Update on Giardiasis: Diagnostics, Treatment, and Management

The protozoan parasite Giardia duodenalis is transmitted in a fecal–oral cycle when cysts shed into the environment by infected hosts are ingested by naïve hosts. Cysts shed in feces are immediately infectious and extremely robust in the environment, making control of environmental contamination difficult. After cysts have been ingested, they excyst in the duodenum, where they become trophozoites that colonize the small intestine using a structure known as the ventral disc to adhere to the epithelial cells. In addition to causing physical damage to the gut epithelium, trophozoites induce molecular changes, which in combination disrupt nutrient absorption and barrier function of the gut epithelium.¹ Trophozoites encyst as they enter the large intestine; cysts usually become detectable within 3 to 10 days of infection. Occasionally, trophozoites may be passed in the feces during acute episodes of diarrhea if organisms do not have time to encyst before being voided; however, trophozoites cannot persist in the environment.²

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Giardiasis more frequently affects young dogs and cats (<1 year of age) than adults.^2-4 The most common clinical signs associated with Giardia infection are malabsorption, malodorous feces, chronic diarrhea, and weight loss or lack of weight gain. Apparently healthy animals (usually adults) may harbor chronic subclinical infections and persistently shed cysts; due to the absence of clinical signs, the incidence of this phenomenon is likely underdiagnosed.^4-6

Public Health Concerns

G duodenalis is found in all parts of the world and infects a variety of hosts, which leads to some concern regarding Giardia infection in companion animals and the possibility of zoonotic pet-to-owner transmission.

Within the past few decades, it has been determined that despite having identical morphology, Giardia recovered from different host species tend to differ genetically.⁷ These genetic differences have led to the classification of Giardia into 8 distinct genetic assemblages (A–H), most of which are highly host specific. In the United States, assemblages C and D are thus far limited to dogs, assemblage F is limited to cats, and assemblages A and B are considered to have zoonotic potential.^7,8 However, even within these potentially zoonotic assemblages, some subassemblages seem to have a broader host range and therefore carry greater zoonotic potential than others. Assemblage A-II is found almost exclusively in humans; A-III and A-IV are found in animals; and A-I has a wider host range, which includes humans, dogs, and cats.^9-11

Of note, recent molecular data indicate that cats may carry zoonotic assemblages more frequently than their canine counterparts. Although nationwide, overall Giardia prevalence among surveyed cats was nearly half that among dogs (7.48% and 14.18%, respectively) and 19.1% of Giardia-positive cats carried potentially zoonotic assemblages, compared with only 3.7% of Giardia-positive dogs.¹¹ However, current evidence indicates that although zoonotic risk exists and may be of greater concern with regard to children or immunocompromised persons, most Giardia infections are host specific and it is unlikely that transmission between companion animals and owners is a common source of Giardia infection in humans.^12-14

Diagnosis of Giardiasis

There are several tests for diagnosing Giardia infection in companion animals, although to optimize diagnosis, they may need to be used repeatedly or in combination.¹⁵

Fecal flotation is less sensitive than molecular-based testing¹⁶; it is commonly used to screen apparently healthy animals and may also be used to diagnose giardiasis in animals with suspected infection. Giardia cysts are best recovered after centrifugal flotation in zinc sulfate solution; addition of Lugol’s iodine stain may enhance visualization of cysts (FIGURE 1).¹⁷ Although sodium nitrate and sugar solutions can also be used for flotation, they can collapse or completely destroy cysts, making identification of any remaining cysts more difficult.¹⁸ Direct smears can also be used to detect trophozoites in diarrheic feces.

Figure 1A. Comparison of Giardia cysts (8–12 × 7–10 μm) (arrows) floated in zinc sulfate solution with Lugol’s iodine stain.

Figure 1B. Comparison of Giardia cysts (8–12 × 7–10 μm) (arrows) floated in zinc sulfate solution without Lugol’s iodine stain.

More and more, the use of patient-side enzyme-linked immunosorbent assays (ELISAs) and the ability to send samples to reference laboratories has lessened the need for general practitioners to run flotations and direct smears. In the United States, 2 patient-side rapid ELISAs are available: the SNAP Giardia Test (IDEXX, idexx.com) and the VETSCAN Canine Giardia Rapid Test (Zoetis, zoetisus.com), which detect cyst-associated antigen in feces. Commercial ELISAs and fluorescence antibody assays used to detect Giardia antigen are also available through reference laboratories and academic institution diagnostic laboratories.

