356 TRANSACTIONS OFTHEROYAL SOCIIXY OFTROPICALMEDICINEANDHYGIENE.Vol. 69. No. 3. 1975. BRIEF THE
INTERMEDIATE
HOSTS
COMMUNICATION
OF SCHISTOSOMA
BOWS IN WESTERN
KENYA
V. R. SOUTHGATE ANDR. J. KNOWLES Department of Zoology, British Museum (Natural History). Cromwell Road, London, S. W.7.
In the Mediterranean and in the Middle East the principal molluscan host for Schistosoma bovis is but in East Africa and parts of the Sudan some members of the B. africanus group serve as intermediate hosts (SOUTHGATEand KNOWLES, 1975). Some reports have also implicated B. forskali but only MCCLELLAND (1955) and TEESDALEand NELSON (1958) confirmed this by recovery of paired adult worms from infections in experimental animals. During a visit to Kisumu, Kenya in October 1974an examination was made of the roles of different bulinid snails in the transmission of S. bovis. It is widely acceptedby malacologiststhat preciseidentification of somepopulations of the B. africanus group in East Africa is extremely difhcult: the group is in need of taxonomic revision (C. A. WRIGHT, personal communication). Thus, it is not always possible to differentiate betweenB. africanus, B. nasutus or B. gtobosus. However, our results have confirmed earlier observations(e.g. DOWDESWELL, 1938;MCCLELLAND, 1956; TEESDALEand NELSON, 1958; KINOTI, 1971) that some members of the B. africanus group transmit S. bovis. On the edgeof the Kano Plain, to the south of Kisumu, 13% of the snails with characteristics intermediate betweenB. africanus and B. globosus, taken from a borrow pit near Urudi School, Nyakach were infected with S. bovis (see Table I for the egg measurementsand prepatent periods of the different isolatesof S. bovis). Approximately 8 km. north west of Kisumu, on the Maseno Road, in a pool near Tiengre School, Kajulu District 76% of the snails with affinities for B. africanus/B. nasutus were infected with S. bovis (Table I). B. ugandae were prevalent on the shore of Lake Victoria at Aram Market, Bondo District, inhabiting the pools of water created by the impressions of cattle hoovesin the papyrus swamp. In Kisumu, B. ugandae were found at the edge of Lake Victoria adjacent to the abattoir. There was no evidenceof B. ugarrdaetransmitting S. bovis out of 212 specimensexamined. It is interesting that those from Kisumu were not infected becausethe concentration of S. bovis miracidia in the surrounding water was probably high due to the daily hosing of the infected faecal material into the lake. Bulinus truncatus,
TABLE I. The egg measurements(in 0.85% saline from hamster liver) and prepatent period of isolates of S. bovis from different intermediate hosts, Western Kenya. Snail host
Egg measurements
Prepatent period
B. africanusiglobosus
Length 241.5 pm & 16.8 Width 70.6 pm f 6 (109 measurements)
53 days (sheep)
Length 223.1 pm f 13-3 Width 66-S pm f 5.7 (77 measurements)
51 days (sheep)
B. forskali
Kajulu District
Length 224.1 pm & 15.3 Width 64-6 pm * 6-2 (29 measurements)
53 days (hamster)
B. truncatus s.1. Bondo Market, South Nyanza Province
Length 224.5 pm f 16.9 Width 62-l pm * 4-6
51 days (sheep)
Urudi, Nyakach B. africanuslnasutus
Kajulu District
B. forskali
(57 measurements)
were found in abundance in the temporary pools situated between the Maseno Road and
the railway line, about 6-8 km. from Kisumu. No other bulinid snails were found in these pools, which are used for watering cattle. 6.7% of the B. forskali were infected with S. bovis.
Y.R.SOUTHGATE
AND R.J.KNOWLES
357
B. truncatus sensu lato were collected from a dam at Bondo Market, about 9 km. south west of Migori, South Nyanza Province, and these snails had an infection rate of 4.4% with S. bovis. This is the first record of B. truncatus s.1. transmitting S. bovis in Kenya. These observations have demonstrated that S. bovis in Western Kenya is transmitted by snaiis belonging to the africanus, forskuli and truncatus groups. SOUTHGATE and KNOWLES (1975) showed that a strain of S. bovis isolated in africanus group snails from the Kisat stream, Kisumu is also compatible with snails of the truncatus, forskali and reticulatus groups. Further work is in progress to establish whether the recent isolates of S. bovis from B. truncatus and B. for&&i are similarly compatible. It seems likely that the unusual development and/or retention of infectivity of S. bovis to four bulinid groups enables transmission of the parasite to occur over a wider area than would be the case if the host-parasite relationship was more restricted. This is relevant because members of all the bulinid groups are found in East Africa but often in different habitats, and therefore the infectivity of the parasite is apparently of adaptive significance. It will be important to establish whether the infectivity of S. haematobium from Western Kenya parallels that of S. bovis. REFERENCES DOWDESWELL, R. M. (1938). Trans. R. Sot. trap. Med. Hyg., 31, 673. KINOTI, G. (1971). Ibid., 65, 637 MCCLELLAND, W. F. J. (1955). Ibid., 49, 295.
(1956). J. trap. Med. Hyg., 59, 229. SOUTHGATE, V. R. & KNOWLES, R. J. (1975). J. nat. Hist., 9, 273. TEESDALE, C. & NELSON, G. S. (1958). E. Afr. med. J., 35, 427.
