Pets Potential Reservoir Hosts Numerous Species Of Parasites Biology Essay


Pets provide many great benefits to human beings. However, they have become a potential reservoir hosts for numerous species of parasites. Close relationship between humans and their companion animals leads to the transmission of animal parasites to humans. Therefore, this paper demonstrates the potential health risk of the three most significant parasitic zoonoses associated with domestic animals particularly dogs and cats, and evaluates the possible factors associated to the emergence of parasitic zoonoses. It can be concluded that parasite adaptation, human behavior and environment changes can promote the survival of new zoonotic parasites.


Pets have played important roles in human daily life, they have been kept as friend and also offer many benefits such as improve development in children (Endenburg and Baarda 1995) as well as therapeutic assistant in elderly people (Hart 1995). Moreover pets can be trained for develop a good habit and provide help in an emergency condition. Cats and dogs are the most popular companion animals with their faithful and playful characteristics. Many family keep cats and dogs as a family members.

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Although pets have many advantages for humans, there are health impacts associated with owning a pet, for instance, animal bites and allergy to pets. However, they have been reported as a reservoir host of a wide range of zoonotic infectious agents (Schantz 1989). Around 30-40% of zoonotic pathogens have been found in dogs and cats (Greene and Levy 2006). Schantz (1989) had also reported that the potential health risk to humans of zoonotic infectious disease carried by pet dogs and cats remains a significant problem in most parts of the world.

Several species of parasite in pets are identified as a zoonotic parasite which can be transmitted to human host accidentally when contacting with infective stages (Glickman and Magnaval 1993, Smyth 1995). Lack of understanding of the importance and the potential risk of zoonotic diseases from pets cause pet owners might not be aware in the risk of parasitic zoonoses transmissible from their pets (Schantz 1989).

The purpose of this project is to demonstrate the potential health risk of the three most significant parasitic zoonoses associated with domestic animals particularly dogs and cats, and evaluates the possible factors associated to the emergence of parasitic zoonoses.

An Overview of Zoonoses

Infectious diseases cause public health problems in humans and animals. The infection of Animals result in the direct effect in animal health as well as production losses in most livestock producing countries of the world (Tomley and Shirley 2009). Moreover, animals can carry various species of infectious agent which can cause human infectious disease.


Zoonoses or Zoonotic infectious diseases are human infectious diseases caused by infectious agents of vertebrate animal which can be transmitted to human (WHO 2008). Such diseases are the serious illnesses and can spread rapidly across the world. These diseases remain a problem in many areas of the developing countries especially in poor households (Robinson 2009 and Dalton).

Due to the definition of World Health Organisation (WHO), "Zoonoses or Zoonotic infectious disease is an infectious disease in vertebrate animal that can be naturally transmitted to human".

Causing Agents

Zoonotic infectious diseases are caused by numerous group of organisms including virus, bacteria, fungi and parasites. Protozoa, helminthes and arthopods have been identified as a member of Parasites in human and animal which some species can cause zoonoses.

Generally, zoonotic agents have a zoonotic potential which means ability to cause disease across species under different condition between animals and human host. Degrees of severity of infection are also different depending on condition in transfer from animal to human (Shakespeare 2002).

Risk groups

The risks of zoonotic infection are related with immune response of each individual. Immune responses for different organisms are different in individuals due to their personal factors like age, sex, ethic group, genetic and frequencies of contact with pathogen. Healthy adults with the normal immune response can acquire zoonotic pathogen every exposure time. In many cases, their immunological response would eliminate the infection, preventing any clinical signs of disease depending on the virulence of organisms (Carithers 2002).

People with high frequency contact with infected animal such as animal handle and food industry operatives are at greater risk of infections, they seem to have lower resistance to infections than those of the hygiene group. Children and elderly person, who are infirm, are more likely susceptible for infections due to incomplete function of their immune system (Shakespeare 2002).

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The high risk group of infection and transmitted disease are the patients with problems of immune system. Both of immunosuppressed and immunocompromised patients are more risk from a wide range of zoonotic infection disease as well as organ failure patients, organ transplants and patient under chemotherapeutics treatments (Wilson et al. 1996). Another risk group, pregnant women are at risk of acquiring specific virulence zoonotic parasites such as Toxoplasma gondii which can transmit through placenta to infect unborn baby (congenital transmission).

