First Record of Contracaecum Spp. (nematoda: Anisakidae) in Fish-eating Birds from Zimbabwe

a INTRODUCTION Fish-eating birds are abundant in freshwater habitats in Zimbabwe as they are in most parts of Africa and the world over


INTRODUCTION
Fish-eating birds are abundant in freshwater habitats in Zimbabwe as they are in most parts of Africa and the world over 13 .With the increase in man-made impoundments in the country, many of the bird species have adapted to breeding in these habitats 13,14 , which also provide a more abundant source of fish than flowing waters 6,9 .Although the ecology of the birds has been studied to some extent in Zimbabwe's major dams 9,11,12,14 , little is documented on their parasitic diseases.These include 1 paper by Beverly-Burton 5 on Diplostomum mashonense, a trematode from the grey heron and a Government Veterinary Laboratory record of the nematode Spiroptera sp. in 2 cormorants found dead on Lake Chivero in 1995 (T Couto, Department of National Parks and Wildlife Management, Zimbabwe,  pers.comm., 2002).
Elsewhere in Africa, the occurrence of parasites in aquatic birds has been determined in Ghana 23 and South Africa 20,21,26 , and they range from clinostomid and diplostomid trematodes to cestodes and nematodes.The anisakid nematode, Contracaecum sp., is one of the most prevalent parasites in southern African piscivorous birds, infecting 5 avian species 20,21 .Some of the Contracaecum species are widely distributed in other parts of Africa, Europe, Asia, Australasia, North and South America 3,27 .It is partly for this reason that the occurrence of Contracaecum spp. was investigated in 4 piscivorous bird species at Lake Chivero, Zimbabwe.This study aimed to specifically identify the Contracaecum species, the adults of which are only found in the bird definitive host and the larvae of which had been found in some fish from the lake 4 and to compare them with those from South African birds to establish their regional distribution.
Lake Chivero lies 37 km southwest of Harare, the capital of Zimbabwe.It is the city's potable water reservoir and is very eutrophic because of sewage and industrial effluent that is discharged into its feeder streams from Harare and its neighbouring town, Chitungwiza 17 .The lake supports a large fish community and about 12 bird species (T Couto, Department of National Parks and Wildlife Management, Zimbabwe, pers.comm., 2002), of which the reed cormorant Phalacrocorax africanus (Gmelin, 1789), the white-breasted cormorant P. carbo (L.), the darter Anhinga melanogaster Lacapéde & Dauchin, 1802, and the grey heron Ardea cinerea (L.), were selected because of their abundance on the lake.

MATERIALS AND METHODS
Four reed cormorants, 4 white-breasted cormorants, four darters and four grey herons were shot with a 0.22 rifle and a 12-bore shotgun firing buckshot at Lake Chivero.Their beaks were sealed with rubber bands to prevent the escape of parasites lodged in the buccal cavity.In the laboratory, the birds were weighed, dissected, and the alimentary canal was opened.Undigested and partly digested fish encountered in the oesophagus and stomach were removed, identified to species level where possible, and weighed.Nematodes either found in the oesophagus or attached to the stomach mucosa were removed, fixed in glacial acetic acid and stored in 70 % ethyl alcohol.Sub-samples of nematodes from each infected bird were set aside for electron microscopy.The remainder was cleared and mounted in lactophenol and the cover slides were sealed with clear nail varnish.
Specimens for scanning electron microscopy (SEM) were cleaned in an ultrasonic cleaner (Branson 3200) to remove debris from the cuticle.They were dehydrated in an ascending series of ethyl alcohol (70 %, 80 %, 90 %, industrial methanol and 100 %) after which they were critical-point dried.They were mounted on stubs and gold-coated for 40 minutes and then examined and photographed under a JEOL 6100 scanning electron microscope between 5-7 kV.Keys by Anderson 3 , Hartwich 10 and Yamaguti 27 were used for identification.

