Giardiasis infection in dogs

Introduction

With the availability of better diagnostic methods, the single-celled parasite Giardia duodenalis is nowadays frequently detected in dogs and cats. However, infection can range in severity, from subclinical to causing a variety of gastrointestinal (GI) signs, and the clinician may have many questions surrounding this parasite – for example, the sensitivity and specificity of diagnostic tests, the need to treat or not, and the best options for treatment. Furthermore, clinical signs may persist after treatment, or a feces test can remain positive despite successful therapy and clinical improvement, so given that this parasite can raise so many questions, it is important that the diagnosis, treatment and management of Giardia infections is clear and unambiguous. Answers – sometimes confounding – can be obtained from different sources: from a parasitologist, a GI specialist, a laboratory, or the manufacturer of drugs registered for the treatment of Giardia. It is therefore not easy to provide general advice that is valid in all situations, but a unified approach to Giardia infections leads to better control and ensures that any other GI problems present can be recognized at an early stage. This article aims to provide information on which to base such an approach.

Epidemiology

The flagellate Giardia duodenalis (syn. G. lamblia, G. intestinalis) occurs worldwide as a protozoan intestinal parasite in mammals (including humans), birds, reptiles and amphibians. With genotyping, the parasite can be divided into 8 groups (known as assemblages A to H), which generally exhibit clear host specificity. Assemblages A and B occur in humans, C and D in dogs, and F in cats. Assemblages A and B are sometimes found in dogs and cats, but C, D or F have to date rarely been demonstrated in humans 1,2.

Giardia prevalence in humans ranges from 0.4-7.5% in western countries to 8-30% in non-industrialized countries 3, and it is estimated that more than 1 billion people worldwide are infected with the parasite 4. Infection rates in humans, dogs and cats vary widely by country, living conditions and testing methods. In Europe, prevalences of 3-7% are reported in domestic dogs, but in kennels it can be as high as 46% 3,5. In non-industrialized countries, the parasite may be found in 10-30% of pet dogs 3. In a Dutch study that looked at 381 dogs without signs, hunting dogs were found to have the highest prevalence of Giardia (65%) while a random group of pet dogs in the same study had a prevalence of 8%. Another study reported a prevalence of 25% in fecal samples sent to a diagnostic laboratory from 192 dogs that did have clinical GI signs 6.

Life cycle

Giardia has a direct life cycle. After ingestion of infectious cysts from food, drinking water or the environment, motile trophozoites excyst in the anterior part of the small intestine and then attach to the mucosa with a ventral suction cup. Asexual multiplication takes place, after which the trophozoites encyst further down the small intestine and are then excreted in the feces in large numbers (sometimes intermittently) for weeks to months (Figure 1). Mobile trophozoites can be seen in fresh, still-warm stools if there is accelerated intestinal passage, (i.e., diarrhea) but these do not survive gastric passage and are therefore not infectious (Figure 2). The immobile cysts are highly resistant (hence the persistently high environmental levels), and are immediately infectious once excreted (Figure 3). Feco-oral contamination occurs by ingestion of cysts via coprophagy, or via fur, food, soil or drinking water contaminated by feces. In humans it has been shown that infection can occur with very low numbers (10-100) of cysts 7. Infection can remain active for weeks to months and be acute, chronic or subclinical. The incubation period in dogs is four to sixteen days, and the minimum time between time of infection and the first opportunity to detect the parasite in feces (the prepatent period) averages seven days. Although cysts can survive for months in the environment, they are sensitive to sunlight and dehydration and are greatly reduced in number by freeze-thaw cycles 5,8; the colder and more humid the environment, the longer the cysts remain infectious.

Asymptomatic carriers of Giardia 9 can infect the environment unnoticed for long periods. It is assumed that infection induces partial immunity to an individual, which reduces clinical signs, and that the infection eventually disappears, after which the host shows limited resistance to reinfection 5.

Clinical signs

Giardia infection is usually subclinical in nature and is often self-limiting, but it can cause chronic intermittent soft or slimy stools and even watery diarrhea. In addition, anorexia, vomiting, emaciation and lethargy may occur, especially in animals with reduced immunity, puppies, in the presence of concurrent infection, or working dogs. Where young puppies are infected, retarded growth and development may be observed. 