Importance of Parasitic zoonoses

Recently, parasitic zoonoses have become the main problems in many parts of the world. These infections are now spreading globally and increasing every year because of many factors such as, human-animal interaction, migration, intensification of agriculture, habitat devastation, and changes in world trade patterns. McCarthy and Moore (2000) pointed that, "the possible reasons for emergence of parasitic zoonoses is the social and environmental change, varieties of diet or culture as well as advancement in diagnostic laboratories which can recognized the neglected infectious agent". An outbreak of parasitic zoonoses have many impacts on public health as well as on the economy in developing countries especially in tropical countries. Some zoonotic parasites can cause production losses in livestock production in most countries of the world leading to the global economic problems.

Parasitic zoonoses are also associated with human diets, behaviors and relationships with different animal species. These zoonotic infections represent significant problems in public health, animal agriculture and conservation, and the meat industry. Parasitic zoonoses include protozoa infection, helminthic infections and arthropod infestation. The most significant parasitic zoonoses are shown in the table 1.

Table 1 The most important parasitic zoonoses

Type of infections


Protozoan zoonotic infections

Toxoplasmosis, Babesiosis, Balantidiosis, Leishmaniasis, Trypanosomiasis, Cryptosporidiosis, Amebiasis

Helminthic zoonotic infections

Echinoccosis, Taeniasis, Toxocariasis, Hymenolepiasis, Trichinellosis, Fascioliasis, Fasciolopsiasis, Angiostrongyliasis, Visceral larva migrans, Cutaneous larva migrans

Zoonotic arthopod infestations

Myiasis, Sarcoptic scabies, Dermatitis

Parasitic Zoonoses of Domestic Animals

Toxoplasmosis, Toxocariasis and Hookworm infections are the three most significant recognised as parasitic zoonoses in cats and dogs, these diseases are mostly found in children, pregnant women and immunocompromised individual with worldwide distribution (Shakespeare 2002).


Toxoplasmosis is one of the major significant parasitic infectious diseases caused by Toxoplasma gondii. It is described as an intracellular parasite of vertebrate animals including humans (Markell and Vogers 1999). T. gondii has a complex life cycle including asexual and sexual reproduction phase which requires different hosts for development (Figure 1).

Life cycle of Toxoplasma gondii

Figure 1 Life cycle of Toxoplasma gondii (CDC, 2010c)

According to figure 1, the definitive host of T. gondii is cat, parasite can develop and shed infective oocysts in cat's faeces. Other animals including humans are infected by ingestion of contaminated oocysts. After ingestion, the parasite transform into cyst in tissue of host. Cats are infected by ingest tissue cyst of intermediate host, and then parasite can continue to develop to the next stage (Markell and Vogers 1999)

Animal infection has no clinical appearance. This protozoa causes a serious condition for humans in all group, especially in pregnant women and immunocompromised patients. Humans can be infected by ingested contaminated oocysts and undercooked infected tissue cysts especially pork and lamb which have been recognized as the most risk of infection. Recently, waterborne toxoplasmosis in humans has been reported as a new route of infection which can contaminate in the environment (Bowie 1997). However, the route of infection in human population is associated with their culture and consumption (Tenter et al. 2000).

T. gondii can be transmitted via placenta leading to congenital infection and abortion (CDC 2010c). In immunocompromised, T. gondii can caused encephalitis as well as visceral organ infection particularly in HIV/AIDS patients. Most of toxoplasmosis cases are associated with cat ownership.

Healthy individual infection is typically asymptomatic whereas 10-20% of acute infection may show flu-like illness which can resolve within a month. Some of them ocular infection can be found along with the visual loss. The disease in immunodeficient patients can be harmful with the infection of central nervous system as well as other systems. Toxoplasmosis in pregnant women is at high risk of direct transmission to unborn babies up to 40% of clinical cases (Shakespeare 2002). Infection in early pregnancy leads to abortion stillbirth. The prevalence of infection in pregnant women in Thailand showed that 31.8% was the cases for women who had cats in their home and 19.3% for those who did not (Chintana et al. 1998).

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Barriga (1997) surveyed the antibodies response to Toxoplasma in population in many continents found that between 16% and 40% of the population in the US and Great Britain and from 50% to 80% of the people in Europe and Latin America have antibodies to the parasite. It can be concluded that they have had an experience of this infection. The prevalence of this infection can be found worldwide, it could be estimated that over one-third of world's population are infected with this parasite (Weiss and Dubey 2009).

