Schistosomes: beware the water

Over 700 million people worldwide are at risk from parasitic worms called schistosomes (blood flukes), which are transmitted via swimming in contaminated water. The worms are a major problem and can cause a severe debilitating disease called schistosomiasis in their hosts, which, sadly, kills nearly 200 million people each year.

This is an adult male blood fluke. Note its highly developed mouth piece on the right, which is uses to 'clamp' onto the inside of blood vessels and anchor itself against the blood flow. Once secured, it feeds on the nutrients carried in the host's blood. The fold that you can see running through the centre of the worm is called the gynacophoric canal, in which the much smaller female worm lies in permanent copulation.

Due to the large distribution of the schistosomes and their prevalence in third world countries that have very little infrastructure, they are of huge concern to the World Health Organisation, who is trying to reduce the levels of infection by educating people in endemic areas of the dangers, improving sanitation and by ensuring that the drug praziquantel is readily available to as many patients as possible.

Unfortunately, praziquantel treatment is extremely unpleasant and results in a range of negative side effects in humans, including: dizziness, somnolence, seizures, arachnoiditis and diarrhoea. To make matters worse, the parasites themselves actually worsen these side effects and patients undergoing treatment can also suffer from urticaria, rashes and pruritus. This is due to the sheer volume of dead parasitic material that is released into the blood at any one time, which can also cause eosinophilia in leukocytes.

Schistosomes are waterborne and infect human hosts when they swim in contaminated water. As the host swims, they leave a trail of fatty acids from their skin in the water behind them. The parasite is attracted to these fatty acids (L-arginine inparticular) and then attaches itself to the host's skin. Once attached, schistosomes secrete acids and enzymes from acetabular glands and literally burn through the skin until they enter the bloodstream! The distinctive and unpleasant rash that results is often called 'swimmers itch'.

Due to the problems associated with praziquantel, many health agencies are beginning to acknowledge that the best way to combat schistosomiasis is to prevent human infection in the first place. The easiest way to do this is to treat infected waters with insecticides that kill water snails – another vector that is essential to the schistosome life cycle. Without water snails, schistosomes will be completely unable to infect humans and the problem of schistosomiasis is thereby solved!

Like most parasites, schistosomes have an extremely complicated life cycle where they morph through a range of different forms and, at different stages, live in more than one host.

One of the reasons schistosomes are such a major problem is because males and females live in permanent copulation with each other inside the human body so can literally release thousands of eggs in a very short space of time. Rather than the parasites themselves, it is actually these eggs that cause the symptoms of schistosomiasis – a chronic, severely debilitating disease that is usually fatal. The exact symptoms of schistosomiasis vary (depending on the species that has infected the host and where the worms prefer to take up residence), but the most common symptoms are given below:

• Intestinal schistosomiasis is caused by the bowel-living species of blood flukes Schistoma mansoni and S. japonicum, and is associated with abdominal pain, diarrhoea and bloody faeces. Liver enlargement is frequently seen in advanced cases, which leads to gross abdominal swelling as fluid is forced into the host's peritoneal cavity as a side effect of abnormally high blood pressure
• Urogenital schistosomiasis is caused by S. haematobium, which lives in the bladder, and is commonly characterised by haematuria (blood in the urine); lesioning and scarring of bladder tissue; ureter and kidney damage; genital lesions that lead to pain during sexual intercourse; and, in advanced cases, can lead to infertility and a variety of cancers

Liver enlargement is one of the most debilitating symptoms of schistosomiasis and is fatal in almost all cases. This is due to the large-scale, irreversible damage it causes to the host's body.

