Vaccines can do wonderful things for people and especially travelers. Scientists are one step closer to creating a vaccine against Giardia, scourge of the traveler and outdoor adventurer. A recent article, published in Nature, looks at a discovery that should play a vital role in the development of a vaccine for Giardia.
Giardia is a protozoan infection that effects the small intestine, most frequently. Symptoms of infection can range from none at all to “explosive diarrhea”, flatulenceand abdominal cramping. This difficult to kill bug is most commonly acquired through drinking infected water or poor sanitation such as “hand to mouth” contact with infected feces, also known as fecal-oral transmission. Unfortunately, giardia can be found worldwide and is responsible for 100,000 to 2.5 million infections per year, in the US, alone.
The life cycle of giardia has two basic forms, a cyst and a trophozoite. The trophozoite is responsible for the symptoms associated with the illness. Trophozoites are a very fragile form and do not live long, outside of a host. The cyst is the method of transportation and is remarkably durable. Cysts can even survive both freezing and boiling, for short periods of time. When a person becomes infected, it is usually through ingestion of cysts, as stomach acid kills the trophozoite form. Once ingested, the cysts mature into the damaging trophozoite form, in the intestines.
Hopefully, most all adventurers know that water purification is an important step in preventing illness. Giardia is one of the reasons why you want to treat or purify your water. While there are many methods to prepare water for human consumption, not all of them are effective against the giardia cysts.
Boiling is effective at killing cysts, if done for a long enough time. Water temperatures of 100 degrees Celsius (212 F) are generally able to kill the cysts on contact. Water at a temperature at only 60 degrees Celsius (140 F) requires 10 minutes and only offers a 98% inactivation of cysts. Also remember that boiling times vary depending on altitude.
Filtration is effective at keeping cysts out of your drinking water, provided the pores in the filter are small enough. 3-5 micrometers is considered the maximum pore size for stopping the cysts. Remember that only a few cysts are required to cause infection in humans. One study showed that only 10-25 cysts caused infection in 8 of 25 people and more than 25 cysts caused 100% infection.
Halogens, such as iodine and chlorine, kill giardia cysts, as well. Some difficulties begin to arise, using these methods, for several reasons. Higher concentrations of halogens are required to kill cysts, than other typical water bacteria. This means more of that unpleasant iodine or chlorine taste in your water. Longer “contact time” is also required, meaning that you will have to wait longer to drink the water your are preparing, giving the halogens more time to work. Lastly, water temperature effects the halogen’s effectiveness. Data for using chlorine, to achieve a 99.9% kill, shows that 0.5 mg per liter concentration requires 6-24 hours at 3-5 degrees Celsius. A solution of 1.5 mg per liter takes only 10 minutes at room temperature.
Clearly, the prevention of giardia is a difficult task and that is just for personal use! A vaccine for giardia prevention would also help alleviate mortality associated with massive dehydration due to diarrhea, in developing nations.
Back to the Vaccine
The difficulty for producing a vaccine against giardia centers around it’s ability to express 190 different surface proteins. This basically allows giardia to have one outer coating, then for a previously unknown reason, switch to another. Anything that has 190 different “doors” is tough to find a key for. The exciting thing about the work being done, by Dr. Lujan and his group, is that they have found a way to force giardia to express all of it’s 190 surface proteins at one time. Basically, the protozoa is made to show all of the defensive adaptations, at one time.
Antigenic variation in Giardia Lamblia is regulated by RNA interference, published in “Nature” issue 456