Parasites and the Gastrointestinal Tract

by Dr. Leo Galland

Parasites and the Gastrointestinal Tract

Do Intestinal Parasites Explain Your Post-Vacation Slump?


After traveling, it’s time to get your GI tract back on track, because you may have brought home a parasite on your vacation.


Your bowel disturbances may have already told you that parasites had arrived in your digestive tract.


And to learn how to stay healthier next time you travel, read my article Pills for Your Upcoming Trip.


Parasites Can Be Acquired Anywhere


You don’t need to travel to get a parasitic infection.  The gastrointestinal tract is vulnerable to getting parasites anywhere, as I will explain below.


Last year I saw a patient who had returned from her Christmas vacation complaining of persistent dizziness and fatigue that had not improved for over a year.


Before visiting my office she had consulted three other doctors but no cause for the dizziness was found.


When I questioned her about the vacation, which took place at a Caribbean resort, I discovered that she had experienced a transient episode of diarrhea there and since returning home had been unusually constipated and gassy.


She had never mentioned these intestinal symptoms to a doctor before, because it was dizziness and fatigue that bothered her the most.


Testing for Parasites


I’ve seen many patients with chronic fatigue caused by intestinal parasites, so I had this woman tested at a specialized laboratory. The test revealed infection with a parasite called Giardia lamblia, a condition called giardiasis.


Healing the GI Tract


After treatment with anti-parasitic medication, her gastrointestinal symptoms slowly resolved, along with her main complaints of dizziness and fatigue.


Learn more about testing for and fighting parasites in Fighting Parasites


Two Types of Parasites


There are two general groups of parasites that infect the digestive tract. The first consists of worms–tapeworms and roundworms–which attach themselves to the lining of the small intestine, causing internal bleeding and loss of nutrients. People infested with gastrointestinal tract worms may have no symptoms or may slowly become anemic.


The second category of digestive tract parasites is the protozoa, one-celled organisms like Giardia, which can cause bowel health symptoms such as acute or chronic diarrhea or can cause symptoms that are not ordinarily associated with parasitic infection like constipation, fatigue, dizziness, joint pain or hives.


Parasite Outbreaks


Epidemics of giardiasis have occurred at ski resorts in the dead of winter because of drinking water contamination. They commonly occur at day care centers that serve toddlers who wear diapers. Twenty to thirty per cent of workers in day care centers harbor Giardia. Most have no symptoms; they are merely carriers.


Parasites at the Salad Bar


Eating at a salad bar increases your risk of food-borne infection. Just observe the unhygienic habits of patrons or the way in which handles of serving spoons and tongs fall into the food.


A study at Johns Hopkins medical school demonstrated antibodies against Giardia in 20 per cent of randomly chosen blood samples from patients in the hospital. This means that at least twenty per cent of these patients had been infected with Giardia at some time in their lives and had mounted an immune response against the parasite. Most were unaware of having been infected.


Parasites: From the Water to the GI Tract


Giardia contaminates streams and lakes throughout North America and has caused epidemics of diarrheal disease in several small cities by contaminating their drinking water.


One epidemic, in Placerville, California, was followed by an epidemic of Chronic Fatigue Syndrome, which swept through the town’s residents at the time of the Giardia epidemic. Possibly, this epidemic was due to failure of some people to eradicate the parasite.


Another protozoan that’s achieved notoriety in the U.S. is Cryptosporidium parvum, which contaminated Milwaukee’s water supply in the 1990’s, causing the largest epidemic of diarrhea in U.S. history, infecting about 400,000 people and causing over one hundred deaths. Most municipal water supplies in the U.S. today are home to protozoa like Giardia and Cryptosporidium and one in five Americans drinks water that violates Federal health standards. Every year, almost a million North Americans become sick from water-borne diseases.