What makes schistosome eggs so problematic is the large spike that protrudes from their surface. This spike is designed to slice through the walls of blood vessels so the eggs can pass into their hosts urine or faeces (and thereby be passed back into water where they hatch and infect more snails), but can become caught in the host’s tissues upon occasional. Once an egg has become lodged, the host’s immune system identifies it as 'foreign material' and a type of leukocyte called a macrophage activates in response. Macrophages quickly form a granuloma around the egg, which is essentially just a tough wall of inert proteins that prevents the egg from interacting with the body’s tissues. Ordinarily, granulomas are beneficial and are eventually removed from the body, but this cannot happen with the trapped egg. Due to this long-term presence, the surrounding tissue begins to inflame and no longer functions correctly. This problem is exacerbated as an ever increasing amount of eggs build up in the organ and are themselves enclosed by granulomas! Eventually, the organ becomes so blocked and damaged that it is unable to function correctly - leading to the symptoms of schistosomiasis!

Scientists are currently working on a waterproof cream that mask the fatty acids on our skin when we swim (which will help prevent schistosomes from invading our body), but this research is still in its early stages and doesn't look likely to have a practical application anytime soon. For now, you should simply avoid swimming in any water where schistosomiasis is prevalent (no matter how inviting it may look). If you are travelling aboard, such as to Africa (where schistosomiasis is a particular problem), I strongly recommend that you consult your doctor and travel professionals to familiarise yourself with the risks of the area before you head out.

The Land Massive

Life is hard for wild animals, being fraught with the perils of predation, starvation and sickness. Only the strongest manage to survive into adulthood and breed, which means that only the best genes are passed into the next generation. This is the major driving force behind evolution and is responsible for the staggering diversity of life on our little planet.

One of the most obvious adaptations that some species have is their tendency to grow to enormous sizes. This is typically a defense against predators and takes advantage of the 'ten times larger' rule. Simply put, this rule states that a terrestrial animal becomes safe from predation if it can grow ten times heavier than their largest predator, since their predators are unable to develop jaws strong enough to kill them.

Obviously, this rule is not absolute. It doesn't take mankind in account, as our technology and sentience allowed us to overcome it as cavemen, nor does it take into account predators growing larger themselves. If all of the prey species in an area begin to grow larger, this will itself drive a size increase in predators. Bigger prey means more meat per kill, which is able to fuel more muscle mass and only the largest predators will be able to make kills (so only the genes for a bigger body size will be passed down into subsequent generations).

This means that predators and prey are locked in a constant arms race where each is trying to become bigger. Over time, the size of each species will grow bigger until size becomes limited by another factor - such as the availability of food - and is therefore no longer advantageous (where upon it's selected against and a 'maximum' size of the species has been reached).

Animals have reached unbelievable sizes in the past, both in the oceans and on land. Dinosaurs are the obvious example of megafauna, with some species becoming so heavy that the ground used to tremble as they walked! Sadly, there aren't animals this impressive anymore (certainly not on land, anyway), but we do have some truly huge beasts roaming our continents. Here's a list of the top ten largest animals that are alive today (sorted by average weight):

10: Wild (Asian) water buffalo: 770kg/3.5m long (max weight: 1, 250kg)

Also called the Asian or Asiatic buffalo, the wild water buffalo, Bubalus arnee,  is endemic to Southeast Asia. As well as its obvious bulk, the bovid is famed for having the widest horn-span in its family, which can be as wide as 2m from tip-to-tip in large males.

Wild water buffaloes are gregarious and live in tight, matriarchal groups as a defense against predators. Each group is called a 'clan' and is made up from related females and their young offspring. A clan can be as large as 30 individuals and often band together with other clans to form large herds (for further defense) that can consist of more than 500 individuals!

To avoid inbreeding, young males actually leave their clan once they become sexually mature and form small 'bachelor' groups. These groups consist of about 10 males and generally spend the dry season away from females. In breeding season, the young males rejoin a herd where they are polygynous and mate with multiple females.

9: Black rhino: 1, 150kg/3.5m long (max weight: 1, 900kg)

Black rhinos have two horns on their snout, which are made from keratin - the same protein as hair and nails. Their horns can grow as much as 3 inches a year and individuals have been found with horns that are longer than five feet! Sadly, their horns have been their downfall as it is coveted for Asian herbal medicines and ornaments in the Middle East, meaning that the black rhino is now classified as 'Endangered' by the IUCN.

