Giardia Antigen Positive But No Cysts. What Does This Mean For Your Dog?

Table of Contents

Introduction to Giardia in Dogs

Giardia is a single-celled parasite that infects the small intestine of dogs and causes an illness known as giardiasis. It is one of the most common intestinal parasites found in dogs. Giardia attaches to the intestinal wall and interferes with the absorption of nutrients, leading to gastrointestinal signs like diarrhea, vomiting, gas, greasy stools, and weight loss. Giardia spreads through the ingestion of cysts that are passed in the feces of infected animals. Giardia cysts can survive for long periods in the environment and are easily transmitted through contaminated food and water sources. While some dogs may not show any symptoms when infected with giardiasis, the parasite can cause acute or chronic gastrointestinal illness in dogs of any age. Diagnosis is made through an antigen test and microscopic examination of a fecal sample.

Giardia Antigen Testing

Giardia antigen tests look for Giardia proteins called antigens in a stool sample from a dog. These tests use antibodies that bind specifically to Giardia antigens. If Giardia antigens are present in the stool sample, they will bind to the antibodies on the test and produce a positive result.

Antigen testing is considered more sensitive and specific for detecting active Giardia infection compared to looking at a stool sample under a microscope for cysts, especially when few cysts are being shed. According to one study, the sensitivity of a rapid antigen test was 94% and the specificity 100% compared to PCR testing, which is considered the gold standard [1].

However, antigen tests may still produce false negative results if the level of antigen shedding is low or intermittent. In dogs with clinical signs consistent with giardiasis but negative antigen results, vets may recommend retesting 1-3 times over several days to increase sensitivity.

Interpreting Positive Antigen, No Cysts

A positive Giardia antigen test indicates exposure to Giardia, even if no cysts are seen on fecal examination (1). There are a few reasons why a dog may test positive for Giardia antigen but have no visible cysts in their stool:

– The Giardia infection is localized to the small intestine and cysts are not being shed yet in stool (1). Antigen shedding can begin 1-2 weeks before cyst shedding starts (2).

– The dog has cleared the infection but antigen remnants are still circulating (1). The antigen can persist for weeks after infection while cyst shedding has ceased (2).

– The fecal sample missed sampling cysts, since shedding can be intermittent (1, 3).

– There are false positive antigen test results, though this is rare (3).

So a positive antigen test indicates current or recent exposure to Giardia, even without visible cysts. Repeated fecal exams and antigen testing may be needed to clarify if active infection is present.

False Positives

A common issue with giardia antigen tests is the potential for false positive results. This occurs when a dog tests positive for giardia antigens, but no active infection is actually present. There are a few reasons why false positives can occur with giardia antigen testing:

Persistent Antigen Excretion: One of the main causes of false positives is that dogs can continue to shed giardia antigens for weeks or even months after the infection has been cleared. This leads to positive test results even though the dog no longer has an active giardia infection (Rishniw, 2010).

Nonspecific Reactions: In some cases, the antigen test can cross-react with antigens from other organisms, causing a positive result even though Giardia is not truly present. This seems to be uncommon but can lead to incorrect diagnosis (Strand, 2008).

Overall, false positives with giardia antigen testing are possible and must be considered when interpreting positive results, especially in dogs with no clinical signs of infection.

Repeated Testing

Guidelines do not recommend retesting dogs that have tested positive for Giardia antigens but negative for cysts. According to the CDC, retesting is only recommended if symptoms persist after treatment (source). Similarly, Cornell University experts state that retesting for antigens is not recommended, as they can remain present after successful treatment, potentially leading to false positives (source).

In dogs with positive antigen tests but no cysts seen, the antigen test is likely detecting remnant proteins from a prior infection that has been cleared. Retesting may yield more false positives, so vets typically recommend proceeding directly to treatment if clinical signs are present. Repeat antigen testing is not useful in confirming active infection in these cases.

Treatment Options

The main treatment options for giardia infection in dogs include metronidazole, tinidazole, and nitazoxanide. Metronidazole is commonly prescribed and is typically administered at 25-50mg/kg once or twice daily for 5-7 days [1]. Tinidazole and nitazoxanide can also be very effective in a single dose, with cure rates over 90% [2]. Other medications may be used as well under veterinary guidance. Treatment is aimed at stopping the infection and preventing cyst shedding to avoid reinfection.

