Imagine a silent saboteur wreaking havoc in your body. This invisible enemy is chronic inflammation — the underlying cause of many chronic diseases.
Parasitic worms (also known as helminths) are soil-transmitted parasites that live in your gastrointestinal tract, while others like schistosomes reside in blood vessels. They escape from hosts through a variety of strategies.
1. Infection
The immune system is a complex organism that must respond quickly and effectively to a myriad of threats; from bacteria, viruses, and fungi to the many different parasites that infect our bodies. Over millions of years of co-evolving with their host species, parasitic worms have developed advanced methods to evade, manipulate or modulate the immune response to survive and fulfil their life cycles.
Many of the strategies used, involve altering the composition of a host’s gut microbiota, which has an influence on inflammation and immune development. For example, a recent study shows that the blood-borne parasite Trypanosoma brucei plays a game of hide-and-seek with its host by constantly changing its protective surface coat and thus evading antibodies and avoiding detection by the immune system.
Parasites are a major contributor to disease in the developing world and are associated with poverty and malnutrition. While this is true, it is also important to recognize that low levels of parasites can often bolster health, at least in the area of immune function. Evidence has shown that individuals with a heavy burden of intestinal helminths present fewer symptoms of diseases such as multiple sclerosis and irritable bowel syndrome.
These unwelcome guests are obtained by ingesting parasite eggs from contact with contaminated surfaces (e.g. bed linens or pet fur), eating contaminated food, or walking on soil with the larvae of certain types of intestinal parasites (e.g. hookworms, Strongyloides, or Ascaris). Some parasites, such as malaria (i.e. Plasmodium), require insect vectors, such as mosquitoes or ticks.
Once inside the host, parasites purposefully break into specific cells to steal what they need. For example, for the parasites that cause malaria, they invade red blood cells to utilize the nutrients in those cells; for other parasites, they invade the skin, in order to reach other tissues in the host, like blood vessels, or invade the lungs to commandeer our respiratory cells.
Some parasites are identifiable, often due to their size, by their visual presence via microscope, and for those they are easily detectable. But for others, like the parasites that infect our intestines, the only way to detect they are infected is long-term monitoring of an individual’s immune responses using a fluorescent marker – which has been done by genetically modifying to express luciferase, a fluorescent protein, and so scientists are able to track immune cell trafficking obtained via in vivo observations.
2. Decreased Immune Function
Although parasites are generally viewed as harmful, they are actually capable of providing several beneficial services. In humans, infection with some parasites provides a means for immune system modulation and can reduce the risk of developing autoimmune disease in a person predisposed to it. Parasite infections can also help identify the presence of bacterial infections or other pathogens by indirectly signaling the immune system to respond to them.
Parasites are also a valuable research tool for immunologists because they can shed some light on how the immune system’s responses to infection and how they can be manipulated in specific circumstances. Furthermore, parasites can provide information about what types of protein antigens, that trigger infection, activate the immune system’s responses. For example, protozoal parasites which elicit mainly Th1 response, such as Leishmania, provoke different responses than helminths, like hookworms or schistosomes, which elicit mainly a Th2 response.
Parasites cause disease primarily by attempting to stifle or stall a host’s immune response. This can be accomplished through the secretion of immunomodulatory molecules which act on the immune cells, or by blocking the expression of pathogen-associated molecular patterns (PAMPs), which are fundamentally important to host immune detection.
Throughout the life cycle of some parasites, there is dramatic alteration in shape and antigen composition particularly with respect to multicellular organisms like flukes and tapeworms. These alterations are critical to avoid immune attack from the host, and predators like nematodes, eskimos, and ticks.
Parasite invasiveness allows researchers to observe how the immune system responds to them. For example, researchers have demonstrated that helminth parasites, such as hookworms Nippostrongylus brasiliensis, can down-regulate the induction of a classic macrophage phenotype during infection. This is likely a factor in the observed impaired immune response in helminth-infected malaria patients in comparison to uninfected controls.
