Let's not gloss over the glossy pillars: Cochliocopa lubrica (Cochliocopidae)

Dicrocoelium dendriticum stages
and intermediate hosts
Acrylic on canvas by Jeba Jesudoss

Cochliocopa lubrica is also known as Cionella lubrica or the glossy pillar. It is a rather beautiful land snail species that is the intermediate host of the ruminant parasite – Dicrocoelium dendriticum and of the nematodes Protostrongylus rufescens, Meullerius capillaris, Elaphostrongylus cervi, Varestrongylus capreoli, Varestrongylus pneumonicus and Skrjabingylus nasicola. 

C. lubrica is a holarctic species found throughout North America, Europe, Africa, parts of Asia and New Zealand. In the United States, it is found on both sides of the continental divide. Being a land snail, it is found in forests and grassy areas, often close to human dwellings.

The snail is about 5 – 7 mm long and 2-3 mm wide. It has a brown ( ranging from off-white to almost amber), smooth, glossy, translucent shell which is bluntly rounded at the apex. There are 5-7 whorls. The opening/aperture is ovate and wide, and lacks denticles.  The soft parts of the body is dark blue-black.

Cochliocopa lubrica
Muséum national histoire naturelle /
CC BY
(https://creativecommons.org/licenses/by/4.0)
via Wikimedia

Experimental studies have shown that C. lubrica can tolerate high temperatures of 37 – 40 C and survive through a drought for 50 -70 days. However, snails infected with Dicrocoelium were less resistant than uninfected snails. 

While many other snail genera can be first intermediate hosts of D. dendriticium in parts of the world, C. lubrica is epizootically important in North America. For example, in a 1952 study on a heavily infected sheep farm in New York,  Mapes C.R. found that only C. lubrica out of 16 mollusc species studied harbored the sporocysts or cercariae of D. dendriticum. He also found that the distribution of the snail was similar to known areas where dicrocoeliasis occurred. 

Interestingly, C. lubrica is small enough to be preyed upon by other predator snails (a phenomenon called malacophagy). In experimental conditions, Nesovitrea hammonis (a snail found in Europe) was shown to be capable of preying upon C. lubrica, but not well enough to be a biological control agent. 

References:

Grewal, P. S., et al. "Parasitism of molluscs by nematodes: types of associations and evolutionary trends." Journal of Nematology 35.2 (2003): 146.

Badie, A., and Daniel Rondelaud. "Influence Du Parasitisme Sur La Résistance De Cionella Lubrica Müller A La Température Et A La Dessication." (1982).

Mapes, Cortland R. "Cionella lubrica (Muller), a new intermediate host of Dicrocoelium dendriticum (Rudolphi, 1819) Looss, 1899 (Treinatoda: Dicrocoeliidae)." Journal of Parasitology 38.1 (1952).

https://www.carnegiemnh.org/science/mollusks/va_cochlicopa_lubrica.html

http://fieldguide.mt.gov/speciesDetail.aspx?elcode=IMGAS11010

Snail Clone wars - Lymnaea cubensis and other look-alikes (Lymnaeidae)

Lymnaea cubensis
Antonio Alejandro Vázquez Perera
& Susana Perera Valderrama /
CC BY 3.0 via  Wikimedia Commons

The next snail in this series has many synonyms in literature. It has been called Galba cubensis, Bakerilymnaea cubensis, Fossaria cubensis and Lymnaea cubensis. I am going to refer to it as Lymnaea cubensis because the malacologists who have authored the book "Freshwater Gastropods of North America" call it that.  From the veterinary standpoint, L. cubensis is important as the first intermediate host of Heterobilharzia americana (along with another snail - Pseudosuccinea columella) and Fasciola hepatica.

The native range of L. cubensis is North and South America. In the United States, the snail is currently confined to the south and south east regions (The Carolinas, Georgia, Florida, Louisiana, Alabama, Texas, New Mexico and California). Due to anthropogenic introductions, L. cubensis was recorded in Spain by Schniebs et al. in 2018, which was the first time that it had been found in the wild in Europe.

