Ascaris Lumbricoides

Ascariasis is an infection caused by a parasitic roundworm, Ascaris Lumbricoides. This is the most common intestinal worm infection. It is found in association with poor personal hygiene, poor sanitation, and in places where human faeces are used as fertiliser. Intake of food or drink contaminated with roundworm eggs causes infection. The eggs hatch and release larvae within the intestine. The larvae then move through the bloodstream to the lungs, exit up through the large airways of the lungs, and are swallowed back into the stomach and intestines.

Cryptosporidium

Cryptosporidium is a coccidian parasite recognised as an important enteric pathogen. The organism causes an acute, though self-limiting, infection in immunocompetent individuals. Incubation periods of 1 to 12 days have been reported, with most oocyst shedding ending by day 21. Symptoms range from mild to severe diarrhoea, with a variety of complications. In immunocompromised patients, the infection is much more severe and may often be life-threatening. Passage of fluid, up to 12 litres per day, has been reported. Multiple pathways of Cryptosporidium transmission have been implicated. These include animal-to-human, water contamination, and person-to-person transmission. The latter may include contact between members of the same household, in day-care centres, and among men.

Entamoeba Histolytica

E. histolytica is the protozoan parasite responsible for the disease amebiasis. Symptoms of acute amebiasis include diarrhoea and colitis. The disease may manifest itself as an acute, chronic, or asymptomatic infection. Additionally, a percentage of the intestinal amebic infections will become extra-intestinal and cause abscesses in various organs. If extra-intestinal amebiasis is suspected, a serology test should be used for diagnosis. By the time abscesses are occurring, the patient’s stools are usually clear of amoebas. The mode of transmission of E. histolytica is typically through the faecal-oral ingestion of cysts, often by drinking contaminated water. Epidemics of amebiasis have been documented in developed nations, but the parasite is quite common in underdeveloped countries. Travellers returning from underdeveloped countries account for the majority of cases in developed countries.

Fasciola

Fascioliasis is a parasitic disease caused by liver fluke species of the genus Fasciola: Fasciola hepatica and Fasciola gigantica. While F. hepatica is found globally, F. gigantica can be found in humans and animals in tropical regions of Africa and Asia. However, in some parts of Africa and Asia, the two species overlap. Today, fascioliasis is considered an important human disease, and several geographic areas have been described as endemic for the disease in humans, including hypoendemic (prevalence < 1%), mesoendemic (prevalence between 1% and 10%), and hyperendemic (> 10%) situations, with intensities ranging from low to very high. Estimates suggest up to 17 million people are infected worldwide. This number may even be an underestimate, considering the lack of data from numerous Asian and African countries. Moreover, in the last two decades, human fascioliasis has been emerging in many regions, a phenomenon partly linked to climate change. Recent studies have shown that the high pathogenicity of this disease is not only restricted to the acute phase but also extends to long-term liver fluke infections, the immune modulation during the acute phase, and their immune suppression effect in chronic and advanced chronic stages. These factors are often observed alongside co-infections with other parasitic and infectious diseases.

Giardia Lamblia

Giardia lamblia is one of the most common human intestinal protozoan pathogens worldwide. The incidence strongly depends on the geographic region, reaching 2-7% in central Europe and exceeding 50% in tropical countries. The life cycle of Giardia lamblia is characterised by two stages: the trophozoite and the cyst stage. The trophozoite is the motile dividing stage and inhabits the upper small intestine. Ascending infections of the gallbladder may also occur. The cyst is the infective form of the parasite. It develops in the intestine and is excreted with the faeces. Cysts are transmitted via contaminated food or drinking water, but also from person to person.

