Porocephalus crotaliNorth American porocephalus

Geographic Range

Porocephalus crotali is found all over the world, including Africa, Asia, The Middle East, and North and South America. It is particularly abundant in North and South America. (Layne, 1967; Penn, Jr., 1942; Tappe and Buttner, 2009)


Porocephalus crotali are parasitic on snakes, and outside their hosts, they are found in terrestrial and swamp areas. Adults live inside the respiratory tract of snakes and larval nymphs live in the viscera of mammals. Nymphs spend much of their time migrating and crawling through the fluid-filled viscera by using their soft and flexible tegument, hooked legs, and penetrating mouth hooks. During their long evolutionary history, adult and larval nymphs have become very adapted to this internal habitat. (Layne, 1967; Riley and Henderson, 1999)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

Physical Description

Porocephalus crotali are bilaterally symmetrical and contain two parietal muscle systems, longitudinal and latitudinal. They are rounded in cross section but tapered at the ends. Their body cavity is a hemocoel with an open circulatory system. They do not have a respiratory system. They have an external, non-living cuticle and the body plan is characterized by a cephalothorax, abdomen, and terminal segment. On the cephalothorax, adults have 4 hooks surrounding a keyhole-shaped mouth and 2 pairs of ventral sensory papillae. They are annulated but not truly segmented. Both males and females average around 35-36 annuli over their entire body length. Both sexes have an abundance of annular pits on the annuli which are used to communicate with specialized epidermal cells that regulate the hydromineral balance of the haemolymph. Sexual dimorphism exists. Male body length ranges from 27-35 mm and female body length ranges from 48-70 mm. Males average 32 mm and females average 65 mm. Larval nymphs are quadripeds and contain 4, doubly hooked legs. At their anterior ends, they have a penetration apparatus and at their posterior ends, a bifurcate tail. Eggs contain an inner, oval-shaped vitelline membrane (< 1 micron) and an outer membranous shell that is a much thicker (3-4 microns), perfectly shaped sphere. Both shell membranes enclose the larvae and can be visually distinguished by a wide and clear, fluid-filled space that separates both layers. The outer shell membrane is flexible and permeable. The inner shell membrane is an "inner shell complex" made up of three distinct layers. (Brookins, et al., 2009; Esslinger, 1962a; Esslinger, 1962b; Penn, Jr., 1942; Riley, 1981; Self and Riley, 1979)

  • Sexual Dimorphism
  • female larger
  • Range length
    Adult male: 27 mm; adult female: 48 to Adult male: 35 mm; Adult female: 70 mm
    to in
  • Average length
    Adult male: 32 mm; Adult female: 65 mm


Eggs ingested by a mammal travel down the alimentary tract and hatch in the small intestine. Within only 1 or 2 hours, the motile, primary larva penetrates the gut wall and migrates to the liver and the lungs of the mammal, eventually encysting and becoming encapsulated in tissues, including the viscera, mesenteries, and walls of the thoracic and abdominal body cavities. At about the seventh day, the primary larva become inactive, then undergo their first molt. Following molts, larvae grow larger in size. Mouth hooks are visible during the third nymphal stage and distint segmentation is visible after the 4th nymphal stage. Sexual differentiation occurs at the fifth nymphal stage. The snake ingests an intermediate mammal host containing seventh (VII) instar infective nymphs that excyst and penetrate the gut wall, migrating through the viscera and directly to the lungs. Within the lung cavities of the definitive host, infective seventh instar nymphs continue to molt and finally mature into male instar (X) and female instar (XI) adults to reproduce and complete the life cycle. Adults with mature eggs are expelled from the trachea and eliminated from the definitive host by oral expulsion. The adults may also be swallowed, resulting in shed eggs in the feces in which another intermediate mammal host will ingest to begin the life cycle again. (Brookins, et al., 2009; Buckle, et al., 1997; Esslinger, 1962a; Esslinger, 1962b; Layne, 1967; Self and Riley, 1979)


