Infectious Disease

Giardia Lamblia (You Better Have a Toilet Nearby)

virtualmedstudent

This little bastard to the left is called giardia lamblia and it is a single celled protozoan. It exits in two forms: trophozoite and cyst. The cyst form is highly resistant to destruction and can survive for weeks outside a host.

The life cycle of giardia is relatively simple. First, cysts are pooped out by a host (ie: animal or human) where they contaminate food and water supplies. They are then unknowingly ingested by a host. Once in the gut of the host, the cyst transforms into the trophozoite form. Trophozoites undergo asexual reproduction in the gut; as they reach the colon they re-encyst themselves. From there they are pooped out again to reinfect another unfortunate soul.

The trophozoite form has two nuclei (the two "eyes" in the photo to the right) and four pairs of flagella that allow it to move. The organism attaches itself to intestinal cells. How it causes symptoms is not entirely known. One possibility is that it causes inflammation of the intestinal cells resulting in a decreased ability to absorb nutrients from food.

Signs and Symptoms

Interestingly, most people infected by giardia lamblia are asymptomatic. However, in some individuals a foul smelling diarrhea results. The foul smell is due to steatorrhea (ie: undigested fat molecules). Many symptomatic people will also have belly pain, fatigue, flatulence, nausea, and possibly vomiting. All of these symptoms can give rise to weight loss.

Symptoms generally begin seven days after exposure. They generally last anywhere from two to four weeks with or without treatment. However, in a subset of patients who are untreated, symptoms may continue for months to years!

Diagnosis

Diagnosis is made most commonly by looking at the stool for proteins made by the organism (ie: giardia antigens). Also by looking at a stool sample under a microscope it is possible to directly see the trophozoites and cysts.

Treatment

Treatment is with an antibiotic known as metronidazole. This antibiotic is taken up by anaerobic bacteria and some protozoans where it is converted into toxic by products. These toxic metabolites damage DNA making the organism unable to divide; eventually, the damage causes cell death.

Overview

Giardia lamblia is a protozoan parasite that exists in two forms: trophozoite and cyst. The cyst is defecated and then ingested by another host. It turns into the trophozoite form and adheres to intestinal cells causing a foul smelling diarrhea in some individuals. Many people remain asymptomatic. Treatment is with an antibiotic known as metronidazole.

References and Resources

Ixodus, Burgdorferi, and Lyme Oh My!

virtualmedstudent

Borrelia burgdorferi is a gram negative spirochete bacterium. Its hosts are mice, deer, and humans. Spread from host to host is via a tick from the species Ixodes. Ixodes scapularis is the most common tick vector in the Northeastern United States, whereas Ixodes pacificus is the most common in the Western United States.

When an infected tick feeds from a human it can pass borrelia burgdorferi bacteria into the bloodstream. From there the bacterium is usually attacked by both the innate and adaptive immune systems. The innate immune system uses toll-like receptors on macrophages to bind to bacterial proteins. The adaptive immune response uses antibodies to bind to bacterial surface proteins, which results in complement activation and bacterial death.

Unfortunately, the immune response is often not enough to entirely eliminate this bacteria. This is because borrelia burgdorferi has the ability to evade the immune system by changing proteins on its cell surface.

Signs and Symptoms

Borrelia burgdorferi causes Lyme disease. Lyme disease manifests in different ways depending on which stage of the disease the patient enters. The first stage of the disease presents with a rash known as erythema chronicum migrans. This rash appears as a red ring around a clear central area (ie: it resembles a bull’s eye). During this stage of the disease many patients also have flu-like symptoms.

If left untreated the disease can progress to stage two. During this stage neurological, joint, and cardiac symptoms can occur. The bacterium can cause cardiac arrhythmias, especially heart block. Facial drooping can be caused by damage to the facial nerve (cranial nerve seven). Meningitis, which ranges in severity, can occur. Finally, a migrating intermittent arthritis can also be seen during this stage of the disease.

If the disease continues to remain untreated, stage three ensues! This stage happens approximately a year or more after the initial tick bite. It is characterized by chronic arthritis, which can cause severe joint damage. Encephalitis (inflammation of the brain tissue) and myelitis (inflammation of the spinal cord) can also occur.

Diagnosis

The diagnosis of Lyme disease is a two step process. A blood sample is taken from the patient and a test known as an ELISA is performed. ELISA stands for “enzyme linked immunosorbent assay”. This test detects antibodies made by the patient against borrelia burgdorferi antigens (ie: it’s surface proteins). If the ELISA is positive, then a second confirmatory test called a "western blot" is performed. This test further separates the different antibodies to borrelia. If both the ELISA and western blot are "positive" the likelihood of Lyme disease is high.

Treatment

Treatment for early stage disease is with an antibiotic known as doxycycline. Children and pregnant women are often treated with amoxicillin. Other medications used include cefuroxime; erythromycin is sometimes used in patients who are allergic to pencillin. Later stages of the disease may need intravenous antibiotics.

If patients do not respond to initial antibiotic treatment it is important to think of co-existent infections with other tick-borne illnesses. Ticks may carry not only borrelia burgdorferi, but also other pathologic entities so treatment may need to be further tailored.

Overview

Lyme disease is caused by the bacterium borrelia burgdorferi. It is transmitted by the Ixodes tick and is common in the northeastern United States. Symptoms include a bull’s eye rash known as erythema migrans. Neurological, joint, and cardiac symptoms can also occur. Treatment for early stage disease is with oral antibiotics, usually doxycycline, although amoxicillin is sometimes used. Intravenous antibiotics may be necessary for later stages.

References and Resources