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
Global political power is often exercised through the use of military deployments to foreign lands. One limitation to the use of military force has been foreign diseases and susceptibility of troops to diseases. Large armies were deployed by global powers in the 19th century. Some of them met catastrophe in the form of tropical diseases such as yellow fever. Some army doctors turned their attention to diseases afflicting their troops. Army doctors were the leaders in developing the science of parasitology and vector biology.
Vector biology originates with the work of Patrick Manson, an English army surgeon. Manson studied filariasis, a disease caused by the nematode, Filaria sanguinis hominis. He discovered that humans could be infected with the worm through the bite of a mosquito & published his findings in 1898. His report stimulated subsequent research that identified mosquitoes as vectors of malaria and yellow fever. By 1904, Vector Biology had a firm scientific basis and strategies based on mosquito biology were being applied that would effectively eliminate Yellow Fever from the Panama Canal Zone.
Annually, over a quarter million people (mostly in the tropics) are estimated to contract yellow fever and about 30,000 die, a rate slightly over 10 percent. This rate does not reflect the experience in all populations. The United States suffered from sporadic yellow fever outbreaks until the early 1900s, when methods of mosquito control and sanitation finally succeeded in disrupting major epidemics. In these epidemics, it was noted anecdotally that far more people of European ancestry died of yellow fever that those of some African ancestry. There was some suggestion that American Blacks were immune to the disease. Recently Blake and Garcia-Blanco* conducted a retrospective study of 6 yellow fever epidemics in the US with adequately detailed records of disease and mortality. They investigated several factors which might be correlated to mortality rates, including genetics. Interestingly, they found no difference in yellow fever incidence in the Caucasian and Non-Caucasion populations. Overall, 58.6% of Caucasions contracting yellow fever died; 8.4% of Non-caucasians contracting yellow fever died in the same epidemics, a rate that is 6.8 times less. Blake and Garcia-Blanco ruled out several factors that are not affecting differences in mortality including: differences in virus strain, acquired immunity, demographic differences in age or sex, and socioeconomic, environmental and cultural factors. All these factors have compelling evidence to negate their influence. This leaves genetic factors related to ability to fight off infection that are present in the Non-Caucasian population but not the Caucasian populations. If genetic differences are the key, future studies could lead to better understanding of how the body fights these disease and perhaps lead to new therapies.
*Lauren E. Blake & Mariano A. Garcia-Blanco. 2014. Human Genetic Variation and Yellow Fever Mortality during 19th Century U.S. Epidemics. 3 June 2014 mBio vol. 5 no. 3 e01253-14.