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Life Cycle of Parasites

Life Cycle of Parasites

What defines the life cycle of one parasite from another?

I bet when somebody even mentions the word ‘parasites’ like Buzz Lightyear, you’d probably cringe like Woody (most people do, and I can’t say I blame them).

Pic: http://3.bp.blogspot.com/-HkX0ml3IzwQ/UNuBaeV9J4I/AAAAAAAAKDA/TTQK1NqxMCI/s1600/disgusting-parasites.jpg
Pic: http://3.bp.blogspot.com/-HkX0ml3IzwQ/UNuBaeV9J4I/AAAAAAAAKDA/TTQK1NqxMCI/s1600/disgusting-parasites.jpg

When it comes to life cycles of parasites, they can differ depending on the following factors:

  1. The environment in which it naturally survives in such as the soil or water supplies.
  2. The parasites’ definitive host i.e. the host in which it can sexually reproduce and create eggs: is sometimes us humans but not always
  3. The number of intermediate hosts i.e. hosts in which the parasite can reproduce asexually (produce clones of itself) but not produce sexually.
  4. The parasites’ infective vs. protective stages (usually the eggs or cysts are protective, and the adult forms or trophozoites are infective)
  5. The way in which it enters and exits the human host: examples of methods of entry include inhaling, swallowing (ingestion) and direct penetration through the skin, and examples of exiting include coughing/sneezing, and through the body’s natural waste systems (urine/faeces).

In this page, I won’t go over every single life cycle of every single parasite that I mentioned in Introduction to Parasites, but I will go over the ones in which humans play a role, either as a definitive or an intermediate host.

Cryptosporidium spp. 

Pic: http://static.ddmcdn.com/gif/flea-1.jpg
Pic: http://static.ddmcdn.com/gif/flea-1.jpg
  • Species of Cryptospordium: C.parvum
  • Definitive host: Humans
  • Intermediate host: None
  • Method of transmission: Faecal-oral

C. parvum is a very common infection in patients that are HIV positive. It is also common in patients that are both healthy and immunocompromised: but it eventually cures itself in healthy patients, whereas it can potentially be fatal for those whose immune systems are not strong enough to fight off the parasite. Symptoms of C. parvum infection include watery diarrhoea, anorexia (refusal to eat), vomiting and abdominal pain. Unlike most other water-borne parasites such as Giardia, it is able to resist the effects of chlorine. The life cycle of C. parvum is demonstrated below.

Smart Art Graphic: Original
Smart Art Graphic: Original

Dipylidium spp.

Pic: http://static.ddmcdn.com/gif/flea-1.jpg
Pic: http://static.ddmcdn.com/gif/flea-1.jpg
  • Species of Dipylidium: D. caninum
  • Definitive host: Humans
  • Intermediate host: Fleas, dogs, cats
  • Method of transmission: Ingestion of fleas

D. caninum is carried via fleas and can be passed onto multiple hosts. It is also known as the flea tapeworm, and can cause restlessness and diarrhoea in children, but does not tend to cause symptoms within dogs or cats. The life cycle of D. caninum is displayed below;

Smart Art Graphic: Original
Smart Art Graphic: Original

 

Echinococcus spp.

  • Species of Echinococcus: E. granulosis 
    Pic: http://www.vippetfoods.com.au/Uploads/group%20dogs2.png
    Pic: http://www.vippetfoods.com.au/Uploads/group%20dogs2.png
  • Definitive host: Dogs and other canines
  • Intermediate hosts: Human, sheep, goats, pigs
  • Method of transmission: as explained below.

E. granulosis is endemic (constantly present) in Australia. This particular parasite prefers dogs to other creatures, and it tends to transfer between dogs and sheep by the dogs eating the meat of the herbivores, and the sheep, goats and pigs eating the contaminated faeces of the dogs. It is passed onto humans when we eat contaminated lamb, pork or goat. When it enters our system, its first target is the liver, and then it migrates up to the lungs and the brain. When it is extracted by surgery, the cyst needs to be killed off before it is removed or anaphylactic shock (drastic drop in blood pressure) can occur due to leakage. The life cycle is demonstrated below:

Smart Art Graphic: Original
Smart Art Graphic: Original

Giardia spp.

