December 25 - Trypanosoma lewisi

On December 25, 1643, Captain William Mynors and his crew aboard the ship the Royal Mary, sailed past a small island in the Malaysian archipelago and dubbed it "Christmas Island." More than 300 kilometers away from the nearest other piece of dry land and uninhabitated by humans or their animals until the 1890's, many of the animals and plants found here were unique to this island. These species included two endemic species of rats, Rattus macleari and Rattus nativitatis. Despite the fact that the first settlers found them to be abundant, within a very short time, i.e. by 1908, the two species had gone extinct. Why? In the early 1900's, a tropical parasitologist had noticed several Rattus macleari individuals acting sickly and he speculated that they had been infected with trypanosomes. This was nothing but a hunch for almost exactly a century at which point molecular diagnostic techniques were brought into the picture. Scientists, including some of my colleagues at the American Museum of Natural History, took rats that had been collected from Christmas Island and deposited as specimens into natural history museums, extracted DNA from them and tested them for trypanosomes. Sure enough, many of the rats collected after humans arrived on the island showed evidence for infection with the parasite, Trypanosoma lewisi. The scientists also tested three rats collected prior to any settlements and none of those tested positive. Thus, it appears that fleas bearing T. lewisi hopped off the black rats (Rattus rattus) on the ship, bit the island's endemic rats and transmitted the parasite. The naïve hosts were likely killed by these parasites and went extinct.

You can read the whole paper here. Image is from this site.

October 6 - Hymenolepis nana

Hymenolepis nana, or the dwarf tapeworm (only 40 mm long!), is thought to be the most common human cestode on the planet. Unusual amongst tapeworms, H. nana does not require an intermediate host, but can be passed simply from one person (or rodent) to another via the ingestion of eggs that are shed in the feces, and auto-infection occurs in parts of the world where the worms are common. This direct life cycle is thought to be a recent adaptation in this species as the life cycle can be completed via an intermediate such as a flour beetle, much like its cousin, H. microstoma that you met way in the beginning of the year.

Image from the CDC Public Health Image Library.

August 14 - Fahrenholzia pinnata

Fahrenholzia pinnata is a species of louse that is found sucking blood on kangaroo or jumping mice in the western U.S. The name of this genus is in honor of Heinrich Fahrenholz, who was a German entomologist who studied lice. The pattern that the evolutionary trees of parasites tend to mirror those of their hosts is now called "Fahrenholz's Rule" (though was coined by Eichler, not by Fahrenholz himself). And, lice are almost always the textbook example of this rule. This is because lice spend almost their whole lives on their hosts and become adapted to clinging to their hair to avoid being groomed away. The only time they have an opportunity to transfer from one host to another is via direct contact - which, if you're a rodent, is likely to only occur between members of the same species. This phenomenon is especially true in gophers. See the post from July 16 for more information on gopher lice.

August 11 - Protospirura numidica

Protospirura numidica (Seurat 1914) can be found inhabiting the stomach or lower esophagus of the deer mouse, Peromyscus maniculatus as well as other small rodents that are commonly found in the Great Basin Desert in Utah including: Ord’s kangaroo rat, western harvest mouse, Great Basin pocket mouse, and the piñon mouse (Grundmann and Frandsen 1960). P. numidica has a complex lifecycle and requires passage through a beetle (Cook and Grundmann 1964). Interestingly, there is an increase in parasite prevalence in the fall in deer mice, likely due to the increase in insect consumption by the definitive host. A mean of 4 nematodes per host is common, however, I found up to 50 individual nematodes in a single deer mouse stomach. Talk about digestion issues…de-wormer please?

Contributed by Craig Gritzen.

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.

January 13 - Hymenolepis microstoma

Tapeworms are one of the main groups of parasitic flatworms, which also include the flukes (e.g. Schistosoma mansoni) and the 'monogeneans' (freshwater and marine fish parasites). Species of Hymenolepis, as shown here, are common parasites of rodents and use flour beetles as intermediate hosts. These species are thus easily maintained in the laboratory and have served as models for understanding tapeworm biology since the 1950s. Ongoing efforts to sequence the complete genome of this animal (see this link) will bring this classical model into the age of genomic research and will thus greatly increase the speed with which investigators can elucidate the genetic mechanisms that underly tapeworm biology.

Contributed by Pete Olson

January 8 - Giardia muris

A cousin to the parasites that cause the disease sometimes affectionately referred to as "beaver fever" in humans, is Giardia muris. Unlike the parasite that plagues people, this species cannot infect humans, but rather only mice and other rodents. It has thus been developed as a laboratory model for studying the parasite of humans. These parasites and their relatives, the diplomonads are also cool in that they have two nuclei.

Image from the CDC Public Health Image Library.