L. truncatula
Francisco Welter Schultes, modified by Michal 
Maňas. Versión actual modified by Veronidae /
CC BY-SA 2.5 via Wikimedia Commons

L. schirazensis
María Dolores Bargues, Patricio Artigas,
Messaoud Khoubbane,  Rosmary Flores,
Peter Glöer, Raúl Rojas-García,
Keyhan Ashrafi, Gerhard Falkner,
Santiago Mas-Coma /
CC BY 2.5 via Wikimedia Commons

The snails are amphibious and are found in the mud in ditches and ponds. It is quite impossible to distinguish L. cubensis from L. truncatula based on shell morphology alone, and malacology training is required to identify the small differences in male reproductive system morphology between the two species as illustrated by Pointier et al. in their 2009 paper.

Interestingly, L. cubensis is not the only snail with striking morphological similarity to L. truncatula. A cryptic species with near worldwide distribution and very similar morphology called L. schirazensis was described in 2011 by Bargues et al. and is very resistant to Fasciola hepatica (i.e. no cercaria are produced). There are some serious complications that result from the identical morphologies of L. schirazensis and L. truncatula. Bargues et al. posit that misidentification of L. schirazensis as L. truncatula has distorted epidemiological data regarding snail susceptibility status to Fasciola. In the future, molecular data would have to be used to confirm  snail morphological identities in cases of cryptic species and the known occurrence of shape and size variations attributable to geography and lineage. PCR + sequencing of the coxI, ITS or 18S genes may help resolve identification problems, since it is well-known that the classification of the Lymnaeidae is fairly complicated (Link here for an explanation).

The capacity of L. cubensis in the transmission of the following is important:
(i)  Heterobilharzia americana: The role of L. cubensis was worked out in the early 1960s by Dr. Hong-Fong Lee. Furcocercous cercariae were shed 4 weeks after infection. Distribution of the trematode closely follows the distribution of its two first intermediate hosts: L. cubensis and P. columella.

(ii) Fascioloides magna: Vignoles et al. found that L. cubensis could be an experimental intermediate host for Fascioloides magna. The role played by the snail in natural infections is still unknown.

(iii) Fasciola hepatica: L. cubensis along with L. viatrix and L. neotropica have been established as the major vectors in Latin America. And while misidentifications of L. cubensis as L. truncatula may not throw-off the snail susceptibility data as much as L. schirazensis, the situation is still not ideal.

So, if you happen to be see a Lymnaeid snail that looks like L. truncatula, remember that identification to species level is very difficult and that the snail before you might not be the species that you think it is.

References:

Schniebs, Katrin, et al. "The first record of Galba cubensis (L. Pfeiffer, 1839)(Mollusca: Gastropoda: Lymnaeidae) from open fields of Europe." Folia Malacologica 26.1 (2018).

Dillon, Robert T. The freshwater gastropods of North America. Freshwater Gastropods of North America Project, 2019. FWGNA press. 

Lee, Hong-Fang. "Life history of Heterobilharzia americana Price 1929, a schistosome of the raccoon and other mammals in southeastern United States." The Journal of parasitology (1962): 728-739.

Vignoles, Philippe, et al. "Lymnaea cubensis, an experimental intermediate host for Fascioloides magna." Folia parasitologica 61.2 (2014): 185.

Bargues, M. D., et al. "Characterisation of Lymnaea cubensis, L. viatrix and L. neotropica n. sp., the main vectors of Fasciola hepatica in Latin America, by analysis of their ribosomal and mitochondrial DNA." Annals of Tropical Medicine & Parasitology 101.7 (2007): 621-641.

Don't pierce holes with this awl - Subulina octona (Subulinidae)

The next species of snail host extraordinaire first came to my attention as the first intermediate host of the feline trematode Platynosomum fastosum. It is known as the miniature awl snail or by its binomial name Subulina octona.

Subulina octona was originally native to the tropical parts of the Americas and the Caribbean. However, the current distribution is extensive due to introductions into other parts of the world. The snail has been reported in Europe (Denmark, Germany, Czech Republic), Sri Lanka and several island nations of Oceania such as Fiji, Samoa, Vanuatu etc. Whether the introductions were accidental or deliberate is currently unknown.