At the intestinal level, little inflammation is seen in the acute stage. Maldigestion, malabsorption and malsecretion explain the clinical picture. The severity of signs is highly dependent on various factors, including the Giardia strain, host immunity, age and nutritional status, and the presence of any other infections. However, if a host also has a worm burden, this seems to inhibit the development of the Giardia population, with a lower number of cysts being found on microscopic examination 10. Treatment with anthelmintics may therefore increase susceptibility to Giardia infection, probably because worm-induced T-helper 2 activity ensures a good immune response against Giardia 11,12. Although the parasite does not invade the mucosa, changes inside and outside the GI tract have been described in humans that can lead to growth retardation and even emaciation, as well as chronic post-infectious changes, again both within the GI tract and elsewhere in the body 13. 

Given that the prevalence of Giardia varies greatly, it can be difficult to predict how frequently it may account for clinical signs in dogs. Infection was not found to be significantly associated with loose stools in a recent study of 1,291 dogs from various backgrounds (namely, household, shelter, hunting and laboratory-kept dogs), although household dogs were significantly more likely to test Giardia-positive if they had diarrhea. Young dogs and dogs in kennels were significantly more likely to be positive for the parasite on testing, and these associations were consistent across different diagnostic testing methods. Young dogs and dogs with clinical signs were noted to excrete the highest numbers of Giardia cysts 10.

Diagnostics

Several tests are available for the detection of the Giardia parasite 6,14; these are: 

• Fecal smear (using still-warm thin stools viewed immediately for the presence or absence of moving trophozoites)
• Passive flotation technique
• Centrifuge-sedimentation flotation (CSF) 
• Rapid (ELISA) point-of-care (benchtop) tests (often based on the detection of cyst wall protein in feces)
• Immunofluorescence test (IFT)/direct immunofluorescence assay (DFA)
• Polymerase chain reaction (PCR) 

The relative sensitivity of each method is shown in Figure 4. Note that IFT and PCR tests can only be performed in specialist laboratories. 

At least one study has reviewed the different tests available. Fecal samples from 573 dogs were examined using a variety of methods (CSF, microscopic analysis, DFA, rapid enzyme immunochromatographic assay, and qPCR) for the presence of G. duodenalis. It was concluded that all tests were highly specific, with the rapid assay showed the highest relative specificity (99.6%) and qPCR the lowest (85.6%). The relative sensitivities were much more variable, with qPCR showing the highest (97.0%) and CSF the lowest (48.2%) sensitivity. DFA was more sensitive than the rapid point-of-care assay, but slightly less specific. Methods that involve microscopy for cyst identification or cyst wall detection should be used in situations where high specificity is required 6, but it is debatable if this is necessary for a diagnosis when investigating clinical cases of diarrhea.

Because the sensitivity and specificity of the various diagnostic tests are not 100%, a negative test does not completely exclude infection, and a positive result does not guarantee the presence of Giardia infection. Trophozoites are mainly detected when diarrhea is severe, and cysts tend to be excreted in moderate numbers and intermittently. On the other hand, a true positive test result does not always mean that Giardia is the cause of the clinical signs, only that the cysts (or cyst wall proteins) are present. In a study of 152 healthy domestic dogs without clinical signs, Giardia cysts were found in the feces in 15% of the animals using CSF 15, whilst in a study of 8,685 dogs with diarrhea or vomiting, 24.8% of animals tested positive for Giardia using a rapid ELISA test 16.

