Parasites: Medical, Forensic, and Antisocial

Summer is the time to take a few hours on a hot sunny day, pack a few drinks and snacks, and head to the beach. Its also the time when you may arrive at your favorite lake to find a sign posted at the water’s edge: “Lake closed to swimming due to swimmer’s itch.”

Two things come together to create this disappointing scenario: water birds and warm water. Migrating water fowl fly in every year, infected with schistosomes – tiny parasitic worms that live in the bloodstream. The eggs of the schistosomes are added to the lake water in the bird’s feces, and these eggs quickly hatch. Emerging larvae, called miracidia, infect snails that are living in the lake, and multiply. Then, when more birds land on the water, or when an unsuspecting human wades in for a cooling swim, the parasites emerge from the snails and plunge into the skin of the swimmer.

Humans have their own species of schistosomes, but the bird species don’t do well in human bodies. They dive into the skin and stop, creating a localized area of inflammation. They don’t survive long, and the body mops up the remains, but not before the immune process causes a nasty itchy rash that is awful enough to keep people out of the water if schistosomes are about. It’s a hard way to do without a favourite summer activity. If you’re lucky enough to live near the sea, you could try the salt water – it’s colder, but safer.

You may think that the transmission of Lyme disease is very simple – you get it from a deer tick bite. That information is essentially true, and if you just want to avoid Lyme disease, that’s all you really need to know. What goes on in the woods, however, is interesting, and it helps to explain why Lyme disease is spreading and becoming more common.

Lyme transmission starts with the White-footed mouse, a common forest creature that carries Borrelia burgdorferi, the organism responsible for Lyme disease. In the most common scenario, newly hatched larval Ixodes sp. ticks feed on the mice and become infected with the organism. The ticks then develop through a nymphal stage to an adult tick, feeding on other hosts as they get older.

At both the nymph and adult stage, ticks are found clinging to the tall grasses in open forests and forest edges, the same type of environment favored by White-tailed Deer. When a deer wanders by, the ticks climb on and take another blood meal. If it’s a human that wanders by, or a dog, a raccoon, or even a bird, the tick is not fussy – deer ticks will readily feed on all of these hosts as well as other warm blooded animals.

Borrelia burgdorferi gets passed on to the new host as the tick feeds. Many animals aren’t affected by the organism and their immune systems simply kill it. White-tailed deer and some bird species are among the lucky animals that don’t get Lyme disease. Humans, however, can suffer a long and debilitating illness.

Deer are responsible for supporting large numbers of ticks; one deer can have hundreds of ticks feeding on it at any one time. Anyone who has driven the roads of North Eastern North America at dusk and dawn in the summer months knows that the White-tailed deer population is booming like never before. Deer graze by the roadsides and wander into cities and gardens. Lots of deer means lots of ticks, and deer in human communities means Lyme transmission to humans.

Deer don’t spread Lyme disease over large geographical distances however – birds do that. When a deer tick bites a bird, two things can happen: if the bird is already infected, having been bitten by an infected tick somewhere else, the new tick becomes infected and can spread the organism to other hosts. If the tick is already infected and the bird is not, the organism gets passed the other way, and because ticks take days to finish their meal, both tick and bird can be many miles away from their starting point before the tick drops off. Both can now continue to Lyme transmission. Migrating birds are thought to have spread B. burgdorferi to many new locales.

A final factor to consider is global warming. Warmer climates have opened up new territory for deer ticks, and they are now surviving and reproducing in areas where they were previously unable to withstand cold winter temperatures. Thus, ticks carried in on migrating birds are more and more likely to survive the trip.

It is interesting to ponder how humans have contributed to Lyme transmission. The global warming factor is a well known one, but the impact that people have had on eastern forests is less well understood. Since Europeans first colonized North America, vast areas of old growth forests have been removed and replaced by young forests characterized by brushy undergrowth. While ancient forests were open and large predators, such as wolves, kept deer populations low, the new growth contains virtually no predators and deer love the low brushy plant growth. The ecological changes brought about by deforestation have thus created ideal conditions for a high number of deer and deer ticks, and the appearance of Lyme disease.

Hookworm disease exists in tropical and subtropical areas of the world where there is sandy soil that contains lots of organic material, and rainfall adequate to keep the soil from drying out too much. Infective larvae are very susceptible to drying and to extremes of temperature; therefore, a person who has not lived in or traveled to such a region is very unlikely to have hookworms. It is important to remember, however, that hookworm infection can be transmitted locally in other areas, during the warmer seasons of the year, if conditions are right.

A light hookworm infection (a couple of dozen worms in the intestine) will go unnoticed and is not likely to compromise health. To describe symptoms of hookworm infection for heavier infections, it’s necessary to divide hookworm disease into two separate time periods: the period immediately after infection when the larvae are migrating and maturing to adult worms, and the period of chronic infection, when the adult worms are established in the intestine.

Hookworm larvae penetrate human skin when people walk bare-skinned on moist soil or through damp vegetation where the larvae are waiting for just such an event. An itchy rash appears at the site of penetration, which may become infected with other organisms if the individual scratches. The larvae, meanwhile, have moved on, traveling to the lungs in the bloodstream and breaking out into the airspaces of the lungs.

The presence of many larvae in the lungs can cause inflammation that can look like a severe pneumonia in extreme cases, but it is usually mild. The most notable symptom at this stage is a slight to severe cough. Pneumonitis clears up as the larvae pass up the bronchi and are swallowed.