Subulina octona
By Luis Ruiz Berti, Creative Commons
CC BY-SA via Wikimedia

If you were to come across this terrestrial, air breathing snail you would observe the following. Subulina octona is an elegant, small snail measuring only 1.4 - 1.7 cms. The thin, glossy, pale yellow to brown shell is narrow, tapering and long with 8 - 11 whorls ending in an ovate aperture.

S. octona live in moist ground litter in forests but are also capable of thriving in greenhouses and hothouses. A distinctive feature of the life history that D'avila et al. record in their 2018 article involves "egg-retaining", which refers to a reproductive phenomenon in which a major part of the embryonic development occurs inside the body of the parent snail, and the egg when laid has a well-developed embryo. This strategy along with a long life span, several reproductive events per year and high survival of juveniles all result in S. octona being a successful invader.

This snail has an impressive ability to host parasites whose identities span both helminth phyla. It plays the role of intermediate host to trematodes, nematodes and cestodes including:

(1) Postharmostomum gallinarum, the cecal fluke of chickens, the life cycle of which involves S. octona and was worked out in Hawaii as early as 1940 by Dr. Joseph Alicata.

Subulina octona
By Bruguière, 1789 - Naturalis
Biodiversity Center,
Creative Commons CC0 via wikimedia

(2) Tamerlania bragai, the kidney fluke of domestic pigeons, the life cycle of which involves S. octona andwas worked out in Puerto Rico in 1945 by Dr. Jose Maldonado.

(3) Angiostrongylus cantonensis, the rat lungworm, which typically uses slugs and snails of other genera. A very interesting paper from Brazil by Caldeira et al. records S. octona as a naturally infected intermediate host of the A. cantonensis, harboring on average 20 L2/L3 larvae per snail.

(4) Angiostrongylus vasorum, the french heartworm: A paper published by Bessa et al. shows that S. octona infected with A. vasorum were capable of infecting a dog in an experimental setting, resulting in a patent infection in 49 days.

(5) Davainea proglottina: Cysticeroids of the poultry cestode Davainea were found in S. octona in Cuba. There was also a unique seasonal variation recorded by Perez et al, who observed cysticercoids only in February, May and August.

The wackiest thing about this snail is that there are sources (listed on the first page of a Google search) which sell awl snail adults for 2 Euros each (as of March 28, 2020). Buying and shipping this snail to your location is a terrible idea because it is a highly invasive species and considered an agricultural pest. There is certainly much ink spilled on the terrifying effects of introducing invasive species to novel non-native habitats, and the practice is not commendable.


References:
Title reference: The title of this post is a play on the common name of the snail (Miniature awl snail) and the small pointed tool used for piercing holes called the awl.

Juřičková, L. U. C. I. E. "Subulina octona (Bruguière, 1798)–a new greenhouse species for the Czech Republic (Mollusca: Gastropoda: Subulinidae)." Malacologica Bohemoslovaca 5 (2006): 1-2.

D’ávila, Sthefane, et al. "Life history of Subulina octona (Brugüière)(Gastropoda: Pulmonata: Subulinidae) based on four-year laboratory observations and a comparative histological analysis of egg-retaining and ovoviviparous subulinids." Journal of Natural History 52.23-24 (2018): 1551-1569.

Alicata, Joseph E. "The life cycle of Postharmostomum gallinum, the cecal fluke of poultry." The Journal of Parasitology 26.2 (1940): 135-143.
Maldonado, José F. "The life cycle of Tamerlania bragai, Santos 1934,(Eucotylidae), a kidney fluke of domestic pigeons." The Journal of Parasitology 31.5 (1945): 306-314.

Caldeira, Roberta Lima, et al. "First record of molluscs naturally infected with Angiostrongylus cantonensis (Chen, 1935)(Nematoda: Metastrongylidae) in Brazil." Memórias do Instituto Oswaldo Cruz 102.7 (2007): 887-889.

Bessa, EC de A., et al. "Biological development of Angiostrongylus vasorum (Baillet) Kamensky (Nematoda, Metastrongylidae) in Subulina octona Bruguière (Mollusca, Subulinidae) in laboratory conditions." Revista Brasileira de Zoologia 17.1 (2000): 29-41.

Perez, A., et al. "Seasonal dynamics of the cysticercoids of Davainea proglottina in the intermediate host Subulina octona." Revista Avicultura, Cuba 24.3/4 (1980): 223-225.