Hookworm infection

The second significant helminthic zoonotic condition is hookworm infection. Hookworm species are widely distributed, mostly in tropical and subtropical area.  Common hookworms of dogs and cats comprise Ancylostoma duodenale, A. braziliense, A. caninum and Necator americanus. The adult hookworms are found in intestine of dogs and cats, female worms produce egg passed in faeces. Larva hatch and then develop to the infective stage in soil, the 3rd stage larva can be transmitted via skin penetration as well as ingestion of contaminated food and drink. After entry, larvae need to migrate to the specific organ for development which depends on host specificity. The hookworm life cycle is shown in the figure 2.

Life Cycle of non-human hookworm species

Figure 2 Life cycle of hookworms of dogs and cats (CDC 2010a)

Hookworm species have different migration due to its pathogenicity which can be classified by clinical signs. These parasites can cause two significant infectious diseases in human which are Ancylostomiasis and Cutaneous larva migrans (Bowman et al. 2010).


A. duodenale and N. americanus are hookworm of dogs contributing to Ancylostomiasis. This disease typically found in tropical area. After penetrating skin or being ingested, larvae begin to migrate to the deep layer of tissue and pass via blood circulation to the lung and return to gut lumen for development to be an adult worm. The worm uses mouth part for attach to the blood vessel in the gut and produce and secrete hyaluronidase at site of attachment. Heavy infection of hookworm result to blood lost due to anemia. Moreover, the attachment site can be injured due to the secondary infection of bacteria. The presence of residual hyaluronidase and hookworm anticoagulant after the hookworm lose their hold or are killed by anthelminthic, blood loss can occur due to the presence of this residual factor. The hookworm larva can develop to be adult worm in gut and produce egg and pass in the stool (Shakespeare 2002).

Human infection can be pneumonia due to the worm migration. The larva can be found in other organ such as cornea, spleen and liver. The symptom of the disease include anemia, nausea, vomiting abdominal pain, bloody diarrhea and weakness. Moreover, female dog can transmit worm to puppies via mammary route and placenta (Bowman et al. 2010).

Cutaneous larva migrans (CLM) or creeping eruption

Larval hookworms of dogs and cats, A. braziliense and A. caninum are responsible for cutaneous larva migrans in human which typically found in children.

Creeping eruption is the condition of skin infection due to the larval stage of hookworm of dogs and cats. Clinical signs begin after the infective stage of larval hookworm penetrating through skin. The initial sign shows itchy reddened spot at the entry site and 2-3 days follow, larvae migrating of larvae through the deep layer of skin resulting a raised red and itchy track with localised swelling. The size itchy track may reach everyday to several millimeters.

These infections are typically found in children associated with the contact of contaminated sand or soil. In 2006, there was an outbreak of CLM in a children camp in Florida associated with the sandbox contamination (O'Connell et al. 2007). In addition, almost reported cases were the travelers with beach activities in the endemic area of hookworm infection in cats and dogs. Public beaches in many countries particularly in tropical zone are reported as the source of infection, for instance, Mexico, Barbados, India, Sri Lanka and Thailand (Bouchaud et al. 2000).

Toxocariasis (Visceral larva migrans and optical larva migrans)

The common ascarids of dogs and cats are Toxocara canis and Toxocara cati respectively. They are the most significant zoonotic parasites causing toxocariasis in companion animals, which have become a serious infection in human host. It has been reported as the most significant zoonotic helminths infection related with dogs and cats.

Figure 3 Life cycle of Toxocara spp. (CDC 2010b)

Toxocara spp. has a complex life cycle associated with animals (dog and cat) and humans (Figure 3). Adult worms reside in intestine of dogs and cats, and then pass egg to the faeces. Eggs develop and contaminate in the environments. These worms can be transmitted by ingestion.

Oliveira-Sequeira et al. (2002) reported that over 60% of dog in Brazil were infected by T. canis. Similar to Martínez-Barbabosa et al. (2003) found that the infection rate of T. cati in cat in Mexico was 25.2-66.2%. The evidence seems to indicate that high prevalence of infection would lead to the contamination of infective egg in the environments.