Because Giardia cysts can be excreted intermittently, fecal examination with CSF can yield a false negative result, especially if an animal is excreting only moderate numbers of cysts. However, reliability increases when feces from three consecutive days are tested. This does not apply to the increasingly common rapid benchtop tests, where one sample is sufficient for a reliable result. An advantage of the CSF method is that it can give information about the presence of other parasites, whereas many rapid tests only detect Giardia. Indeed, studies have shown that mixed infections are often found in dogs with diarrhea 7, so a general fecal examination may therefore add value. In addition to worm infections (Figure 5), protozoan infections such as Cystoisospora spp. should also be considered (Figure 6). With the rapid test, a false positive result may occur if cyst wall protein is still present in the intestine even when no viable cysts remain; the proteins can still be detectable for 1 to 2 days after infection has passed (due to the transit time from the small intestine to fecal excretion). In practice, positive rapid benchtop tests are sometimes seen in animals with no clinical signs following treatment, when other diagnostic tests return a negative result. 

Because Giardia can also be present in dogs without signs, the clinical picture should always be the deciding factor; if trophozoites, cysts or cyst wall proteins are detected in a healthy dog that shows no clinical signs, initiating therapy is not necessary, although it is useful to monitor the animal. However, the situation may be very different if an asymptomatic animal which has tested positive is introduced into a negative population of susceptible animals.

For animals with trophozoites detected on fecal smear, or where there is a positive CSF or rapid benchtop test with matching clinical signs, therapy is a useful course of action. Dogs in shelters and kennels will often show persistent or recurrent signs suggestive of a Giardia infection and will frequently test positive. There is no clear correlation between the number of cysts found in feces and the severity of infection. The same applies to the presence of trophozoites in feces, and detection of such does not necessarily indicate serious infection, but does indicate accelerated transport through the intestine (i.e., diarrhea), which may either be due to the presence of Giardia or other causes. 

Treatment

Household dogs

A three-day course of fenbendazole (50 mg/kg q24H) is licensed in some countries for treating Giardia infections in dogs, and is the drug of first choice 17,18,19; however, three days of therapy can be insufficient in some cases, and there is also the risk of autoinfection/re-infection. Treatment of longer duration (e.g., up to 10 days) is therefore sometimes recommended, although note this may be outside the manufacturer’s recommendations in many countries 20. When dealing with clinical infections, the patient’s perineum, hindquarters and hind legs can be washed (e.g., with a chlorhexidine shampoo) to remove cysts from the hair. This is particularly useful if the risk of reinfection from the environment is very low and therefore the likelihood of auto-infection from an animal washing itself is relatively high. Cleaning up feces as soon as possible after toileting is always recommended.

If there is insufficient improvement after a week on treatment, and other causes such as concurrent infections have been ruled out, therapy can be repeated. If necessary, an alternative option is metronidazole (25 mg/kg q12H for five days, or as advised by the data sheet). With this drug, keep in mind that neurological side effects can sometimes occur, but there is some evidence that a dose of 25 mg/kg q24H is also effective and greatly reduces the risk of side effects 5,16. Metronidazole should be used with caution due to antibiotic resistance issues.

If multiple dogs are present in a household, it should be borne in mind that reinfection can be caused not only by the patient itself or from the environment, but also by an asymptomatic carrier in the house, and it may be prudent to test the other animals in the household as well. 

In addition to medication, an easily digestible gastrointestinal diet can help support recovery, especially where metronidazole has been used, as it can have a negative impact on the intestinal flora. Some publications will recommend a low-fiber, low carbohydrate, high protein diet to prevent rapid growth and multiplication of Giardia and Clostridium spp. However, improvement is sometimes seen with a diet that has higher crude fiber content and is less digestible. The substrate change, together with the host’s immunity and possible medication, can slow the growth of Giardia to such an extent that the immune balance reverses and the host controls the infection by itself. 

The prognosis for giardiasis is usually good, but in young, dehydrated or older animals, and for those with reduced immunity, there is an increased risk of complications; for instance, attachment of the Giardia trophozoites may lead to breakdown of the tight junctions of the intestinal epithelium, potentially resulting in secondary bacterial infections 21. Experience shows that despite all measures, some chronic cases can persist with little or no response to therapy. If the animal has been treated and Giardia is no longer present, yet clinical signs persist, then further investigation is advised – for example, to look for other protozoal infections, chronic intestinal inflammation and food allergies. 

