Parasitic zoonoses or zoonotic diseases are the type of infectious parasitic diseases and infections that are spread between animals and man. The World Health Organization (1951) zoonosis definition states that they are “diseases and infections that are naturally transmitted between vertebrate animals and man.”

According to the Centers for Disease Control report, more than 60% of the known infectious diseases that affect people originate from animals. Parasitic zoonotic diseases like cryptosporidiosis, schistosomiasis, and leishmaniases have dire global health and economic consequences worldwide that require urgent attention.

We have already had a one-on –one experience or witnessed the adverse impacts with the recent zoonotic COVID 19 pandemic, right? Over 6 million mortalities, lockdowns, slowed global economic growth, and on the bright side, the rapid innovation of vaccines.

Before we delve deeper into the types of parasitic zoonoses, let me take you through the introduction bit of the zoonotic diseases. I will provide you with a preview on the origin, categories, and the terms useful in the classification of zoonotic diseases.

Let’s go!

What is the origin of zoonoses?

Zoonosis (singular) or zoonoses (plural) word is a combination of two Greek words, zoon meaning animals and noson, meaning disease.  Zoonosis was coined by Rudolph Virchow, the father of cellular pathology in the 19^th century while he was designating human diseases of animal origin. Pathology is a branch of medical science that focuses on the study of the nature and causes of diseases or injuries.

 Categories of zoonotic diseases

The causes of these diseases can be bacteria, fungi, viruses, prions or unconventional etiologic agents. Scientists classify zoonotic diseases in terms of the pathogenic/causal organism, animal reservoir, and the how the disease is contracted: direct contact or accidental ingestion. Zoonotic disease transmission mode can either be direct or indirect.

I will give you examples of the diseases in each category for elaboration purposes:

• Causal organism– Coronavirus disease (viral); Lyme borreliosis (bacterial), toxoplasmosis (parasitic), bovine spongiform encephalopathy in cattle (prion); candidiasis(fungal).
• Animal reservoirs– domestic dogs, foxes, cattle, bats and skunks(rabies); humans(cysticercosis); horse bats (COVID-19 virus); deer (Lyme disease)
• Transmission mode– direct contact (rabies, brucellosis), accidental ingestion in foods (toxoplasmosis, cryptosporidiosis, trichinellosis; Inhalation (pssitacoccis and ornithosis).

  Terms useful in the classification of zoonotic diseases  

• Orthozoonoses are types of zoonotic diseases that results from direct contact exposure with an infected animal, fomite, or a mechanical vector. Examples include human rabies and brucellosis.
• Saprozoonoses require both vertebrate and inanimate development site or reservoir for their transmission to occur. The inanimate reservoir includes organic matter, plants, soil, or food. Examples of zoonotic diseases are listerioisis, and histoplasmosis.
• Metazoonoses/ pherozoonoses are the ones that require both vertebrate and the invertebrate hosts like mollusks and crustaceans for the completion of their infectious developmental stages. An example is schistosomiasis or bilharzia disease that requires freshwater snails.
• Cyclozoonoses are the types of zoonoses that require more than one vertebrate or animal hosts to complete its life cycle. An example is cystericosis caused by Taenia solium. solium requires both pigs (intermediate hosts) and humans (definitive hosts).

Below are top 5 parasitic zoonoses of global health concern that you should know:

  1. Leishmaniasis

Leishmaniases are a group of complex protozoan diseases that are caused by more than 30 Leishmania species. The female phlebotomine sandflies are the primary vectors of the leishmaniases. the clinical forms of the disease ranges from the self-resolving cutaneous to the fatal visceral form.

Factors like the Leishmania species, geographical distribution, clinical manifestations, transmission cycle reservoirs, and epidemiologic characteristics are vital in the classification of the Old World and the New World leishmaniasis

 Parasitic Zoonotic cutaneous leishmaniasis (ZCL)

ZCL is prevalent in sub-Saharan Africa, Mediterranean basin, Middle East, and Central Africa.

Leishmania species responsible for ZCL are L. donovani and L. aethiopica in the Old World. The New World species include L.mexicana, L. brazieliensis, L. peruviana, L.guyanensis, and L.  pananensis.

Although the mild form of cutaneous leishmaniasis is self-limiting, it can result in devastating health consequences when left untreated. These include

• disfiguring lesions and scars,
• ulcerated lesions,
• lifelong scars
• leishmaniasis-related disabilities resulting in low productivity of patient
• psychosocial problems -stress. impacts- low self-esteem, stress, depression, anxiety, and social stigma.