Slender walkers are moonwalkers: Pomatiopsis lapidaria (Pomatiopsidae)

Continuing our intellectually profitable exercise of studying snail intermediate hosts brings us to our next snail - Pomatiopsis lapidaria, also known as the slender walker. This freshwater amphibious snail is the first intermediate host of the lung trematode of the mammals of North America - Paragonimus kellicotti.

Pomatiopsis lapidaria
by John Slapcinsky under CreativeCommons 
CC BY-NC-SA 3.0
via boldsystems.org

Members of the genus Pomatiopsis are restricted to the temperate portions of North America and there are atleast five species : P. lapidaria, P. californica, P. cincinnatiensis, P. chacei and P. binneyi. The distribution of P. lapidaria is skewed and populations occur mostly in the eastern United States with the western borders of the distribution being in Iowa, Missouri, Kansas, Texas and New Mexico.  

 Pomatiopsis lapidaria
by Smithsonian Institution NMNH, 
under CreativeCommons
CC BY-NC-SA 3.0 via eol.org

P. lapidaria are small, thin, operculate (having a small lid that closes the aperture of the shell) dark brown-colored snails, on average about a fifth of an inch in length but never more than 8 mm. According to the original species description published by Say in 1817, the shell is turreted with a raised spire with six revolutions/whorls. The coiling is right handed (dextral). The shell surface has wrinkles/growth-lines across it and the sutures are impressed. The opening or aperture is ovate with a lip that is simple or slightly reflected. Gills may be seen in live specimens. These snails are different from the Lymnaidae in that they are sexually dimorphic. Males are more slender and may have more whorls than females.

It is interesting to note that P. lapidaria are said to have a characteristic "loping" movement, which involves large arching waves of motion that results in unusually rapid progression. This seems to be a characteristic of other land snails as well. Additionally, these are amphibious snails, and controversies have raged in the past about their terrestrial/aquatic habitat preferences. The species also seems to be nocturnal in its habits, hiding under leaves on bright days and being active on warm humid evenings in areas with moist marshy soils. They are also able to withstand long periods of dessication and are capable of being laboratory reared for experimental purposes.

As a veterinary parasitologist, I find it incredibly important to understand the vectorial capacity of this snail. These are several mentions of experiments from the world war II era in which attempts were made to determine if P. lapidaria was a vector of Schistosoma japonicum (the "Oriental" blood fluke). The basis for these experiments stemmed from the ecological and morphological similarity of P. lapidaria to Oncomelania, another member of the Pomatiopsidae, found in east Asia, which is known to be a competent vector of S. japonicum. However, it was found that only 5 out of 2000 experimentally infected P. lapidaria could shed S. japonicum cercaria, making the snail a poorly adapted vector.  However, the snail is intermediate host extraordinaire for Paragonimus kellicotti, Nudocotyle novicia (a bile duct fluke of meadow mice) and Euhryhelmis monorchis (a mink trematode).

From a public health perspective, it is important to note that Paragonimus kellicotti is an endemic trematodiasis that can affect humans in North America who consume raw or undercooked crayfish containing the infective metacercaria of the fluke. While the trematode can complete its life-cycle in humans resulting in patent infections, ectopic migration can cause trematode larval migrans (when the flukes migrate in subcutaneous tissues) and cerebral paragonimiasis, which are both rare in dogs, cats and other animals. The second intermediate hosts are crustaceans (crayfish) of the genera Oronectes and Cambarus, in which the metacercaria have a predilection for the crustacean heart. Humans infections have been reviewed in a fantastic paper by Diaz J. in the journal Clinical microbiology reviews (Link here). 

References:

Title reference: The title of this post is a play on the common name of the snail (Slender walkers) and their ability to move by loping, which is a motion almost as unique as the dance move called "moon-walk" of wide pop-cultural fame. The snails are also nocturnal, which helps with the moon walking theme.

Say, Thomas. Description of Seven Species of American Frech-water and Land Shells, Not Noticed in the Systems. 1817.

Dundee, Dee Saunders. "Aspects of the biology of Pomatiopsis lapidaria (Say)(Mollusca: Gastropoda: Prosobranchia)." (1957).