Among the newest additions and improvements to the arsenal of Giardia diagnostics are in-clinic artificial intelligence (AI) microscopes and increased accessibility to polymerase chain reaction (PCR) testing. Two AI microscopes are now on the market: the VETSCAN IMAGYST (Zoetis, vetscanimagyst.com) and the Element AIM (Heska, heska.com), both of which can identify Giardia, among other parasites. PCR detection of Giardia in the feces is also becoming more widely available; diagnostic testing is offered by a number of academic institution laboratories and is included as part of larger PCR panels offered by commercial laboratories, such as IDEXX (Diarrhea RealPCR Panels) and Antech (KeyScreen GI Parasite PCR, antechdiagnostics.com). Although PCR panels are more expensive than traditional microscopy and patient-side tests, they offer the advantage of testing for a variety of targets with high sensitivity in a relatively short amount of time. The Antech KeyScreen GI Parasite PCR additionally identifies the most common, potentially zoonotic, assemblages of Giardia and is currently the only commercially available test that includes assemblage typing as part of the results. Note that ELISA and PCR testing are not recommended for routine screening but are intended for testing animals with suspicious clinical signs.¹⁷

Giardiasis Treatment

Treating giardiasis can be frustrating for veterinarians and clients alike, for several reasons. Occasionally, initial treatment does not resolve clinical signs, which may result from poor client compliance or lack of adherence to the veterinarian’s treatment and management recommendations but may also result from unrealistic expectations associated with the performance of specific drugs or parasite resistance to certain drugs. Furthermore, and perhaps more commonly experienced, treatment may resolve clinical signs but the infected animal continues to shed cysts in low numbers or have positive PCR or antigen test results. To reduce frustration with these cases and avoid unnecessary repeated drug therapy, the ultimate goal of the treatment plan should be considered.

The primary goals of treatment should be resolving clinical signs, educating clients regarding their pet’s infection status, and creating an appropriate management plan with clients. Although total clearance of infection is certainly an ideal goal, for many patients it is unattainable. All treatment regimens should involve an integrated approach, should not rely on drugs alone, and should include measures such as picking up feces, monitoring where the pet defecates, washing bedding, decontaminating hard surfaces, routinely bathing the pet, and promoting overall gut health with nutritional therapies including a high-fiber diet and probiotics.¹⁹

Until recently in the United States, there were no drugs labeled explicitly for treatment of Giardia in companion animals, although there are a few drugs that have conventionally been used to treat giardiasis and are recommended by the Companion Animal Parasite Council (CAPC) (TABLE 1). These treatment regimens should always be considered the first choice as they have the most documented evidence of efficacy for resolving clinical signs and for safety. There is now an oral suspension formulation of metronidazole which has been approved by the U.S. Food and Drug Administration for treatment of giardiasis in dogs and can be integrated into existing treatment regimens where appropriate. However, reports of drug resistance, although primarily anecdotal and perception-based at this time,²⁸ are increasing, which is not surprising considering that drug resistance in cases of human giardiasis has also begun to be detected.

Authenticating perceived drug resistance is very difficult in naturally infected pet populations because it is nearly impossible to rule out reinfection within the home environment or underlying immunosuppressive conditions. The true cause of perceived drug resistance is most often reinfection; however, there are cases in which reinfection has been ruled out and clinical Giardia infection persists. In these cases, when first-choice treatments fail to eliminate clinical signs, some alternative therapeutics, primarily sourced from drugs used to treat human giardiasis, have been experimentally used with effectiveness similar to that of conventional treatments and with variable adverse reactions.²⁹ Follow-up testing at the appropriate time after completion of drug therapy should be considered an essential part of the treatment process. The CAPC recommends retesting by centrifugal fecal flotation 24 to 48 hours after completion of the drug regimen.¹⁷ Correctly timed follow-up testing with the appropriate diagnostic tools is crucial for determining whether persistent infections are the result of reinfection or true drug resistance.

Lifestyle Management of Persistently Infected Pets

Although use of drugs usually minimizes or eliminates clinical disease and in some cases completely eliminates cyst shedding, no drug is consistently 100% effective. In practice and in experimental treatment studies, some animals continue to shed cysts at low levels even after treatment and resolution of clinical disease. However, persistent, subclinical Giardia infection does not have to drive a wedge in the human–animal bond. In such cases, lifestyle management and client education are essential for preventing the spread of infection to other pets, minimizing environmental contamination, and promoting public understanding of zoonotic risk.

In the age of molecular science, the commercial availability of zoonotic assemblage typing is a major advancement and a powerful tool that can be used to assess zoonotic risk and to counsel owners accordingly. Methods of physical infection control—including bathing to remove cysts from the coat, promptly picking up feces, having an adequate number of litter boxes in multicat homes, and routine disinfection of surfaces—play a key role in minimizing the risk for transmission to other pets and are of minimal to no cost to clients. Veterinarians should evaluate the management of subclinical chronic cyst shedders on a case-by-case basis, carefully considering the health of the patient, client, family, and community, as well as drug stewardship. If concern regarding zoonotic transmission is minimal and intraspecies spread can be avoided, then managing subclinical cyst shedders by lifestyle adjustment should be considered.²⁷