RESULTS
All the cormorants and darters were infected with at least 4 nematodes in their alimentary canal (prevalence 100 %) while only 1 grey heron (25 %) was infected (Table 1).The parasite intensity was greatest in the darter compared the reed cormorant, the white-breasted cormorant and the grey heron.The nematodes were 11-29 mm long (mean 17 mm) and the majority (73 %) were male.The Zambezi bream, Pharyngochromis acuticeps (Staindachner), and the Nile bream, Oreochromis niloticus (L.), constituted the most abundant species in the diet of the birds (Table 2).The darters and herons preferred O. niloticus while the cormorants were not very specific.
Four species of adult Contracaecum Raillet and Henry, 1912 were identified: C. rudolphii Hartwich, 1964, C. carlislei Ortlepp, 1938, C. microcephalum (Rudolphi, 1809) and C. tricuspis (Gedoelst, 1916).Generic diagnosis using light microscopy was based on the presence of the ventricular appendix and intestinal caecum as well as length measurements, while SEM revealed the detailed structure of the head and tail, the size and number of caudal papillae and spicules in males, which were used for species identification.
Contracaecum carlislei (Fig. 2) Three rectangular lips with bilobed anterior surfaces.Large curved interlabia.Dorsal lip with 2 single papillae and subventral lips with a double and a single papilla.Males with 26-34 pairs of precloacal papillae and 5 pairs of postcloacal papillae.Hosts: P. africanus and P. carbo.
The worms from the grey heron (A.cinerea) were not identified to species level because they were still in their 4th larval stage.

DISCUSSION
The results obtained from this study indicate that darters and cormorants are the most common hosts of Contracaecum species in Lake Chivero.Because of permit restrictions on the size of the sample that could be collected (maximum of 5 per species), samples were too small to make concrete conclusions on the prevalence of this parasite.However, the distribution of the parasites is similar to other related studies where host sample size was low (Table 3).
The Contracaecum spp.found in Zimbabwe were all previously described in South Africa 19,21 , indicating a possible regional distribution.Contracaecum carlislei, for example, was first described by Ortlepp in 1938 from the stomach and oesophagus of P. africanus and was thought to be indigenous to South Africa only 19 .More data should be gathered to ascertain the distribution of C. carlislei on the continent.
Contracaecum rudolphii is one of the most widely studied species with a cosmopolitan distribution 27 .In Africa, it infects the Pelecaniformes (cormorants, darters and pelicans) 1 6 , 1 9 , 2 1 , 2 6 and has also been reported from European cormorants 22,25 .It was therefore not surprising to find it in cormorants from Zimbabwe.Contracaecum microcephalum also has a worldwide distribution 3,27 .In southern Africa, it infects the herons Ardea cinerea and    A. melanocephala as well as cormorants and darters 21 but it was not found in the grey herons from Lake Chivero.Contracaecum tricuspis was originally described from a heron in the Democratic Republic of the Congo 20 , but in this study and other related studies in southern Africa, it was found in the darter 20,21 .Stomach content analysis shows that the cichlids, O. niloticus and P. acuticeps are dominant in the diet of the birds from Lake Chivero, both numerically and by weight (Table 2).This suggests that these fish are most likely the 2nd intermediate hosts of Contracaecum.In fact, a number of cichlid fish species and the barbel, Clarias gariepinus, have been shown to successfully host the parasite 1,2,7,8,15,16,18,20,24 .
It would be interesting to link specific fish species from Lake Chivero to adult nematodes in the birds.So far, larval Contracaecum have been found in Clarias gariepinus from Lake Chivero 4 , but this fish forms a very insignificant part of the birds' diets 9 .Previous studies on the feeding ecology of cormorants and darters in Zimbabwe showed that cichlids constituted over 60 % of their diet, of which P. acuticeps and Pseudocrenilabrus philander (Weber, 1897) were the most numerically dominant 6,9 .Moreover, Hustler 11 showed that cormorants and darters preferentially fed on prey weighing less than 100 g and this corresponds to the prey obtained in this study (Table 2).This would exclude C. gariepinus, which reaches a weight of more than 500 g at maturity, from the birds' diet.

Table 1 : The infection statistics of Contracaecum species found in the alimentary canal of four species of piscivorous birds at Lake Chivero.
*MI = mean intensity, P = prevalence.