- Email: [email protected]
Parasite: Extra larvae and extra bites on the Net
418
Forum
TRENDS in Parasitology Vol.18 No.9 September 2002
ParaSite
Extra larvae and extra bites on the Net Mosquito and malaria discussion groups Internal parasites of mosquitoes – potential for biological control?
Both discussion groups were e-mailed by Rob Anderson (University of Winnipeg, Canada), and responses came from both, after he described a visit to a student’s Culex tarsalis colony: fourth instar larvae were turning dark brown to black and sinking to the bottom of the pans. A few weeks later, his own Anopheles stephensi followed suit. He found ‘both dead and living larvae teeming with small mobile parasites that had a slightly corkscrew shape and appeared to have dark bodies (spores perhaps?) within them…could they be microsporidians perhaps or something else?’ Immediately, Peter Singfield from Belize replied: ‘Advise continuing this line of research with the utmost effort possible! Biological warfare meets the mosquito?’ Jo Lines (London School of Hygiene and Tropical Medicine, UK) was not excited. Infections by bacteria, fungi and microsporidians that cause catastrophic mortality are well known to those who run mosquito colonies and are difficult, if not impossible, to cure. ‘Good hygiene – general cleanliness as well as autoclaving the pans – can help…good nutrition and rearing practices are also important – crowded and hungry larvae are much more susceptible…eradication is hard… recovery comes slowly and gradually.’ Genetic selection and adaptation are probably important in the gradual recovery. WHO has backed research into possible biological control by microbial pathogens without much success. Microsporidians often have complicated life cycles requiring intermediate hosts that may be absent from natural breeding sites and, whereas pathogens can cause temporary population crashes when first introduced, they either die out quickly or reach a kind of equilibrium, causing some mortality but not enough. Frequent repeated introductions are needed for sustained adequate control; however, http://parasites.trends.com
Bacillus sphaericus ‘has proved successful and persistent enough (when used in field habitats against Culex mosquitoes) to be made into a commercial product’. Finding all the breeding sites is exceedingly difficult, though this is less important with a biological control agent that can disperse by itself. ‘The key point is that in normal circumstances, 90% or more of the baby larvae die before they reach maturity, but do so in a densitydependent fashion. In other words, if you kill off most of the competition in the puddle, the remaining few surviving larvae have a greatly improved probability of surviving to adulthood. So, a pathogen that kills 95% of the baby larvae in a puddle may have little impact (50% or less) on the number of emerging adults.’ Jerry Vanderberg (New York University, USA) said that the microsporidian Nosema algerae is the most common pathogen of mosquito colonies but is neither corkscrew-shaped nor motile. Large numbers of big immobile spores collect in the haemocoel and attach to the outside of the gut, the malphighian tubules and the fat body. Mark Wiser (Tulane University, New Orleans, LA, USA) thought that large size and motility suggested an Ascogregarina, but gregarines are more common in Aedes and other Culicidae mosquitoes than in Anopheles, and ‘gregarines do exhibit some host specificity’. Stephen Doggett (Westmead Hospital, New South Wales, Australia) thought that protozoans were common in dead larvae and were unlikely to be the cause of the die off. ‘The only parasite that I have encountered was the fungus Culicinomyces, we got rid of this by not using field-collected water, only deionised.’ He had suffered when his larval Oc. vigilax colony reached the pupal stage and then all died, although the larvae looked well, but he had traced the problem to a new batch of food. Finally, Anderson decided his might be a Nosema infection and hoped that surviving reproducing females would lead to a stronger and more resistant colony of Anopheles stephensi. ‘Certainly the issue of larval mortality that ends up producing a few, but very fit vector individuals as adults is a fascinating area of mosquito ecology that has not had nearly enough attention.’
Mosquito discussion group (mosquito-l@ iastate.edu) Preferential biting: why me and not you?
