I have always been impressed by the ingenuity of parasites. A parasite depends upon another living organism on its survival. Sometimes a parasite can live inside the host for many years without too many ill effects. Other parasites might change the behaviour of the host to enable the parasite to complete its life cycle. For many parasites the life cycle involves more than one host and perhaps different forms of the parasite itself.
The liver fluke, Fasciola hepatica, is a classic example of a life cycle that contains two hosts. The adult fluke resides in the liver of a mammal, usually a sheep or cow. The fluke lays eggs which enter the animals digestive system, presumably via the bile duct, and are then egested onto the ground with the faeces. A embryonic form of the fluke called a miricidium then invades a snail and grows and develops eventually being released in a form called cercariae that then attach to aquatic vegetation waiting to be eaten by another sheep or cow. Humans can become infected by liver fluke by eating plants such as watercress containing the cercariae developmental stage.
Liver flukes can live inside humans for up to 30 years. Symptoms include vomiting, diarrhoea, jaundice, enlarged liver, liver stones, abdominal pain and the possibility of internal infections. Many people live with these symptoms and by the time they seek medical advice the mature flukes have done lasting damage.
Of course parasites don't have to be animals. At this time of year we might try and sneak a kiss under the mistletoe. Mistletoe itself is a parasite, growing in the branches of a tree absorbing nutrients directly from the host plant. They are spread form tree to tree via birds carrying their sticky seeds.
For all you cat lovers out there a more common and potentially serious parasite is very commonly found and may already be in you. Cats frequently carry a single celled parasite called Toxoplasma gondii. In cats the parasite rarely show any symptoms although infection has been linked to lethargy, fever and loss of appetite. However in humans infection can lead to flu like symptoms, and potentially cause damage to eyes, brain and other organs. Infection during pregnancy can lead to miscarriage or congenital defects.
The life cycle of Toxoplasma gondii has two distinct parts. The sexual stage occurs in the primary host, the cat, whereas the second asexual phase can occur in warm blooded animals including mice, birds or humans. The sexual stage lives inside the small intestine and releases 'eggs' called oocysts. These then are shed with the faeces. If these are ingested by another animal a series of asexual divisions occurs resulting in a number of cysts that will then be released in the faeces, with the hope of reaching another cat.
As humans live closely with cats and many have litter trays in their houses the chances of a human becoming the secondary host is high. According to an article in the Independent up to 40% of the British population have been infected, 1000 a day or 350,000 a year!!
But here is the interesting bit, studies tentatively suggest that infection might cause changes in the brain leading to behavioural changes. In fact there is a suggestion that in infected rodents the behaviour change increases the chances of being caught by a cat making the parasites complex life cycle more likely. The Independent article also details a study by scientists that suggesting the parasite can stimulate the production of a neurotransmitter called GABA. One of GABAs effects on the body is the inhibition of fear and anxiety. Infected rodents don't fear cats.
Perhaps humans infected with this parasite are more likely to take a risk. Watch out for cat owners they may be unpredictable.