Cat With Flea Dermititis
Historically, morphological characters have been used to identify insect species. Often morphological differences between species are slight and require careful examination by trained experts. For example, the Cat Flea, Ctenocephalides felix,
and the Dog Flea, Ctenocephalides canis,
must be viewed with a high resolution microscope to confirm the species identification. Why is species identification important? Fleas can transmit pathogens to people and our companion animals. The ability to transmit a disease and the diseases transmitted vary among flea species. Knowing the flea species narrows the list of possible diseases.
Morphological characters have a molecular basis. The characters we can see are the products of genes and proteins. Structural differences are the products of gene or protein differences. Postive identification can just as well be based on molecular characters as morphological characters. A group of scientists* applied mass-spectrometry (MS) to fleas. MS of extracts from fleas with abdomens removed gave consistent profiles according to speices examined. Profiles of cat fleas and dog fleas are easily distinguished by this method. The MS method works best on fresh specimens and less well on fleas preserved in alcohol. Molecular analysis of flea species could potentially be used as an aid for diagnosing diseases that require a flea vector. Analysis and identification can be done quickly by a technician trained in standard lab techniques. This would be faster than shipping the fleas to a trained expert for identification.
*Amina Yssouf, Cristina Socolovschi, Hamza Leulmi, Tahar Kernif, Idir Bitam, Gilles Audoly, Lionel Almeras, Didier Raoult, & Philippe Parola. Identification of flea species using MALDI-TOF/MS. Comparative Immunology, Microbiology and Infectious Diseases, Volume 37, Issue 3, May 2014, Pages 153-157.
The bubonic plague is quiescent for long periods, then suddenly erupts as a major disease. One of the puzzles is why it does this. Part of the answer is development of resistance in host populations. The plague kills those least resistant. Those that survive carry genes that confer resistance to plague and those genes are present in high frequency among the decendents.
Another question, “How did the first plague originate?” Plague is caused by the bacteria, Yersinia pestis. Modern genetic analysis has determined that Yersinia pestis is closely related to another bacteria, Yersinia pseudotuberculosis Yersinia pseudotuberculosis is capable of colonizing the hindgut of a flea but not other areas of the digestive system. It can be transmitted from flea to flea by a fecal elimination/ oral ingestion route. Transmission to humans requires that Yersinia colonize the midgut. Yersinia pestis produces a phospholipase D which Yersinia pseudotuberculosis does not. This change allows Yersinia pestis to colonize the midgut. Efficent transmission to the vertebrate host requires that Yersinia produce a biofilm in the mid and fore guts that blocks entry of blood. Yersinia pestis has lost 3 genes that are present in Yersinia pseudotuberculosis. The genes prevent biofilm formation. Their loss allows Yersinia pestis to produce biofilm and block the gut of the flea. The flea feeds more often because it is starving but regurgitates blood and Yersinia pestis because its gut is blocked. The changes in 4 genes are the difference between an infectious agent of minor importance and an epidemic disease that kills millions.
Yi-Cheng Sun, Clayton O. Jarrett, Christopher F. Bosio, B. Joseph Hinnebusch. Retracing the Evolutionary Path that Led to Flea-Borne Transmission of Yersinia pestis. Cell Host & Microbe. Volume 15, Issue 5, 14 May 2014, Pages 578–586.
The Human Flea, Pulex irritans
The most studied of the bubonic plague vectors is the Oriental rat flea, Xenopsylla cheopis.
This flea is considered the primary vector of bubonic plague in urban areas. The island of Madagascar, the site of a worrisome number of plague cases
, is considered to have endemic plague. The plague was first detected on the island in 1898 after visits from rat infested ships that had sailed from plague infested areas.
Madagascar has also received another imported pest, the human flea, Pulex irritans. Pulex irritans is likely a native of South America where it infests guinea pigs. In Madagascar, it bites a variety of animals including cats, dogs, chickens and humans. A group of scientists* collected fleas from areas reporting plague including the house of a plague victim. They found plague bacteria, Yersinia pestis in 9 P. irritans individuals. No plague was detected in the other 4 other flea species including the rat flea and dog flea.
The human flea is not found on rats in Madagascar and is probably not responsible for plague transmission from the rodent population to humans. However, once humans are infected, the human flea could transmit plague from person to person. Control of human fleas may be important in stopping the spread of plague in some areas.
