Those that are a little squeamish may want to skip today’s blog post, as it is all about a particularly nasty human parasite, the hookworm.
Hookworms make their home in the small intestine of their human hosts, where they feed on blood and shed eggs that are passed in human stool. Once outside in the environment, hookworm eggs mature into larvae that remain in the soil awaiting someone to walk through with bare feet. Once the opportunity arises, larvae penetrate the skin of the unsuspecting person and migrate through their tissues to eventually wind up in the small intestine and start the whole cycle over again. Disgusting, right?
It gets worse. Hookworm is a part of the vicious cycle of poverty. It is the most common parasitic infection after malaria and chronically infects approximately 700 million of the world’s poorest people. Hookworm is a major cause of iron deficiency anemia for people living in developing countries, and the chronic anemia caused by hookworm can have long term affects on child development and socioeconomic productivity. In other words, people that live in poverty and poor sanitation are likely to be infected with hookworm, which then further limits economic development. And thus the cycle of poverty continues.
Currently, hookworm infection is combated through the regular mass administration of anthelmintic drugs, generally to school-aged children. But, this strategy has its consequences. Mass administration of drugs is not a long term solution because of the potential of drug resistance in humans. Widespread use of these anthelmintic drugs makes it likely that in the future these drugs will no longer be useful against parasites, but unfortunately, they are currently the most effective weapon in the public health arsenal. [See previous blog posts for more on how the development of drug resistance]
However, researchers are currently working hard to develop a vaccine against hookworm. If successful, this vaccine would revolutionize the fight against hookworm parasites by preventing moderate and heavy infections with the parasite. Eliminating heavy infections of hookworm will greatly reduce the amount of environmental contamination with parasite eggs and therefore reduce transmission. The vaccine is being initially targeted for at risk children under the age of 10 years.
The war against hookworm is not won yet, however. The hookworm vaccine has a fair number of challenges ahead of it. Developing a vaccine against a worm is quite different than vaccines that exist for bacteria and viruses. Hookworms are complex multicellular organisms that are capable of evading immune responses of their hosts. First, the vaccine candidate must successfully complete phase 1 clinical trials where the vaccine is tested for safety in healthy adult volunteers. Next, the vaccine will be tested for efficacy in preventing hookworm infections in phase 2 and 3 studies in areas of Brazil and sub-Saharan Africa where hookworm is a problem. Finally, adult volunteers without hookworm will be vaccinated and then experimentally challenged with hookworm infection. That’s right; some people will voluntarily be infected with hookworm after vaccination. Now that is dedication to scientific progress.
Because we are all optimists, let’s fast forward to a future where the hookworm vaccine has passed all trials with flying colors. Now for the logistical challenges…How do you get a vaccine that requires storage temperatures of 2 – 8⁰C out to rural parts of sub-Saharan Africa? How often will the vaccine be useful for? It is unlikely that the hookworm vaccine will induce lifelong immunity to the parasite, and children will need to be re-vaccinated. Who is going to pay for the vaccine? Remember, hookworm is largely a problem among the world’s poorest- people living on under $1-2 dollars per day. Chances are some serious external funds will be required.
In summary, the hookworm vaccine is in the pipeline, but it will still be several years until it becomes a reality. And when the much anticipated vaccine finally arrives, it will be a useful tool for the management of hookworms. However, let’s not forget the transmission cycle of this nasty parasite. People are infected by contact with fecal contamination in soil. Yes, drug and vaccine treatments may assist in reducing the problem of hookworm, but without basic sanitation and hygiene improvements, we still have a long way to go.
To read a full review of the human hookworm vaccine, please read “The Human Hookworm Vaccine” by Hotez et al. (2013).