If clinical signs resolve but cysts are still seen, additional treatment may be needed. Chronic cases may require longer treatment duration. Supportive care like fluids, anti-nausea medication, probiotics, or a specialized diet may help manage symptoms.

Overall the main treatment options include metronidazole, tinidazole, and nitazoxanide. Cure rates are typically very high with a proper course of medication, but chronic infections may need extended treatment.

Prevention

There are several ways to help prevent giardia infection in dogs:

Clean the dog’s environment frequently. Giardia cysts can survive for months in the environment, so regular disinfection of kennels, crates, floors, and other surfaces is important. Use a cleaner containing quaternary ammonium or steam cleaning when possible (CDC).

Pick up and dispose of dog feces promptly. Since giardia cysts are passed in feces, removing waste quickly can reduce environmental contamination (TAMU).

Bathe dogs after treatment. Bathing can help remove any remaining cysts from the coat. Use a shampoo containing chlorhexidine if possible (Cornell).

Prevent exposure to feces-contaminated sources. Avoid letting dogs drink from puddles, streams, or sources contaminated with animal feces.

Practice good hygiene. Wash hands and disinfect surfaces after handling dog feces to prevent human infection.

Environmental Control

Cleaning and disinfecting the environment is crucial for controlling the spread of giardia. The parasite can survive for weeks to months in cool, moist environments like puddles, mud, soil, and dog feces (1). Thorough cleaning and disinfection helps remove giardia cysts from the environment and prevent reinfection.

First, remove all visible fecal material with disposable gloves. Any items soiled with feces should be cleaned and disinfected as well. Use a quaternary ammonium disinfectant or bleach solution to kill giardia cysts (2). Diluted bleach can disinfect surfaces at a concentration of 1 part bleach to 32 parts water. Allow surfaces to remain wet for at least 10 minutes when disinfecting.

Pay special attention to areas like food and water bowls, bedding material, floors, and outdoor areas where the dog spends time. Rinse off disinfectants thoroughly since they can be harmful if ingested by pets. Repeated disinfection of contaminated environments helps reduce the spread of giardia between pets.

Sources:

(1) https://www.cdc.gov/parasites/giardia/prevention-control-pets.html

(2) https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/health-info/giardia-infection-treatment-and-prevention

Prognosis

The prognosis is generally good for dogs treated for giardia. With appropriate treatment and environmental control, most dogs make a full recovery within 2-4 weeks according to the American Kennel Club (AKC) https://www.akc.org/expert-advice/health/giardia-in-dogs/.

After treatment, dogs should have another fecal test done in 2-4 weeks to confirm the infection has cleared. Some dogs may require a second round of medication if the parasite persists.

While giardia itself is not life-threatening, dogs who are very young, old, or immunocompromised are at higher risk for dehydration and malnutrition due to the infection. With prompt veterinary care, these dogs generally recover fully as well.

According to Cornell University, dogs with competent immune systems are able to clear giardia infection on their own within a few weeks, even without medication https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/health-info/giardia-infection-treatment-and-prevention. However, treatment is still recommended to help resolve symptoms and prevent spread.

With appropriate treatment, prevention measures, and monitoring, the prognosis for dogs with giardia is very good. Most dogs go on to live normal, healthy lives after recovery.

When to See a Veterinarian

If your dog tests positive for giardia antigen but has no cysts seen on fecal analysis, it is still recommended to have your dog evaluated by a veterinarian (1). Giardia can cause acute or chronic gastrointestinal signs like diarrhea, vomiting, weight loss, or poor growth in puppies (2). Even if your dog is not currently showing symptoms, giardia can be spread and infect other animals and humans. Bringing your dog to the veterinarian will allow for a full clinical examination and discussion of potential treatment options.

Veterinary evaluation is especially important for puppies, older dogs, and immunocompromised dogs, as they are at higher risk of developing severe clinical illness from giardia. Puppies with giardia may fail to thrive and have poor growth (3).

If your dog has had recurring positive giardia antigen tests but negative fecal exams, a veterinarian can help determine next steps, such as repeat testing, empirical treatment, or advanced diagnostics like an ELISA test (3). Your veterinarian may recommend re-testing your dog 2-4 weeks after initial treatment to ensure the infection has cleared (1).