Zoonotic visceral leishmaniasis (ZVL)

ZVL caused by Leishmania infantum is an important parasitic zoonosis that affects humans and their companion dog pets.  Infected domestic dogs and rodents are the animal reservoirs maintaining the transmission. However, recent research suggests that cats are also affected in endemic regions.

Humans get infected when they are in close proximity with their dogs or cats.   Affected domestic dogs may fail to present any clinical signs or symptoms (asymptomatic). In symptomatic cases, the dogs suffer from: swollen lymph glands, swollen lungs and spleen (hepatosplenomegaly). The conditions arise due to host immune response to Leishmania invasion into the reticuloendothelial system.

Infected humans, on the other hand may experience severe ZVL if they are suffering from HIV/AIDS or bacterial co-infections. Patients may exhibit body mass wasting, chronic fever, pancytopenia, massive bleeding, renal failure, thrombocytopenia, hepatosplenomegaly, or even death due to lack of treatment.

  2. Schistosomiasis/bilharzia/snail fever

Schistosomiasis is a parasitic infection that is caused by blood flukes or trematode worms.  It is the second leading disease with huge socioeconomic importance after malaria.

People can get this parasitic zoonosis when they come in contact with infested water with Schistosoma parasites. The exposure may happen while one is carrying out his/her routine activities like irrigation, swimming, or fishing

There are four key Schistosoma species of medical importance:

• Schistosoma mansoni, Schistosoma mekongi, and Schistosoma japonicum cause intestinal schistosomiasis.
• Schistosoma haematobium causes urogenital schistosomiasis

Intestinal schistosomiasis following S. mekongi and S. japonicum infection is a type of parasitic metazoonosis affecting many individuals in Asia. S. mekongi is common or endemic to South East Asia, Cambodia, Indonesia, and the Lao People’s Democratic Republic. S. japonicum is endemic to China, Philippines, East Asia, and South East Asia.

The animal reservoirs of S. japonicum are dogs, cats, horse, rodents, pigs, or goats. By contrast, dogs are the reservoirs for S. mekongi. Aquatic freshwater snails belonging to the Oncomelania species and Neotricula aperta serve as the invertebrate hosts for S. japonicum and S. mekongi respectively.

Intestinal schistosomiasis account for 4.5 million cases of disability adjusted years (DALYS) globally and result in over 200,000 annual deaths. People with intestinal schistosomiasis may suffer from:

• abdominal pain,
• chronic diarrhea,
• hepatopathy,
• hepatosplenomegaly,
• anal pain,
• pass blood in stool
• chronic colitis due to polyp formation
• severe infection increases the risk for liver fibrosis

Young children bear the heaviest brunt of the parasitic zoonosis. They can present stunted growth, anemia, and impaired cognitive abilities. In school going children, the parasitic zoonosis is responsible for poor academic performance, high absenteeism, and poor concentration in class.

  3.  Cystericosis

This is a type of cyclozoonosis which occur following infection with the larval cysts (cysterici) of Taenia solium tapeworms. Pigs are the intermediate hosts while humans are the definitive hosts. The parasitic zoonosis is prevalent in Africa, Latin America, most parts of Oceania, and Asia regions where pigs are reared as the principal food source.

High disease transmission rates are strongly linked with humans living in close proximity to pigs and poor sanitation.  Transmission of cysticercosis can occur in two ways:

• The oral-fecal route whereby people get parasitic zoonosis by ingesting fecal-contaminated foods and water or coming in contact with surfaces containing the gravid proglottids. These are the infections or embryonated T. solium eggs.
• The auto-infection route means that T. solium carriers facilitate person-to-people disease spread through the fecal-oral route. Autoinfection is highly facilitated by poor hand hygiene.

Cystericosis affects different parts of the body including the brain, muscles, eyes, spinal cord and other tissues. The severity of the disease depends on the location of the larval cysts and its location. The occurrence of larval cysts in the brain lead to neurocystericosis, the most fatal form of the disease. Neurocysticercosis is the common cause of epileptic seizures, severe headaches, intracranial hypertension, hydrocephalus, poor coordination, stroke, or death.

   4.  Lymphatic filariasis / Elephantiasis

Lymphatic filariasis is a type of a rare parasitic infection that is responsible for causing disability in patients. It is caused by three types of microfilaria worms namely Wuchereria bancrofti, Brugia malayi, and Brugia timori and spread by different types of mosquito vectors. They include   different mosquito species belonging to the Aedes, Culex, Mansonia, and Anopheles genera as well as Coquillettida juxtamansonia

Elephantiasis affects individuals from the underdeveloped tropical and subtropical regions worldwide such as Africa, Oceania, South America, East and South East Asia.

