Trichobilharzia regenti is considered a member of the blood flukes of the Class Trematoda, but it belongs to the Schistosomatidae family. It is a flatworm parasite that has great importance to both veterinary and medical fields ( Leontovyč et al., 2016). The T. regenti parasite has a wide geographical distribution, making it also highly prevalent, spanning areas such as Russia, Iran and New Zealand. The parasite penetrates the skin of a definitive vertebrate host, but unlike many members of the Schistosoma family, the T. regenti invades and migrates through the skin of its host, and establishes itself within the nasal mucosa (Leontovyč et al., 2016). T. regenti is known to cause Cercarial dermatitis in man, as with other certain species of the Schistosoma family, although generally, schistosomes are known within the scientific community to be more prevalent within mammals and birds, but due to some chemotropic reactions, they are not very host specific (CDC, n.d.). One unique aspect of T. Regenti species, unlike other species of schistosomes, is that its eggs are produced within the nasal cavity from where they are released into the environment, as opposed to other species that thrive in the mesenteric and portal vessels of their bird hosts, being excreted into the environment through fecal matter (Jothikumar et al., 2015). This research will offer more information on the T. regenti parasite, including its lifecycle, its pathophysiology once in the body of its host, the disease epidemiology and any treatments if possible.
Lifecycle and Contraction of T. regenti
The T. regenti is considered to be dioecious, just like most blood flukes of the Schistosoma genus, meaning that it has a two host lifecycle. The parasite has an invasive furcocercarial stage, which is responsible for the penetration of the skin of the vertebrate host (Leontovyč et al., 2016). After the mating of adult flukes, which often occurs in the nasal mucosa of its host, mainly birds, the females hatches the eggs with miracidia, which are motile due to the presence of cilia; these later on become sporocysts. Once the eggs have been hatched in the nasal mucosa, they usually leak from the bird while the host is feeding such as when drinking water. At this stage of the parasite, the cilia enable the miracidia to swim through water seeking out its first host, which are aquatic snails such as the lymnaeid snail (Leontovyč et al., 2016).
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The Radix genus of snails is the most common intermediate host for the T. regenti species (Leontovyč et al., 2016). The intermediate aquatic host provides the necessary environment for the miracidia to develop into primary and secondary sporocysts, which then develop into cercariae. The cercariae stage is then shed from the aqcuatic intermediate snail host and it goes back into the water, where they can only survive for a very short time without finding its definitive host. During this time, the cercariae survive on glycogen reserves it had obtained from the intermediate host. When the cercariae make contact with the skin of a definitive host, they release secretions that contain peptidases (proteolytic enzymes) from their postacetabular and circumacetabular penetration glands that give the cercariae tissue penetration capabilities ( Leontovyč et al., 2016 ).
During this penetration period, the cercariae are able to transform to schistosomula within a period of roughly 12 hours. The transformation has several effects on the schistosomes, which includes the loss of their tail, the formation of a double membrane (heptalaminar) that covers the tegument, as well as reductions of surface glycocalyx ( Leontovyč et al., 2016 ). The schistosomes additionally switch to anaerobic metabolism from aerobic, but this is dependent on the glucose levels in the host and the activation of metabolic processes in the gut of the parasite (Ibid.). Unlike other human schistosomes, the schistosomula of T. regenti do not drift directly to the host’s blood vessels, but first enter peripheral nerves, then migrate to the brain and spinal cord of the host. During this migration, the parasites feed on neural tissue.
While in the meninges of the host, the schistosomes reach the pre-adult stage, and they start feeding on blood; it is also at this time that they migrate to the nasal cavity majorly through intravascular routes. The schistosomes then grow into adults while in the nasal cavity, where mating occurs between adults and the life cycle of the parasite begins all over. During the migration of the parasite’s schistosomula, there are major damages to the nerve tissue, which may lead to disorientation, behavioral changes, paralysis and even death of the host (Leontovyč et al., 2016).
