**3. Non-encapsulated** *Trichinella* **species**

## **3.1** *Trichinella pseudospiralis*

The first species to be discovered in the non-encapsulated clade was *T. pseudo spiralis* andsubsequently the fourth one in the genus *Trichinella*. **Figure 2** represents the timeline of discovery of the three non-encapsulated species. *T. pseudospiralis* was isolated from *Procyon lotor*, a raccoon caught in Krasnodar region. Significant differences were observed from the other species, which has been recorded earlier. Firstly, it can infect both mammals as well as birds, secondly, it was devoid of any collagen capsule and lastly, the adult worms and larvae had a smaller size from other species [8, 13]. It exhibits a cosmopolitan distribution in America, Asia, Australia and 20 different European countries, highlighting the importance of birds in carrying and spreading the parasite to new areas [14, 18]. The larvae of *T. pseudospiralis* is shown in **Figure 3.** In this case, the striated muscle fibres that surround the nurse cells are dense and branching longitudinally, but there is no netting pattern as seen in *T. spiralis* [20].

Since its discovery in 1972, this species has been detected in 249 animals, 237 of which were single infections and 11 of which were mixed infections with other *Trichinella* species. It has been discovered in 18 mammalian and eight avian species [14]. The parasite

#### **Figure 2.**

*Timeline of discovery of* Trichinella *species.*

**Figure 3.** T. pseudospiralis *larvae in the muscle of domestic pig [19].* *Non-Encapsulated Trichinella Species:* T. pseudo spiralis*,* T. papuae *and* T. zimbawensis *DOI: http://dx.doi.org/10.5772/intechopen.105680*

has also been detected in raccoon dogs in Germany [21–23], American mink in Poland [24], cougars from Colorado, United States [25], red foxes in Poland [26], raccoon dogs in Central Europe [27], wolverines from the Canadian north [28], Eurasian blackbird from Armenia [29], wild boars in Estonia [30], red kite from Italy [31], wolf from Central Italy [32], bobcats from Oklahoma [33]. *T. pseudospiralis* infection in red-eared sliders was found to be influenced by environmental temperature, as the infection was successful in turtles maintained at 38°C compared to those reared at 32°C and 28°C [32]. In order to identify the larvae, artificial digestion tests are preferred over trichinoscopy [1, 18].

Various studies have been carried out to elucidate the immune response of *T. pseudospiralis*. Matrix mettaloproteinases 9 and 2 have been identified as the markers for inflammatory response of both *T. spiralis* and *T. pseudospiralis* infection [33]. Infection with *T. pseudospiralis* also resulted in a reduction of follicular T helper cell differentiation [34]. A serpin gene, on the other hand, was discovered to play a key role in infection by activating the M2-polarised signaling pathway [35]. The parasite's excretory-secretory proteins can be used for early detection and the development of a vaccine candidate [36].

#### **3.2** *Trichinella papuae*

An examination of domestic and wild swine from Papuae New Guinea, along with eighty-three wild animals, was conducted after the detection of non-encapsulated *Trichinella* larvae in five domestic female swine in the settlement of Balamuk in 1988. Six wild pigs were then tested positive in the Bula Plain in the years 1988 to 1998. The larvae detected in the diaphragm muscles of pigs can be seen in **Figure 4**. However, none of the 83 wild animals tested, including domestic pigs, had any larvae. The larvae from one of the wild pigs were then characterised and classified by Edoardo Pozio [12]. In Western Province, near Indonesia, 8.8% of the wild pig population was shown to be infected. Intake of infected wild pig meat was the source of infection [37].

It was also found in PNG's saltwater crocodiles and the source of infection was improper feeding of wild pigs to them [38, 39]. Varans, caimans, pythons and turtles have also been infected with *T. papuae* and *T. zimbawensis* in an experimental setting where varans were found to have the highest reproductive capacity rating of all the species. Despite receiving a high infection dose, just a small number of larvae were found in pythons and turtles. Furthermore, no clinical indications of the infection have been reported indicating that they do not play a substantial role in epidemiology. Only these two *Trichinella* species can complete their life cycle in both cold- and warm-blooded animals. As a result, they could trigger distinct physiological processes depending on the host they are infecting [40]. Further infection was investigated in the equatorial freshwater fishes *Serrasalmus nattereri* and *Serrasalmus rhombeus*, but no larvae or adult worms were found in any organ, implying that, despite being a food source for reptiles like crocodiles, they have no role to play in the epidemiology due to the entozoic habitat of these fishes, which is not suitable for these two *Trichinella* species [41]. **Table 2** lists the natural and experimental hosts of *T. papuae*.

