Ixodes holocyclus

There are 14000 known living species of blood-sucking animals, but while drinking blood has become a staple in many different lineages of animals,  some of nature's vampire can be quite picky about which animals they feed on. Even for those that drink from a variety of different animals, they might have preference for certain bouquets of blood over others.

Left: Female (top) and Male (bottom) Ixodes holocyclus, Right: Engorged female after feeding
Photos by Alan R Walker from here and here

Ixodes holocyclus is a species of hard tick native to Australia. It can infect a wide range of different animals including various Australian native marsupials, bird and reptiles. But over the last two hundred years, many other species of mammals have been introduced to the Australian continent, and I. holocyclus has eagerly taken to those new hosts as well. But while I. holocyclus is capable of drinking from both Australian native marsupials and the more recently introduced placental mammals, that does not mean that they are equivalents from the tick's perspective.

A group of researchers in Sydney conducted a study to look at the distribution of I. holocyclus on native and introduced mammals, in particular the long-nosed bandicoots and introduced black rats from areas around the Northern Beaches of Sydney, Australia. They captured these small mammals with cage traps, then briefly inspected them for ticks before letting them go free.

They found that on average, bandicoots had about three to four times as many I. holocyclus as rats, but most of those ticks were found on an unlucky few that were each infected with over 30 ticks. The ticks also distributed themselves different on the bodies of those animals. On the bandicoots, I. holocyclus spread themselves out pretty evenly across the host's body, clinging to the bandicoot's head, legs, belly, flanks, and there were even a few around the genital region. But on the rat they mostly hung around the head and neck region of the animal.

So even though I. holocyclus would happily drink blood from both bandicoots and rats, it seems they would much prefer a bandicoot. Compared with bandicoots which have co-evolved with I. holocyclus for a long time, rats are relatively recent interlopers. So while the ticks can infect them, rats are just not comparable to the native marsupials that they are more used to.

Ticks have specialised mouthparts for clinging to and feeding from their host, and even though I. holocyclus is a generalist that can drink blood from many different animals, its mouth part might not work equally well on them all. So whereas they can comfortably access all areas on the bandicoot, on a rat they stick to the sweet spot around the head to get their fill of blood.

This has important consequences when it comes to quantifying parasite abundance in a given environment. For example, if you are trying to find out about tick abundance in a given region, you might get vastly different results depending on which animals you decide to examine. Parasites are not evenly distributed across the landscape, across hosts, or even across different hosts' bodies. For a tick like I. holocyclus the host's body is an entire landscape in itself, and when in unfamiliar territory, it is better to stick to a well-trodden path.

Reference:
Lydecker, H. W., Etheridge, B., Price, C., Banks, P. B., & Hochuli, D. F. (2019). Landscapes within landscapes: A parasite utilizes different ecological niches on the host landscapes of two host species. Acta Tropica 193: 60-65

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.

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 3 - Xenopsylla cheopis

As the quote on the top of the pages says, big fleas have little fleas. Well, the Oriental rat flea, Xenopsylla cheopis Rothschild 1903 (Arthropoda: Siphonaptera) isn't exactly crawling with other fleas - but it is often an important vector for other parasites that infect it. Not only does this jumping insect parasitize rats and other mammals by feeding on their blood, it can also be infected by Yersinia pestis, the bacteria that cause the bubonic plague and serve as the intermediate host for certain tapeworms. Doesn't that make you itch just thinking about it?