Parker, George Howard. "The loping of land-snails." The Biological Bulletin 72.3 (1937): 287-289.

Ameel, Donald J. "Observations on the natural history of Pomatiopsis lapidaria Say." American Midland Naturalist 19.3 (1938): 702-705.

DeWitt, William B. "Pomatiopsis lapidaria, its occurrence in the Washington, DC area and its laboratory rearing in comparison to that of Oncomelania spp." The Journal of parasitology 38.4 (1952): 321-326.

Walker, Bryant, Charles Keene Dodge, and Edward Bruce Williamson. A Synopsis of the Classification of the Fresh-water Mollusca of North America, North of Mexico: And A Catalogue of the More Recently Described Species, with Notes. No. 1-6. The University, 1916.

Diaz, James H. "Paragonimiasis acquired in the United States: native and nonnative species." Clinical microbiology reviews 26.3 (2013): 493-504.
https://explorer.natureserve.org/Search#q

Let none say again that Dwarf pond snails are grasping and ungracious: Galba truncatula and Pseudosuccinea columella (Lymnaeidae)

A key feature of the life-cycle of trematodes of veterinary and medical importance is the use of snails as the first intermediate host. Typically, one learns the names of the various snails and goes on one's merry way. I thought it would be an intellectually engaging exercise to learn a little bit more about these intermediate hosts par excellence.

Galba truncatula 
By Peter Glöer & Vladimir Pešić
[CC BY 3.0  (https://creativecommons.
org/licenses/by/3.0)],
via Wikimedia Commons

Pseudosuccinea columella
By Francisco Welter Schultes ,
Public Domain,
https://commons.wikimedia
.org/w/index.php?curid=14726645

The intermediate hosts for the famous ruminant trematodes Fasciola and various Paramphistomes is an air breathing freshwater pond snail belonging to the genera Galba (previously Lymnaea) in the Family Lymnaeidae. Galba truncatula, commonly known as the dwarf pond snail, is found in North and South America, Africa and Asia, and are notably absent in Australia. In the United States and Australia, Fasciola is transmitted by another snail called Pseudosuccinea columella. Another snail called Fossaria bulimoides can also host Fasciola. The vectorial capacity of Pseudosuccinea columella is marked because it can also be the first intermediate host of Heterobilharzia americana, the etiological agent of canine schistosomiasis in North America.  

Like so many other organisms, the taxonomy of members of the family Lymnaeidae is under dispute, since Lymaeid snails are known to exhibit diversity and plasticity in shell appearance. Only 8 species inhabiting the Neotropics are considered as intermediate hosts of Fasciola hepatica, as described in a 2011 paper published in the journal Infection, Genetics and Evolution by Correa et al. These are Galba cousini, G. cubensis, G. neotropica, G. truncatula, G. viatrix, Lymnaea diaphana, L. rupestris and Pseudosuccinea columella. It is also interesting to note that Lymnaea fuscus (Stagnicola fuscus) is partially resistant to infection with Fasciola hepatica while Stagnicola elodes is completely refractory.

Stagnicola fuscus which is partially
 resistant to infection by F. hepatica. 
Image attribution : Francisco Welter Schultes /
 Public domain via wikimedia.

Stagnicola elodes which is completely

 refractory to infection with F. hepatica. 

Image attribution: Naturalis Biodiversity Center / CC0 via wikimedia

In general, Galba spp. can be identified by their turriform yellow to brownish shell of less than 7 - 10 mm height, an oval aperture less than 0.5 times the height of the shell, short spire, 5-6 inflated stepped whorls (the spiral structures of mollusc shells) and straight columellar edge. There may be striations on the shell.

Pseudosuccinea columella, also known as the American ribbed fluke snail, has a thin translucent shell that is brown with fine striations. It has 3.5-4 weakly convex whorls that end in a point. The aperture is oval with a straight columellar margin that is reflected only in the upper part. These snails may have white spots and measure 1.5 - 2 cm.

However, given that morphological identification beyond genus level may be hard, the use of molecular techniques is recommended. In the same paper, Correa et al. characterized the 18S, ITS-1, ITS-2 and CoxI genes of the 8 Neotropical species for ease of identification in cases where morphological characteristics are confusing/inadequate to identify a given lymnaeid specimen to species level.

