**5. Diagnosis**

The diagnosis of giardiasis is commonly established by microscopic identification of cysts or less commonly trophozoites in faecal specimens stained with trichrome (**Figure 3**) or iron

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

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

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

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

*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

The diagnosis of giardiasis is commonly established by microscopic identification of cysts or less commonly trophozoites in faecal specimens stained with trichrome (**Figure 3**) or iron

among a large number of dogs [54, 91].

**4.2. Horses**

46 Current Topics in Giardiasis

the infection rates [38, 110].

young and older horses [115].

**5. Diagnosis**

or by contamination of food and/or water supplies.

kennel or cattery setup, where there is overcrowding [106].

**Figure 3.** *Giardia* cysts stained with trichrome stain (source: https://www.cdc.gov/dpdx/giardiasis/index.html).

haematoxylin. This follows the application of faecal concentration techniques, especially zinc sulphate flotation and centrifugation [119]. Direct smear or wet mount examination for trophozoites can also be performed. However, because of the cyclical nature of cyst excretion, several samples need to be examined to detect the organism [120]. The disadvantage of microscopy is that it is of limited epidemiological value as it does not indicate the source of the infection [6].

Faecal immunoassays have been developed and these have improved the sensitivity of detecting the parasite in faecal specimens. The sensitivity and specificity of different assays is reported to range from 87 to 100% [121, 122]. Enzyme-linked immunosorbent assay (ELISA) is the mostly used immunoassay and it has enhanced the detection of the parasite in field samples and a number of kits are commercially available [120]. Furthermore, the development of direct immunofluorescence microscopy (antigen detection) has generally improved the sensitivity of detecting and quantifying faecal *Giardia* cysts and may allow for more accurate determination of prevalence rates and cyst excretion intensities compared to the conventional microscopy [46]. However, despite antigen detection being more sensitive than conventional microscopy, the method cannot discriminate between species or morphologically similar organisms. The other disadvantage is the need for a fluorescent microscope which is costly [123].

To overcome the non-discriminatory nature of the conventional microscopy, molecular techniques, particularly PCR-based procedures have been developed and have greater sensitivity and specificity than the techniques that rely on microscopy and/or immunodiagnosis [98]. For example, in a survey of dogs in India, microscopy detected only 3% prevalence compared to 20% with PCR [101]. The molecular methods are also able to provide information on the genotypes and species of *Giardia*, information that is necessary for determining the source of infection. PCR, when combined with restriction fragment length polymorphism (RFLP) analysis is faster when compared to sequencing which is also costly [124]. Although PCR has high sensitivity, results may be affected by amplification inhibitors and barriers to DNA extraction in faecal samples [125]. Moreover, PCR assays are very costly for diagnostic laboratory use [126] and are therefore commonly used in research.

Serodiagnosis cannot be used to differentiate between present and previous infection and is therefore not useful for the diagnosis of giardiasis.

## **6. Treatment**

Treatment of giardiasis in livestock is through use of fenbendazole and albendazole, which have been shown to be effective in the elimination of *Giardia* from both housed and range calves [32, 127–129] as well as improving the mucosal microvillus structure and function within a week [129]. In sheep, treatment with fenbendazole at a dose of 10 mg/kg for three consecutive days, has been shown to successfully clear the infection. In an outbreak of giardiasis on a sheep farm, *Giardia*-infected lambs (30–90 days of age) presenting with malabsorption, decreased weight gain, and reduced feed efficiency recovered rapidly from the symptoms and poor weight gain after treatment with fenbendazole [69]. Similarly, in calves experimentally infected with *G*. *duodenalis* and treated with fenbendazole, a significant difference in weight gain was noticed between fenbendazole-treated and untreated calves. Animals in the treatment group gained on average 2.86 kg (equal to 102 g per day) more than the animals in the control group [27]. However, in some other treatment studies where fenbendazole or paromomycin sulphate were used, differences in mean body weight, average daily weight gain, or feed intake between the control and treated groups were not significant, although there was a slightly higher weight gain and lower occurrence of diarrhoea in the treated groups [12, 42].

In dogs and cats, fenbendazole is the commonly used therapy, normally given once daily for 3–5 days. Albendazole can be used but it has been associated with bone marrow suppression in both dogs and cats, and so no longer being used in both animal species [130]. Vaccines for *Giardia* in dogs and cats have been developed and they are reported to have the ability to reduce the duration of shedding of cysts which may subsequently reduce environmental contamination [131]. A prolonged treatment up to 5 days was shown not to be statistically better than treatment for three consecutive days [132]. On the other hand, metronidazole has been used to treat giardiasis in horses, with resolution of clinical signs after treatment [112].

## **7. Conclusions**

*Giardia* infections are prevalent in livestock and companion animals. A number of studies have reported and genotyped *Giardia* in domestic animals, particularly livestock and companion animals, and have found that they may be infected with zoonotic or species-specific genotypes. However, there is still limited information on infection rates in pigs and horses. Further, the role of these animals and dogs in the zoonotic transmission of *Giardia* still needs further investigation. Studies reporting the existence of zoonotic assemblages in both animals and humans in the same locality (e.g. for dogs) emphasise the need for further studies on zoonotic transmission of *Giardia*. Such information will assist in further highlighting the public health significance of *Giardia*. Increased interaction and the nature of the interaction between companion animals and their owners can determine whether zoonotic infection occurs or not.

Economic implications of the disease in terms of treatment costs that the farmers have to incur cannot be overlooked especially in livestock (particularly dairy calves). Giardiasis adversely affects production; and chronic cases may impact negatively on the performance of affected animals resulting in reduced weight gain, impaired feed efficiency and delayed maturity. These loses translate into food loses.

Unfortunately, giardiasis in humans is not a health priority in most countries but the effect of the parasite in terms of patient well-being and its effect on quality of life have been highlighted by many authors, highlighting its impact on human health.

A better understanding of the disease in animals (livestock and companion animals), the species and transmission patterns is necessary for appropriate prevention and control strategies which should result in increased livestock production and reduced treatment costs for the farmers or animal owners. More molecular epidemiological studies are required especially in areas where these have not been conducted such as sub-Saharan Africa to understand and probably be able to relate human and animal infections. Treatment of *Giardia* infection in both livestock and companion animals is recommended whether or not they are clinically ill, because of the potential for zoonotic transmission.
