Parasitic diseases are not considered as contagious diseases meaning that they are not usually transmitted by direct contact with an infected person or with its secretions. That also means that these diseases can theoretically be contained more easily in case of an outbreak.
Parasitic diseases have many transmission modes. However, they are mainly acquired through insect vector or by ingestion of contaminated food or water. As mentioned in the next paragraphs, other modes of transmission can also occur, but they are less common.
Some insect vectors can transmit parasitic diseases, mainly protozoan parasites. They can be an accidental carrier of the disease, for example when a fly came in contact with animal or human faeces and carries the parasite on its body, or they can be an intermediate host for the parasite. In the latter case, the insect species is a specific host for the parasite species, meaning that one type of parasites infects only one or a few specific insect species. Then, the insect can infect humans (or sometimes other mammals that acts as intermediate or final hosts) by two principle means. Firstly, insect can transmit parasites such as Plasmodium and Leishmania through its saliva while taking a blood meal on a human. Finally, insect can also defecates faeces containing parasites such as Trypanosoma cruzi on the human host’s skin immediately after taking their blood meal.
Other parasites can also be transmitted by contact with pets. The diseases that can be caught by contact with animals are generally called zoonotic diseases. As an example, cats and dogs, especially when they are young, are often carrying soil-transmitted hookworms or nematodes. Humans can then be infected by close contacts with these pets.
A wide range of protozoan parasites or helminths can be acquired by eating contaminated food. Infection can occur from many ways. These food-borne parasitic diseases are seen occasionally in developed countries, but are widespread in undeveloped or developing countries.
Some zoonotic parasitic diseases, such as the raccoon parasite Baylisascaris, can be caught by accidental ingestion of contaminated soil by animal faeces. It is one of the reasons why it is best to wash fruit and vegetables before eating them. This parasite mode of transmission is also called fecal-oral transmission. It is worth noting that food can also be contaminated by parasites by lack of hygiene from infected workers in food service, notably if they omit to wash their hands after going to the washroom. Furthermore, infection can be acquired if contaminated water comes in contact with food, for example if it is used to wash fruit or vegetables.
Eating raw or not enough cooked meat, fish or seafood can also be a source of a wide range of parasitic infections. The best-known example is the nematode Trichinella that is present in wild animals and pork and is able to infect humans ingesting raw or uncooked pork. However, because of the regulation standards in the pork industry, infection related to pork consumption with this parasite is now rare in Western countries, but still very present in other parts of the world, such as Eastern Europe and Asia. This type of contamination is also called predator-prey transmission.
Drinking or swimming in contaminated water is also a source of parasites contamination. While this mode of transmission is less common in developed countries, it is a real global problem because it is widespread in undeveloped and developing countries. Travelers should be aware of this mode of transmission and take necessary precautions. Examples of parasites that can contaminate water include Schistosoma, Giardia, Cryptosporidium and Dracunculus medinensis responsible for the Guinea worm disease.
Some parasites can be transmitted vertically from the mother to the unborn child. The most notable one is the protozoan parasite Toxoplasma gondii that causes toxoplasmosis. This infection is usually benign or asymptomatic, but it can be very dangerous and even fatal when acquired for the first time during pregnancy. It is a cause of miscarriage and of health complications for newborns. This parasite is commonly acquired by contact with soil contaminated with animal, mostly cats, faeces and raw meat. It can also be acquired by inhalation of cysts present for example in contaminated cats’ litter dust. It is the main reason why pregnant women should avoid cleaning their cats’ litter, wear gloves while gardening and avoid eating uncooked meat.
Some helminth larvae are also able to penetrate directly the skin. This mode of transmission is called transdermal. This is the case of soil-transmitted nematodes, like Ascaris species, that can be transmitted by contact with contaminated soil occurring, for example, when walking barefoot. It is also interesting to note that some parasites (or parasites’ cysts or eggs) can also penetrate in their human host by a wound that come in contact with contaminated soil.
It is noteworthy that the amoebae Acanthamoeba can cause a keratitis, which is a serious infection of the eye cornea. In developed countries, this disease is transmitted mainly by the utilisation of contact lenses that are not always washed and wear as recommended by the manufacturer.
Although parasitic diseases are not usually considered as sexually transmitted infections, there is one notable exception. Effectively, Trichomonas vaginalis is a protozoan parasite causing trichomoniasis, a sexually transmitted infection of the penis, the urethra or the vagina.
Finally, although it occurs in very rare occasions, some parasites living in the human bloodstream, such as some Babesia and Plasmodium species, can be acquired from blood transfusion, needle sharing or organ transplant.
Source: ARC/NHMRC (Australian Research Council/National Health and Medical Research Council) Research Network for Parasitology; Centers for Disease Control and Prevention