The article calls for a multidisciplinary approach, international cooperation, & enhanced surveillance systems to tackle these challenges effectively and safeguard public health against parasitic diseases.

Over three billion individuals worldwide are afflicted with one or more parasites, resulting in variable illness and mortality. Parasites are being increasingly recognised as important pathogens with significant global economic, environmental, and public health impacts.

While the last couple of centuries saw significant strides in counter-measures against parasitic infections, the changing dynamics of host-parasite interactions continue to give rise to newer challenges whilst the older problems continue to elude reliable solutions.

Neglected tropical diseases and opportunistic parasitic infections

In the global scenario, NTDs are estimated to affect close to two billion people with a collective disability-adjusted life years (DALYs) burden that was equivalent to other diseases like HIV/AIDS, TB, and malaria which gained more attention.

1. 11 out of the 21 diseases notified by WHO as Neglected Tropical Diseases are parasitic infections. NTDs are more prevalent among people who live in poverty, without access to clean water, adequate sanitation, and quality health care. Most NTDs are chronic illnesses that have the ability to impair rather than kill, leaving a trail of social effects.
2. Limited resource allocation and lack of political interest have contributed to NTDs remaining a major health concern despite being both preventable and treatable. India experiences a significant burden of global NTDs. It has been estimated that the most common NTDs could become diseases of the past if only a 10% share of funding for HIV, TB and Malaria is allocated for all NTDs.
3. The global pandemic of HIV coupled with advancements in the treatment of Autoimmune Diseases and Cancer Therapeutics has given rise to a large immunocompromised population which has become a fertile ground for parasitic infections. The prevalence of immunosuppression from health conditions and medication use among US adults is reported to be about 3% while chronic immune-mediated inflammatory diseases (CIMIDs) affect around 5-8% of the world population. Both are associated with a high risk of opportunistic infections.
4. The parasitic species that most often cause opportunistic infections in the human body are Toxoplasma gondii, Cryptosporidium parvum, Isospora belli, Cyclospora cayetanensis, Microsporidia, Cryptococcus neoformans, Pneumocystis carinii or jiroveci, and Entamoeba.
5. Immunosuppression often increases the overall risk of opportunistic infections, but certain medications, depending on their mechanism of action, can increase the risk of specific illnesses. On the other side, abruptly discontinuing these drugs can lead to immunological reconstitution inflammatory syndrome.
6. Immunosuppression can also lead to serious disease and complications like Strongyloidiasis Hyperinfection Syndrome and Disseminated infections in otherwise benign or asymptomatic infections like Strongyloidiasis and Scabies.

Spread to infection-naïve areas

The era of globalisation has ushered in a hitherto unseen blending of humans and their ecosystems, not limited to a single species or genus. While such interactions have made the world a smaller place and have contributed to geopolitical and economic progress, it has come with a pinch of salt.

The World Health Organization (WHO) recently estimated that there are 1 billion migrants in the World. Of these, 258 million are international migrants and 763 million are internal migrants. Migrant public health implications vary across a spectrum from the Healthy Migrant Effect to the transfer of geographically limited pathogens, limited healthcare access and disease outbreaks especially when driven by poverty or conflicts.

Screening protocols for parasitic infections remain deficient in light of geopolitical and socio-economic constraints. While a risk-based approach with separate protocols for refugees and non-refugees has acted as a useful barrier, recent experiences with COVID and other zoonotic pandemics prove that these mechanisms remain failure-prone in light of high susceptibility scenarios.

For instance, Trypanosoma cruzi infection, which may cause latent chronic infections and subsequent complications and may also be transmitted outside endemic areas, remains neglected in many European countries.

Emerging and reemerging parasites

Parasitic zoonoses are found all over the world and are frequently connected with foodborne diseases and insect bites, although they receive little public attention. Zoonotic parasites, which seldom infect people, are becoming more well identified in Asia and impacting tourists.

Newly recognised zoonoses in humans, previously considered strictly animal pathogens like Sarcocystosis cause myositis in humans and can be acquired from eating undercooked meat:

• Sarcocystis hominis
• From beef
• Sarcocystis suihominis
• From pork

A species from the reptilian host, Sarcocystis nesbitti, can be acquired from ingesting feces-contaminated water or food from endemic areas.

Other emerging zoonotic parasites include the raccoon roundworm, Baylisascaris procyonis, Onchocerca lupi, Dirofilaria species, and Trypanosoma evansi. B. procyonis is widespread in raccoons but infects a wide range of vertebrates and has been increasingly recognised to cause infections in humans, predominantly young children, for more than a decade. This parasite, which is prevalent in North America, can cause severe visceral larva migrans, as well as neurological and ophthalmic complications.

O. lupi is a filarial nematode that lives in the scleral conjunctiva of dogs, wolves, and cats.The first human case of O. lupi was reported in Europe in 2001 and the first case in the USA in 2013. Unlike the cases reported in other countries, American patients have not had subconjunctival nodules but presented with more invasive diseases, such as spinal, orbital, and subdermal nodules.

Trypanosoma are major zoonotic parasites recognised to cause substantial disease burden in sub-Saharan Africa [sleeping sickness] and Latin America [Chagas disease], but other less known animal species can occasionally cause human disease [T. lewisi, T. congolense, and T. evansi]. T. evansi was first reported to cause human infection in India in 2005 and more recently in Vietnam, with nonspecific febrile illness. However, with a high burden of disease in bovids and horses [approaching 30%], human infections may be more common than recognised.

Dirofilariasis is also regarded as a developing parasitic zoonosis in Europe. Dirofilariasis is caused by D. immitis, a dog heartworm carried by mosquitoes that causes pulmonary nodules in humans, or by D. repens, which causes subcutaneous nodules or ocular abnormalities.

An infection or disease is said to re-emerge when there is an increased incidence of human infection or disease associated with a parasite of wildlife origin after a period of reduced incidence. Multiple countries spanning across continents have reported an increase in incidence of previously-endemic parasitic infections which had reduced with improving hygiene and living conditions.

Multiple factors like globalisation, climate change, changing lifestyles, development of drug resistance, increased prevalence of immunocompromised individuals due to the HIV pandemic and increased usage of immunosuppressive agents, ecosystem disruptions due to urbanisation and parasite evolution and natural selection are some of the factors implicated in the re-emergence of parasitic infections and infestations.

There is a need for global surveillance to monitor for emerging and reemerging parasitic zoonoses.

Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr.Vyom Agarwal is a Senior Resident, MBBS/MD(General Medicine), at Government Medical College, Korba.