Pic: http://www.syncytiabeta.org/~syncyt5/syncytiabeta/images/c/ce/Giardia.jpg
Pic: http://www.syncytiabeta.org/~syncyt5/syncytiabeta/images/c/ce/Giardia.jpg
  • Species of Giardia: G.lambia
  • Definitive host:  Humans
  • Intermediate hosts: None
  • Method of transmission: Swallowing

Giardia survives in fresh water supplies that have been contaminated with human faeces. Unlike a lot of other parasites, humans are the only species that can contract Giardia. Symptoms of a Giardia infection include malnutrition (because Giardia absorbs the nutrients from our food), abdominal cramping and fatty diarrhoea. It is very easily spotted under the microscope, as it can appear like it has two ‘owl eyes’.  Here is its life cycle in full:

Smart Art Graphic: Original
Smart Art Graphic: Original

Hookworm spp. 

  • Species of hookworm: N. americanus, A. duodenale 
  • Definitive host: Humans
  • Intermediate host: None
  • Method of transmission: Direct penetration of the skin

I talked about hookworms before in Introduction to Parasites, so I won’t talk about them much here. They have a tendency to infect dogs. The life cycle of hookworms is demonstrated here: it is very similar to Strongyloides, but far less complex:

Smart Art Graphic: Original
Smart Art Graphic: Original

Leishmania spp.

Pic: http://www.creepycrawlies.info/images/sand-fly.jpg
Pic: http://www.creepycrawlies.info/images/sand-fly.jpg
  • Species of Leishmania: too many to mention!
  • Definitive host: Sandflies
  • Intermediate host: Humans, rodents, dogs
  • Method of transmission: Blood meal from female sandfly

There are over 30 separate species of Leishmania (hence why I didn’t bother to list them, there are simply too many!) They are carried by female sandflies and  are transferred to other hosts when the sandfly takes blood from another species: whilst it eats, it secretes saliva contaminated with the parasite. This parasite is responsible for the disease of leishmaniasis: there are several subtypes of the disease and its symptoms include: scarring (cutaneous form), skin lesions (diffuse cutaneous form), skin ulcers (mucocutaneous form) and fever, weight loss, fatigue, and hepatosplenomegaly (swelling of the liver and spleen.) The life cycle of Leishmania is below (click to enlarge).

Smart Art Graphic: Original
Smart Art Graphic: Original

Nematoda spp.

Pic: http://www.qmedicine.co.in/top%20health%20topics/R/images/demo/Roundworm.jpg
Pic: http://www.qmedicine.co.in/top%20health%20topics/R/images/demo/Roundworm.jpg
  • Species of Nemtoda: E. vermicularis, T. trichiura, A.lumricoides
  • Definitive host: Humans
  • Intermediate hosts: none
  • Method of transmission: Direct penetration of skin, swallowing

As I mentioned before in Introduction to Parasites, round worms are the most common species of worms in Australia. T. trichiura is also known as whipworm, E. vermicularis is also known as the pinworm or threadworm, and A. lumricoides is also known as the giant roundworm. Each of these mature within the intestine of humans and are excreted in faeces. The life cycle of roundworms in general is illustrated in the Smart Art Graphic below.

Smart Art Graphic: Original
Smart Art Graphic: Original

Schistosoma spp. 

Pic: http://4.bp.blogspot.com/-0X_Mm-Y-8XE/T7zuSSUaokI/AAAAAAAAKSo/q68sumXb6MM/s1600/Schistosoma_mansoni.jpg
Pic: http://4.bp.blogspot.com/-0X_Mm-Y-8XE/T7zuSSUaokI/AAAAAAAAKSo/q68sumXb6MM/s1600/Schistosoma_mansoni.jpg
  • Species of Schistosoma: S.mansoni, S.japanicum and S. haematobium 
  • Definitive host: Humans
  • Intermediate hosts: Snail
  • Method of transmission: Direct penetration

There are many stages involved in the life cycle: below is a condensed version of the cycle. Depending on the species of Schistosoma, they are excreted from humans either in urine or in faeces: S. mansoni (as shown here) and S. japanicum are expelled in faeces, and S. haematobium is expelled in urine. The full cycle goes like this:

  1. Sporocysts hatches from excreted egg
    Smart Art Graphic: Original
    Smart Art Graphic: Original
  2. Sporocysts infects snails
  3. Many cercariae are released from infected snails
  4. Humans are infected by contact with cercariae in water
  5. Cercariae migrate to liver and mature to adulthood
  6. Adult Schistosoma mate in the liver and migrate to the intestines or urinary tract
  7. Eggs released from adults and are expelled in the faeces in the urine.