The final stage of the worm’s lifecycle, and of hookworm disease, begins as the larvae pass into the small intestine. Each larva attaches itself to the intestinal lining by taking a mouthful of tissue and hanging on. The tissue breaks down and blood begins to flow and feed the young worm that will soon be mature. At this stage, the victim may suffer fatigue, abdominal pain, diarrhea, vomiting, and bloody stools. Small children and individuals in an already weakened state of health are in grave danger at this point, especially if the number of worms is very high.

Finally, chronic hookworm disease sets in, characterized by listlessness and anemia, pale skin tone, and swollen face and feet. The adult worms continue to draw blood, and blood loss continues from their sites of attachment in the intestine. Female worms produce many eggs, which are passed in the stool.

During the early stages of infection, diagnosis of hookworm disease depends on clinical symptoms along with appropriate travel or exposure history. In the absence of symptoms, an individual who has traveled in an endemic area may desire screening for the infection, but it will have to wait. The definitive test is examination of stool for hookworm eggs, and it takes about five months for worms to mature and begin producing eggs. In some cases it may take even longer.

Little know facts about hookworm infection include the fact that at least one species can remain dormant in the muscle tissue of common domestic animals such as beef cattle and pork: if these animals are slaughtered and the meat consumed without being thoroughly cooked, the larvae will infect the new host. It also appears that the same species is capable of passing from mother to fetus during pregnancy – some people may be born with hookworm infection.

Hookworm infection, the presence of hookworms in the intestine, is very common in humans, especially in tropical climates. There is a difference, however, between hookworm infection and hookworm disease: a few hookworms generally do not cause any symptoms or illness in a human host, whereas large numbers of the worms cause hookworm disease.

In order to understand the symptoms of hookworm disease, it's important to understand how hookworm infection occurs. You do not catch this parasite through direct contact with another infected human. Rather, you pick it up when your skin comes in contact with soil contaminated with the feces of an infected person. Hookworm larvae, the stage that infects people, are present in warm moist soil where an infected person has defecated outside on the ground.

Hookworm larvae do best in loose soil that contains plenty of organic material. They don't do well in dry sandy soils, or soils that contain a lot of clay, and they don't develop at temperatures below about 17 degrees Celsius. Freezing kills them. Because they are so sensitive to cold temperatures, hookworms are basically confined to warm climates and to warm microclimates such as soil inside mines. Hookworm infection does not occur in temperate northern and southern climates.

When human skin comes in contact with live hookworm larvae in soil, the larvae penetrate the skin and start to migrate. If there are a lot of them, there may be a skin reaction, and inflammation where they penetrate the blood vessels. The body's immune system causes this inflammation as it tries to kill the larvae. Hookworm infection may end at this stage, if the immune system is successful; larvae that survive get carried off to the heart and lungs. In the lungs, large numbers of larvae can cause the first serious sign of hookworm disease.

Hookworm larvae break out of blood vessels into the alveoli of the lungs (the sac-like spaces that fill with air when you breath in). Each larva breaking through causes a tiny bit of bleeding into the lungs. A few larvae won't even be noticed, and massive numbers breaking out at once are rare, but when it does happen, a pneumonia-like illness occurs. From the lungs, the larvae travel up the windpipe to the throat and then down through the stomach to the intestine, where they grow to adult worms. At this stage, they may cause abdominal pain and diarrhea.

Again, a couple of dozen hookworms in the intestine is a light infection that won't cause any symptoms; up to 100 worms will cause mild hookworm disease, while hundreds will seriously compromise host health, and thousands may kill. Serious symptoms of chronic (ongoing) hookworm disease include stunted growth, anemia, weakness, fatigue, protein deficiency, and learning impairment. In regions where people struggle with nutritional deficiencies already, a high incidence of hookworm disease has grave consequences for the society as a whole.

Hookworm is one of most successful human parasites, having been around for many thousands of years and today residing in the intestines of close to a billion human beings. The two main species are Ancylostoma duodenale and Necator americanum.

Hookworm would not be so prevalent were it not for several persistent habits of human beings. The first is the habit of defecating outside on the ground, and the second is the practice of using untreated human sewage as fertilizer for crops. These two things, so ingrained in the cultural habits of many societies around the world, account for the majority of hookworm infection worldwide.

Deposition in the soil plays right into the scheme of the hookworm lifecycle. Adult hookworms are seldom seen because they are quite tiny (a female is only about 1 cm long, and the males are even smaller); they remain in the intestine clinging to the lining with their wide mouths and grasping teeth. Females produce many eggs, which are passed out with the feces onto the ground. There the parasite will infect its next host if conditions are right.

In warm moist conditions, hookworm larvae emerge from the eggs and develop quickly to infective larvae. They wait at the surface of the soil as the feces gradually break down, waving their bodies in the air in anticipation of the opportunity to infect a new host if one wanders by – the next stage of the hookworm lifecycle is to penetrate the skin of an unsuspecting human, then travel through the body via the bloodstream, to the heart, then the lungs, and finally the intestine. Intestinal hookworm infection is the end result of this complex journey.

Hookworm infection is usually not a fatal disease, but the worms suck blood as they hang by their mouths from the lining of the intestine. Bleeding into the intestine can also occur. Individuals infected with many hookworms are initially likely to suffer from nausea, vomiting, diarrhea, bloody stools, fatigue, and weakness. Lethargy continues and anemia develops over time. Victims often suffer from swelling of the feet and face, and enlargement of the heart. Growth and learning ability is often affected in children.

With symptoms like that, one can imagine how hookworm would take a heavy toll on a society in the long term – and one can see how easy it should be to break the chain of transmission.