The impacts of elephantiasis are largely disabling –oriented culminating in overall lower quality of life and lower productivity. The microfilaria targets the lymphatic system of the patients resulting in impaired immune system, thus, increased susceptibility to secondary bacterial infections and opportunistic infections.

The worms also do block the lymphatic system as the move around in the body leading to lymphedema (fluid accumulation in the lymphatic vessels), lymphadenopathy (swollen lymph nodes), and lymphangitis (bacterial infection related complications in the lymphatic vessel).

The hallmark clinical feature of lymphatic filarisasis is elephantiasis. This arises due to the gross extreme swelling of the arms and legs as a result of lymphedema. The skin of the affected regions also thickens and become dry resembling the elephant’s leg.

The parasitic zoonosis may affect the groin of both males and females. In males, it affects the scrotum resulting in hydrocele. Patients have retracted penis under the skin and experience burning sensation, pain, and increased risk of male infertility. In females it affects the vulva and patients can suffer from lymphadenopathy.

Lymphatic filariasis further causes pulmonary eosinophilia, particularly in the Asian population. In this case, patients can manifest one of the following signs and symptoms: wheezing, coughing, or difficulty in breathing.

    5. Cryptosporidiosis

Do you love water recreational activities in swimming pools or water parks? Or have you taken your child to day care at a certain point in your busy schedule?

Cryptosporidiosis is one of the parasitic cyclozoonoses responsible for the global waterborne and food gastrointestinal illnesses outbreaks between January 2004 and December 2010.  It is the second leading cause of diarrheal infections in young children after rotary virus.

CDC reports also indicate that the source of cryptosporidiosis outbreaks occurrence in U.S. between 2009 and 2017 was strongly linked to swimming pools (35%), cattle (15%), and day care centers (13%).

The fact that Cryptosporidium species are resistant to disinfectants, like chlorine, which is useful in treatment of swimming pools and water parks raises great public health concerns.

The common causative agents of human cryptosporidiosis belong to the Cryptosporidium species. They are Cryptosporidium hominis and Cryptosporidium parvum. C. parvum is a zoonotic parasite with a wide range of animal reservoirs like domestic livestock, wild animals, reptiles, humans, amphibians, and birds.  C. hominis have high preference for human hosts.

Cryptosporidiosis is water-borne diarrhea that affects both immunocompetent and immunocompromised individuals worldwide. Cryptosporidiosis is the second leading cause of diarrheal disease in young children after the rotary virus.

The disease’s pathogenesis arises following the ingestion of sporulated oocysts of Cryptosporidium species in swimming pools, food, drinking water sources, or child-care settings contaminated with fecal matter. Patients with mild cryptosporidiosis can have watery foul-smell diarrheal, fever, nausea, vomiting, dehydration, fatigue, weight loss, and stomach pains.

Cryptosporidiosis is considered an opportunistic infection in immunocompromised patients (HIV/AIDs, those under immunosuppressive treatments anti-cancer, or those from organ transplant surgeries). These cohorts more likely to suffer from severe cryptosporidiosis. They may exhibit the following clinical signs and symptoms:

• intractable /chronic diarrhea
• malabsorption
• body mass wasting
• biliary tract obstruction
• papillary stenosis
• life threatening malnutrition, growth defects, or even death (young children).

Bottom line

Parasitic zoonoses are infections with substantial global burden that requires urgent attention for their elimination and eradication globally. Schistosomiasis, leishmaniasis, lymphatic filariasis, cryptosporidiosis and cystericosis are grouped under Neglected Tropical Diseases. They affect low and middle-income countries.

The group of infections receives lesser attention. Nonetheless,  the emerging and reemerging of these parasitic zoonoses and their devastating health impacts is becoming a global health concern. To prevent ourselves from these parasitic zoonoses when can apply the following prevention and control measures:

• Maintaining high standards of personal and environmental hygiene. We should promote the use or construction of toilets in areas where they are lacking
• Promote public health education to raise awareness of the parasitic zoonoses
• Protecting oneself from mosquito bites by wearing long-sleeved clothes, use of insect repellents, bush clearing, and sleeping under insecticidal-treated bed nets
• Seeking treatment while sick to prevent misdiagnosis self-treatment and rise of disease resistance through the over the counter drugs. Above all let us stick to the 3 Ts of disease treatment and management mantra (Test, Treat, and Track).
• Governments particularly countries from low and middle-income regions ought to improve their governance in spearheading the management of these parasitic zoonoses.