Mike Muller (Brisbane City Council Mosquito and Pest Services, Queensland, Australia) had been asked by his education officer, who delivers basic mosquito education to primary schoolchildren (and their teachers), for information about why (or if) some individuals are more attractive to mosquitoes than others. He knew, in general, about differences in carbon dioxide and lactic acid output, that some people notice and react to bites more than others, and ‘even the old stories about different blood groups’, but he wanted a fact sheet or similar on the state of current knowledge. David Kelly, who works in an Insect Behaviour and Sensory Ecology group (University of Oxford, UK), referred him to an article accessible on the BioMedNet website at www.http://news.bmn.com/hmsbeagle/113/ notes/feature12 [This was in fact an article entitled ‘Why are some people bitten more than others?’ by one Kelly, D.W. (2001) Trends Parasitol. 17, 578–581.] He added, modestly, it ‘tries to look beyond the question of how mosquitoes discriminate between different hosts, to ask why, in an evolutionary sense, it might be an advantage to bite one person in preference to another’. Malaria discussion group ([email protected]) Why ‘vivax’?
Plasmodium vivax parasites exhibit greater amoeboid movement than the other species because they tend to infect reticulocytes which are larger than mature red blood cells, said Jerry Vanderberg (New York University, USA), so the early Italian malariologists, who described this parasite, named it ‘vivax’ (alive or lively). Richard Donaldson-Alves (Mae Ai Malaria Research Centre, North Thailand) thought differently: ‘I understand that Camillo Golgi, a Roman Physician in 1886 when describing P. vivax, gave it the name ‘vivax’ (long-lived, lasting, enduring) because that was the nature of the disease’. Can malaria parasites be transmitted from an infected mosquito to her egg?
This was asked by Peter Singfield from Belize. David Warhurst (London School of Hygiene and Tropical
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(02)02335-8
Forum
Medicine, UK) replied with a categorical ‘No’. Vanderberg added, however, that whereas not known for malaria (‘but one must keep an open mind’), transovarial or vertical transmission of pathogens is
TRENDS in Parasitology Vol.18 No.9 September 2002
common among ticks and can occur in mosquitoes in the case of Yellow Fever and West Nile viruses, and with microsporidia.
419
ParaSite was compiled from the Internet by Janice Taverne ([email protected])
Published online: 6 August 2002
Book Review
A portal to the field The Encyclopedia of Arthropod-transmitted Infections edited by M.W. Service and R.W. Ashford, CABI Publishing, 2002. US$185.00 (hbk) (600 pages) ISBN 0 851 99473 3
Is there a need for a hard copy encyclopedia given the rapid searching availability of the internet? Entering ‘arthropodtransmitted infections’ into PubMed yields quite a superficial list of 12 results, although entering ‘arthropods and infections’ results in an overwhelming 4744 hits. By contrast, The Encyclopedia of Arthropod-transmitted Infections provides an easy entrée into specific infections, even if the query starts with only the name of a disease, an infectious agent or an arthropod vector. Unlike more specialized texts focused on specific arthropod vectors or disease agents, this Encyclopedia does not require the user to know if the vector is an insect or a tick, or if the pathogen is a virus or a bacterium. Uncommon in most scientific reference texts, the entries are listed alphabetically, rather than by any taxonomic separations of the vector, pathogen or host species. However, this works well for the user because the entries are extensively cross-listed. For example, if the user starts their search using the terms ‘plague’, ‘Yersinia pestis’, or ‘fleas’, they will be directed to the most appropriate comprehensive entry, which is listed under the disease name. Once directed to the appropriate entry, the user has access to a summary of the vector, pathogen, and disease, and to a selected bibliography. Armed with this introductory knowledge, the search capabilities of the internet now become useful. http://parasites.trends.com
The entries themselves are written by experts in their fields – I found that most of the entries on subjects of which I had little knowledge rapidly provided me with information to dig further into the topic. Moreover, I found that many of the entries for topics of which I have some previous knowledge were well written and, at the survey level appropriate for an encyclopedia, comprehensive. The most useful entries were those written by individuals who work on both the arthropod-vector and the pathogen. For example, the section on Anaplasmosis, written by Katherine Kocan (pp. 28–33), is contemporary and succinctly covers the mammalian infection and development of the pathogen within the tick vector. However, other entries were less comprehensive and