May 31 - Rickettsia prowazekii

On Memorial Day, we honor the many soldiers who fought for our country. While bullets and other forms of arms are certainly the primary concern for soldiers at war, diseases that are spread under the conditions of warfare have taken their share of casualties as well. One of the most important ones, from the times of the Peloponnesian Wars in ancient Greece up through World War II, was epidemic typhus, caused by the bacterium, Rickettsia prowazekii. The bacteria are transmitted from person to person by the human body louse (Pediculus humanus humanus) and causes high fevers, rashes, headaches. muscle pains and delirium. Typhus has played a major role in history: the disease killed more of Napoleon's soldiers than the Russians did and then in World War I, over 3 million Russians died from typhus. The disease was rampant in Nazi concentration camps - both Anne Frank and her sister died of it.

The photo, from Wikipedia, shows two soldiers demonstrating a DDT gun, which was used to kill lice.

May 30 - Ascaris suum

Although most commonly found infecting pigs, Ascaris suum, a large roundworm nematode, can also infect humans. This species has the same lifecycle as Ascaris lumbricoides, and also like its cousin, has eggs that are extremely durable in the environment. Although they were long popular in biology classes as a large, easy-to-dissect representative of Nematoda, because the eggs are so resistant and easily spread, the represent a health risk and their use has largely been discontinued. You can virtually dissect one here.

May 29 - Protogynella sp.

While it is far from a universal rule that the largest worms are found in the largest hosts, it is true that the longest tapeworm known is from a whale. And this may be the candidate for the smallest tapeworm- Protogynella sp. from a shrew, Sorex vagrans. It is only 250 microns (a quarter of a millimeter) long. Shrews are among the smallest mammals (some weigh less than 5 grams) and most of their parasites (trematodes, cestodes, nematodes) are correspondingly tiny. The darker staining structures are the eggs, which probably infect an arthropod intermediate host since shrews are primarily insectivorous.

Contributed by Mike Kinsella.

May 28 - Eutrombicula alfreddugesi

If you’ve ever had chiggers, you know they’re really, really irritating. These are the parasitic larval stages of free-living mites, and Eutrombicula (Trombicula) alfreddugesi is the most familiar in North America. Chiggers perch on foliage, climb aboard passing pedestrians, and find their way to the new host’s skin. Once there, the tiny mite positions itself atop a hair follicle or pore, and secretes highly digestive saliva that liquefies the host’s skin cells. The surrounding cells harden in defense, forming a stylostome. But it’s a poor defense – the stylostome helps the parasite by functioning as a straw for the chigger to slurp his slurry of dead cells. Moreover, the stylostome contributes to that really, really irritating inflammatory (i.e., itchy) response in the host’s skin. Thankfully, E. alfreddugesi is not a vector for disease, and the discomfort subsides after a few days.

Eutrombicula alfreddugesi isn’t picky, and infects numerable hosts besides humans. It’s a parasite of many vertebrates, including birds, mammals, and reptiles (the photo shows several chiggers on the dewlap, or throat fan, of an anole lizard). In fact, host habitat is probably more important that host taxonomy for this species. Chigger infestations usually occur in warm, shady, and moist environments, as this is best for the soil-dwelling and detritus-eating adult stages (meaning that this is where most eggs are laid). Similarly, these chiggers prefer certain areas on the host, from the undergarment areas of humans, to the “mite pockets” of some lizards. Mite pockets are small, but relatively deep, cavitations that are typically located just behind the lizard’s forelimbs. When these lizards have mites, most are in the pockets, leading some scientists to speculate (and others to dispute) that a pocketful of mites is advantageous for the lizard. The chiggers are bright red, and on a drab lizard, they present a flashy spot of color that can be turned on and off as the lizard moves its legs.

Contributed by Bryan Falk.