So the next time you see a snail in stagnant waters/ water-associated environs such as reeds/mud, you can try to see if it is Galba or Pseudosuccinea.

References:

Title reference: The title of this post is a play on the common name of the snail (Dwarf pond snail) and a reference from the Lord of the Rings. Galadriel says " Let none say again that Dwarves are grasping and ungracious" to the parting fellowship (Ref here).

Correa AC, Escobar JS, Noya O, Velásquez LE, González-Ramírez C, Hurtrez-Boussès S, Pointier JP. Morphological and molecular characterization of Neotropic Lymnaeidae (Gastropoda: Lymnaeoidea), vectors of fasciolosis. Infect Genet Evol. 2011 Dec;11(8):1978-88. 

Animal base: http://www.animalbase.uni-goettingen.de/zooweb/servlet/AnimalBase/search

On improving our contributions to science

While several thousand primary research articles are published every year, secondary literature such as reviews are used to collect large amounts of research data into coherent, manageable pieces. They provide excellent sources of information for novices or for the seasoned veteran. Review writing is not just an art but a rigorous scientific task that must be systematically undertaken. In an article titled “On the benefits of systematic reviews for wildlife parasitology”, published in the journal International Journal of Parasitology Parasites and Wildlife in 2016, authors Haddway and Watson actively encourage authors of review articles to avoid non-systematic reviews.

They start by pointing out that systematic reviews require significant investments of time and resources to ensure that reliable methods are used to aggregate literature and understand the subject under study to avoid biases. In contrast, non-systematic reviews may exhibit biases including biases in publication selections, lowering their intended reliability. The authors provide an ideal example of how to conduct a systematic review in their paper. 

The best part of the paper is perhaps table 1 that lists the different types of reviews that they identified in wildlife parasitology, an aggregative field that combines ecology and parasitology.

1. “Configurative narrative integrative reviews” typically analyze qualitative metrics with little attention paid to quality and ill-defined criteria for inclusion or exclusion in the review. These are useful introductory notes to an area of work.

2. “Aggregative scoping reviews” typically analyze literature by vote-counting or categorizing but pay little attention to quality and possess ill-defined criteria for inclusion or exclusion. These provide a precursory evaluation of the scope of research in the field.

3. “Aggregative full literature reviews” typically are exhaustive searches that analyze quality data, placing them in categories based on some metric. These add to research in a field by synthesizing evidence provided in empirical data into coherent arguments.

4. “Aggregative meta-analysis reviews” typically use well-defined quantitative methods following exhaustive literature searches to synthesize statistically important arguments from multiple sets of empirical data, overcoming biases that arise from homogenous populations.

5.“Aggregative systematic reviews” are high in detail with quantitative and qualitative appraisals of all data available in a given area including grey literature, with assessments of quality and liability.

The authors expand on “domains” that affect the quality of reviews and give some advice for avoiding landmines that I think are best implemented during the writing process. I have summarized them here, but I suggest that the original paper be read for context (Link).

1. Transparency: To ensure that science remains repeatable and verifiable, the authors suggest that information on search strings and databases be provided.

2. Comprehensiveness: To avoid publication selection biases given the power of reviews in driving decision-making, the authors suggest that multiple databases be used with attention paid to non-significant results in grey literature.

3. Vote-counting: The authors posit that categorizing data into positive, negative or non-significant bins, aka vote-counting, should be avoided so that magnitude and variability of data that arises from heterogeneity and effect size are captured.

4. Critical-appraisal: Studies included in an review should be assessed for their reliability to ensure that only good quality data is included. The authors provide examples for why accuracy, validity and biological relevance may vary for each study based on its experimental design (sample size, control matching and inherent genetic variability).

5. Evidence for effects: The authors caution fellow writers on the over-arching tendency to declare the lack of evidence of effect as the evidence of no effect. They recommend extreme care in the interpretative phase so that a lack of studies is not interpreted as a lack of effect. 

I found that the advice prescribed in the article to be broadly applicable to review article writing in the biological sciences. It is especially relevant to graduate students writing their literature review chapters for their dissertations, and having recently written my own recently, I have found ways to improve it before final submission. Let us continue to make efforts to contribute to reliable science!