occasionally inaccurate, probably reflecting a bias towards the disease in an animal or human host, the arthropod or the pathogen. For example, the statement in the section on the Rickettsiales that ‘insect transmission occurs for Rickettsia and Ehrlichia’ (p. 424) is misleading because most of these pathogens are transmitted by ticks; ticks are in a different class of the Phylum Arthropoda to insects. The most significant deficiency in the Encyclopedia is a lack of consistency in the depth of presentation among the entries. Some entries, such as the coverage of Theileriosis written by Patricia Preston (pp. 487–502), are extensive and accompanied by elucidating graphics, tables and photomicrographs. Other entries, however, are minimal in coverage and either do not include needed illustrations, or incorporate uninformative and, occasionally, simply bizarre photographs. Throughout the Encyclopedia, there are multiple pictures of individuals spraying insecticide along the walls of huts, or similarly impoverished dwellings in tropical regions – it is possible there would be a reader who could not envision how spraying would occur, but it seems a low return on investment when many entries are in need of a photograph or graphic
illustration. The most baffling selection of a photograph has to be the picture of the automobile (p. 410) used to illustrate a natural focus of Powassan virus! (The main point might be that the initial outbreak was described in 1958, which would match the apparent vintage of the car.) Although the difficulties of compiling the efforts of many authors can be appreciated, the Encyclopedia would have benefited from a much stronger editorial hand in standardizing entry information and illustration. The Encyclopedia of Arthropodtransmitted Infections could well find its best use in serving the needs of those outside the field of arthropod-transmitted infections. Scientists and faculty with expertise in the field will direct chapterlevel enquiries towards more specialized texts focused on the specific pathogen, vector, or epidemiology and pathogenesis of the disease. Although the Encyclopedia will be of some help to newcomers to the field (e.g. entering graduate students or post-doctoral students re-directing their investigation to arthropod-borne pathogens), these individuals will also need a more comprehensive understanding of the biology of the host–vector–pathogen triad than can be gained from an encyclopedic approach. By contrast, however, the Encyclopedia will be an excellent addition for those engaged in general biology instruction or introductory-level microbiology and entomology teaching, in which selection of specific examples of vector-borne diseases is needed. Similarly, physicians, nurses, veterinarians and other scientists with involvement in public and animal health might find the Encyclopedia an important information portal for arthropod-transmitted infections. Guy H. Palmer Program in Vector-borne Diseases, Washington State University, Pullman, WA 99164-7040, USA. e-mail: [email protected] Published online: 6 August 2002
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
Forum
TRENDS in Parasitology Vol.18 No.9 September 2002
ParaSite
Extra larvae and extra bites on the Net Mosquito and malaria discussion groups Internal parasites of mosquitoes – potential for biological control?
Both discussion groups were e-mailed by Rob Anderson (University of Winnipeg, Canada), and responses came from both, after he described a visit to a student’s Culex tarsalis colony: fourth instar larvae were turning dark brown to black and sinking to the bottom of the pans. A few weeks later, his own Anopheles stephensi followed suit. He found ‘both dead and living larvae teeming with small mobile parasites that had a slightly corkscrew shape and appeared to have dark bodies (spores perhaps?) within them…could they be microsporidians perhaps or something else?’ Immediately, Peter Singfield from Belize replied: ‘Advise continuing this line of research with the utmost effort possible! Biological warfare meets the mosquito?’ Jo Lines (London School of Hygiene and Tropical Medicine, UK) was not excited. Infections by bacteria, fungi and microsporidians that cause catastrophic mortality are well known to those who run mosquito colonies and are difficult, if not impossible, to cure. ‘Good hygiene – general cleanliness as well as autoclaving the pans – can help…good nutrition and rearing practices are also important – crowded and hungry larvae are much more susceptible…eradication is hard… recovery comes slowly and gradually.’ Genetic selection and adaptation are probably important in the gradual recovery. WHO has backed research into possible biological control by microbial pathogens without much success. Microsporidians often have complicated life cycles requiring intermediate hosts that may be absent from natural breeding sites and, whereas pathogens can cause temporary population crashes when first introduced, they either die out quickly or reach a kind of equilibrium, causing some mortality but not enough. Frequent repeated introductions are needed for sustained adequate control; however, http://parasites.trends.com