Leptospira

The clinical manifestations of leptospirosis can range from a mild catarrh-like illness to icteric disease with severe liver and kidney involvement. Natural reservoirs for leptospirosis include rodents, as well as a wide variety of domesticated mammals. The organisms reside in the lumen of nephritic tubules in their natural host and are shed into the urine. Human infection typically results from direct exposure to infected animals (such as veterinarians, abattoir workers, or dairy workers) or by exposure to environments contaminated by animal carriers (e.g., agricultural workers). Bathing or swimming in water sources contaminated by livestock has been shown to pose an infection risk. The organisms enter the host through skin abrasions, mucosal surfaces, or the eye. The incubation period can range from 3 to 30 days, but it is usually between 10 and 12 days. Antibodies may become detectable by the 6th to 10th day of illness, generally reaching peak levels within 3 to 4 weeks. Antibody levels then gradually decline but may remain detectable for years.

Schistosoma Mansoni (Bilharziosis)

Schistosomes belong to the class of distomas (trematodes) and are among the most frequent pathogens. It is estimated that more than 200 million people are affected by these parasites. The mature parasites range from 6 to 22 mm in length. The most important species are Schistosoma mansoni, S. japonicum, and S. haematobium. Schistosoma mansoni is common in Africa, South America, and the Middle East.

Schistosomiasis (also known as bilharzia) is, depending on the species and location of the parasites, a disease affecting the intestine, liver, spleen, or urinary passages. Humans are (re)infected through contact with fresh water contaminated with ova from infected urine or faeces. When larvae penetrate the human skin, an initial transient skin reaction occurs (itching, with exanthema or erythema). Repeated infections may lead to cercarial dermatitis.

Strongyloides

Strongyloides is a genus comprising around 50 species of obligate gastrointestinal parasites of vertebrates. Strongyloides stercoralis is the scientific name of the human parasitic roundworm responsible for causing strongyloidiasis. The S. stercoralis nematode can parasitise humans. The adult parasitic stage resides in tunnels within the mucosa of the small intestine. S. stercoralis is typically found in tropical and subtropical climates, although cases also occur in temperate regions, particularly in rural areas. The prevalence of S. stercoralis is very low in societies where faecal contamination of soil or water is rare. Many infected individuals remain asymptomatic initially.

Symptoms of the infection include dermatitis, characterised by swelling, itching, larva currens, and mild haemorrhage at the site where the skin has been penetrated. If the parasite migrates to the lungs, the chest may feel as though it is burning, and wheezing and coughing may occur, often accompanied by pneumonia-like symptoms (Löffler’s syndrome). The intestines may eventually be affected, resulting in burning pain, tissue damage, sepsis, and ulcers. In severe cases, oedema may cause obstruction of the intestinal tract and loss of peristaltic contractions.

Toxoplasma Gondii

Toxoplasma gondii is a small intracellular parasite, with a life cycle consisting of both a sexual and an asexual phase. Sexual development occurs exclusively in the intestinal cells of cats (likely the only hosts), where oocysts are formed and excreted. Due to their resistant cell walls, the oocysts may remain infectious for up to a year under favourable conditions. Animals and humans serve as intermediate hosts for the asexual proliferation of T. gondii; once ingested, the parasites proliferate explosively within host cells, eventually lysing them. The parasites spread throughout the body via the bloodstream and lymphatic system, potentially infecting any type of cell.

Trichinella Spiralis

Trichinosis (also known as trichinellosis) is caused by nematodes (roundworms) of the genus Trichinella. In addition to the classical agent Trichinella spiralis, which is found worldwide in many carnivorous and omnivorous animals, four other species (T. pseudospiralis, T. nativa, T. nelsoni, and T. britovi) are recognised. Trichinosis is acquired by ingesting meat containing cysts of Trichinella. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa, where they develop into adult worms (females measuring 2.2 mm in length, males 1.2 mm). After one week, the females release larvae that migrate to the striated muscles, where they encyst. Encystment is completed in 4 to 5 weeks, and the encysted larvae may remain viable for several years. Ingestion of the encysted larvae perpetuates the cycle.

Trichinosis occurs worldwide but is most common in parts of Europe and the United States. Light infections may be asymptomatic. For mild to moderate infections, most symptoms subside within a few months, while fatigue, weakness, and diarrhoea may last for several months. In severe cases, death can occur.