Humans are infected Toxocara spp. by consuming encysted egg contaminated in food and drink. After ingestion, egg will be hatch and larva then penetrate through the intestine to the body tissue. Almost cases of toxocariasis have no serious condition. It was be estimated that around 20 cases of infection per year have been reported in UK. Such incidence is believed that was underestimated. This disease is at risk in children case with severe symptoms (Mani and Maguire 2009). In humans, larva could not develop to be adult worm, larva try to migrate to find the organ with suitable condition. Therefore, the larva migration could lead to two conditions in human host; Visceral Larva Migrans (VLM) and Ocular Larva Migrans (CLM).

Visceral Larva Migrans (VLM)

The disease develop when the larva hatch and move to the body of patient. There was reported that larva can persist to migrate in human host for up to 6 months frequently found in multiple tissue including lung, liver, brain, eye and other tissue. The result of larva migrans is the granulomatous development with approximately 1 cm in diameter. The pathology effects of larva movement produce multiple abscesses, hepatomegaly and pneumonitis. Symptoms are coughing, nausea, vomiting, fever, wheezing, and enlargement of the spleen and lymph nodes

Ocular Larva Migrans (OLM)

This condition is rare but usually serious eye infection resulting in ophtamologic damage as well as visual loss. In this infection, larval worm migrate to the eye and encapsulates causes localised inflammatory reaction and scar on the retina. Retinal pathology can be misdiagnosed with retinoblastoma. Larva can be some time re-emerge and migrate in the eye.

Factors contributing to the emergence of parasitic zoonoses

There are three main factors associated with the emergence of parasitic zoonoses

Human factors

Currently, the number of world population is increasing. The migration of population seems to increase everyday due to the economic as well as tourism growth, the zoonotic parasites are now spreading globally and have introduced to many parts of the world (Stohr and Meslin 1997). The growths of human population rate and insufficiency sanitation have led to the contamination of zoonotic parasites in environment resulting to an outbreak of some zoonotic parasites such as Toxoplasma, Giardia, Cryptosporidium. A further factor associated with human is genetic factor for susceptibility of parasite infection.

Parasites factors

Similar to other zoonotic agents, zoonotic parasite has a zoonotic potential to infect across species between animals and humans. Better adaptation to diverse condition could lead the parasite spread widely. Virulent strains are associated with the high potential of adaptation in wide ranges condition. Cristina et al. (1995) points out that the degree of pathogenicity in Toxoplasma species is different corresponding to genotype and phenotype expression. This hypothesis has been confirmed in the experimental animal.

Environmental factors

In terms of environmental factors, the major topic is the climate change which has brought many detrimental impacts on physical and biological changes. Higher temperature can be directly affected to the development of parasites. Some species can adapt themselves to survive and develop free-living stage without host which can help parasite develop to infective stage in a short time. These can also increase the number of generation of parasite leading to high population of parasite (Harvell et al. 2002). Croese et al. (1995) points out that climate change has a significant influence to the increase of human infection of animal hookworms. High incidence of infection has been found in tropical climates. This change can leads to a new route of transmission (Hoberg et al. 2008).


In conclusion, pets provide many benefits to human beings. However, they can act as potential reservoirs for numerous species of zoonotic parasites. The three most significant zoonotic parasites associated with domestic animals are Toxoplasma gondii, Hookworms and Ascarid nematode. Such parasites are common parasites in pets and could be transmitted to human host directly. The increase level of pet ownership as well as an increase in number of infected dogs and cats leads to the distribution of infective stages which are then further contaminated in the environment and becomes a major source of infection (Glickman and Schantz 1981). Although, infections in healthy individuals sometimes do not have any expressed symptom, the progress of disease could be serious in some specific high risk groups including children, pregnant women and immunocompromised (Juckett 1997). High risk of zoonoses was also found in households with pet ownership.

With regard to the emerging of a new parasitic zoonose, some of the associated factors have been discussed by many research groups. These can be divided into three main areas; zoonotic parasites, human and environmental factors. Parasite factors include the adaptation of parasites in a wide range of host and mode of transmission, and this has encouraged the virulence and distribution of parasites. Human population and their activities and environmental changes can promote the survival of new zoonotic parasites. A better understanding of host parasite relationship under environmental changes would lead to improved strategies of controlling parasite transmission and pathogenesis.