الفهرس 
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Abstract
Fish is an important food stuff allover the world. However, salt and freshwater fish can serve as sources of parasitic infections to man. Fish-borne parasitic diseases tend to go unnoticed, and hence under recorded, not only because the symptoms are often quite general, but also because they may be mistaken with a range of other infectious diseases.
Fish-borne parasitic trematodes include many species of the families Heterophyidae and Opisthorchiidae. A well established fish-transmitted cestode is Diphyllobothrium latum. Capillaria philippinensis and anisakids are the well known fish-transmitted nematodes reported from different parts of the world. Microsporidia and Cryptosporidia species are two groups of intestinal protozoa that were found in fish and other aquatic animals. Human infection by these parasites is initiated by ingestion of raw or imperfectly cooked fish containing their infective stages.
Egypt, especially coastal cities as Alexandria, is rich with different types of salt and freshwater fish which may be a source of parasitic infections to man. The presence of parasites in the consumed fish can reduce its commercial value, as well as, be a threat to human health.
Hence, the aim of the present study was to identify parasites of medical importance in fish and study their infectivity to laboratory mice. To fulfill this aim, commonly consumed fish in Alexandria were examined for the presence of parasites in their intestines and flesh. Fish chosen were Tilapia zillii (Bolti), Mugil cephalus (Bori), Clarias gariepinus (Karmout), Brycinus nurse (Sardinet) and Atherina boyeri (Bessaria). Fish samples were obtained from the well known markets in Alexandria. A total of 300 fish were examined in the present study. Thirty fish from each type were obtained during the cold months (November 2006-April 2007) and another 30 during the warm and hot months (May 2007-October 2007).
Intestinal contents were examined directly under a binocular microscope at (X40) to detect helminthic parasites. Smears stained with MZN, safranin-methylene blue and modified trichrome stains were used to detect protozoan parasites. Positive samples were preserved in 2.5% potassium dichromate at 4oC to maintain the viability of protozoan parasites for infection of experimental mice.
Flesh snips were examined between two slides under a binocular microscope at (X40) to detect encysted larvae of helminthes. Isolation of the metacercariae from fish flesh was done by invitro pepsin- hydrochloric acid digestion. For identification of protozoa, parts of fish flesh were cut into small pieces and shaken with phosphate buffer saline with the aid of a vortex mixer and then centrifuged. Smears from the sediment were stained with modified trichrome and MZN stains.
Our results showed that: Tilapia zillii was the most infected fish, where all parasites detected were present namely, Microsporidia (53 fish), metacercariae of Pygidiopsis genata (45 fish), Cryptosporidia (23 fish) and Cyclospora (7 fish).
This was followed by Clarias gariepinus, where three parasites were observed Microsporidia (55 fish), Cryptosporidia (20 fish) and Cyclospora (6 fish). Atherina boyeri and Brycinus nurse were similar. Both were infected mainly with Microsporidia (25 and 19 fish respectively). Only one sample of each type of fish contained Cryptosporidia.
The least infected fish was Mugil cephalus, where only one parasite was found, namely, Microsporidia (18 fish).
Our results also showed that the number of infected fish was higher in the warm and hot months than the colder ones. Statistical analysis revealed that the percentage of positive samples in the hot weather was as follows: Cryptosporidia (71%), Cyclospora cayetanensis (69%), Microsporidia (59%) and metacercariae of Pygidiopsis genata (58%).
Infection of experimental mice was done to study the infectivity of the detected parasites and their pathological outcome. This was achieved by examination of the stools of mice before their sacrifice and by histopathological examination of their intestinal sections. Heterophyids obtained from the intestines of mice infected with metacercariae were examined after clearance in lactophenol and staining with carmine stain. Morphological features of juvenile and adult worms led to the identification of the parasite as Pygidiopsis genata.
The detected parasites were found to be infective to experimental mice. This was proved by observation of the previously mentioned protozoa and heterophyid eggs in their stool samples. Histopathological changes seen in their intestinal sections also gave a clue for the infectivity of the detected parasites.
Thus, it could be concluded that:
1) Fish harbour infective parasites in their flesh and intestine that can have great impact on the human health. This problem is especially expanding in areas where fish is eaten raw, under-salted or imperfectly cooked.
2) Tilapia zillii and Clarias gariepinus had higher percentages of parasites compared to other types of fish in the present study. This is most probably because these fish inhabit polluted water sources. On the other hand, Mugil cephalus in our markets is mostly obtained from aquacultures, thus, it showed the least parasitic infection. Therefore, proper aquacultures would reduce fish-transmitted parasitic diseases.
3) Parasites detected in fish were infective to mice and hence can be transmitted to man leading to various infectious diseases.
4) Infective stages of the detected parasites may also be excreted by fish in their water environment and thus increasing its pollution.
5) Fish is usually a hidden element in food-borne parasitic diseases due to the multiple routes of transmission which pass unnoticed and under-recorded. Thus, fish as a direct cause of parasitic infections is under-estimated most of the time.
6) Sewage disposal in Alexandria is a public health problem which affects the health of the population, not only by contamination of drinking water, but also by ruining fish and other seafood highly consumed by people. Thus, adequate treatment and proper disposal of sewage are top priorities to decrease the incidence of parasitic infections among our citizens.