Reference:

Haddaway, Neal R., and Maggie J. Watson. "On the benefits of systematic reviews for wildlife parasitology." International Journal for Parasitology: Parasites and Wildlife 5.2 (2016): 184-191.

A story of rotten apples and golden death!

What did a rotten apple in Paris, France and a rotten fig in Bangalore, India have in common?

They harbored common microscopic saprophytic worms that were host to the most remarkable worm-digesting bacteria.

 Biocontrol is the holy grail for environmentally conscious researchers. Biocontrol agents come in a wide variety of types, but the most common of them are predators, pathogens and parasitoids. Predators are of greatest use in pest control. Pathogens are very useful in weed control. Parasitoids have shown great success in lepidopteran insect control. 

Biological control has been endorsed by many as one of the ways to overcome resistance to anthelmintic drugs in livestock nematodes. Extensive research has been done with the microfungus Duddingtonia flagrans, which traps larvae in the pasture, and the bacterium Pasteuria penetrans, which has been used to control the root-knot nematodes of certain plants. The most recent event in this subfield of parasite control has been the isolation of a new bacteria called the golden death bacillus by a team of researchers based in Glasgow, Scotland. (Read the paper here).

Two isolates of the Golden death bacteria were isolated from worms found in a rotten apple obtained from Paris, France, and a rotten fig from Bangalore, India. The bacteria formed golden, mucoid colonies on agar and had a pungent odor. Microbiological testing and phylogenetic analysis of the whole genomes of the isolated bacteria revealed that they were very closely related but still distinct, and the name Chryseobacterium nematophagum was conferred on them, to describe both their color and their activity. 

The biological activity of Chryseobacterium nematophagum was experimentally studied in the model non-parasitic nematode C. elegans intially. C. elegans is a fantastic model for this study because it is a bacterivorous nematode that can be grown on bacterial lawns in the lab. Compared to control cultures of E. coli and a closely related isolate of Chryseobacterium gallinarum from a chicken, Chryseobacterium nematophagum was able to completely kill all L1 larvae placed on them within 7 hrs. As few as 200 bacteria were able to kill larvae in 24 hrs. C. elegans larvae were attracted to the bacteria in cultures, which is remarkable as they are repelled by other pathogens such as Serratia marcescens and Pseudomonas fluorescens. The larvae could not be killed by other related bacteria of the same family (Flavobacteriaceae). 

The authors observed that Chryseobacterium nematophagum multiplies in the pharynx of the C. elegans larvae and digested them from the inside out, which was demonstrated by the degradation of collagen and chitosan, but not beta-actin. The authors hypothesized that the effect of bacteria was due to chitinases and collagenases, and by comparative genomics of related bacteria were able to show that Chryseobacterium nematophagum indeed had copies of several nematode killing genes that encoded collagenases, chinases, cytolysins, hemolysins and others. Unique to Chryseobacterium nematophagum are slo- genes that code for a thiol activated cytolysins. Pertussis toxin S1 type secretion systems associated with motility and virulence were also found.

Chryseobacterium nematophagum was also able to kill bacterivorous, free-living larval stages of the important ruminant nematodes of the genera Haemonchus, Ostertagia, Cooperia and Trichoctrongylus, equine cyathostomins, and canine hookworm Ancylostoma among others. The bacteria, however, was not able to kill the potato nematode Globodera pallida or mosquito larvae.

Concerns on the safety of the bacteria to humans and animals have to be addressed before the bacteria can be deployed in full force in the field. We do not yet know if there are wild type Chryseobacterium nematophagum or similar bacteria already circulating in the pastures of the world, keeping nematode numbers in check. However, it is undeniable that the discovery of the golden death bacillus - Chryseobacterium nematophagum is a remarkable achievement.

References:  

Page, Antony P., et al. "The golden death bacillus Chryseobacterium nematophagum is a novel matrix digesting pathogen of nematodes." BMC biology 17.1 (2019): 10.

Stiling, Peter, and Tatiana Cornelissen. "What makes a successful biocontrol agent? A meta-analysis of biological control agent performance." Biological control 34.3 (2005): 236-246.