A Study on Chronic Fatigue Syndrome and Giardia


In 1991, my colleagues and I published a study of two hundred patients with Chronic Fatigue Syndrome and demonstrated active Giardia infection in 46 per cent. Most of the patients with giardiasis had only minor gastrointestinal symptoms but were really ill with muscle pain, muscle weakness, flu-like feelings, sweats and enlarged lymph nodes. In fact, 61 per cent of fatigued patients with giardiasis had been diagnosed elsewhere as suffering from chronic fatigue immune dysfunction syndrome (CFIDS), compared to only 19 per cent of fatigued patients without giardiasis. Cure of giardiasis resulted in clearing of fatigue and related ‘viral’ symptoms (muscle pain, sweats, flu-like feelings) in 70 per cent of cases, some reduction of fatigue in 18 per cent, and was of no benefit in only 12 per cent.


In 1990 I presented a paper before the American College of Gastroenterology which demonstrated Giardia infection in about half of a group of two hundred patients with chronic diarrhea, constipation, abdominal pain and bloating. Most of these patients had been told they had irritable bowel syndrome, which is commonly referred to as "nervous stomach". I reached two conclusions from this study: (1) Parasitic infection can be a common event among patients with chronic gastrointestinal symptoms. (2) Many people are given a diagnosis of irritable bowel syndrome without a thorough evaluation.


My presentation was reported by numerous magazines and newspapers, including the New York Times. My office was flooded with hundreds of phone calls from people who were suffering with chronic gastrointestinal complaints. Most of them had been given a diagnosis of Irritable Bowel Syndrome (IBS) by their physicians. The standard treatment for this syndrome had not helped them. All they had received was a label. Many had been told there was no cure.


In evaluating these patients, I found that the majority had intestinal parasites, food intolerance or a lack of healthy intestinal bacteria. (For more information about intestinal bacteria, see Probiotics or Friendly Bacteria)


These conditions were not mutually exclusive. Many patients had more than one reason for chronic gastrointestinal problems. Treating these abnormalities as they occurred in various patients produced remarkably good therapeutic results. A year later, researchers in the Department of Family Medicine at Baylor University in Houston reported findings similar to mine.


Despite the research and the news reports about parasites, I still find that intestinal parasites are a common and often unsuspected cause of mystery illness.


For more info on a gastrointestinal health, see my article Do you Have Leaky Gut Syndrome?


Resolving parasites and other digestive problems and improving gastrointestinal health are vitally important to healing. Learn how to assess your digestive function and the presence of intestinal toxicity in my book Power Healing.


Now I’d like to hear from you…

Have you traveled recently?

Did you experience any stomach upset?

Have you taken anything for it, and what helps?

Please let me know your thoughts by posting a comment below.


Best Health,

Leo Galland, MD


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Galland L, “Intestinal Protozoan Infestation and Systemic Illness”, in Textbook of Natural Medicine, 3rd Edition, Volume 2, J. Pizzorno and M. Murray, editors, Churchill Livingstone Elsevier, St. Louis, 2005, pp. 655-660.


Galland L. “The Effect of Systemic Microbes on Intestinal Immunity” Post-Viral Fatigue Syndrome (Myalgic Encephalomyelitis) R. Jenkins and J. Mowbray, editors. John Wiley & Sons Ltd. London. 1991. pp 405-430.


Galland, L., Lee M., Bueno H. and Heimowitz C. (1990) “Giardia lamblia infection as a cause of chronic fatigue.” Journal of Nutritional Medicine, 2, 27-32.


Galland, L. (1989) “Intestinal protozoan infection is a common unsuspected cause of chronic illness.” Journal of Advancement in Medicine, 2, 529-552.



30 thoughts on “Parasites and the Gastrointestinal Tract

  1. Cathy DiFlorio

    Hi Dr Galland, I have been following your diet for a few years now and love it! Within the past year I have had bowel and stomach issues. I have had a colonoscopy, endoscopy and a cat scan. They found nothing wrong with me. I still do not feel like myself. I did travel to mexico before these problems started. And I did have them test for a parasite, but the results came back negative. I excercise and eat very healthy, still following your diet for the most part. I’m really stuck on what I should do, maybe you have a suggestion for me. Thank you and looking forward to hearing from you. Cathy Di Florio

  2. Mary Rezeski

    Hi Dr. Galland,

    First of all I think your articles are very informative an I appreciate the time you are taking to provide this information to us. My question is in regard to parasites. I have had chronic headaches, dizziness and fatigue for many years now and these symptoms are greatly relieved by allergy shots. Shortly before I got sick I had a period of severe stomach pain just under my diaphram on the right side. Could I have had a parasite that set some process in motion causing immune system compromise that in turn resulted in all these allergies ? If so, should I be tested for parasites ? It has been years since I had a test.

