Giardiasis and “Post-Infectious Disease Syndrome”

“The impact of an infectious disease extends far beyond the acute phase characterized by a majority of symptoms presented, to a multitude of adverse events and sequelae that follow viral, parasitic, fungal, and bacterial infections.

Juan S. Izquierdo-Condoy, et al., Frontiers in Cellular and Infection Microbiology, January 30, 2024

“While it is commonly assumed that people either recover or die from infections, long-term symptoms—or sequelae—are a possible outcome as well.”

Jan Choutka, et al., Nature Medicine, May 2022

Giardiasis, caused by the parasite Giardia duodenalis, is a major diarrheal disease found throughout the world. A recent study showed that a significant subset of Giardia patients developed chronic, debilitating symptoms months or years after the initial infection had cleared. (Miko, S., et al., “ Post-infectious Syndromes and Long-Term Sequelae after GiardiaInfections,” Emerging Infectious Diseases, December 2025).The Centers for Disease Control and Prevention (CDC) subsequently added giardiasis to a growing list of acute infectious diseases that can precipitate a long-term, post-infectious illness akin to Long COVID.

While Giardia duodenalis is an interesting and common pathogen, most Americans are probably not aware of it nor are they aware that chronic disability can follow not only on the heels of giardiasis but also following a number of other acute infectious diseases as well. In today’s post, I provide a brief review of giardiasis, summarize the current thinking about the pathogenesis of post-infectious syndromes, and end with a bit of good news— a treatment recently found to benefit patients suffering with the post-infectious disease syndrome Long COVID.

What is giardiasis? Giardia duodenalis (also known as G. intestinalis or G. lamblia) is a flagellated parasitic protozoan microorganism of the genus Giardia that colonizes the small intestine, causing a diarrheal condition known as giardiasis. According to the CDC, G. dueodenalis affects roughly 2% of adults and 6% to 8% of children in developed countries annually, and is considered the most common intestinal parasite in the U.S., where it causes an estimated 1.2 million cases of illness per year.

G. duodenalis spreads via the fecal-oral route when Giardia cysts excreted with feces contaminate food or water. (Stephen, T., Tobin E.H., Jeurgens, A.L, Giardiasis, StatPearls Publishing, January 2026). It is a leading cause of waterborne disease often transmitted by drinking contaminated lake, stream, pond, or river water while hiking. It can also spread by person-to-person transmission especially in child care settings where handwashing practices may be poor or through community sources.

According to the CDC, those at greatest risk of infection are people who;

• work in childcare settings;

• are in close contact with someone who has the disease;

• have contact with feces such as during sexual activity;

• have weakened immune systems;

• get their household water from a shallow well; and

• have contact with infected animals or animal environments contaminated with feces.

Also at risk are: travelers to areas that have poor sanitation; backpackers or campers who drink untreated water from springs, lakes, or rivers; and swimmers who swallow water from swimming pools, hot tubs or from springs, lakes or rivers.

Symptoms usually appear 1 to 2 weeks after becoming infected with the parasite and can last for weeks or months if untreated. They typically include diarrhea, along with abdominal cramps, bloating, and nausea. As a result of intestinal epithelial damage, acquired lactose deficiency develops in up to 40% of patients. But, approximately 15% of people infected with G. duodenalis are asymptomatic.

Giardiasis has been traditionally diagnosed by using microscopy to identify trophozoites or cysts in stool samples. Microscopy remains in use today but more sensitive and convenient methods are now also widely available. These include molecular assays, enzyme-linked immunosorbent assays and rapid immunochromatographic cartridge assays.

Treatment is not always necessary for patients with giardiasis as most symptomatic cases resolve on their own within 2 to 4 weeks. When treatment is indicated, the antimicrobial drug metronidazole is usually recommended.

Post-infectious syndrome following giardiasis. Most Giardia infections are self-limiting. A subset of symptomatic and asymptomatic people, however, experience infection-associated chronic conditions that can affect multiple body systems. (Miko, S., et al., “ Post-infectious Syndromes and Long-Term Sequelae after Giardia Infections,” Emerging Infectious Diseases, December 2025).

The post-infectious, chronic conditions that can follow acute giardiasis include impairment of cognitive function, fatigue, joint pains, fibromyalgia, and dysfunction of daily activities. All of these conditions can persist for months or even years and impair daily function and quality of life.

The mechanisms underlying these chronic sequelae are unknown, as is the case for patients with the chronic illness called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) or Long COVID. But as the researchers said: “We synthesized what is known about giardiasis-associated chronic conditions and illnesses to improve recognition of those complications and ensure appropriate management that can improve the well-being of persons affected.”

What is the cause of post-infectious disease syndromes? The precise mechanisms underlying the chronic illness that can follow on the heels of a number of acute infectious diseases (designated “post-infectious disease illness”) are under active investigation, and an increasing number of researchers are contributing to our understanding of the pathophysiology. The list of pathogens that share the ability to cause this chronic illness is growing and includes viruses (SARS-CoV-2 and Epstein-Barr Virus), bacteria (Borrelia burgdorferi, Coxiella burnetii, Brucella, Bartonella, and Babesia), and now G. duodenalis, a parasite, has been added.

Long COVID is the most commonly recognized form of post-infectious disease syndrome. It is a debilitating chronic illness that is precipitated by SARS-CoV-2, the virus that causes COVID-19. The clinical features of Long COVID overlap with the idiopathic illness referred to as ME/CFS and include, but are not limited to, fatigue, pain, sleep disturbances, and cognitive impairment.

Treatment of post-infectious disease illness. Many billions of dollars have been invested in research on the chronic illnesses that follow an acute infectious disease and sideline so many people. This is especially true in the case of Long COVID. Nonetheless, an effective treatment has yet to materialize. That may be about to change given the results of a recent clinical trial of the selective serotonin reuptake inhibitor, fluvoxamine (Luvox). (Reis, G., et al., “The Effect of Fluoxamine and Metformin for Fatigue in Patients With Long Covid: An Adaptive Clinical Trial—A Long Covid Repurposed Drug Study,” Annals of Internal Medicine, March 31, 2026).

Fluvoxamine is an antidepressant, a selective serotonin reuptake inhibitor (SSRI), that goes under the brand name Luvox. (Wilson, F.P., “The Special Antidepressant that Treats Long COVID, Medscape Impact Factor, March 30, 2026). Like other SSRIs, it treats depression “at the level of the serotonin receptor with generally modest efficacy.” It has, however, two things that set it apart from other SSRIs.

First, it “is the most potent stimulator of the sigma-1 receptor in the brain.” The sigma-1 effect “theoretically reduces brain inflammation.”  And, among all the SSRIs, fluvoxamine “is the most potent inhibitor of the liver enzyme CYP1A2, an enzyme that is responsible for breaking down, among other things caffeine—dramatically increasing the half-life of caffeine from 5 hours to 30 hours.” As Dr. Wilson said, “In a study where the primary outcome is fatigue, this seems like a potentially important mechanism. It was not explored in the paper.”

Whether fluoxamine is truly beneficial in treating Long COVID needs to be verified by additional studies. If it is, its role in other post-infectious disease syndromes—like that following acute giardiasis—also needs to be studied. But I do agree with Dr. Perry, the mechanism of benefit is more of an academic question. For people suffering from post-infectious disease syndromes, “the reason the drug works may be less important than the fact that the drug works.”