Parasites can also be helpful tools for tracking illegal wildlife trade. In a recent study, biologists used the parasites present when snakes or other endangered species are confiscated from illegal trade to trace them back to their original source to determine if laws have been broken.
3. Inflammation
Parasites living in your digestive system produce toxins that cause irritation to the intestinal wall. This irritation can lead to inflammation and symptoms of nausea, vomiting, bloating, constipation / diarrhea, malnutrition and dehydration in serious cases, and an increased likelihood of developing certain cancer types.
Inflammation is also a factor in neurodegenerative disease (dementia such as Alzheimer’s and Parkinson’s ). Parasites can disrupt the brain-blood barrier, permitting pathogens to start infecting the brain. Not only can these toxins alter the function of glial cells that support and protect the blood vessels of the brain, but this can also increase inflammation and the loss of function of the brain.
Parasites have some clever strategies of their own to survive in the body. For example, certain parasites can elicit an immune response that suppresses Th1 and Th2. This allows them to escape the sterilizing effect of the Th1 response, while getting redirected to an even more self-destructive Th2 response; creating autoimmune diseases.
For this reason individuals living in areas where parasitic infections are more common demonstrate lower rates of inflammatory bowel disorders (i.e., Crohn’s and ulcerative colitis). Epidemiologic studies have also demonstrated that carriers of whipworm or strongyloides (i.e., parasites that are a type of roundworm living in the gut) have lower rates of IBD than uninfected controls.
In a report by NYU Langone researchers, the team infected mice with whipworm and associated intestinal changes in direct observation. The researchers reported that whipworm parasites stimulated an immune response that accelerated mucus production, allowing for a class of good bacteria (Clostridia) to multiply, potentially emphasized by outcompeting parasitic bacteria (Bacteroides), entering a symbiotic relationship to restore gut homeostasis, otherwise called microbiome restoration. A healthy microbiome can fight and prevent intestinal parasites while keeping them in check. Following an eating pattern rich in probiotics, Vitamin C and sulfur, and some of those foods would be garlic, beetroot and pumpkins seeds, while avoiding dairy, processed food and sugars that also fuel parasites.
4. Decreased Growth
The parasites that most people probably know are single-celled, microscopic organisms, such as the protozoa which can cause giardiasis or cryptosporidiosis, or multicellular worms like Taenia saginata (beef tapeworm) or Taenia solium (pork tapeworm). They can be transmitted by ingesting infected food or water, by contact with contaminated soil, or through an insect vector.
These parasites also contribute to a substantial burden of disease in many developing countries. The helminths that cause intestinal parasitic infections, such as roundworm or hookworm, have a particularly profound burden in poor resource-limited situations by degrading cognitive ability and causing retarded growth and development in children. These diseases are called neglected tropical diseases, and they impose a significant economic burden on the countries in which they occur.
Other parasitic diseases such as malaria, lymphatic filariasis and onchocerciasis can also take a huge toll on the lives of their human hosts. These diseases are often considered to be endemic in low-income countries, but they can also occur in people living in resource-rich countries where the infection is rarely detected because of lack of screening and treatment services.
Even a single parasitic infection can decrease linear growth in children, and multiple parasites can have an additive effect. This is why researchers are focusing their attention on understanding how to control the spread of parasitic diseases. Education in personal hygiene and sanitation, deworming programs, nutrient supplementary programs and regular evaluation of infection and nutritional status are all possible strategies to control parasitic diseases and improve linear growth in children.
Parasites are not just bad news for humans: they can sometimes act as useful scientific tools. For example, when Museum curator Eileen Harris discovered a nematode worm in an alligator skin handbag bought from an illegal dealer of endangered animals she used the worm to help authorities track down the seller and convict them for breaking the law. Parasites are also helpful in investigating cases of animal and human neglect. For example, a myiasis-causing worm in a live frog can help investigators find out who was responsible for the frog’s neglect by helping to identify the fly larvae that caused the myiasis Read More.