  3. Kathleen

    I became very anemic and after months of iron treatment was still anemic. I finally did a CDSA (comprehensive digestive stool analysis) and was flabbergasted to find parasites. I realized I had a shallow well and horses upstream over 12 years ago and had a bout of severe abdominal pain and diarrhea. I have since had some IBS symptoms, but was taking lots of supplements and probiotics for years, which keep me functioning fairly well. The parasites eventually tapped my system, despite the fact I have been extraordinarily healthy. So yes, you can be doing everything to take care of yourself, but if you don’t eliminate the parasites, you won’t get to the root of your health imbalance. I took the medication Flagyl, along with many immune boosting nutrients, antioxidants (to treat the oxidative effects of iron) and high dose B12 and folate.

  4. Mike

    Hi, I have fibromyalgia with chronic fatigue being dominant symptom. I have bad abdominal bloating that so far hasn’t been helped by going dairy & gluten free. I have been on probiotics, prebiotics,nystatin,anti-parasitic herbs,L-glutamine,polysacarides etc and so far nothing seems to help really.
    I see a ‘Functional & Wellness’ Doctor in New Zealand, but can you suggest something else to try??????? thanks

  5. Elaine Broderick

    I’ve had a severe bout with entomeda histolitica (sp?)found by you in about 1984, you treated me and cleared me, (thank you); moved to Fla for many years. Before my return north (MA) had much weight loss, negative stool tests for recurrence of tropical parasite, developed left bundle branch block & continued anemia along with much muscular pain w/ osteopeonia, a developed breathing problem on exercise. Since returning to MA, have been weak, anemic and have had a right breast lumpectomy for lobular carcinoma, much muscle pain arthritis, a monitored pulmonary nodule along with tree in bud inflammation in lungs and continued breathing difficulty on exercise. At 82 , I am still seeking improvement, (many specialists)mostly helped by your nutritional, etc help in ’84

  6. LP

    Dear Dr. Galland,

    Thank you very much for saving my life. You are not only an excellent physician, but a kind and caring person. I was fortunate to have had you as my doctor.

  7. Shannon H.


    I was diagnosed with Giardia over 25 yrs. ago. I had done a lot of hiking at Mr. Rainier in Washington State and there was an outbreak of giardia back then. I walked around for over 2-yrs miserable with IBS type symptoms. It would come and go. My doctor did every test but a stool sample and I believe because he became frustrated with me diagnosed me with depression and being a hypochrondriac and put me on anti-depressants. I continued of course to be ill and FINALLY he ordered a stool sample test and of course I had high levels of the parasite. After walking around for two yrs with this parasite taking every good nutrient from my body, I was diagnosed and placed on Flagyl. I guess I was clear from the disease, but quite frankly I am not sure now, because my doctor said it was not necessary to reorder another test. I can not believe I did not speak up back then. A few yrs later I was diagnosed with Chronic Fatigue Syndrome. All this occurred in my early 30’s. I am in my 50’s now. I have had many health issues, including my gallbladder being removed. I have terrible IBS type symptoms that come and go. Bloating, intestinal spasms, chronic fatigue, gas, greasy, yellow bile type stool, pain, but also I have developed terrible arthritic pain all over my body. I thought I might have fibromyalsia, but very few doctors will talk about this and the doctor I have now just says because I am in my 50’s it is normal and common to have arthritis. Well, this pain is severe and all over my body. I do have a bad hip and osteoarthritis, but this pain is overwhelming. I always feel tired, with flu like symptoms and I know my bowel is suffering. Is it possible to still have giardia? After research and more research, I find that I am NOT alone, but my life long issues are probably due to the giardia from yrs ago. What are my options. Where can I get your book? What do you recommend. I would love to talk to you or others with this kind of history. How can I get in touch with anyone else like this? Please help. Articles are limited. I just happened to stumble on this site. I can provide you with many stories on myself, but also others that lived in the NW. Primarily, Washington, Oregon, Tahoe, Idaho, where giardia became epidemic back in the 80′ and 90’s. I feel I need to be tested again. I am afraid that I may have never gotten completely rid of this terrible parasite that can cause so much damage. Shannon

