Parasites and Parasitic infections in Australia: causes, prevention and treatments

Members of the Protozoa are tiny, single-celled organisms commonly found in soil and water. While most Protozoa are free-living and do not cause parasitic infections in humans, there are various species that represent significant health burdens. For example,

• Giardia – this protozoan is the causative parasite behind giardiasis, described above. This microorganism has a two-stage life cycle, with an initial stage known as trophozoite, where the parasite can move and swim within the inner workings of the small intestine. During this stage, this parasite feeds on nutrients It finds in its environment. In the second stage of this parasite, Giardia becomes a non-moving cyst, which is commonly expelled in the feces of animals or humans. When these cysts reach water supplies or food, it becomes a risk for any human consuming the food or water. Human to human contact, and unsanitary living arrangements are common causes of parasitic infection by the Giardia Some common sites if parasitic infection for Giardiasis include:
  • Childcare centres, where poor hygiene may occur when handling dirty nappies from infected children.
  • Contaminated water supplies. Here, the parasite spreads when people drink or use contaminated water to wash fruits. Swimming in contaminated water bodies is another common route of infection²¹.

In Australia, there are about 600,000 cases of giardiasis diagnosed every year, and this parasitic infection affects a disproportionately large number of children in poor and remote Indigenous communities³⁴.

• Plasmodium – this protozoan is the parasite responsible for malaria, described above. The Plasmodium parasite first develops in mosquitoes, which then go on to infect humans. When a parasite-infected mosquito bites a human, the parasite enters the body, starting the parasitic infection. Once in the human body, this parasite destroys red blood cells, impairing the function of multiple organs⁸.

 Parasitic infections by these worms, as by many other worm parasites, require the ingestion of eggs, which can occur via the faecal-oral route. The primary cause behind parasitic infections by worms involves poor sanitary practices, particularly when sharing living facilities.  

Soil-transmitted helminth infections are one of the most common parasitic infections worldwide. They include species like roundworms (Ascaris lumbricoides), the whipworm (Trichuris trichiura) and hookworms (Necator americanus and Ancylostoma duodenale)¹².

The leading cause behind helminth parasitic infections is the transmission of eggs found in the faeces of infected people. When egg-infested faeces contaminate the soil, they can reach other persons through various routes:

• eggs can become attached to vegetables, which can transmit the egg when not properly washed, peeled or cooked;
• ingestion of contaminated water sources;
• ingestion by children who play in contaminated areas;

In some species of parasitic worms, like hookworms, the eggs hatch in the soil, releasing a larva that matures into an adult form. The adult can penetrate the skin, and people become infected when walking barefoot in contaminated soil¹². Learn more about these parasites on the WHO website.

The main cause behind parasitic infections by this parasite is a fecal-oral transmission. However, the different life stages of this parasite are not fully understood, so specific details of the infection process are not known, such as the infectious stages of the parasite. So far, it is thought that the cysts form of this parasite represents the infectious stage. However, the most common form of this parasite, found in human stools, is called the central body or vacuolar and can be much bigger than the cysts. There are also other different forms of the parasite that have been found in stools. Currently, it is not clear what is their biological function, within the life cycle of the parasite²².

Besides humans, Blastocystis has been detected in the stool of many non-human animals, such as canids, swine, primates, rodents, birds and other species. Read more about the causes behind Blastocystis infections here.

Despite being a known parasite, causing parasitic infections in humans for more than 100 years, several aspects of the biology of this single-celled protozoan parasite are unknown. For example, there is no clear understanding of this parasite’s host distribution or even its complete life cycle. Currently, there are two leading hypotheses for how this parasite infects and spreads to new human hosts. One hypothesis suggests oral-faecal transmission, involving handling food or objects contaminated with faecal matter containing trophozoites of the parasite (a stage in the parasite’s life cycle). Another hypothesis is that D. fragilis is transported to a new human host through pinworms or other helminth worms, acting as vectors for this parasite. However, the evidence for the role of worms as vectors is not clear, and there are conflicting reports^22,36. A potential life cycle of this parasite can be found here.

Transmission of head lice and scabies occur via person-person contact. According to one review, it only takes 20 minutes of physical or sexual contact to transmit these ectoparasites³⁶ successfully.  There may be more than 200 million people with scabies worldwide, and up to 6 million people have head lice in the USA. In Australia, as many as 60% of school-age children.