Hookworm infection is caused by the nematode parasites Necator americanus and organisms within the genus Ancylostoma. Soil-transmitted helminths (STHs) include:

- N. americanus (primary organism that causes hookworm infection)
- A. duodenale
- Roundworm
- Ascaris lumbricoides
- Trichuris trichiura 


Etiology of Hookworm

The global disease burden, as analyzed through disability-adjusted life years, overwhelmingly exceeds other tropical diseases other than malaria, leishmaniasis and lymphatic filariasis. Hookworm infections are correlated with increased school absences and, as a result, a decreased future socioeconomic productivity. This research reflects that hookworm, similar to other NTDs, promotes long-term disability and increases the likelihood that a marginalized population will remain in impoverished conditions. 

Global Disease Burden of Hookworm

This parasitic infection results from the exposure of infective larval stages through the skin. Following entry of the pathogen, the larvae travel through the body to the lungs before ending in the gastrointestinal tract. The worms reproduce to form more than 30,000 eggs per day, which exit the host through the expulsion of feces. 

Clinical Features of Hookworm

Despite a lack of surveillance mechanisms, extensive review of literature places the number of global cases as approximately 740 million people since 1990. The highest prevalence occurs in sub-Saharan Africa and eastern Asia. Higher transmission rates are correlated with impoverished areas in southern China, the Indian subcontinent and the Americas. The neglected status of hookworm is associated with high hookworm prevalence rates among the world’s poorest populations. 

Global Distribution of Hookworm

Hookworm morbidity is proportional to the number of adult parasites inhabiting the patient. The severity of infection is determined through quantitative fecal egg counts and ranges from modern infections to heavily intense infections. 
- Moderate-intensity infections: 2,000-3,999 eggs per gram of feces
- Heavy-intensity infections: 4,000 or more eggs per gram of feces

Unlike other STH infections, hookworm infections portray an age-intensity profile where the severity of disease rises with age or plateaus in adulthood. Although this is largely observed in patients, the biological basis for this phenomenon is unknown. 


Epidemiology of Hookworm

Current control strategies target the reduction of morbidity rates through mass treatments (deworming) of populations afflicted with the parasite. These mass drug campaigns include the distribution of anthelmintic drugs, benzimidazoles (BZAs), levamisole and pyrantel. Continued investment in deworming should be complemented by sanitation and access to safe drinking water. The World Health Organization has led campaigns to distribute doses of these drugs to schools with the highest rates of STH infections. The deworming of students has many health benefits including: 
- Improved iron and hemoglobin levels
- Physical growth and fitness
- Cognition

There are also a number of societal benefits including 
- Advancements in education
- Reduced community transmission of ascaris and trichuris infections


Hookworm Control Strategies

The Human Hookworm Vaccine Initiative (HHVI) is in the process of developing a functional and safe vaccine. HHVI is a non-profit collaboration at the George Washington University, London School of Hygiene and Tropical Medicine and the Oswaldo Cruz Foundation (vaccine development has been sponsored by the Sabin Vaccine Institute). Hookworm vaccine development involves research, process development, vaccine manufacturing and control and pre-clinical/clinical testing. 

Na-ASP-2 Hookworm Vaccine

Adult hookworms may destroy the host through intestinal hemorrhage. These parasites ingest the blood rupturing erythrocytes and degrading hemoglobin in the process. Hookworm infections cause the disease silent blood loss, which leads to anemia and malnutrition. Generally, there is a positive correlation between parasite population and destruction of RBCs in the host (women of reproductive age, children and other populations with low iron). In children, chronic infections may lead to growth retardation and cognitive impairments. Infections in pregnant women may lead to negative maternal-fetal outcomes. 

Progression of Hookworm Infection

Hookworm is a common intestinal nematode infection caused by _Necator americanus_ (New World hookworm) and _Ancylostoma duodenale_ (Old World hookworm). Symptoms include abdominal pain, diarrhea, loss of appetite, weight loss, fatigue, and anemia.

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- _Ascaris lumbricoides_
- _Necator americanus_
- _Ancylostoma duodenale_
- _Enterobius vermicularis_
- _Toxocara canis_ and _T. cati_
- _Trichinella spiralis_
- _Dirofilaria immitis_

Parasitic species of Roundworms (Nematodes)

Centers for Disease Control and Prevention. (n.d.). Centers for Disease control and Prevention. Centers for Disease Control and Prevention. https://www.cdc.gov/. 

Centers for Disease Control and Prevention

Hookworm: “The Great Infection of Mankind”
Hotez PJ, Bethony J, Bottazzi ME, Brooker S, Buss P (2005) Hookworm: “The Great Infection of Mankind”. PLOS Medicine 2(3): e67. https://doi.org/10.1371/journal.pmed.0020067

Hookworm: "The Great Infection of Mankind"

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

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Hookworm

_Ascaris lumbricoides_ is a widespread nematode and the largest intestinal parasite found in humans, with females reaching lengths exceeding 1 meter. Although it is now relatively uncommon in the United States, it is common globally and can cause symptoms ranging from a mild cough and abdominal pain to severe complications like intestinal obstruction and impaired growth.

*Ascaris lumbricoides*

The pinworm, caused by _Enterobius vermicularis_, is the most prevalent nematode infection in the United States. It typically causes nighttime sleeplessness and intense itching around the perianal area, where female worms migrate to lay their eggs during the night.

*Enterobius vermicularis*

_Necator americanus_, also known as the New World or North American hookworm, is a nematode parasite that causes hookworm infection. Symptoms of infection include abdominal pain, diarrhea, appetite and weight loss, fatigue, and anemia.

*Necator americanus*

*Toxocara canis* and *T. cati* are the causative nematodes for toxocariasis infections, which can be transmitted from dogs/cats to humans. 

 *Toxocara canis* and *T. cati*

_Trichinella spiralis_ is the parasitic nematode that causes trichinellosis (or trichinosis). It is contracted by eating undercooked meat containing larvae, which are released in the body and encyst within muscle tissues. Symptoms include fever, muscle pain, and digestive issues, while severe cases can result in coordination loss, respiratory or cardiac difficulties, and death.

*Trichinella spiralis*

_Dirofilaria immitis_ is the nematode parasite responsible for heartworm disease in dogs and other animals. Transmitted through mosquito bites, the infection manifests with fatigue and coughing, and can lead to death if left untreated.

*Dirofilaria immitis*

_Ancylostoma duodenale_, also known as the Old World hookworm, is a species of parasitic nematode that causes hookworm infection in humans. Symptoms include abdominal pain, diarrhea, loss of appetite, weight loss, fatigue, and anemia.

*Ancylostoma duodenale*

Whipworm (*Trichuris*) is a parasitic nematode that can infect both humans and dogs. Globally, it is a significant public health concern, causing infections in approximately 604 to 795 million people.

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