  8. K. D.

    I travelled to Jamaica at the end of Jan. 2011 & when I arrived home I immediately noticed changes to my stool. Two weeks later I had a severe case of diarrhea. I have also had an excess amount of gas & bloating. My digestive system is very active & noisy just before I have a bowel movement. I say an alligator is in my bowels. I am very tired all the time.
    I wondered if I may have picked up parasites on the trip. I have seen 2 Drs. concerning this & both have taken stool samples & one had blood work taken. Nothing has shown up.
    I also did a parasite cleanse from the health food store & it seemed to clear up some of the symptoms. I still have a severe case of diarrhea & am to see a Gastroentrologist this week. I am assuming I will have a colonoscopy. My Dr. said these symptoms are a coincidence & do not have anything to do with the trip to Jamaica. He is suspicious of IBS. I do not agree. I went away very healthy & came home to this. I have no pain & no bleeding. The Tropical Disease Dr. said the diarrhea will clear up on its own by Dec. What do you say about this?
    I am interested in your thoughts & any comments you may have.
    I am also wondering if there is a prescription medication that I could try. Something that would not cause more problems.
    Would Giardia show up on the cultures that have already been taken?
    I am going to the specialist on Wed. of this week & will speak to him about your information.
    Thank you for your consideration to my case. KD

  9. Susan Parker

    Dr. Galland,
    My family is suffering from a mystery illness. We are all extremely tired, sweating, severe pain in joints and legs, bloating, diahrea, dizzy, confused, and the list of symptoms goes on. We have been diagnosed with everything from fibramyalsia, chronic fatigue and IBS. We recently found out that we all have a urinary tract infection (even our dog). The UTI’s are from bacterias that are found in human waste (enterococcus, strep and e-coli). We moved from our house when we found that our septic was crushed and our yard completely contaminated. We are on antibiotics and took one dose of mebendazole. After one week of complete recovery, the symptoms are returning. Our physicians have no idea what to do and I am scared for my children. What can we do to find out exactly what parasite we may have? Susan

  10. Woodrow Dunn

    I live in Honduras and take anti-parasitic medicine every four months. (I usually take mebendazole). After taking the medicine I have huge amounts of gas and sometimes diarrhea. Does that probably mean that I have a high amount of parasites. I treat my water and wash my vegetables in water with chlorine. Any advice?

  11. Lauren

    I have been diagnosed with CFS and can not find a doctor in the Portland, Oregon area to test me for parasites. Do you have any colleagues that you know of that test for this in my area? I have been suffering for years and believe that it is a parasite! I have fatigue, achy joints, enlarged lymph nodes and brain fog

  12. aubrey

    i have been severely fatigued for over a year. gradually is becoming worse and worse. i am thirty one years old and can’t get through an eight hour day without comming home and taking a two hour nap. i have had numerous blood tests and no one can seem to find anything wrong with me. they say i have to much stress. i have rounds of diarrhea, and gastric upset, bloated and acid reflux ( only in spells) Help could it be parasites and how do i get my doctor to test for them. and is there a natural cure

  13. Maureen Ryan

    About one week after returning from Mexico I experienced flu like symptoms: chills, headache, nausea. It has been one more week and I am left with unrelenting nausea and fatigue. I have experienced some weight loss. I have not had diarrhea, just a few small soft stools initially. Help!

  14. Lisa

    I found this article after searching “extreme fatigue after Mexico travel”. I have been googling for answers for 3 years now. I have also visited several doctors and undergone many blood test with no answers. One week after my return from Cancun, I became miserably fatigued. The Doctor didn’t feel there was any coorelation. My once active and outgoing life has diminished to a sedentary one that spends a lot of time at home with new aches, pains and symptoms, daily. The idea that this could be parasite related is, at least, a new avenue to pursue. I miss myself and hope to see “me” again soon.

