Pentastomiasis: Life Cycle and Morphology

Transmission

Life Cycle

Inside the definitive host:

Adults pentastomids reside in the lungs of a snake, or other definitive host organism. They are attached using hooks, and subsist on blood, lymph, mucus, and epithelial cells, which they suck into their digestive tract. These adults undergo internal fertilization, and produce eggs.

The eggs pass from the nasal passages of the host via nasal discharges, saliva, or feces.

Inside the intermediate host:

Infection: Humans accidentally ingest the ova via snake saliva or excreta, usually in contaminated water or food. Humans also sometimes eat uncooked snakes, and the pentasome ova within.

Inside the human, the ova reach the intestinal tract. The egg hatches and a viable embryo emerges. It develops into a first-stage larva.

 
 

The first-stage larvae penetrate through gut wall and migrate in the peritoneum and pleura. After a few days, they become encysted in tissue, such as the liver, spleen, mesentery, or pleura. (This is the first infectious stage for a definitive host.)

This is the end of the road for most pentastomes. The larvae inside the cysts usually die in approximately two years.

 

 

Again, inside the definitive host:

If the intermediate host (usually an animal besides man, such as a rat, monkey, or other wild or domesticated animals) is eaten by a snake or reptile, the encysted larvae enter the snake’s intestine, and develop into adult parasites. The adults in the snake reproduce and the ova are present in saliva, excreta, and lungs. And so the cycle continues...

 

Humans: a possible definitive host?

Encysted larvae in human tissues can (rarely) develop further in the cyst. This occurs as a series of 9 molts. After about 6 months of this molting, the now third-stage larvae emerge and wander throughout the peritoneal cavity. (This is the second infective stage for a definitive host.)

These larvae can migrate to the nasopharynx or lung. This is known as the Halzoun Syndrome. The larvae molt a few more times, and the mature adults can mate and lay eggs. In this instance, the human is now considered the definitive host.

A Stressed Host

Pentastomes can also escape from the host in seemingly unexpected ways, when the host is undergoing stress. The snake below was stressed, and pentastomes are emerging through the scales and nostril.

 

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Morphology:

There are approximately 70 species of pentastomes. However, differentiation among various species is much less important than differentiating pentastomes from helminths and mites. These pentastomes range from a few millimeters to more than 15cm in length, depending on species and sex.

 

Pentastomes possess four hooks surrounding a projection that is the true mouth, giving the appearance of a “5-mouthed” organism (hence the name “penta-stome.”) They have straight-tube digestive tracts and ventral nervous systems, but do not have any form of a circulatory or respiratory system.

Linguatulidae have a flattened, tongue-like body, and Porocephalidae are more cylindrical in shape.

   
 

The adult pentastome is translucent, whitish, or yellow in color. The body is an elongated cylindrical vermiform body with a blunt tail. They have a characteristic “string of beads” look due to rings called annuli, all down the length of the body.

 

Females are always much larger than males, approximately 90 to 120 mm long and 5 to 9 mm wide. Males range from 30 to 45 mm in length and 3 to 4 mm in width. These adults resemble helminthes, whereas the first larval stage resembles a mite.

Although smaller, nymphs inside cysts are coiled in a flat spiral and resemble the adults in shape and structure.