Approach

Patient’s medical history, clinical signs, nutrition, and immune system status should be taken into consideration when treating Giardia infections.[72]

Standard therapy

First-line therapy is tinidazole, metronidazole, or nitazoxanide.[1][54][72]​​​​​​​ Some guidelines recommend tinidazole or nitazoxanide as the preferred options.[53][66]​ Metronidazole is associated with a higher incidence of gastrointestinal adverse effects, but it is commonly used and widely available, although its use for giardiasis is off-label in the US.[66] [ Cochrane Clinical Answers logo ] ​

Tinidazole can be offered as a single dose and has an efficacy of 90% to 98%.[66][73]​ Nitazoxanide requires multiple doses and is 85% to 91% effective.[74][75]

Metronidazole is given for 7 days and has an efficacy of 80% to 95%.[76]

Several other drugs including paromomycin, quinacrine, and furazolidone, may also be used for the treatment of diarrhea associated with giardiasis.[1][72]​​[77][78]​​ Quinacrine and furazolidone are not available in the US.

Treatment should, in addition to standard therapy, include advice regarding hygiene and prevention measures, and correction of dehydration and electrolyte abnormalities as required. Rehydration is of increased importance among infants, as dehydration due to diarrhea can be life-threatening.[72]

Intolerance of standard therapy

Some patients treated with nitroimidazoles (tinidazole, metronidazole) develop significant gastrointestinal symptoms such as nausea, vomiting, metallic taste, diarrhea, dyspepsia, and dry mouth. Nitroimidazoles may have disulfiram-like effects if alcohol is consumed during treatment, making them unsuitable for some patients, but data are contradictory.[79] 

Albendazole, a benzimidazole, has shown excellent activity in vitro against Giardia duodenalis isolates.[80] It is as effective as metronidazole for the treatment of diarrhea associated with giardiasis.[81] Albendazole requires multiple doses. 

Pregnancy

No therapeutic agent combines optimal efficacy and safety in pregnancy.

In the first trimester, treatment should only be given when adequate hydration and nutritional status cannot be maintained. If drug treatment is required in the first trimester, paromomycin, a nonabsorbed oral aminoglycoside, is the drug of choice, due to its limited absorption when given orally. Metronidazole can be used in the second and third trimesters but it is not approved for this indication. If there have been previous episodes of infection earlier in the pregnancy, advice regarding hygiene and prevention measures should also be given.

Rehydration is of increased importance among pregnant women as they may be at higher risk for dehydration due to diarrhea.[72]​

Treatment failure

Treatment failures have been reported with all of the common anti-Giardia agents; however, it is important to differentiate true drug resistance from cure followed by reinfection, post-Giardia dietary/lactose intolerance, or irritable bowel symptoms after therapy. There is no clinical test for drug resistance.

The first step in evaluating persistent symptoms after treatment is to send a stool sample for Giardia antigen, ova and parasites detection, or polymerase chain reaction (PCR)-diagnostics to confirm ongoing infection.[82] Positive results should direct a careful history to assess the likelihood of reinfection and an exploration for reinfection risk factors, including underlying immunocompromise. Reinfected patients should respond to the original therapeutic agent.

Treatment should, in addition to standard therapy, include advice regarding hygiene and prevention measures for the patient and all household members.

Treatment refractory infection

True treatment failure could mean infection with a drug-resistant isolate of G  duodenalis. Clinically resistant strains have often been treated with longer repeat courses or higher doses of the original agent.​​[83]​​ However, data indicate that the most efficacious means of eradicating these infections is to use a different class of drug to avoid potential cross-resistance.[84]

Combination regimens using metronidazole plus albendazole, metronidazole plus quinacrine, or other active drugs, or giving a nitroimidazole plus quinacrine for courses of at least 2 weeks, have been shown to be highly successful against refractory infection.​​​​[84]​​​ Quinacrine is not available in the US. Sometimes, several different combinations or approaches will be necessary to produce a cure. Little data exist for comparative efficacy between different combination regimens, and the choice of treatment is often determined by medication availability.

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