Strongyloides spp.

  • Species of Strongyoides: S stercoralis
  • Definitive host: Humans
  • Intermediate host: None
  • Method of transmission: Direct penetration into the skin

S. stercoralis is more commonly known as threadworm, and infection by S. stercoralis is lifelong unless it is treated. Only the rhabdiforms are seen in human faeces, and they take up to three weeks to fully mature into adults once they reach the intestines. Since it enters the skin, it takes a convoluted route to reach its final destination, as the Smart Art graphic shows below. Symptoms of this infection include dermatitis (skin inflammation) at the site of puncture, a burning sensation at the lungs, wheezing and coughing, and ulcers and sepsis within the intestines.

Smart Art Graphic: Original
Smart Art Graphic: Original

Taenia spp.

  • Species of Taenia: T. saginata, T. solium
  • Definitive host: Human
  • Intermediate: Cows, pigs
  • Method of transmission: Eating contaminated pork or beef products.

T. saginata and T.solium are mentioned in Parasite and Prion Infections, so I won’t go into too much detail. The way that these parasites are excreted from the human hosts is via small segments known as either eggs or gravid proglotids, as shown in the picture above. The picture below is the life cycle of the Taenia spp.

Smart Art Graphic: Original
Smart Art Graphic: Original

 

Toxoplasma spp.

Pic: http://jasonlefkowitz.net/wp-content/uploads/2013/07/cats-16140154-1920-1080.jpg

Pic: http://jasonlefkowitz.net/wp-content/uploads/2013/07/cats-16140154-1920-1080.jpg

  • Species of Toxoplasma: T.gondii
  • Definitive host: Cats
  • Intermediate host: Humans, pigs, sheep
  • Method of transmission: Eating raw contaminated pork products, touching contaminated cat faeces.

I won’t go into too much detail here since I’ve already discussed Toxoplasma in Parasite and Prion Infections. They present themselves within tissue samples as bradyzoites (or tissue cysts). The life cycle of Toxoplasma is demonstrated below.

Smart Art Graphic: Original
Smart Art Graphic: Original

Trichomonas spp.

Pic: http://www.trichomoniasis.org/gfx/t_vaginalis_trophozoites.jpg
Pic: http://www.trichomoniasis.org/gfx/t_vaginalis_trophozoites.jpg
  • Species of Trichomonas: T. vaginalis
  • Definitive host: Humans
  • Intermediate host: None
  • Method of transmission: Sexual contact

T. vaginalis is a sexually-transmitted disease. Infected males generally don’t show any symptoms, however women may show symptoms of trichomoniasis if it ends up in the vaginal tract. Symptoms of trichomoniasis include inflammation of the cervix, vagina and urethra and a yellow-green discharge from the vagina.  The life cycle is very simple as it is always exists in the trophozoite form:

Smart Art Graphic: Original
Smart Art Graphic: Original

Trypansoma spp.

Pic: http://ergodd.zoo.ox.ac.uk/livatl2/images/tsetse.gif
Pic: http://ergodd.zoo.ox.ac.uk/livatl2/images/tsetse.gif
  • Species of Trypanosoma: T. cruzi
  • Definitive host: Humans
  • Intermediate host: Tsetse fly
  • Method of transmission: Bite from tsetse fly

T. cruzi is the parasite which is responsible for African sleeping sickness. Symptoms of this condition include lesions and lymphadenopathy (disease of the lymph nodes) in the early stages and in the late stages, mental confusion and coma. If left untreated, it can be fatal. It gets spread by bites from the tsetse fly. The life cycle of T.cruzi is demonstrated below. Apologies if the names are confusing, unfortunately most of the names for these things are in Latin!

Smart Art Graphic: Original
Smart Art Graphic: Original