**2. Transmission and clinical disease**

widely used for different purposes, including companionship, life-saving actions, security as well as hunting and farming [2, 3]. Other than entertainment and sports, horses are also being kept for companionship. These livestock and companion animals are however hosts to many parasites, some of which have detrimental effects on the health and productivity of those affected. Protozoa such as *Giardia duodenalis* affect a wide range of domestic and wild animals,

*G. duodenalis* (syn. *Giardia lamblia*, *Giardia intestinalis*), a flagellate protozoan parasite, and the aetiological agent of giardiasis, is one of the most prevalent and widespread intestinal parasite in humans and several vertebrate animal species worldwide [4]. The taxonomy of the genus is mainly based on morphology and genetic evidence. According to these criteria, six species have been recognised in the genus *Giardia* and these include *G. duodenalis* in humans and other mammals, *G. agilis* in amphibians, *G. muris* and *G. microti* in rodents, *G. psittaci* and *G. ardeae* in birds. In recent years, phylogenetic analysis and enzyme electrophoresis have revealed the existence of eight assemblages A–H within the species *G. duodenalis* [5–7]. *Giardia* from humans appears to fall exclusively into Assemblage A and B while C and D are dog specific assemblages. Assemblage E is isolated from hooved animals, a characteristic of isolates from sheep, goats, cattle and pigs [8]. Cats are hosts F or Assemblage F while rats are hosts for Assemblage G [9, 10]. Assemblage H has been

*G. duodenalis* is a frequently encountered intestinal parasite of domestic animals, especially livestock, dogs and cats. *Giardia* infections have been reported widely in livestock and companion animals with varying prevalence in different parts of the world, but high frequency was mostly in dairy calves [12–16]. As a parasite, *Giardia* has a broad host range, however, the adverse consequences of infection and its pathogenic potential are best recognised in

Latin America, about 200 million people have symptomatic giardiasis with some 500,000 new cases reported each year [18]. Its simple life cycle involving an environmentally resistant cyst (**Figure 1**) provides greater opportunities for the parasite to be transmitted directly from one infected individual to another, or indirectly through contamination of the environment or

**Figure 1.** *Giardia* cyst: wet smear stained with iodine (source: https://www.cdc.gov/dpdx/giardiasis/index.html).

human cases per annum [17]. In Asia, Africa and

with serious clinical consequences especially in young animals.

reported in the grey seal [11].

40 Current Topics in Giardiasis

food [4].

humans [6]. It causes an estimated 2.8 × 10<sup>8</sup>

The cyst is the infective stage and represents the resting stage of the organism. Its rigid outer wall protects the parasite against changes in environmental temperature, dehydration and chlorination, all of which would destroy the trophozoite [6, 19, 20] . Transmission occurs by the faecal-oral route, either by direct contact with an infected host, or through contaminated food or water [21, 22]. Mechanical transmission of the parasite through insect vectors has also been reported [23]. Factors that facilitate infection include overcrowding, the high excretion of cysts by infected animals and the low infectious dose (between 10 and 25 cysts) [24, 25].

*Giardia* is not invasive and therefore lives and multiplies by asexual multiplication on the luminal surface of the small intestine of the vertebrate host [6]. Although the pathogenesis of *Giardia* is not completely understood, the pathophysiological process is initiated by infection with the parasite resulting in variable clinical signs such as abdominal pain, diarrhoea and weight loss [26]. A rise in numbers of intraepithelial lymphocytes increases epithelial permeability. Activation of T-lymphocytes has also been observed in *Giardia* infections [27, 28]. Trophozoite toxins and T-cell activation initiate a diffuse shortening of brush border microvilli and decreased activity of the small intestinal brush border enzymes, particularly lipase, proteases and disaccharidases [29–31]. The microvillus shortening leads to a decrease in overall absorptive area in the small intestine and an impaired uptake of water, electrolytes and nutrients resulting in malabsorptive diarrhoea [29, 32]. The steatorrhoea and mucous diarrhoea usually observed in giardiasis are attributed to reduced activity of lipase and increased production of mucin by goblet cells [33]. Severity of the disease is dependent on factors like developmental, nutritional and immunity of the host as well as virulence factors of the parasite [30, 34, 35]. Although gross intestinal lesions are rarely observed, microscopic lesions consisting of villous atrophy and cuboidal enterocytes may be reported [33].

## **3. Giardiasis in livestock**

#### **3.1. Cattle**

In cattle, *Giardia* is considered an important emerging parasite of dairy cattle and also as a cause of zoonotic disease with negative effect on public health [19]. Calves have been reported to be infected with *G. duodenalis* as early as 4 days of age, and the highest intensity of cyst excretion of 10<sup>5</sup> –10<sup>6</sup> cysts per gram of faeces between the ages of 1 and 3 months has been documented [36, 37]. A periparturient rise in cyst excretion has also been demonstrated [37]. Transmission occurs among infected calves as well as chronically infected adults [12, 38, 39] and is particularly high among dairy calves [38, 39]. There are four main proposed cycles of transmission that are believed to maintain host-specific and zoonotic assemblages of *Giardia* in mammalian hosts: human cycle, livestock cycle, dog/cat cycle and wildlife cycle (**Figure 2**).

**Figure 2.** Transmission cycles of *Giardia duodenalis* (*frequency of transmission is unknown*).

The livestock cycle is thought to maintain Assemblage E within the livestock group [6, 40, 41]. The other cycles maintain the assemblages in the specific hosts. For example, assemblages A and B can be maintained by direct transmission between humans, assemblage C and D between dogs (e.g. puppies in a breeding kennel) and wildlife genotypes among various wildlife species. Some assemblages, however, infect other animal species and humans. The frequency of transmission is however not very clear and still under debate. Zoonotic species have been reported in wildlife, but their role as a potentials reservoir for human infection still requires further molecular epidemiological research [4].