There are four species of black rhinos (of the genus Diceros), which are actually grey. Their skin looks darker because they frequently wallow in mud to help keep cool through the hottest parts of the African day. Their size and rotund shape means that they actually retain body heat, so black rhinos are usually inactive during the hottest hours and rest beneath shade if the cannot find shallow water. During gloaming, when it's cooler, the rhinos feed by browsing shrubs and the lower branches of trees.

8: Walrus: 1, 200kg/3.4m long (max weight: 2, 150kg)

Odobenus rosmarus - the Latin taxonomic name for the walrus - literally means "tooth-walking sea-horse". This name stems from the tendency of walrus' to drag themselves onto pack ice using their tusks for purchase. It should be noted, however, that walrus tusks are actually elongated canines. These teeth are so large that they can grow as long as 1 meter in big males!

Walrus' are found throughout the Northern Hemisphere in the Arctic Circle and some subarctic regions. Walrus' once covered most of the frigid north, but their numbers were decimated by hunting in the 19th Century and their populations are now discontinuous; being limited to certain regions. That being said, they can still be found through most areas of shallow water, where they like to dive and crawl across the bottom of the sea bed to feed off crustaceans, molluscs, amphipods and slow-moving fish.

7: Giraffe: 1, 400kg/6m tall (max weight: 2, 150kg)

There are nine distinct species of giraffe, which are classified under the genus Giraffa. Growing to heights of 5 - 6m, giraffes are the tallest living animal and have unique patterns of spots in their fur. Their spotting is very similar to human fingerprints and can used to identify an individual. Interestingly, their spots also reveal the relative age of a giraffe as they grow darker as throughout their lives.

Giraffes are herbivores and use their long necks to reach leaves that are high up in the canopies of trees. This allows them to access food that many animals are unable to get at, thus allowing them to exploit a very specific niche. Despite this being an obvious use of their long neck, many scientists are skeptical that this is why it first evolved. Their neck is simply too exaggerated for access to slightly more food to have driven its appearance and many scientists now believe that the length of their neck was initially a sexually selected trait.

During mating season, male giraffes fight for the right to mate with females. They have an interesting way of dueling called 'clubbing', where they literally use their heads as bludgeons to strike their opponent with. As explained by the mechanisms of physics, a giraffe's head will hit harder the longer its neck is. Thus, having a longer neck would have given a giraffe an advantage in battle, meaning that it is more likely to win and pass on the 'long neck' gene so becomes selected for. Being able to reach an abundance of previously unattainable food would have then acted as a secondary use of their long necks and would have helped to reinforce its selection.

6: Gaur: 1, 600kg/4m long

Gaur, Bos gaurus, are characterised by a very prominent dorsal hump and have a long dewlap that hangs from their chin. Other than mankind and tigers, their large size means that they are rarely preyed upon and their lives are relatively safe for a herbivore.

Gaur are typically diurnal, gregarious animals where the females live matriarchal herds with their close relatives and offspring. To avoid inbreeding, a young male leaves its herd once it has sexually matured and spends much of the year in solitude. During the breeding season, polygynous males rejoin herds where they compete for mates by showcasing their size and emitting loud calls. 

Interestingly, scientists have noticed that many gaur become nocturnal in areas with high levels of human activity. But whatever hours gaur keep, their behaviour remains the same and they seem to actively avoid water, going to sources only sparingly to drink. They rarely wash or wallow and spend much of their time browsing on a wide variety of shrubs, flowers and grasses. 

5: Hippopotamus: 2, 500kg/1.2m long (max weight: 3, 400kg)

The hide of the hippopotamus, Hippopotamus amphibius, is so thick it can make as much as a half a ton in large males! Interestingly for an animal that lives in an extremely hot environment, hippos have neither sweat nor sebaceous glands. Instead, they have unique glands that secrete a viscous red fluid. This has given rise to the urban myth that hippos 'sweat blood'.