C?
INTERMEDIATE
HOSTS
COMMUNICATION
OF SCHISTOSOMA
BOWS IN WESTERN
KENYA
V. R. SOUTHGATE ANDR. J. KNOWLES Department of Zoology, British Museum (Natural History). Cromwell Road, London, S. W.7.
In the Mediterranean and in the Middle East the principal molluscan host for Schistosoma bovis is but in East Africa and parts of the Sudan some members of the B. africanus group serve as intermediate hosts (SOUTHGATEand KNOWLES, 1975). Some reports have also implicated B. forskali but only MCCLELLAND (1955) and TEESDALEand NELSON (1958) confirmed this by recovery of paired adult worms from infections in experimental animals. During a visit to Kisumu, Kenya in October 1974an examination was made of the roles of different bulinid snails in the transmission of S. bovis. It is widely acceptedby malacologiststhat preciseidentification of somepopulations of the B. africanus group in East Africa is extremely difhcult: the group is in need of taxonomic revision (C. A. WRIGHT, personal communication). Thus, it is not always possible to differentiate betweenB. africanus, B. nasutus or B. gtobosus. However, our results have confirmed earlier observations(e.g. DOWDESWELL, 1938;MCCLELLAND, 1956; TEESDALEand NELSON, 1958; KINOTI, 1971) that some members of the B. africanus group transmit S. bovis. On the edgeof the Kano Plain, to the south of Kisumu, 13% of the snails with characteristics intermediate betweenB. africanus and B. globosus, taken from a borrow pit near Urudi School, Nyakach were infected with S. bovis (see Table I for the egg measurementsand prepatent periods of the different isolatesof S. bovis). Approximately 8 km. north west of Kisumu, on the Maseno Road, in a pool near Tiengre School, Kajulu District 76% of the snails with affinities for B. africanus/B. nasutus were infected with S. bovis (Table I). B. ugandae were prevalent on the shore of Lake Victoria at Aram Market, Bondo District, inhabiting the pools of water created by the impressions of cattle hoovesin the papyrus swamp. In Kisumu, B. ugandae were found at the edge of Lake Victoria adjacent to the abattoir. There was no evidenceof B. ugarrdaetransmitting S. bovis out of 212 specimensexamined. It is interesting that those from Kisumu were not infected becausethe concentration of S. bovis miracidia in the surrounding water was probably high due to the daily hosing of the infected faecal material into the lake. Bulinus truncatus,
TABLE I. The egg measurements(in 0.85% saline from hamster liver) and prepatent period of isolates of S. bovis from different intermediate hosts, Western Kenya. Snail host
Egg measurements
Prepatent period
B. africanusiglobosus
Length 241.5 pm & 16.8 Width 70.6 pm f 6 (109 measurements)
53 days (sheep)
Length 223.1 pm f 13-3 Width 66-S pm f 5.7 (77 measurements)
51 days (sheep)
B. forskali
Kajulu District
Length 224.1 pm & 15.3 Width 64-6 pm * 6-2 (29 measurements)
53 days (hamster)
B. truncatus s.1. Bondo Market, South Nyanza Province
Length 224.5 pm f 16.9 Width 62-l pm * 4-6
51 days (sheep)
Urudi, Nyakach B. africanuslnasutus
Kajulu District
B. forskali
(57 measurements)
were found in abundance in the temporary pools situated between the Maseno Road and
the railway line, about 6-8 km. from Kisumu. No other bulinid snails were found in these pools, which are used for watering cattle. 6.7% of the B. forskali were infected with S. bovis.
Y.R.SOUTHGATE
AND R.J.KNOWLES
357
B. truncatus sensu lato were collected from a dam at Bondo Market, about 9 km. south west of Migori, South Nyanza Province, and these snails had an infection rate of 4.4% with S. bovis. This is the first record of B. truncatus s.1. transmitting S. bovis in Kenya. These observations have demonstrated that S. bovis in Western Kenya is transmitted by snaiis belonging to the africanus, forskuli and truncatus groups. SOUTHGATE and KNOWLES (1975) showed that a strain of S. bovis isolated in africanus group snails from the Kisat stream, Kisumu is also compatible with snails of the truncatus, forskali and reticulatus groups. Further work is in progress to establish whether the recent isolates of S. bovis from B. truncatus and B. for&&i are similarly compatible. It seems likely that the unusual development and/or retention of infectivity of S. bovis to four bulinid groups enables transmission of the parasite to occur over a wider area than would be the case if the host-parasite relationship was more restricted. This is relevant because members of all the bulinid groups are found in East Africa but often in different habitats, and therefore the infectivity of the parasite is apparently of adaptive significance. It will be important to establish whether the infectivity of S. haematobium from Western Kenya parallels that of S. bovis. REFERENCES DOWDESWELL, R. M. (1938). Trans. R. Sot. trap. Med. Hyg., 31, 673. KINOTI, G. (1971). Ibid., 65, 637 MCCLELLAND, W. F. J. (1955). Ibid., 49, 295.
(1956). J. trap. Med. Hyg., 59, 229. SOUTHGATE, V. R. & KNOWLES, R. J. (1975). J. nat. Hist., 9, 273. TEESDALE, C. & NELSON, G. S. (1958). E. Afr. med. J., 35, 427.
C?