Bacillus sphaericus ‘has proved successful and persistent enough (when used in field habitats against Culex mosquitoes) to be made into a commercial product’. Finding all the breeding sites is exceedingly difficult, though this is less important with a biological control agent that can disperse by itself. ‘The key point is that in normal circumstances, 90% or more of the baby larvae die before they reach maturity, but do so in a densitydependent fashion. In other words, if you kill off most of the competition in the puddle, the remaining few surviving larvae have a greatly improved probability of surviving to adulthood. So, a pathogen that kills 95% of the baby larvae in a puddle may have little impact (50% or less) on the number of emerging adults.’ Jerry Vanderberg (New York University, USA) said that the microsporidian Nosema algerae is the most common pathogen of mosquito colonies but is neither corkscrew-shaped nor motile. Large numbers of big immobile spores collect in the haemocoel and attach to the outside of the gut, the malphighian tubules and the fat body. Mark Wiser (Tulane University, New Orleans, LA, USA) thought that large size and motility suggested an Ascogregarina, but gregarines are more common in Aedes and other Culicidae mosquitoes than in Anopheles, and ‘gregarines do exhibit some host specificity’. Stephen Doggett (Westmead Hospital, New South Wales, Australia) thought that protozoans were common in dead larvae and were unlikely to be the cause of the die off. ‘The only parasite that I have encountered was the fungus Culicinomyces, we got rid of this by not using field-collected water, only deionised.’ He had suffered when his larval Oc. vigilax colony reached the pupal stage and then all died, although the larvae looked well, but he had traced the problem to a new batch of food. Finally, Anderson decided his might be a Nosema infection and hoped that surviving reproducing females would lead to a stronger and more resistant colony of Anopheles stephensi. ‘Certainly the issue of larval mortality that ends up producing a few, but very fit vector individuals as adults is a fascinating area of mosquito ecology that has not had nearly enough attention.’
Mosquito discussion group (mosquito-l@ iastate.edu) Preferential biting: why me and not you?
Mike Muller (Brisbane City Council Mosquito and Pest Services, Queensland, Australia) had been asked by his education officer, who delivers basic mosquito education to primary schoolchildren (and their teachers), for information about why (or if) some individuals are more attractive to mosquitoes than others. He knew, in general, about differences in carbon dioxide and lactic acid output, that some people notice and react to bites more than others, and ‘even the old stories about different blood groups’, but he wanted a fact sheet or similar on the state of current knowledge. David Kelly, who works in an Insect Behaviour and Sensory Ecology group (University of Oxford, UK), referred him to an article accessible on the BioMedNet website at www.http://news.bmn.com/hmsbeagle/113/ notes/feature12 [This was in fact an article entitled ‘Why are some people bitten more than others?’ by one Kelly, D.W. (2001) Trends Parasitol. 17, 578–581.] He added, modestly, it ‘tries to look beyond the question of how mosquitoes discriminate between different hosts, to ask why, in an evolutionary sense, it might be an advantage to bite one person in preference to another’. Malaria discussion group ([email protected]) Why ‘vivax’?
Plasmodium vivax parasites exhibit greater amoeboid movement than the other species because they tend to infect reticulocytes which are larger than mature red blood cells, said Jerry Vanderberg (New York University, USA), so the early Italian malariologists, who described this parasite, named it ‘vivax’ (alive or lively). Richard Donaldson-Alves (Mae Ai Malaria Research Centre, North Thailand) thought differently: ‘I understand that Camillo Golgi, a Roman Physician in 1886 when describing P. vivax, gave it the name ‘vivax’ (long-lived, lasting, enduring) because that was the nature of the disease’. Can malaria parasites be transmitted from an infected mosquito to her egg?
This was asked by Peter Singfield from Belize. David Warhurst (London School of Hygiene and Tropical
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(02)02335-8
Forum
Medicine, UK) replied with a categorical ‘No’. Vanderberg added, however, that whereas not known for malaria (‘but one must keep an open mind’), transovarial or vertical transmission of pathogens is
TRENDS in Parasitology Vol.18 No.9 September 2002
common among ticks and can occur in mosquitoes in the case of Yellow Fever and West Nile viruses, and with microsporidia.
