Is Gnathostoma turgidum an annual parasite of opossums? Drastic seasonal changes of infection in Didelphis virginiana in Mexico

Molecular Identification of the Etiological Agent of Human Gnathostomiasis in an Endemic Area of Mexico

Human gnathostomiasis, which is endemic in Mexico, is a worldwide health concern. It is mainly caused by the consumption of raw or insufficiently cooked fish containing the advanced third-stage larvae (AL3A) of Gnathostoma species. The diagnosis of gnathostomiasis is based on epidemiological surveys and immunological diagnostic tests. When a larva is recovered, the species can be identified by molecular techniques. Polymerase chain reaction (PCR) amplification of the second internal transcription spacer (ITS-2) is useful to identify nematode species, including Gnathostoma species. This study aims to develop a duplex-PCR amplification method of the ITS-2 region to differentiate between the Gnathostoma binucleatum and G. turgidum parasites that coexist in the same endemic area, as well as to identify the Gnathostoma larvae recovered from the biopsies of two gnathostomiasis patients from Sinaloa, Mexico. The duplex PCR established based on the ITS-2 sequence showed that the length of the amplicons was 321 bp for G. binucleatum and 226 bp for G. turgidum. The amplicons from the AL3A of both patients were 321 bp. Furthermore, the length and composition of these amplicons were identical to those deposited in GenBank as G. binucleatum (accession No. JF919679), corroborating our previous morphological finding that G. binucleatum is the etiological agent for human gnathostomiasis in the endemic area of Sinaloa, Mexico.

Histopathological changes in the liver and stomach of Didelphis virginiana (Didelphimorphia: Didelphidae) during natural infection with Gnathostoma turgidum (Nematoda: Gnathostomidae)

Gnathostoma turgidum is a nematode parasite that exploits the stomach of Virginian opossums, Didelphis virginiana, in Latin America. The opossum is the definitive host of G. turgidum in the wild. Intrahepatic growth and maturation of the parasite, subsequent migration to the stomach and spontaneous expulsion are common. However, the histopathological lesions caused by G. turgidum are poorly described. A better understanding of the life cycle of this parasite and the pathological changes in natural host-parasite interactions could help to clarify the progression of human infections caused by Gnathostoma binucleatum. The aim of this work was to study morphological changes in the liver and stomach of D. virginiana during natural infection and adult worm expulsion. Three opossums naturally infected with G. turgidum were captured from an endemic area of gnathostomosis. Three uninfected opossums captured from a non-endemic area were used as controls. The opossums were sacrificed at different stages of infection (March, May and December), and a histopathological study of their livers and stomachs was conducted. Injuries in livers were observed by histopathology - areas of necrosis and collagen septa were identified. Parasites caused nodules with necrosis on the periphery of lesions, and collagen fibres were also observed in stomachs. Collagen septa may be caused by antigenic remains of the parasite. Further immunological studies are necessary to verify that stimulation is caused by these factors.

Helminths of three species of opossums (Mammalia, Didelphidae) from Mexico

From August 2011 to November 2013, 68 opossums (8 Didelphis sp., 40 Didelphisvirginiana, 15 Didelphismarsupialis, and 5 Philanderopossum) were collected in 18 localities from 12 Mexican states. A total of 12,188 helminths representing 21 taxa were identified (6 trematodes, 2 cestodes, 3 acanthocephalans and 10 nematodes). Sixty-six new locality records, 9 new host records, and one species, the trematode Brachylaimadidelphus, is added to the composition of the helminth fauna of the opossums in Mexico. These data, in conjunction with previous records, bring the number of taxa parasitizing the Mexican terrestrial marsupials to 41. Among these species, we recognized a group of helminths typical of didelphids in other parts of the Americas. This group is constituted by the trematode Rhopaliascoronatus, the acanthocephalan Oligacanthorhynchusmicrocephalus and the nematodes Cruziatentaculata, Gnathostomaturgidum, and Turgidaturgida. In general, the helminth fauna of each didelphid species showed a stable taxonomic composition with respect to previously sampled sites. This situation suggests that the rate of accumulation of helminth species in the inventory of these 3 species of terrestrial marsupials in the Neotropical portion of Mexico is decreasing; however, new samplings in the Nearctic portion of this country will probably increase the richness of the helminthological inventory of this group of mammals.

Spontaneous cure after natural infection with Gnathostoma turgidum (Nematoda) in Virginia opossums (Didelphis virginiana)

Seasonality of the nematode Gnathostoma turgidum in Virginia opossums (Didelphis virginiana) in the wild has been reported; however, the mechanisms involved in deworming are unknown. We monitored the parasitologic and biologic changes in four Virginia opossums naturally infected with G. turgidum by coproparasitologic examination and abdominal ultrasonography. Eggs became detectable in the feces of opossums in May, peaked in July and August, and suddenly decreased in October. Adults of G. turgidum were expelled in the feces mainly in September. Ultrasonography of the liver showed slight damage during May. Lesions in the stomach appeared in April and persisted until September. The abnormalities of the liver and stomach were resolved in November. These data suggest that G. turgidum is likely expelled as a result of host immunologic mechanisms, although termination of a natural life span cannot be definitively excluded.

Description of advanced third-stage larvae of Gnathostoma lamothei Bertoni-Ruiz et al. 2005 (Nematoda: Gnathostomatidae) from experimental hosts and contributions to its life cycle

The advanced third-stage larvae (AdvL(3)) of Gnathostoma lamothei was obtained from experimental hosts. Frogs Lithobates heckscheri and snakes Nerodia fasciata pictiventris were compatible hosts allowing optimal larval development. AdvL(3) are 4,487.94 μm long, have two lateral cervical papillae between rows 10 and 16 and an excretory pore at row 23. The average counts of the cephalic bulb hooklets from the four rows are 39.3, 43.3, 44.2, and 47.3. Larvae show an esophagus that represents 40 % of the body width. These findings indicate that amphibians and reptiles could be involved as G. lamothei natural hosts; nevertheless, their role as etiological agents of human gnathostomiasis is uncertain. This paper reports for the first time the taxonomic description of G. lamothei AdvL(3) obtained from experimental hosts and contributes to the understanding of its life cycle.

Intrahepatic growth and maturation of Gnathostoma turgidum in the natural definitive opossum host, Didelphis virginiana

Gnathostoma turgidum is a gastric nematode parasite of opossums found in the Americas. We recently found that G. turgidum juveniles appear in the liver of the opossums where they become mature adults and almost synchronously move to the stomach during certain months of the year, suggesting the importance of the liver for the growth and maturation of this species in the final hosts. In this study we attempted to detect G. turgidum larvae in the liver of opossums, Didelphis virginiana that are the natural final hosts. The results show that tiny (<3mm in length) third stage larvae (L3) appeared in the liver of opossums around November and December. Also in the liver, we found large L3 of up to about 10mm in length together with juveniles and mature adults from February to March. In spite of their length, large L3 have 4 rows of hooklets, and their gonads remained undeveloped. Morphological features of the small and large L3 of G. turgidum are described including scanning electron microscope images. The seasonal switching of the several growth stages of G. turgidum from small L3 to adult worms in the liver and eventual migration to the stomach in opossums suggests the unique feature of G. turgidum utilizing the liver as the maturation site.