Gnathostoma infection in fish caught for local consumption in Nakhon Nayok Province, Thailand. II. Deasonal variation in swamp eels

From August 2000 to August 2001, 1844 swamp eels (Monopterus albus) were purchased from several local markets in Nakhon Nayok Province, Thailand, and examined for the presence of Gnathostoma advanced third-stage larvae. The overall prevalence was 30.1% and the mean number of larvae/eel (infection intensity) was 10.0. The highest infection rate (44.1%) was found in August 2000 and the lowest (10.7%) in March 2001. The greatest mean number of larvae/eel (75.1) was found in August 2000, whereas the fewest (2.3) was in July 2001. It is suggested that the prevalence and intensity of infection decreased within two months after the end of the rainy season and started to rise again about two months after the next rainy season began. A total of 5,532 Gnathostoma larvae were recovered from 555 infected eels, with a maximum number of 698 larvae/eel. The highest rates of Gnathostoma infection according to eel body length and weight were 87.5% in the group 91-100 cm, and 100% in groups of 901-1100 g, respectively. There were significant correlations between eel body lengths and infection rates, body lengths and infection intensities; eel body weights were also significantly correlated with infection rates and infection intensities. It was noted that the longer/ heavier the eels were, the higher would be the infection rates and the greater the infection intensities. Tissue distributions of Gnathostoma larvae in the livers and muscles of swamp eels were as follows: 43.0% of the total number of larvae were found in the muscles and 57.0% were in the liver; 29.7, 51.7, and 18.6% were in the anterior, middle, and posterior parts, respectively; 35.1% were in the dorsal part, while 64.9% were in the ventral part; 9.0, 18.7, 7.4, 20.6, 33.1, and 11.2% were in the anterodorsal, mediodorsal, posterodorsal, anteroventral, medioventral and posteroventral parts, respectively. Of the 5,532 Gnathostoma larvae examined, 1101 (19.9%) were found to possess morphological variants or abnormal cephalic hooklets. The most common unusual feature was that there were few to numerous extra rudimentary hooklets below row 4 and between the 4 rows of hooklets (7.6%), the presence of a fifth row of hooklets (3.5%), abnormal hooklets in any of the 4 rows of hooklets (5.2%), spiral arrangement of the 4 rows of hooklets (1.8%), and larvae having only 3 rows of hooklets (0.3%).