Despite the absence of a collagen capsule, the larvae can thrive in a tropical climate, making them more likely to be e transmitted to a new host [45]. Infection with *T. papuae* was reported to reduce the severity of dextran sulphate sodiuminduced colitis in mice. The absence of 57% of *T. papuae* lipids in humans indicates variations in lipid metabolism, which could aid in the development of innovative treatments [46].

#### **Figure 4.**

*In the village of Balamuk, larvae of Trichinella papuae were discovered in the diaphragm of an infected female pig (PNG), 1988 [12].*


#### **Table 2.**

*List of natural and experimental hosts of* T. papuae.

#### **3.3** *Trichinella zimbawensis*

*Trichinella* larvae were identified in crocodile muscles in Zimbabwe in 1995. This was the first time *Trichinella* was found to naturally infect a reptile [47]. In an epidemiological survey, a farm near Victoria Falls was revealed to be the source of infection. The larvae isolated from crocodiles were able to infect domestic pigs and laboratory rats [48]. In the year 2002, Edoardo Pozio was the first to characterise and describe the larvae. This species has been found to infect both mammals and reptiles [11, 40]. Morphology of adults and larvae was determined to be comparable to that of *T. papuae. T. Zimbabwensis* males and females can procreate in both ways with *T. papuae* adults. As a result, the F1 offspring produces less viable F2

*Non-Encapsulated Trichinella Species:* T. pseudo spiralis*,* T. papuae *and* T. zimbawensis *DOI: http://dx.doi.org/10.5772/intechopen.105680*

larvae [11]. **Figure 5** shows the *T.zimbawensis* larvae in the muscles of mice after four months of infection.

The parasite was then discovered in monitor lizards and Nile crocodiles in Zimbabwe and Mozambique, marking the first time to be found in wild reptiles [49]. Later, 38.5% prevalence rate was also been detected in wild Nile crocodiles in South Africa [15]. Natural infection has also been seen in mammals. In another experimental set-up baboons and vervet monkeys were also infected with the larvae. The most prevalent symptoms were fever, diarrhoea and muscular soreness. The infection was treated with ivermectin, but two baboons and two monkeys died as a result of the trial [50]. **Table 3** lists the experimental and natural hosts of *T. zimbawensis.*

Host age was found to have no effect on the distribution of parasites in various segments of intestine in case of golden hamsters and Balb C mice [51]. Also, increased progesterone levels in pregnant mice had a parasiticidal effect on the newly born larvae [53]. Co-infection of *Plasmodium berghei* with *T. zimbawensis* resulted in increased parasitemia in mice, which could further lead to severe malaria infection [54]. Various experiments have been carried out to study the immune response of *T. zimbawensis* infection. Non-encapsulated species have shown reduced inflammation and nitrosylation levels [55]. In an ELISA devised to detect the humoral response, *T. zimbawensis* was shown not to elicit a substantial immunological response in Nile crocodiles in terms of antibody titres and antibody persistence [56]. Another study revealed the same results, with the infection intensity not correlating with the amplitude of the humoral immune response [57]. The Th1, Th2 and T regulatory responses that are induced

**Figure 5.** *T. Zimbawensis larvae in muscles of mice after four months of infection [11].*


#### **Table 3.**

*List of natural and experimental hosts of* T. zimbawensis.

during the different stages of infection were also shown to have significant variations [58]. This species has also been observed to affect metabolic parameters by inducing compensatory feeding in the host. During chronic infection, it was found to influence the host's Th1/Th17 immunological response [59]*.* The larvae were observed to invade the predilection muscles nearest to their release point in the small intestine first. The parasite load was found to be the highest in the fore and hind limb muscles. The use of biopsy samples from the dorso-lateral areas of the tail has also been recommended for surveillance purposes [60].

#### **Figure 6.** *Hypothetical transmission cycle of* T. zimbawensis in *Kruger National Park [61].*

*Non-Encapsulated Trichinella Species:* T. pseudo spiralis*,* T. papuae *and* T. zimbawensis *DOI: http://dx.doi.org/10.5772/intechopen.105680*

A hypothetical transmission cycle for *T. zimbawensis* has been presented by Louis J. La Grange and Samson Mukaratirwa as shown in **Figure 6**. Recently, leopard and hyaena have been added as the apex predators along with few mesopredators [61].

The green arrows indicate the original hypothesised mode of transmission, while the blue arrows represent the modified way of transmission [61].