419
ParaSite was compiled from the Internet by Janice Taverne ([email protected])
Published online: 6 August 2002
Book Review
A portal to the field The Encyclopedia of Arthropod-transmitted Infections edited by M.W. Service and R.W. Ashford, CABI Publishing, 2002. US$185.00 (hbk) (600 pages) ISBN 0 851 99473 3
Is there a need for a hard copy encyclopedia given the rapid searching availability of the internet? Entering ‘arthropodtransmitted infections’ into PubMed yields quite a superficial list of 12 results, although entering ‘arthropods and infections’ results in an overwhelming 4744 hits. By contrast, The Encyclopedia of Arthropod-transmitted Infections provides an easy entrée into specific infections, even if the query starts with only the name of a disease, an infectious agent or an arthropod vector. Unlike more specialized texts focused on specific arthropod vectors or disease agents, this Encyclopedia does not require the user to know if the vector is an insect or a tick, or if the pathogen is a virus or a bacterium. Uncommon in most scientific reference texts, the entries are listed alphabetically, rather than by any taxonomic separations of the vector, pathogen or host species. However, this works well for the user because the entries are extensively cross-listed. For example, if the user starts their search using the terms ‘plague’, ‘Yersinia pestis’, or ‘fleas’, they will be directed to the most appropriate comprehensive entry, which is listed under the disease name. Once directed to the appropriate entry, the user has access to a summary of the vector, pathogen, and disease, and to a selected bibliography. Armed with this introductory knowledge, the search capabilities of the internet now become useful. http://parasites.trends.com
The entries themselves are written by experts in their fields – I found that most of the entries on subjects of which I had little knowledge rapidly provided me with information to dig further into the topic. Moreover, I found that many of the entries for topics of which I have some previous knowledge were well written and, at the survey level appropriate for an encyclopedia, comprehensive. The most useful entries were those written by individuals who work on both the arthropod-vector and the pathogen. For example, the section on Anaplasmosis, written by Katherine Kocan (pp. 28–33), is contemporary and succinctly covers the mammalian infection and development of the pathogen within the tick vector. However, other entries were less comprehensive and occasionally inaccurate, probably reflecting a bias towards the disease in an animal or human host, the arthropod or the pathogen. For example, the statement in the section on the Rickettsiales that ‘insect transmission occurs for Rickettsia and Ehrlichia’ (p. 424) is misleading because most of these pathogens are transmitted by ticks; ticks are in a different class of the Phylum Arthropoda to insects. The most significant deficiency in the Encyclopedia is a lack of consistency in the depth of presentation among the entries. Some entries, such as the coverage of Theileriosis written by Patricia Preston (pp. 487–502), are extensive and accompanied by elucidating graphics, tables and photomicrographs. Other entries, however, are minimal in coverage and either do not include needed illustrations, or incorporate uninformative and, occasionally, simply bizarre photographs. Throughout the Encyclopedia, there are multiple pictures of individuals spraying insecticide along the walls of huts, or similarly impoverished dwellings in tropical regions – it is possible there would be a reader who could not envision how spraying would occur, but it seems a low return on investment when many entries are in need of a photograph or graphic
illustration. The most baffling selection of a photograph has to be the picture of the automobile (p. 410) used to illustrate a natural focus of Powassan virus! (The main point might be that the initial outbreak was described in 1958, which would match the apparent vintage of the car.) Although the difficulties of compiling the efforts of many authors can be appreciated, the Encyclopedia would have benefited from a much stronger editorial hand in standardizing entry information and illustration. The Encyclopedia of Arthropodtransmitted Infections could well find its best use in serving the needs of those outside the field of arthropod-transmitted infections. Scientists and faculty with expertise in the field will direct chapterlevel enquiries towards more specialized texts focused on the specific pathogen, vector, or epidemiology and pathogenesis of the disease. Although the Encyclopedia will be of some help to newcomers to the field (e.g. entering graduate students or post-doctoral students re-directing their investigation to arthropod-borne pathogens), these individuals will also need a more comprehensive understanding of the biology of the host–vector–pathogen triad than can be gained from an encyclopedic approach. By contrast, however, the Encyclopedia will be an excellent addition for those engaged in general biology instruction or introductory-level microbiology and entomology teaching, in which selection of specific examples of vector-borne diseases is needed. Similarly, physicians, nurses, veterinarians and other scientists with involvement in public and animal health might find the Encyclopedia an important information portal for arthropod-transmitted infections. Guy H. Palmer Program in Vector-borne Diseases, Washington State University, Pullman, WA 99164-7040, USA. e-mail: [email protected] Published online: 6 August 2002
1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.