The resultant giardiasis from *G. duodenalis* infection can result in diarrhoea that does not respond to treatment with antibiotic or anti-coccidia drugs [33, 36, 42]. *Giardia* has been implicated as an aetiological agent alone and in combination with other enteric pathogens in calf diarrhoea [36, 38, 43, 44]. Infection may also result in numerous diarrhoea episodes which in turn adversely affects production and result in economic loses for farmers [45]. In younger calves, especially below 6 months of age, the excretion of watery faeces with a mucoid appearance may be the only indication of infection with the parasite. Chronic cases of giardiasis in calves may impact negatively on performance which may be reflected in reduced weight gain, impaired feed efficiency and decreased carcass weight. This was demonstrated in experimentally infected lambs [43].

*Giardia* has been found in both beef and dairy cattle throughout the world with varying prevalence. Infection rates can be as high as 100% [36–38, 46–50]. The infection pattern of *Giardia* appears similar between beef and dairy cattle [36, 37] with cysts appearing in the faeces at approximately 4 weeks of age [12, 36, 38] . Both dairy and beef calves may harbour more than one genotype of *G. duodenalis*, which can be of zoonotic significance [12, 51, 52]. Assemblages A, B and E have been detected in cattle; Assemblages A and B also infect humans [53, 54]. As calves infected with *Giardia* shed large numbers of cysts, there is concern that cattle could represent a reservoir of *G. duodenalis* with the potential to cause disease in humans either through direct contact or by contamination of food and/or water supplies [36]. Because of the risk of contamination of water supplies by water borne parasites such as *Giardia*, it is normally recommended that animal facilities should be located away from streams, lakes, dams and rivers whenever possible, and waterways should be fenced-off in pasture lands in order to prevent possible run-off into these water sources [55].

#### **3.2. Sheep**

The livestock cycle is thought to maintain Assemblage E within the livestock group [6, 40, 41]. The other cycles maintain the assemblages in the specific hosts. For example, assemblages A and B can be maintained by direct transmission between humans, assemblage C and D between dogs (e.g. puppies in a breeding kennel) and wildlife genotypes among various wildlife species. Some assemblages, however, infect other animal species and humans. The frequency of transmission is however not very clear and still under debate. Zoonotic species have been reported in wildlife, but their role as a potentials reservoir for human infection still

The resultant giardiasis from *G. duodenalis* infection can result in diarrhoea that does not respond to treatment with antibiotic or anti-coccidia drugs [33, 36, 42]. *Giardia* has been implicated as an aetiological agent alone and in combination with other enteric pathogens in calf diarrhoea [36, 38, 43, 44]. Infection may also result in numerous diarrhoea episodes which in turn adversely affects production and result in economic loses for farmers [45]. In younger calves, especially below 6 months of age, the excretion of watery faeces with a mucoid appearance may be the only indication of infection with the parasite. Chronic cases of giardiasis in

requires further molecular epidemiological research [4].

42 Current Topics in Giardiasis

**Figure 2.** Transmission cycles of *Giardia duodenalis* (*frequency of transmission is unknown*).

The prevalence of *G. duodenalis* infection in sheep varies considerably and may be as high as 38% in adult sheep and 68% in lambs [56–60]. In a study in central China [61], the prevalence of *G. duodenalis* was 12.36% in pre-weaned lambs and 5.74% in post-weaned sheep [61]. Other studies have also reported great variability in *Giardia* prevalence: in Canada, prevalence of giardiasis was higher in lambs (57%) than in adults (9%) [62]; in Brazil, lambs had a 32% infection rate while that for ewes was 2%; [63]; and in Mongolia, China, lambs had a significantly higher infection rate than ewes (8.6 versus 0.9%, respectively) [64]. All the findings from these studies suggest that the infection rates of *Giardia* tend to decline as the age of the animals increases. However, the opposite has also been reported. In some studies in Australia, a much higher prevalence of was detected in post-weaned lambs and sheep (44%) than in pre-weaned sheep (11.1%) [56, 65]. In a study in Maryland, USA, the prevalence of giardiasis was higher in postparturient ewes (12%) than in lambs (4%) [59]. Host age and immune status of the host affect the severity of the disease [6] but other factors such as the number of specimens examined, the age structure of the herds, management procedures and the health status of the animals may account for the discrepancies or variations in the infection rates in the different populations [61].

Because of the unexpectedly high levels of infection in sheep, sheep have long been considered a reservoir of human infections [56, 61, 66–68]. In most cases, infections are asymptomatic but infected animals are carriers shedding large numbers of cysts into the environment [58]. Even if most infections are asymptomatic, infections in lambs may result in a malabsorption syndrome, decreased feed efficiency and subsequently a decreased weight gain and sometimes death [19, 43, 69]. In the study by [69], excretion of malodorous and poorly formed faeces was observed. Furthermore, giardiasis may have a negative effect on time to slaughter of the sheep [19, 43] therefore, negatively affecting producers' income.

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

#### **3.3. Goats**

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

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

#### **3.4. Pigs**

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 asymptomatic with no evidence of illness.

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

Since pigs also harbour the zoonotic assemblage A, they should be considered as potential sources of infection. One case–control study in eastern England found an association between giardiasis and exposure to farm animals, pigs included [89].