Kennels

For larger groups of animals under kennel conditions with proven Giardia infection, it is advisable to divide the animals into smaller groups or to house them individually. In this situation it is also useful to treat asymptomatic animals which test positive for Giardia in order to reduce environmental contamination as much as possible. At the end of the treatment period the animals should be washed and transferred to a clean, dry and disinfected pen. Care must obviously be taken to ensure that the animals have no access to potentially contaminated natural water sources afterwards.

A study has described the control of Giardia in a group of test dogs 22, where individually kept beagles were treated with a combination of febantel (a precursor of fenbendazole)/praziquantel/pyrantel for three days. The pens, constructed from stainless steel with a smooth epoxy floor, were disinfected daily with quaternary ammonium compounds or sodium hypochlorite solution (bleach). The latter substance is commonly used, but an insufficient effect is regularly reported. Despite this, Giardia was found to be present again in several dogs after 17-24 days, and all dogs developed diarrhea again. When the dogs were washed on the last day of re-treatment and then moved to a clean pen, the problems disappeared.

Environment

When recurrences occur (and especially in kennels), attempts can be made to reduce the infection pressure by treating the environment wherever possible. Reinfection can easily occur if a dog eats feces from other animals, or drinks from an infected garden pond, or from puddles of water. The plan of action therefore depends on the situation. 

After cleaning smooth surfaces within the household and allowing them to dry thoroughly, it is essential to render cysts inactive via disinfection with a quaternary ammonium compound. However, these are not widely available for private use and only work in a clean environment without soap residues. For effective disinfection, the agent must be in contact with the surface for a sufficiently long period of time, often at least five minutes, so the manufacturer’s package leaflet should always be consulted. Carpets and fabric furniture can be treated with hot water or steam (5 minutes at 70°C or 1 minute at 100°C) 8, but advance testing to ensure that the surfaces can withstand these methods is recommended. Hot water (dishwashers, washing machines) can also be used to disinfect clothing, bedding, toys and food bowls, and again treatment duration depends on the temperature; water at 45°C disinfects in 20 minutes, but at 70°C only requires 5 minutes. A tumble dryer and sunlight can also contribute significantly to the disinfection process. If any cars are used to transport affected dogs (including e.g., a dog-walking service), these should also be disinfected. Since Giardia can also cause infection via surface water, dogs should be prevented from drinking water from outside and licking the grass etc., as far as possible. 

Follow-up and possible re-infection

Given the prepatent period of at least seven days, repeating fecal testing in clinical infections no sooner than eight to ten days after the end of treatment is recommended. Re-testing is really only useful if the animal still presents with clinical signs or if the animal is transferred to a closed, Giardia-free (susceptible) population. If signs persist, the possibility of another cause should be considered, but recurrence is a potential problem, either by reinfection or possibly due to insufficient efficacy of the initiated therapy or lack of compliance. A summary for the various diagnostic and treatment procedures to be followed is given in Figure 7.

Is Giardia a zoonotic disease?

It is often mentioned that dog and cat Giardia spp. can also infect humans, but is this parasite a true zoonosis? In fact, the risk of transmission from dog or cat to humans is very low 23,24. The specific Giardia assemblages (C and D) from dogs and (F) from cats are rarely found in humans 3. Conversely, human assemblages can circulate in dog and cat populations, and humans can apparently be the source of infection for dogs and cats, after which these animals in turn pose a zoonotic risk. In cases where both family members and pets show symptoms suggestive of Giardia infection, mutual transmission of a human assemblage A or B is possible. In cases of diagnosed Giardia infection in a pet, the veterinarian may inquire if GI signs are present in any family member and, if the answer is positive, can advise consulting a doctor.

Conclusion

Fecal examination, centrifuge sedimentation flotation and rapid benchtop tests can all be used to diagnose Giardia in dogs presented to the clinic. If clinical signs are present and a Giardia test is positive, fenbendazole is the first-choice therapy, but a healthy animal that tests positive yet has no signs usually does not require treatment. Where there is recurrence, it may be useful to wash the hindquarters of the patient, and it is also important to clean and disinfect the environment, food and drinking bowls, and to prevent dogs from drinking surface water from the environment or eating feces. The risk of transmission of Giardia from dogs to humans is, however, very low.