  15. Kathleen

    Hi there. I recently went to playa del Carmen via cancun. I found this site as a googled trying to find something why I feel so fatigued upon my return. I left Amanda on the 8th of march and that evening was very ill with diarreah and actually had vomiting. This was a Monday. I did not start feeling a little better until Thursday. I did not drink any water except bottled water and stayed at a reputable hotel. I had one margarita on the Monday and then that was it ….the fun began shortly after. I have been exercising daily previous to leaving waking at 5 am to go to gym. Now I can barly get up just to get to work. And feel like a need a nap during the day. This happened once many years ago too when I went to Paris. Several of us on a tour went down on the third day there with a 24 hour episode. Can you shed some light on this.

  16. e

    Hi i just came upon your site. I have been having diaherra for over a year having it for 3 to 4 times a day i feel gassy and bloated the other day i had stomach cramps and signs of arthritis i know its some kind of parasite im almost 31 yrs old and am very healthy.i do not have health insurance i have very little money. Where can i go? Or what can i do to see if i do have a parasite? I am in portland oregon please help.

  17. Cali Girl

    I went to Mexico for about 7 days and returned on June 11 late, I had one bout of sickness there but I thought it might be from the unprocessed food I ate. I stayed away fromt he water, even when cooking and washed all fruits and veggies really well, Upon my return about a day after I have been struggling with stomach aches before a bowel movement almost like constipation and also bloaty and gassy, I am afraid I picked up a parasite, I was in Jalisco and things are that clean we did eat out a few times at street tacos. I am taking a B12/6/1 combo I purchased at the pharmacy there I am fatigue and weak and its not really helping..I have no insurance any home remedies you can suggest, plkease help.

  18. Jeannie

    I have been home from a Cancun trip for 3 weeks. Symptoms of diarrhea started 5 days into the trip and continued for 1 week. I have suffered extreme bloating and elevated BP for 2 weeks. Will f/u with my PCP but am concerned about the chronici of the sx’s described here as well as the lack of diagnosis and treatment.

  19. crystal

    i just got back from honduras, sat. i woke up feeling like i wanted to throw up! never did, but i had severe stomach pain. im home now and i have stomach pain,a lot of gas, and belching. i was given pepto and zofran, but they don’t help. What can i do? never had runny stools i have constipation.

  20. Robin

    I we recently went on vacation to Steamboat Springs Colorado coming from sea level in Florida. 1 week later I’m still exhausted and weak and dizzy / light headed at times. At first, I thought it was just vacation fatigue because I’ve experienced this before but 7 days have gone by and I’m still super tired. I’m not having stomach problems even though I did eat some uncooked hamburger meat and was at several salad bars. Not sure this could be a parasite or not. Would like your advice and what type of doctor could figure this out?
    Thank you. PS I’ve been taking B12 for 3 days with no change and this morning I took a multivitamin with iron in case I was anemic. I do take a protein drink smoothie every morning except did not do this while on vacation.

  21. Avijit Ghatak

    from past 2 months i am having chronic constipation, gas,bloating and flatulence. First doc prescribed sompraz d20,mebiz sr and saril. For 2 days i had clear evacuation. But again its started pain in the lower left back, sharp pain when pressing the lower left abdomen.Then doc prescribed sompraz d20,joxcy 500 and semi ultracet. The pain got reduced but the bloating and constipation still persists.Feels like sctraching my eyes,light joint pain.please help.

  22. srimrudunjayee.

    sir i was sick of intolerable stomach pain but intermittantly,with nausea and for that hospitalised at a regular intervals of 8 months each from 2011 onwards. diagonised as colitis, acute gastritis and food poisoning ,different investigation each time. medicines prescribed this time :SAMPRAZ-D20,RIFAGUT-550,MEBIZ-SR for 10 days. now controlled. Kindly precribe the specific diet
    (purevegetarian) do’s and dont’s so that i can get rid of.

    description of pain: severe stomach pain with discomfort + slight pain when pressed, in the lower abdomen, nausea/vommitting ctscan,bllodtest in detail and other investigations,were NORMAL. with respects,-

  23. Crystal Ennen

    I just got back from Riviera Maya Mexico yesterday. I felt fine until I got home. Then I had diarhea really bad and started getting cold/hot flashes. Every muscle in my body was tight and in pain. I took several aleve and the pain of my muscles are gone today but my stomach pains are still there. Please advise what I need to do. Thanks in advance!!

