Intestinal parasitic infections affect more than one billion people worldwide. While they have traditionally been associated with developing regions, they’re increasingly being encountered in developed countries as well. According to Dr Froomes, Gastroenterologist, globalisation, international travel, and large-scale migration have contributed significantly to this shift.

In developed countries, intestinal protozoal infections are more commonly encountered than helminth (worm) infections¹. These include organisms such as Giardia lamblia, Entamoeba histolytica, Dientamoeba fragilis, Blastocystis hominis, and Cryptosporidium.

As Dr Froomes explains, exposure doesn’t necessarily require travel to a remote destination. Drinking untreated or inadequately filtered water, eating raw or undercooked meat, consuming unwashed fruits and vegetables, contact with soil contaminated by animal or human faeces, and even sharing a household with infected children can all increase the risk of infection.

In parts of Asia, Africa, Latin America, and the Pacific, environmental contamination of soil and water sources remains an ongoing concern. Travellers returning from these regions may carry infections for months or even years, sometimes with only mild or non-specific symptoms. These infections can continue to affect digestive health, energy levels, and overall wellbeing long after a person has returned home.

Several intestinal parasites can cause anaemia² and other nutritional deficiencies.

Hookworm is one of the best-known examples. These worms attach themselves to the lining of the small intestine and secrete anticoagulant substances that facilitate feeding on blood³. Each individual worm can consume approximately 0.3 to 0.5 mL of blood per day, and in heavy infections, this ongoing blood loss can lead to significant iron-deficiency anaemia and nutritional impairment⁴.

Other parasites that may contribute to iron deficiency through blood loss, impaired iron absorption, or intestinal inflammation include whipworm (Trichuris trichiura), Schistosoma species, Strongyloides stercoralis, and Giardia lamblia⁵.

Some parasites can affect nutrient status in different ways. The fish tapeworm (Diphyllobothrium latum), for example, competes with the host for vitamin B12 absorption in the small intestine and can lead to vitamin B12 deficiency and megaloblastic anaemia.

Given these effects, unexplained iron deficiency, anaemia, vitamin B12 deficiency, or other blood count abnormalities may warrant further investigation, particularly in individuals with relevant travel, environmental, dietary, or exposure histories.

Raw or undercooked foods, particularly fish and seafood, are often associated with concerns about parasitic infections. Dr Froomes explains that some of these concerns are well-founded, while others may be overstated.

Among the highest-risk foods are raw or undercooked fish and seafood. One example is anisakiasis, also known as herring worm disease, which is caused by nematodes that can attach to the walls of the oesophagus, stomach, or intestine after the consumption of contaminated raw or undercooked fish or squid. A study from the University of Washington reported a 283-fold increase in the global abundance of Anisakis worms since the 1970s, raising potential public health concerns amid growing consumption of raw seafood worldwide.

Another example is the fish tapeworm (Diphyllobothrium latum), one of the largest known parasites affecting humans, capable of reaching 2 to 15 metres in length. It’s acquired through the consumption of raw or undercooked freshwater or saltwater fish, including salmon, and can survive in the intestine for many years⁶.

Other food-related sources of parasitic infection include undercooked pork and beef. Undercooked pork can transmit Taenia solium (pork tapeworm) and Trichinella, while undercooked beef may harbour Taenia saginata. Unwashed raw vegetables irrigated with contaminated water can also serve as a route of infection for Giardia, Cryptosporidium, and soil-transmitted helminths in endemic regions.

However, Dr Froomes notes that the belief that all sushi and sashimi carry a high risk of parasitic infection is often overstated. Studies from Hawaii found no cases of fish-borne helminthic infection linked to the consumption of raw tuna or skipjack, species commonly eaten raw across Pacific Island and Asian cultures⁷.

In addition, reputable sushi establishments in developed countries are generally required to freeze fish before it’s served raw, a process that kills most parasite larvae. The practical takeaway is that the risk of parasitic infection from seafood is real, but it’s species-specific, preparation-dependent, and heavily influenced by how the food is sourced and handled.

Intestinal parasites can disrupt gut function through physical damage to the intestinal lining, immune activation, changes in gut motility, and alterations to the gut microbiome. Even after the parasite itself has been cleared, these downstream effects can persist for months or even years, a condition increasingly recognised as post-infectious irritable bowel syndrome (PI-IBS)⁸.

In susceptible individuals, parasitic infections can trigger significant shifts in the gut microbiota, leading to prolonged changes in host-microbiota interactions. These changes may affect the gut-brain axis and visceral sensitivity, disrupt the intestinal barrier, alter neuromuscular function, trigger persistent low-grade inflammation, and contribute to IBS symptoms long after the initial infection has resolved⁹.

In practical terms, this means that a person may test negative for a parasite yet continue to experience bloating, diarrhoea, abdominal cramping, and fatigue. Rather than indicating an ongoing infection, these symptoms may reflect lingering changes to the gut environment and its interactions with the immune and nervous systems.

This growing understanding of post-infectious gut disorders helps explain why some people continue to struggle with digestive symptoms long after an acute infection has apparently resolved.

Parasites are frequently discussed online alongside parasite cleanses and self-diagnosis. Dr Froomes notes that one of the most common misconceptions is that parasite cleanses are an effective, evidence-based response to a suspected infection. They aren’t.

Physicians have repeatedly warned that there’s little to no evidence that herbal cleanse products effectively treat parasitic infections, and some may cause harm. A particularly misleading example involves products that contain strong laxatives, which can cause the intestinal lining to shed and produce long, rope-like material that may be mistaken for worms. This can lead people to believe they are expelling parasites when, in fact, they aren’t.

Another common misconception is that anyone experiencing fatigue, bloating, or general digestive discomfort is likely to have parasites. While parasitic infections can cause these symptoms, they’re far more commonly associated with other conditions, including gut microbiome imbalances, dietary factors, small intestinal bacterial overgrowth (SIBO), and functional bowel disorders. Assuming parasites are the cause without proper testing can delay an accurate diagnosis and appropriate treatment.

Dr Froomes emphasises that the most appropriate approach is to seek medical evaluation rather than self-diagnose. Depending on the clinical situation, doctors may use stool examinations, antigen or PCR-based tests, blood tests, and, in some cases, imaging or endoscopy to identify a specific parasite.

When a parasitic infection is confirmed, most can be treated effectively with prescription antiparasitic medications, sometimes with a single dose. Follow-up testing may then be used to confirm that the infection has cleared. In some individuals, additional assessment may be needed to address lingering digestive symptoms or gut health changes that persist after the infection has resolved.

The key message is that parasitic infections are real, often under-recognised, and deserve proper medical assessment rather than reliance on unverified online remedies or self-diagnosis.