The Pathophysiology of the Disease Once In the Host
When the cercariae of T. regenti get into contact with the skin of its definite host, as with most other mammalian and avian schistosomes, it dies, thus leading to an inflammatory immune response (Jothikumar et al., 2015). The cercariae die after penetrating the human skin because human beings are not appropriate hosts for the species to develop into the adult stage (Jothikumar et al., 2015: CDC, n.d.). After dying however, these cercariae cause dermatitis. Some of the initial symptoms of the disease (cercarial dermatitis) include cutaneous itching and erythema (Turjanicová et al., 2015). It is from this physiopathology that the term swimmers itch emanates (CDC, n.d.). Other pathophysiological that follow after are macular as well as papular eruptions (Latwon et al., 2014), which are similar to small discolored pimples on the infected individual’s skin.
In other hosts such as the avian species, the pathophysiology is different due to the fact that the cercariae do not die but migrate severally and grow within this definitive host to become adults. Majorly, the changes that occur here are related to the destruction of nerves and other parts of the central nervous system, including the spinal cord. The damage to nerves and the CNS as stated earlier has the effect of causing symptoms such as change in behavior, disorientation and paralysis. Such hosts can also suffer from anemic conditions as their blood levels are bound to be affected by the parasites, which at the pre-adult stage and in their migration through the host’s body tend to feed on blood. In cases where their numbers are large within the host, then this loss of blood can have detrimental effects on the health of the host.
The Epidemiology of the Disease
As with all other avian schistosomes, T. regenti have a worldwide distribution ( Lawton et al., 2014: Kolářová et al., 2013 ). Nevertheless, there are higher reported incidences of cercarial dermatitis (swimmers itch) in America and Europe, with swimmers itch now being considered an emerging and re-emerging disease in these areas (Lawton et al., 2014). Studies have shown especially negative implications on local economies of various areas that are dependent on recreational water use for tourism in Europe. The cases of T. regenti and other such schistosomes has however been noted all across the world, with the only exception being Antarctica (CDC, n.d.).
In terms of hosts, the T. regenti species is majorly an avian and mammalian parasite. It has been noted mostly among birds where the adults live within the nasal cavity ( Leontovyč et al., 2016 ). It has however been noted to be a major cause of swimmer’s itch in humans, especially within areas where there are marked increases in the numbers of the intermediate host (aquatic snails). Humans are nonetheless not suitable definitive hosts for the species, and they only attach to the human skin due to the attraction caused by general fatty acids contained both within mammalian and avian skin ( Jothikumar et al., 2015 ). Since they cannot survive in the human body, they die after entering the skin, and cause cercarial dermatitis. Therefore, the T. regenti species has a resilient epidemiology that allows the species to be found both at diverse locations globally, and to infect a wide range of organisms.
Signs and Symptoms of the Host
When the host of T. regenti is human, the symptoms illustrated as a cutaneous inflammation that has an itching effect. The exposed area also reddens (CDC, n.d.). After 12 hours of exposure, the pruitic papules can become vesicular, and any scratching by the infected individual may lead to secondary bacterial infections (CDC, n.d.). In other hosts such as the avian species, there is little pathogenic effect to give symptoms, unless in cases where the host is extremely immuno-compromised. In such cases, signs may include neurologic disorders such as leg paralysis ( Jothikumar et al., 2015 ). However, in many cases, the parasite is destroyed by host immune responses. If they do get to the adult stage at within the avian nasal mucosa, then there can be extensive focal hemorrhages.
Medicines
Due to the sufficiency of immunological responses in dealing with these parasites in humans, treatment is often not necessary. However, antihistamines such as hydroxyzine may hasten the healing process and the reduction of symptoms.
In conclusion, T. regenti is of major interest due to its wide epidemiology that allows it to infect both avian and mammalian species. Thus, the interest in the parasite is warranted as it enables an understanding of the life cycle of the parasite, and how it can be controlled through controlling the intermediate hosts. Due to the low implications of the parasite in terms of mortality, it is not a major cause of concern in terms of disease causing parasites, but still, it is necessary to study this parasite.
References
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https://www.cdc.gov/dpdx/cercarialdermatitis/index.html
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