  24. Erica Bachman-Brown

    I recently returned from the Dominican Republic. On the first night, my husband and i dined at the “sushi” restaurant. Most if the rolls were made of diffetent vegetables as well as “crab” meat. But i also drank a half glass of iced tea. The second night during dinner, I was overcome by heat (as if I was spontaneously combusting),nausea and stomach pain. I returned to my room and in the middle of the night, I started to get diarrhea. Throughout the rest of my honeymoon, I stayed away from alcohol and drank mostly soda and gatoraid when coukd fint it, but the random stomach pains and diarrhea continued. By the end of the trip, I needed to purchase pepto but it only helped to mask the pains. Before I left, I had persistent constipation; since I returned I am now regular. Could this be correct??

  25. erin

    I’m 26 and I’ve had stomach bloating, hives, diarrhea, fatigue and really bad stomach cramping. These symptoms have been going on for about 4 year’s with not much help from my PCP. I’ve never traveled out of the US and I’m fairly active. Please I’m open to any advise. Thanks for taking the time to read my comment.

  26. Cynthia Hale

    I was diagnosed with Irritable Bowel Syndrome at the age of 19. Have had alternating bouts of constipation and diarrhea ever since, mostly constipation. Now I am 62, returned from a 7 night cruise to the Caribbean the second week of September, 2014. Visited Roatan, Belize, and Cozumel. Had lunch in Cozumel, drank Mexican Coca-Cola, and also remember drinking a Coca-Cola in Belize. Since returning from the trip, experienced severe constipation for the first 2 months, followed by fairly severe diarrhea in the last month. The smell is foul and the texture appears unlike any diarrhea I experienced pre-trip. My stomach makes all kinds of growling, “digesting” noises loud enough for my husband to hear them; it is making the noises as I am typing this. I have had a loss of appetite (noticeable decrease) and have lost five pounds since September. I have also experienced muscle pain and weakness, especially in my arms, shoulders, and upper thighs. I can no longer sleep on my left side because of this pain. Part of this may be caused by stress, as I lost my job at the end of July and have no prospects of another. Unfortunately, with the loss of my job I also was left without health insurance of any kind. If you can advise me in any way, please know that I will be very grateful. Thank you so much! My email address is

  27. Cynthia Hale

    I was diagnosed with Irritable Bowel Syndrome at the age of 19. Have had alternating bouts of constipation and diarrhea ever since, mostly constipation. Now I am 62, returned from a 7 night cruise to the Caribbean the second week of September, 2014. Visited Roatan, Belize, and Cozumel. Had lunch in Cozumel, drank Mexican Coca-Cola, and also remember drinking a Coca-Cola in Belize. Since returning from the trip, experienced severe constipation for the first 2 months, followed by fairly severe diarrhea in the last month. The smell is foul and the texture appears unlike any diarrhea I experienced pre-trip. My stomach makes all kinds of growling, “digesting” noises loud enough for my husband to hear them; it is making the noises as I am typing this. I have had a loss of appetite (noticeable decrease) and have lost five pounds since September. I have also experienced muscle pain and weakness, especially in my arms, shoulders, and upper thighs. I can no longer sleep on my left side because of this pain. Part of this may be caused by stress, as I lost my job at the end of July and have no prospects of another. Unfortunately, with the loss of my job I also was left without health insurance of any kind. If you can advise me in any way, please know that I will be very grateful. Thank you so much!

  28. Mike

    I had gal bladder out 5 years ago. I have bad acid reflux and diahrea ever since. I tried every acid reflux medication out there. From Tums, Roland’s gaviston pepto to 75 mg Zantac to 150 mg Zantac, divol tablet, liquid divol, proton pump inhibitors, like nexium, tecta now dexilant. I’m now taking fibre supplements as well as probiotics. This is helping a bit on and off. Im just at a loss on what to do. Can you give me some advice. Be highly appreciated. Thanks so much

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