasite to be widely prevalent in dogs and cats in the USA. The prevalence of *Giardia* in these companion animals is however, believed to be underestimated because of the following reasons: the low sensitivity of the conventional detection methods, cyst excretion is intermittent and the disease is usually subclinical [98].

In most of the studies that have been conducted in dogs, puppies, free-roaming dogs, and shelter dogs have been shown to be at higher risk for infection than adult dogs and owned dogs [15, 94]. Transmission of the parasite appears to be maintained within the dog/cat cycle (**Figure 2**) as evidenced from the host specific assemblage C/D and F commonly isolated in dogs and cats respectively [15, 100]. However, zoonotic transmission of *Giardia* between humans and dogs in the same household has been reported previously [101]. In another study in Brazil, zoonotic assemblage A1 was isolated from dogs and children in the same locality suggesting the existence of a zoonotic cycle of the parasite in that community [102], and a study in Thailand revealed that dogs were a potential source of *Giardia* infections for humans [103]. In this study [103], assemblages A (79%) and B (21%) in addition to the dog specific assemblages C (12%) and D (31%) were isolated from the 104 dogs tested. In the United States, one study reported that 28 and 41% of client-owned dogs presenting with infection with *Giardia* to veterinary clinics had potentially zoonotic assemblages A and B, respectively, while 15 and 16% had host specific assemblages C and D, respectively [104]. The findings from the American study suggest the possibility of the potential for transmission of non-canine-specific assemblages from owners to their dogs as well as zoonotic transmission from dogs to humans. Furthermore, such reports highlight the possibility of two transmission cycles existing in domestic urban environments, that is, transmission of dog-specific assemblages among dogs and the possible transmission of assemblage A between pets and humans. However, it has been reported that in household dogs, the frequency of dog-to-dog transmission may be lower because they are less crowded than kenneled dogs where prevalence is normally higher due to intensive contact among a large number of dogs [54, 91].

Although *Giardia* is common in dogs and cats, it is rarely associated with clinical disease and affected animals suffer minimal consequences of the disease, but may act as a source of zoonotic infection [103, 104]. However, complications such as persistent infections and impairment of growth and development may occur especially in young animals such as puppies and kittens [105]. Such infections with manifestation of clinical signs are usually associated with kennel or cattery setup, where there is overcrowding [106].

#### **4.2. Horses**

There is very few data on *Giardia* in horses and giardiasis is an uncommon condition in these animals. However, the parasite may be commonly found in faeces of asymptomatic animals. The parasite was first reported in horses in South Africa in 1921 [107]. Since then a number of reports have been made regarding the presence of the parasite in horses of all age groups. Relatively high rates of giardiasis among foals (17–35%) and lactating mares (1.9–27.8%) have been documented using the fluorescent antibody method [38]. Lower rates have been observed in weanlings (0–9.1%) [108]. Varying prevalence of giardiasis has been reported in different geographic areas and they differ considerably between locations [62, 109, 110] with age and physiological status of the animal playing an important role in the infection rates [38, 110].

Although giardiasis in horses has been found to be associated with diarrhoea, poor hair coat, ill thrift and weight loss [111, 112], infected horses rarely show any clinical signs [108] and no subclinical consequences have been reported previously. However, infected horses may show signs ranging from a mild and self-limiting to, occasionally, severe diarrhoea (with heavy infections). These are commonly seen in young and aged or immunologically suppressed horses [110, 113, 114]. However, some studies have reported no shedding of *Giardia* cysts in young and older horses [115].

*G. duodenalis* assemblages A, B, and E have been detected in horses [110, 116]. A study in Italy also confirmed the presence of both animal and human sub-assemblage of *G. duodenalis* in horses [117]. However, assemblage E appears to be more common in these animals [110]. Because assemblages A and B are known to infect humans [6, 118], horses could represent a reservoir of *G. duodenalis* with the potential to cause disease in humans through direct contact or by contamination of food and/or water supplies.