May 27 - Pinnotheres pisum

Some crabs, like hermit crabs, live in the discarded shells of mollusks, but some, like the pea crab Pinnotheres pisum, just can't wait for the resident to move out first and move in as a roommate -- but the kind of bad roommate that steals your food and damages your house. P. pisum, which is at most about half and inch wide, lives in the mantle cavities of bivalves such as mussels and clams where it picks the food off their gills and can severely damage them in the process (kind of like you might imagine if something with 10 legs lived in your fridge full-time.) I chose this photo from wikipedia (click on it for a blown-up view), which shows a pea crab that has fallen out of the clam that an otter is eating. Better go find another landlord, little crab!

May 26 - Balantidium coli

It is thought that Balantidium coli is the only ciliate (think Paramecium) that is parasitic in people (causing the disease Balantidiasis), however in humans, these protozoans are zoonotic, with pigs as the primary reservoirs. To our porcine friends, the ciliates appear to not cause symptoms, but humans can suffer severe GI distress if they become infected by ingesting contaminated water or food. This parasite has been found worldwide, but is not a common pathogen of humans, except in the Philippines. Recent work identified the same ciliate in several other kinds of mammals including other primates and rodents, so a better epidemiological understanding is clearly in order.

May 25 - Taenia taniaeformis

This photo may exceed the “yuck factor” for some. Taenia taniaeformis is the common tapeworm of house cats and is also found in other species of felids. The eggs pass out in the feces and are ingested by various species of rodents, where they develop into cysts, usually in the liver. Pictured is a cotton rat, a very common rodent in the southeastern United States. The dissected liver has some large white cysts. If a cyst is opened, it contains what looks like an adult tapeworm with a fluid-filled bladder at the end. This stage is called a strobilocercus. When the rodent is ingested by a cat, only the head of the tapeworm survives, which then develops into an adult. Because of this type of life cycle, a house cat with tapeworms poses little danger to its owners. Thanks to Dr. Christine Miller of the Miami Metrozoo for providing this (yucky) photo.

Contributed by Mike Kinsella.

May 24 - Anisakis simplex

If you suddenly experience a sharp abdominal pain and have recently eaten sushi, ceviche, or pickled herring, there's a chance you might have just become infected with Anisakis simplex. This species is a nematode that primarily uses marine mammals as its hosts. Eggs are excreted in feces, where they will then infect small crustaceans. There they mature into what are called L3 stage larvae. When the crustacean is eaten by a squid or a small fish, they will migrate to the muscle tissue and wait there for a mammal to eat that fish. If another fish eats it, they just repeat the process, and wait in the muscles of the second fish. When a whale, seal or dolphin eats the fish, the larvae mature into adults, mate, and lay eggs to begin the cycle all over again. Because a human body is - at least to these nematodes - about the same as a seal's, the worms can infect a person who eats an infected fish or squid. The result is pain, nausea, and/or vomiting, but as we humans are dead-end hosts, the treatment is usually just to relieve those symptoms and wait for the adult worms to die and pass. However, if individuals are sensitive to Immunoglobin E, however, ingestion of Anisakis can cause anaphylactic shock. Luckily these worms are fairly rare in the U.S., but cases do occur more frequently in Scandanavia, Japan, and western South America.

The photo is of L3 larvae in a herring.

May 23 - Proteocephalus pinguis

Proteocephalus pinguis parasitizes pike (how about them P's??). These are tapeworms (cestodes) that alternate between their fish hosts and copepod intermediate hosts. They have extremely simple scoleces and are small even as adults. The genus contains many other species that have similar life cycles, alternating between vertebrates and copepods. If the first vertebrate host gets gobbled up by a bigger one, then the tapeworms simply infect this host instead.

The photo is from this website.

May 22 - Placobdella parasitica

Today's parasite, the leech Placobdella parasitica is an excellent parent. Not only can it perform both motherly and fatherly duties simultaneously (it's a hermaphrodite), it also takes good care of its young. These leeches spend most of their lives on turtles, particularly snapping turtles, where they will feed on the blood primarily around the leg pits and tail (where the turtle's formidable jaws can't reach them.) They brood their young on their ventral surface protecting them there until they are big enough to feed on their own, which sometimes means transporting them to their first meal. Despite the caring nature of the leeches post reproduction, the conception of those young is actually quite violent. Leeches will stab small spermatophores into each other in a process known as traumatic insemination. Yipes.

Photo by Mark Siddall.