There is limited scientific literature concerning the mating systems of Porocephalus crotali. This parasite is dioecious, and copulation is between terminal instar males and females. Copulation occurs only once in the lifetime of females. Females will mate with one male when the spermathecae are fully developed but the uterus may still be undeveloped. Adult males have spicules that may be used to hold open the female reproductive opening during copulation. Males have seminal vesicles for storing sperm and when copulation does occur, male cirrus threads from the cirrus sac penetrate the spermathecal ducts of females to deliver sperm to the spermathecae. (Buckle, et al., 1997; Riley, 1981)

Reproduction occurs in the lungs of the definitive host and adult males and females reach sexual maturity approximately 16 days after infection of the VII larval instar. Copulation between adult males and females occurs approximately 75-86 days post-infection of the definive hosts. Females are able to store sperm in the spermathecae for several years. Females have enormous fecundicity. Egg production is continuous over 6-10 years. Egg production, however, does not begin until 230-250 days post-infection of the definitive host. There is a higher prevalence of infection during the warmer months of the year when hosts are more mobile in a warm environment. (Buckle, et al., 1997; Layne, 1967)

  • Breeding interval
    Year-round breeding, however, more prevalent during warmer months.
  • Average number of offspring
    1,000,000 eggs
  • Average age at sexual or reproductive maturity (female)
    16 (p.i.) days
  • Average age at sexual or reproductive maturity (male)
    16 (p.i.) days

Porocephalus crotali females do not exhibit any type of specific parental investment.


Upon ingestion of an egg, Porocephalus crotali nymphs are able to remain viable and encapsulated within rodent tissues for several years. Upon ingestion by a definitive host, larvae penetrate the stomach wall and body cavity within 24 hours. Within a few days, there is establishment in the lungs. Adults may survive in their definitive hosts from up to 6-10 years. Eggs are extremely resilient to environmental stress. Experiments found in vitro adults capable of surviving anywhere from 8-30 days. Furthermore, in vitro eggs were able to withstand dessicated conditions for up to 2 weeks and when kept alone in water, were viable for up to 6 months. Remarkably, the addition of water to dry eggs restores the original shape of the egg within seconds. It is difficult to document the longevity of Porocephalus crotali in the wild, as hosts are mobile and difficult to track. Thus, there is limited scientific literature on the viability of P. crotali in the wild. (Buckle, et al., 1997; Esslinger, 1962a; Riley and Henderson, 1999)

  • Average lifespan
    Status: wild
    6-10 years
  • Typical lifespan
    Status: captivity
    8 to 30 days


As an obligate endoparasite with a heteroxenous life cycle, Porocephlus crotali has both sessile and mobile stages. Nymphs are mobile during migration within intermediate host tissues and become sessile when encapsulated in the viscera. In the definitive host, nymphs are mobile during migration to the respiratory tract and also mobile in the snake's lungs. (Riley and Henderson, 1999)

Communication and Perception

Porocephalus crotali have a simple nervous system connected to sensory papillae found on the annuli which helps the worm sense its immediate environment. They also have an abundance of annular pits on the annuli which are used to communicate with specialized epidermal cells that regulate the hydromineral balance of the haemolymph. Nymphs contain a large number of stigmatal pores between annuli that are associated with parietal glands that may be osmoregulatory and function in immunological protection. It is unknown as to how these pentastomids communicate with one another. (Christofferson and Almeida, 1999; Hollis, 1979)

Food Habits

Porocephalus crotali is haematophagus, meaning it is strictly a blood feeder. Both adults and nymphs feed using an anterior sucking mouth. Nymphs feed on white blood cells, or eisinophils of the intermediate host. Adults feed on blood and tissue fluid within close proximity to the definitive host's lungs. Adults obtain blood by generating sucking pressure with the pharyngeal pump that works in conjunction with an oral papillae or tongue. Both nymphs and adults secrete a stage-specific surfactant that coats the cuticle. This membrane coat reduces inflammation caused by the host immune response and increases the viability of the parasite when it is feeding. (Buckle, et al., 1997; Riley and Henderson, 1999)

  • Animal Foods
  • body fluids


There are no known predators of Porocephalus crotali.

Ecosystem Roles

Porocephalus crotali is a parasite because it benefits at the expense of another organism. It is an obligate endoparasite of snakes and small mammals. It is not known to harbor any disease agents and it does not play an important role as a mortality factor within host populations. Infected mammals and snakes do not tend to show any visual signs of weakness or emaciation, or any evidence of poor condition that may be attributed to the parasite. (Layne, 1967)

Cases of this parasite have also been reported in humans and dogs. (Brookins, et al., 2009; Tappe and Buttner, 2009)

Species Used as Host
  • Pit vipers, Crotalinae
  • Small rodents, Rodentia

Economic Importance for Humans: Positive

Porocephalus crotali has no known benefits to humans.

Economic Importance for Humans: Negative

Porocephalus crotali causes human visceral pentastomiasis. The majority of cases have been reported from Africa, Malaysia, and the Middle East. It is an unusual parasitic zoonosis, in which humans may accidentally serve as intermediate hosts. Infection occurs when eggs are ingested from respiratory secretions or feces of the parasite's definitive host. Larval nymphs hatch and penetrate the host gut where the adults become encapsulated in the viscera. Symptoms are highly dependent on affected organs and include abdominal pain, chronic cough, and night sweats. Heavy infections may cause death, however, most human infections are asymptomatic. Prevention of visceral pentastomiasis from Porocephalus crotali can be done by the avoidance of snake meat consumption and washing the hands. (Tappe and Buttner, 2009)

Conservation Status

Porocephalus crotali is not an endangered species.


Michael Hoang (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.


bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).


Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.


uses smells or other chemicals to communicate


used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

delayed fertilization

a substantial delay (longer than the minimum time required for sperm to travel to the egg) takes place between copulation and fertilization, used to describe female sperm storage.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.


a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


Having one mate at a time.


having the capacity to move from one place to another.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).


an animal that mainly eats blood

scrub forest

scrub forests develop in areas that experience dry seasons.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


lives alone


mature spermatozoa are stored by females following copulation. Male sperm storage also occurs, as sperm are retained in the male epididymes (in mammals) for a period that can, in some cases, extend over several weeks or more, but here we use the term to refer only to sperm storage by females.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

year-round breeding

breeding takes place throughout the year


Brookins, M., J. Wellehan Jr., J. Roberts, K. Allison, S. Curran, A. Childress, E. Greiner. 2009. Massive visceral pentastomiasis caused by Porocephalus crotali in a dog. Veterinary Pathology, 46 (3): 460-463.

Buckle, A., J. Riley, G. Hill. 1997. The in vitro development of the pentastomid Porocephalus crotali from the infective instar to the adult stage. Parasitology, 115: 503-512.

Christofferson, M., W. Almeida. 1999. A cladistic approach to relationships in Pentastomida. The Journal of Parasitology, 85 (4): 695-704.

Esslinger, J. 1962. Development of Porocephalus crotali (Humboldt, 1808) (Pentastomida) in experimental intermediate hosts. The Journal of Parasitology, 48 (3): 452-456.

Esslinger, J. 1962. Morphology of the egg and larva of Porocephalus crotali (Pentastomida). The Journal of Parasitology, 48 (3): 457-462.

Hollis, P. 1979. The neuroanatomy of Porocephalus crotali, Humboldt, 1811 (Pentastomida). Transactions of the American Microscopical Society, 98 (1): 114-122.

Layne, J. 1967. Incidence of Porocephalus crotali (Pentastomida) in Florida mammals. Journal of Wildlife Diseases, 3 (3): 105-109.

Penn, Jr., G. 1942. The life history of Porocephalus crotali, a parasite of rhe Louisiana muskrat. The Journal of Parasitology, 28 (4): 277-283.

Riley, J. 1981. An experimental investigation of the development of Porocephalus crotali (Pentastomida: Porocephalida) in the western diamondback rattlesnack (Crotalus atrox). International Journal for Parasitology, 11 (2): 127-131.

Riley, J., R. Henderson. 1999. Pentastomids and the tetrapod lung. Parasitology, Volume 119: 89-105.

Self, J., J. Riley. 1979. On the systematics of the pentastomid genus Porocephalus (Humboldt, 1811) with descriptions of two new species. Systematic Parasitology, 1 (1): 25-42.

Tappe, D., D. Buttner. 2009. Diagnosis of human visceral pentastomiasis. Public Library of Science: Neglected Tropical Diseases, 3 (2): 1-7.