Due to their weight, which makes them cumbersome and ungainly on land, hippos spend much of their time in water where their body weight is decreased. Hippos mainly venture onto land to feed at night, where they are able to climb even very steep banks in their search for grass. Most hippo attacks on humans happen during the night, when a person accidentally walks between the hippo and water. The hippo panics and then charges, bulling over the person as it tries to get back to safety.

Hippos are actually the most dangerous animal to humans in Africa and should be avoided if ever seen in the wild. They are extremely territorial and (in addition to what was mentioned above) may attack people on shores and knock us from boats if we venture into their 'space of water'. With hugely powerful jaws that can open almost 180 degrees, hippos are reportedly able to bite animals as big as Nile crocodiles clean in half with a single bite!

4: White rhino: 2, 350kg/3.8m long (max weight: 3, 850kg)

The white rhino, Ceratotherium simum, spends large amounts of its day wallowing in the wet mud around rivers, streams and lakes. The mud acts as a natural 'sun cream' and helps to deter parasitic insects.

Unlike their cousins, black rhinos, white rhinos are grazers and have differently shaped mouths that are adapted to trimming grass off the ground. They have poor eyesight, but have excellent hearing and even better smell. Due to this, they often walk in single file so they can follow the scent trail of the rhino ahead of them.

3: Southern elephant Seal: 3, 000kg/5m long (max weight: 4, 000kg)

There are actually two types of elephant seal, but it is the southern elephant seal, Mirounga leonina, that grows the largest. Surprisingly, their name does not stem from their enormous size, but rather from their inflatable trunk-like snouts!

Living in the brutal Antarctic waters, Southern elephant seals are superb divers and can go as deep as 1.5 miles, staying submerged for up to 2 hours at a time. While underwater, southern elephant seals hunt fish and squid, which form the staples of their diet. Both male and female seals spend months at sea at a time, where they migrate vast distances in search of food.

Elephant seals return to rookeries during their breeding season, where males compete with each in other in brutally violent displays for the right to mate with females. Successful males can form harems that consist of 40 - 50 females, whom they mate with exclusively!

2: Asian elephant: 4, 200kg/6m long (max weight: 5, 200kg)

Like their African cousins, the Asian elephant, Elephas maximus, has an extremely long gestation period and their pregnancies last for 22 months! Although this isn't surprising really, considering newborn calves can weight as much as 90kg!

Asian elephants have been domesticated by humans over thousands of years for a variety of tasks. Most often, the elephants are used to carry or move heavy objects and as taxis, where they carry people around on saddles or carriages called howdahs. Less commonly, but perhaps more famously, the elephants have been used for combat on occasion by warlords such as Hannibal, who took the elephants over the Pyrenees and the Alps in his campaign against the Roman Empire!

1: African  elephant: 8, 500kg/6.7m long (max weight: 13, 000kg)

Although African elephants are noticeably bigger than their Asian cousins, you can also use the shape of their ears to tell them apart. Rather bizarrely, the shape of an African elephant's ear looks similar to the continent of Africa, while the shape of an Indian elephant's ear looks a bit like India! 

There are actually two types of African elephant of the genus Loxodonta: the African bush elephant and the slightly smaller African forest elephant. Both are larger than the Asian elephant and can roam huge distances in search of food. The elephants eat roots, grasses, fruit and bark, which they attain using their deceptively dextile trunk and their tusks, which they use for scraping trees and digging.

Unfortunately, as with Asian elephants, their trunks have also proved to be their downfall and they were almost hunted almost to extinction by those in the ivory trade. Since the international trading ban on ivory that was placed in 1990, elephant numbers have recovered somewhat (although illegal poaching is still a problem). It's estimated that there are now as few as 700, 000 elephants left in Africa and they are officially classified as 'threatened' by the IUCN. Sadly, less than 20% of the elephants' known range is under formal protection (which is largely due to budget constraints and the instability of many Africa governments).