*Jocelyn Ratovonjato, Minoarisoa Rajerison, Soanandrasana Rahelinirina, and Sébastien Boyer. Yersinia pestis in Pulex irritans Fleas during Plague Outbreak, Madagascar. Emerg Infect Dis. Aug 2014; 20(8): 1414–1415.
Eye of an Ant
Image: Noah Fram
2014 Nikon Small World Honorable Mention
This image of an ant’s eye by Noah Fram won Honorable Mention at 2014 Small World. The visual units (ommatidia) are hexagonal, have a lens that is secreted by the underlying cells and contain multiple receptors that are tuned to different wavelengths of the color spectrum in most insects. In the image at left, the shape and size of the ommatidia are consistent. At the edges of eye, a narrow boundary region can be see between the ommatidia and the undifferentiated cuticle of the head.
Some ants in the Genus Caponotus (Ex: Carpenter Ant) have size variation among workers. Workers with larger heads have a greater number of ommatidia than those with smaller heads. Greater numbers of receptors can produce images with greater detail. All ants are social and create nests. Desert ants, active at night are known to use landmarks including the stars of the night sky to navigate when returning to their nests.
The bubonic plague
has been reduced to a low incidence level in most of the world. Our understanding of disease transmission and the importance of controlling both the rodent population and the flea vectors have dampened the large uncontrollable outbreaks that have had major affects on world history. The plague is still with us and a recent outbreak on the island of Madagascar demands attention. Since August, over 100 people have been diagnosed with plague
with 40 deaths. The plague has recently spread to the capital city, Antananarivo with a population of a quarter million. A serious risk of a plague outbreak in a densely populated area must be addressed. The World Health Organization and International Red Cross have stepped up efforts. Most worrisome is insecticide resistance in the flea population that makes flea control less effective. It is hoped that efforts to control rats and fleas can limit the number of cases. Underinvestment in health care facilities in poor nations leaves the populations at greater risk.
Caterpillar Dorsal Vessel (arrows) is visible through the cuticle
Insects have pumps to make body fluid flow between areas of the body. The largest of these fluid pumps is the Dorsal Vessel, or insect heart. The Dorsal Vessel has rhythmic contractions that pump fluid from the abdominal cavity into the head. Bioengineers desire to create microelectromechanical systems that can pump fluids to make their systems function. Engineers have explored biological material with rhymic contractions, especially heart muscle tissue. Compared to insect heart tissue, vertebrate heart tissue is more difficult to culture. Vertebrate muscle must be kept at optimum temperature and the culture medium frequently changed. Insect muscle is better adapted to a wide range of temperatures (insects are “cold blooded”) and the culture medium requires changing less often.
A team of Japanese researchers* has investigated the use of Dorsal Vessel tissue from the caterpillar, Ctenoplusia agnate. They can culture the cells of the Dorsal Vessel muscle. The cells maintain their contractile properties. The cells can be attached to a micro pillar substrate for ease of manipulation. Their preparation actively contrated for over 90 days at room temperature & they could control the contractions by electrical stimulation. They conclude that insect dorsal vessel muscle has good potential as a bioactuator.
*Yoshitake Akiyama, Kikuo Iwabuchi, Yuji Furukawa, and Keisume Morishima. 2008. Culture of Insect Heart Muscle Tissue and Its Applicability to Bio-Actuators. Mater. Res. Soc. Symp. Proc. Vol. 1096 © 2008 Materials Research Society
Drawing of Culex pipiens eggs Image: Image: Beament and Corbet
Mosquitoes in the genus Culex
lay eggs in groups that form rafts and float on the water. An ovipositing female mosquito will manipulate the eggs as they are laid such that the anterior end of the egg is facing downward. The anterior end contains the corolla, a structure that is wettable on the bottom and hydrophobic on top. Thus, the wettable portion of the corolla is below water level and anchors the egg. The hydrophobic portion resists wetting and floats due to the surface tension of the water. Individual eggs are top heavy and likely to tip. However, the female mosquito packages them in to egg rafts, clusters of several dozen eggs that cling together. The sides of the eggs contain tubercles, arm-like extensions of the egg that intercalate with each other. This holds the eggs in a cluster that has a stable base resistant to tipping.
Culex Mosquito making an egg raft Image: Sean McCaan