At a recent briefing on Capitol Hill entitled, “Making the Case for Cost-Effectiveness of Vaccines for Global Health,” Peter Hotez, MD, PhD, President of the Sabin Vaccine Institute, talked about a little-known infection called schistosomiasis. A disease caused by parasitic worms, Hotez called it “the most important cause of HIV you’ve never heard of.”
Moving from George Washington University to Baylor College of Medicine at the beginning of August, Hotez is a professor of pediatrics and molecular virology and microbiology at Baylor and is also chief of a new Section of Pediatric Tropical Medicine and founding Dean of the National School of Tropical Medicine. He is the current president of the American Society of Tropical Medicine and Hygiene, and is the Texas Children’s Hospital Endowed Chair of Tropical Pediatrics. Science Speaks spoke with Hotez to find out more about the parasitic infection, who is most susceptible and how it increases HIV transmission.
What is Schistosomiasis?
Schistosomiasis is a parasitic worm infection caused by a blood fluke, and the fluke lives in veins surrounding certain organs. So urinary tract schistosomiasis lives in the veins that surround the bladder, and liver schistosomiasis lives in the veins that surround the lower intestines. These are the major types of schistosomiasis, urinary tract schistosomiasis caused by Schistosoma haematobium and intestinal/liver schistosomiasis caused by S. mansoni, as well as an Asian schistosome (S. japonicum), we won’t get into the other minor types. Not so much the adult worm, but the eggs have a spine on them and the spine has the ability to traverse through the wall of the vessel and cause ulcers, granulomas (an inflammatory mass) and bleeding.
The medicine praziquantel is used as treatment and it has an excellent safety profile, but not enough of it is being donated to those who need it. It is available for about $.08 a tablet generically; and three to four tablets are used per treatment, so we’re looking at $.24 to $.32 per treatment.
You do need to treat schistosomiasis on an annual basis, because it can come back.
What does the prevalence of schistosomiasis look like?
We have new prevalence numbers now. Charles King at Case Western Reserve University now has revised the estimates to between 400 and 600 million people living with schistosomiasis around the world, more than 90 percent of whom live in sub-Saharan Africa. It’s one of the most common infections on the African continent and you have both forms of the disease there – urinary tract and intestinal – while in Latin America and China you have just the intestinal variety.
At a recent briefing on Capitol Hill discussing the cost effectiveness of vaccines, you called schistosomiasis “The most important cause of HIV you’ve never heard of.” Can you explain why that is?
So what happens is the eggs that are being deposited in the urinary tract, Schistosoma haematobium, it turns out that up to 75 percent of girls and women with that infection also have those spine-shaped eggs deposited in their cervix and lower genital tract, where they cause ulcers, bleeding and pain. This is also linked to depression.
And now two very important studies have implicated its role in HIV transmission. In 2006 a study in Zimbabwe by Erin Kjetland from the University of Oslo found a three-fold increase in heterosexual HIV transmission among those infected with urinary tract schistosomiasis. A second study came out this year in Tanzania by Jennifer A. Downs from Weil Cornell Medical College. She saw a four-fold increase in horizontal HIV transmission among those with urinary tract schistosomiasis.
We hear so much about herpes as a co-factor in increasing HIV susceptibility or the importance of medical male circumcision to preventing infection. This is even more dramatic in many ways. Schistosomiasis may be one of the most important co-factors in Africa’s AIDS epidemic that’s not well known.
Why is it not better known?
They call schistosomiasis a neglected tropical disease. It just hasn’t gotten a lot of attention in the AIDS community yet.
But we’re advocating for it. In June I co-authored a paper in the New England Journal of Medicine discussing, among other things, how to think about schistosomiasis control as a key AIDS prevention strategy. Because the cost to treat schistosomiosis is so modest (see Africa’s 32 Cents Solution for HIV/AIDS), treating girls on an annual basis to prevent onset of those genital lesions is a potential effective strategy to prevent AIDS. Should the Global Fund to Fight AIDS, TB and Malaria or PEPFAR [The U.S. President’s Emergency Plan for AIDS Relief] for instance consider adding praziquantel to their package of interventions and doing mass drug administration?
The problem with using this medicine is you have to use it every year or every other year, because it looks like once the lesions develop even treating them doesn’t really make a big impact on HIV transmission, because the inflamed cells and ulcers are already there, so the conduit for the virus is already there. We have to use praziquantel in what we call preventative chemotherapy.
We’d like to make a schistosomiasis vaccine, and we have a little bit of money to start that project, though we are very modestly funded. I think making a vaccine for schistosomiasis is potentially less daunting than making an AIDS vaccine, not that it’s either or.
Are there any organizations or projects currently working to treat or prevent schistosomiasis? Where can we get more information?
Yes, our Global Network for Neglected Tropical Diseases (visit www.globalnetwork.org to learn about mass drug administration approaches at fifty cents) in addition to the Schistosomiasis Control Initiative (www.schisto.org) has some new funding from the British Department of International Development for this purpose.
I also write about it in my book Forgotten People and Forgotten Diseases, and also at the end of this month last week in July, in PLoS there will be a paper on how to integrate schistosomiasis control into programs like PEPFAR and the Global Fund, how to go about doing it.
Information on where the co-epidemic is most prevalent can be seen the chart below from Africa’s 32 Cents Solution for HIV/AIDS: