Evidence from the lab of Raffi Aroian, PhD, shows that short-term human hookworm infection, even at low levels, can cause rapid, acute and measurable cognitive impairments in spatial memory among a mammalian animal model.
These findings, published in Scientific Reports, highlight the global importance of hookworm elimination and suggest that additional, specific, spatial memory studies be carried out.
“These parasites have a real impact on brain function in rodents, independent of long-term developmental delays, that can be observed very quickly after initial infection,” said Dr. Aroian, professor of molecular medicine.
“This means we’ve very likely been underestimating the impact these parasites have on the brain in both children and adults, but particularly in very young children.”
Hookworms are a species of parasites that live in the small intestine and infect via the skin, circulation and lung.
Hookworm infection is most common in the developing world where proper hygiene and sanitation are a challenge.
Long-term infection is known to cause severe developmental and cognitive impairments.
The Centers for Disease Control and Prevention estimates that infection occurs in 576 million to 740 million people worldwide.
Hookworm infection is one of the leading causes of anemia, stunted growth and malnutrition.
The worms can live in the small intestine for a few years typically, but for as long as 10 years, consuming blood and robbing their hosts of iron and protein, interfering with absorption of critical nutrients, and suppressing the immune system.
Infection can lead to chronic anemia, as well as permanently stunt the physical and intellectual development of infected children.
Three species of hookworms can infect humans.
People most commonly infected typically reside in poverty-stricken areas with poor sanitation and are exposed to the worm through contaminated soil or food.
Common in the American south 100 years ago, hookworms were virtually eradicated in the United States through improvements in sanitation and public health efforts during the early part of the 20th century.
According to Aroian, prior research into this area has been hindered by an assumption that cognitive impairments happen due to developmental delays and malnutrition caused by long-term infection.
While it is clear that heavy infection loads over time do impact development and brain function, said Aroian, it is less clear if infection can cause acute cognition impairment independent of stunting.
Cognition studies on patients have also been plagued by a lack of accurate data due to the impoverished state of infected populations and the sheer number of environmental variables that may or may not influence cognitive development.
Some studies have shown significant deficits in working memory and cognition performance while others have found only minimal deficit.
Meanwhile, interventional studies on hookworms have produced conflicting results on the effect of clearing the infection.
Some studies have shown cognitive improvement after treatment while others have shown no improvement.
In order to study the acute effects of hookworm infection on brain function in a controlled laboratory setting, Aroian and co-investigators developed a series of experiments to test spatial memory and object memory in rodents.
To mimic acute infection, rodents were infected with juvenile Ancylostoma ceylanicum, a species of hookworm that can infect humans, dogs, cats and rodents.
Aroian and colleagues completed a series of tests designed to assess whether the animal could recognize a new object placed in its environment (object memory) or an object that had been moved from one place to another within its environment (spatial memory) within 17 to 22 days of infection.
Surprisingly, researchers found that the ability to recognize a new object placed in the rodents’ environment was unchanged regardless of hookworm infection or intensity.
However, in a measure of spatial memory, rodents were compromised even at low levels of infection in their ability to detect an object that had been moved.
At higher levels of infection, the rodents failed the task and showed no preference for the moved or unmoved object.
These results could not be attributed to lethargy or lack of interest generated by infection from the parasite as the tested animals performed normally on object memory tests and activity levels were indistinguishable from noninfected rodents.
“There is mounting evidence that changes in the microbiome in the gut, by altering neuropeptides, neurohormones and neuroactive active substances that can cross the blood-brain barrier, can modulate brain functions and impact spatial memory and learning,” said Aroian.
Hookworms are a species of parasites that live in the small intestine and infect via the skin, circulation and lung. Hookworm infection is most common in the developing world where proper hygiene and sanitation are a challenge. Long-term infection is known to cause severe developmental and cognitive impairments. The Centers for Disease Control and Prevention estimates that infection occurs in 576 million to 740 million people worldwide. The image is in the public domain.
In order to investigate a link between hookworm infection and the brain, Aroian investigated whether hookworm infection caused changes in the microbiome of the rodents.
Genetic sequencing of gut bacteria in infected and noninfected rodents showed a decrease in microbial diversity and changes in microbial taxa at the family and genus level.
These changes provide one possible mechanism through which hookworms may impact cognitive function and contribute to spatial memory defects.
“Our work shows that these parasites have a real impact on cognition, independent of long-term developmental delays, that can be observed within a very short period,” said Aroian.
“It may also help explain some of the inconsistencies of past studies as only spatial memory may be impacted. These results demand additional, more finely tuned research in order to understand how infection affects cognitive function and development.
“More importantly, it supports the importance and urgency of global deworming efforts to eliminate hookworm in order to alleviate poverty and save the brains of impoverished children.”
Hookworms are voracious, blood-feeding, soil-transmitted nematode parasites. Adult hookworms infect the small intestine, causing iron-deficiency anemia and other complications. Hookworms are among the most disabling parasites of the developing world. Drugs are useful for controlling hookworm disease.
However, because people often get reinfected rapidly and parasites can develop drug resistance, a vaccine that provides long-term protection would improve control and help lead to eradication.
At present, there is no licensed hookworm vaccine, and progress towards a vaccine has been limited. We identified a cysteine protease in the intestine of the human hookworm Ancylostoma ceylanicum that is among the most strongly expressed genes during blood feeding and that may help digest blood and be essential for hookworm survival. Vaccination of hamsters with this cysteine protease gave high levels of protection when antigen-specific antibodies in the blood were induced.
These antigen-specific antibodies also made hookworms less mobile in culture. This cysteine protease is a promising candidate for further investigation as a human hookworm vaccine antigen.
Human hookworms (Necator americanus, Ancylostoma duodenale, and Ancylostoma ceylanicum) are soil-transmitted nematodes (STNs) that infect the small intestine and feed on blood .
Human STNs encompass three phylogenetically distant parasites: hookworms, large roundworms (Ascaris lumbricoides), and whipworms (Trichuris trichiura).
Among human STNs, hookworms carry the highest disease burden . In children, infection by hookworms causes significant growth stunting, cognitive deficiencies, malnutrition, iron-deficiency anemia and hypoproteinemia; in adults infection results in adverse birth outcomes (e.g., low birthweight babies) and reduced productivity [3–5]. It is estimated that hookworms infect ~500 million people worldwide , and although concentrated in Latin America, sub-Saharan Africa, and Southeast Asia, even people in impoverished regions of the United States (US) still get infected [1,7]. Once at 40% prevalence in the southern US (circa 1911), hookworm infections led to an estimated 43% reduction in future earnings of children infected and were responsible for 22% of the income gap and 50% of the literacy gap between North and South .
The elimination of hookworms via treatment campaigns, improved sanitation, education, and economic development undoubtedly had a major impact on the vitalization and success of the South today. Currently, hookworm disease is estimated to cause 4.1 million disability adjusted life years (DALYs) and US$139 billion in indirect economic losses each year .
Hookworms are the second most important parasitic cause of global anemia after malaria .
Mass drug administration (MDA) of benzimidazoles (albendazole, mebendazole) in school-aged children is the current control measure for hookworms [11,12].
From 1990–2013, MDA reduced hookworm prevalence by only 5.1%, compared to 25.5% reduction for A. lumbricoides .
Additionally, poor efficacy of mebendazole against hookworms is well known  and poor or reduced efficacy of albendazole against hookworms is being reported in multiple locations around the world (e.g., egg reduction rates as low as 0% in Ghana  and cure rates as low as 36% in Lao PDR ).
Veterinary parasites phylogenetically and biologically similar to hookworms (e.g., the blood-feeding Haemonchus contortus) develop resistance to anthelmintic drugs frequently, rapidly, and broadly [16,17].
Water, sanitation and hygiene (WASH) is being explored as a control strategy to combine with MDA .
Improvements in WASH (water, sanitation, and hygiene), although important, are insufficient to tackle the enormous STN problem alone [18–20].
Having a vaccine to prevent infection from occurring in the first place would be of tremendous benefit.
Although it is widely accepted that a hookworm vaccine is needed , there is only a single phase 1 clinical trial underway testing two individual recombinant hookworm proteins formulated on the Th2 adjuvant Alhydrogel .
There are no other candidates in advanced preclinical development .
Because targeting infectious third staged larval (L3i) antigens carries the risk of triggering allergic reactions in previously exposed people , efforts are focused on adult stage antigens, namely an aspartic protease (APR1) and a glutathione S-transferase (GST1) .
Both APR1 and GST1 localize to the adult canine hookworm Ancylostoma caninum intestine (and non-intestinal tissues); these enzymes are thought to help digest hemoglobin and detoxify heme, respectively [25–28].
In canine and hamster models, recombinant protein immunogens from A. caninum gave 33–53% decreased hookworm burdens [26,27,29]. However, in a phase 1a trial, although rNaGST1 was safe and immunogenic in human volunteers , IgG had negligible neutralizing effect on rNaGST1 enzymatic activity, despite the observation that IgG from immunized mice were highly neutralizing ; these results suggest a decreased potential for this vaccine in human trials.
It remains crucial that further and expanded efforts be undertaken to develop new hookworm vaccines.
Development of hookworm vaccines, however, have been limited and lag far behind more concerted efforts, such as against malaria [23,32].
This may be in part because full genomes for human hookworms were formerly unavailable, which prevented large-scale reverse vaccinology  against hookworms.
This situation has recently been addressed for all three species of human hookworms [34–36].
Genomics and transcriptomics for A. ceylanicum hookworm infections in hamsters is being used to identify new and potent vaccine antigen candidates .
Syrian hamsters are the only laboratory rodent permissive for the human hookworm life cycle, and A. ceylanicum infections in hamsters are an excellent model for hookworm infections in humans .
We previously identified two classes of A. ceylanicum genes, that are strongly expressed and upregulated during blood feeding, as encoding potential antigen candidates : cathepsin B cysteine proteases (CPs) and small Kunitz-type protease inhibitors (SKPIs). Here, we explore the vaccine efficacy of two CPs and one SKPI (AceyCP1, AceyCPL, AceySKPI3) using the A. ceylanicum hookworm—hamster model system, and investigate functional and immunological aspects of protection with one of these vaccine candidates.
Jim Fessenden – UMass
The image is in the public domain.
Original Research: Open access
“Cognitive and Microbiome Impacts of Experimental Ancylostoma ceylanicum Hookworm Infections in Hamsters”. Samuel C. Pan, Doyle V. Ward, Yunqiang Yin, Yan Hu, Mostafa A. Elfawal, Robert E. Clark & Raffi V. Aroian.
Scientific Reports doi:10.1038/s41598-019-44301-4