<unk>  is strictly caused by chigoe fleas (the term transmission does not apply because Tunga penetrans is itself responsible for the disease.) The preponderance of<unk>  lesions on the toes may be because chigoe flea is a poor jumper, attaining only a high of 20 cm. But the reality is more complex; for example, the jumping ability cannot explain why hands are the second-most affected body part. Lesions on the hands are better explained by playing in the sand and noting that hands are often used to remove sand from other parts of the body. The occurrence of<unk>  lesions on the toes, between them, and on the soles can be easily explained because most of the victims are poor, walk barefoot, and live in places where the sand (home to chigoe fleas) constitutes the floor. Rate of incidence therefore is greatly increased in poor communities and populations because of the lack of adequate housing. This occurs in significantly higher proportions during the peak of the dry season in local communities. Reservoirs and transmission T. penetrans has been documented to use various warm-blooded animals as reservoir hosts, including humans, pigs, dogs, cats, rats, sheep, cattle, donkeys, monkeys, birds, and elephants. These hosts directly propagate the disease by being the origin of the next generation of fleas. Once the female flea expels 100–200 eggs, the cycle of transmission begins again. Lifecycle T. penetrans eggs, on average, are 604 μm long, The larva will hatch from the egg within one to six days, assuming the environmental conditions (e.g., moisture, humidity, etc.) are favorable. After hatching, the flea will progress through two instar phases. This is unique in that most fleas go through three, instead. Over the course of that development, the flea will first decrease in size from its just-hatched size of 1,500 μm to 1,150 μm (first instar) before growing to 2,900 μm (second instar). About 6 to 8 days after hatching, the larva pupates and builds a cocoon around itself. Because it lives mostly on and below the surface of the sand, sand is used to stabilize the cocoon and help to promote its development. An environmental disturbance such as rain or a lack of sand have been shown to decrease incidence, most likely due to decreasing the environmental factors (i.e., sand) on which the flea depends for overall growth. Barring any disturbances to the cocoon, an adult flea will emerge from the puparium after 9–15 days. In the adult phase, the flea will occasionally feed on unsuspecting animals. Only once the female burrows into the skin can reproduction occur, as the male and female show no interest in each other in the wild. The male flea dies after copulation. The female flea continues in vivo ectodevelopment, described in stages by the Fortaleza classification of<unk> . Fortaleza classification In a seminal paper on the biology and pathology of Tunga penetrans, Eisele et al. (2003) provided and detailed the five stages of<unk> , thereby detailing the in vivo development of the female chigoe flea for the first time. In dividing the natural history of the disease, the Fortaleza Classification formally describes the last part of the female flea's life cycle where it burrows into its host's skin, expels eggs, and dies. Due to the nature of the discussion, overlap with other sections, particularly the one on symptoms, is unavoidable. Stage 1 is characterized by the penetration of the skin by the female chigoe flea. Running along the body, the female uses its posterior legs to push its body upward by an angle of 45–90 degrees. Penetration then starts, beginning with the proboscis going through the epidermis. By stage 2 (day 1–2), penetration is complete and the flea has burrowed most of its body into the skin. Only the anus, the copulatory organs, and four rear air holes in fleas called stigmata remain on the outside of the epidermis. The anus will excrete feces that is thought to attract male fleas for mating, described in a later section. The hypertrophic zone between tergites 2 and 3 in the abdominal region begins to expand a day or two after penetration and takes the appearance of a life belt. During this time, the flea begins to feed on the host's blood. Stage 3 is divided into two substages, the first of which being 2–3 days after penetration is complete. In 3a, maximum hypertrophy is achieved and the flea's midsection swells to the size of a pea. Due to the expanding flea, the outer layer of the skin is stretched thin, resulting in the appearance of a white halo around the black dot (rear end of the flea) at the center of the lesion. In 3b, the chitin exoskeleton of tergites 2 and 3 increase in thickness and gives the structure the look of a mini caldera. Egg release is common in substage 3b, as are fecal coils. The eggs tend to stick to the skin. At about the 3rd week after penetration, stage 4 begins, which is also divided into two substages. In 4a, the flea loses its signs of vitality and appears near death. As a result, the lesion shrinks in size, turns brown, and appears wrinkled. The death of the flea marks the beginning of substage 4b (around day 25 post-penetration) as the body begins to eliminate the parasite through skin repair mechanisms (e.g. shedding and subsequent skin repair). At this phase, the lesion is seen as brown or black. By the 5th stage of<unk> , the carcass of the T. penetrans flea has been expelled and there are circular skin residues of the infection that remain. There are only lingering symptoms at this time, described in the next section. Morphology In a study of 1000 freshly ejected T. penetrans eggs, it was found that females are generally smaller than males for all criteria. In some cases, though, females had a bigger epipharynx and maxillar palpus. Due to its burrowing activity, the chigoe flea has developed a well-developed lacinia and epipharynx that is used to penetrate the skin. Overall, the fleas’ head is relatively flattened, which again aids in burrowing through the epidermal and dermal layers. Investigators have also found that adult T. penetrans have different morphologies with respect to the shape of their head. Some have a rounded head, others have head shapes that resemble ski ramps more than anything else; still, others demonstrate head shapes that are very linear with a slight bulge at the nose. These morphologies were seen to be host-specific, as only fleas of some head-types were found in specific hosts. This, along with genetic differences among the T. penetrans fleas that infect different host animals, may suggest that there are several species of closely related species have been grouped taxonomically under one binomial nomenclature. Though the chigoe flea resembles most others in morphology, the flea has a hypertrophic region between tergites 2 and 3. As stated in Eisele et al. (2003), tergites 2 and 3, as well as the abdominal sternites, stretch considerably and are bent apart. Chitinous clasps that are built for the abdominal enlargement surround these regions and hold onto the hypertrophic zone, giving them the appearance of a three-leafed clover. (See image 7 of the life cycle diagram.) Surprisingly, the rest of the flea, including the head and the thorax, do not change in shape., With the rapid expansion of the flea, the morphology of the flea is now vastly different. It has gone from the smallest flea in the world to a bulging mass that measures 5–10mm in diameter. This results in a volume that is 2000 to 3000 times what it used to be. Reproduction Females have a depression or groove at their abdominal end whereas the males have their protrusive copulatory organs in that same region. These morphological differences reflect the way the male and female copulate. In the first step toward copulation, the female penetrates an organism in an ungravid state. It is only there that the male will find her and copulate. Copulation of adults has not been observed in the wild. With the female reproductive organs pointing outward, the male will place his reproductive organs "in direction to the upright abdominal end of the female" to copulate. Having copulated for only a few seconds to 2 minutes, the male will then begin to search for another female. After copulation is complete, the male will die, although sometimes he will take a blood meal before doing so. Eggs will be expelled whether or not they have been fertilized. The chigoe flea eggs’ average length is 604 μm and the just hatched larvae, in their first instar, have an average length of 1,500 μm. At the second and last instar (T. penetrans is unique among the fleas in that it only has two, instead of three, instars.) the larvae decrease in size to 1,150 μm after growing to at least 2,900 μm. The development from instar 1 to instar 2 lasts less than one day. On the whole, Tunga
Tungiasis
Naegleriasis
Trichiasis
Scarabiasis
Trichinosis
Necatoriasis
Dracunculiasis
Trichuriasis