How are most parasites transmitted
Danger to humans and animals
Hookworms live in the small intestines of dogs and cats. If the eggs of these helminths are excreted with the feces, larvae hatch from them and can actively attack the next host by penetrating the skin. Under favorable environmental conditions, the larvae can survive for months (4). They can also affect humans when they come into contact with the skin, but they get stuck in the epidermis on their migration and usually continue to move in the skin as cutaneous larva migrans.
Dog puppies are often infected at an early age with larvae of the dog roundworm (genus Toxocara), which are transmitted to the offspring via the uterus (intrauterine) or via breast milk (lactogenic) (table). Due to hormonal influences during the pregnancy of the bitch, encapsulated larvae become active again. The infection rate shortly after birth is 70 to 100 percent in Western Europe. Human children get infected quickly via their hands or mouth; Typical symptoms are fever, cough, gastrointestinal disorders, skin changes up to damage to the eyes and the involvement of other organs by the visceral larvae.
The prevalence of Toxocara infection gradually decreases with the age of the dogs and averages 4 to 7 percent in dogs and cats. In German animal shelters, however, roundworms were found in more than a quarter of the cats. Not to be neglected are the dangers posed by the coat contamination of dogs with zoonotic helminths. Sandboxes and play boxes that are freely accessible are often contaminated, and Toxocara eggs remain infectious for up to four years (5).
Dirofilaria is a nematode that can grow up to 30 cm long in the adult stage and infects dogs and cats as a heartworm. Mosquitoes transmit larval stages to the animal. The distribution area of the Dirofilariae extends over southern and southeastern Europe. There are cases in which people contracted an infection through close contact with their non-dewormed dogs or cats, whereby implantation in numerous organs is generally possible (6).
The fox tapeworm is also brought to humans by dogs. Small mammals such as field vole, bank vole and water vole are intermediate hosts for Echinococcus multilocularis. If dogs eat these intermediate hosts, they become infected. Problematic with fox and dog tapeworms: Even if humans are infected in childhood, symptoms can only appear after many decades. Possible complaints vary depending on the organ involvement, for example fatigue, pain, weight loss, jaundice and even liver cirrhosis.
The most important parasitic protozoa in the small intestine of dogs and cats are Giardia (Giardia duodenalis). 15 to 23 percent of dogs and cats in Germany are infected. Significantly higher values were found in animal shelter studies in other European countries. Scientists are currently investigating the potential danger that the Giardia genotypes of domestic animals pose to humans. However, it is believed that giardia can be transmitted from pets to humans (and vice versa). Among other things, they can cause acute and "chronic" diarrhea in humans. Infection of humans is notifiable according to the Infection Protection Act (IfSG).
In the gastrointestinal tract of mostly young cats with chronic colon diarrhea, a greasy, strongly smelling diarrhea with blood and mucus, the single-cell Tritrichomonas fetus has been found more frequently in recent years. This is the causative agent of the cover disease in cattle.
In the case of rabbits with tilted heads, encephalitozoonosis comes first in the differential diagnosis. In addition to neurological symptoms, infected animals can also develop chronic kidney failure. These microsporidia, such as Encephalitozoon cuniculi, are also often diagnosed in asymptomatic animals. In a study in Austria, up to 60 percent of the rabbits were infected, but without symptoms. In recent years, cases of encephalitozoon in cats and dogs with sometimes severe symptoms have become known. People with immunodeficiency can also experience a number of symptoms, including kidney failure.
Closely related coccidia, namely Isospora species, are also found more frequently in dogs and are associated with blood diarrhea.
Dogs are the major host animals for leishmanias, which are transmitted by ectoparasitic sand flies. The protozoa parasitize in mononuclear phagocytes, i.e. immune cells that actually (should) serve to defend against pathogens. In dogs, Leishmania infantum infection is more chronic and sometimes fatal. The incubation time until clinical symptoms appear is often long, which complicates the diagnosis of canine leishmaniasis, which is usually imported. In dogs, ulcers are found on the nasal mirror and ears, as well as inflammation in the area of the eyes. Abnormal claw growth (onychogryposis) is another symptom. In some dogs, the infection is subclinical; these represent dangerous reservoirs. The pathogens reach humans via sandflies and can trigger cutaneous, mucocutaneous and visceral forms of leishmaniasis, depending on the species.
The autochthonous ("native") dog babesiosis, often called "dog malaria", is also regionally widespread in Germany and is increasing. In Saarland, for example, around 4 percent of the Dermacentor reticulatus ticks (alluvial forest ticks) that are collected in which both males and females suck blood are "loaded" with Babesia. When infected with these protozoa, the dog develops a febrile illness with movement disorders, vomiting and paresis up to acute kidney failure. In humans, the infection usually takes an inapparent course, which, however, can be more severe and even life-threatening in immunosuppressed patients.
Toxoplasma gondii uses cats as final hosts, but has a wide range of warm-blooded animals as intermediate hosts. Humans become infected (among other things) through mature oocysts, for example from cat feces. If a woman becomes infected in early pregnancy, this can lead to severe damage to the embryo and even abortion. This particular risk of infection can be easily prevented: If a cat lives in the household, a pregnant woman who does not yet have antibodies against Toxoplasma gondii should never clean the litter box. The infectious stages (oocysts) released with the cat feces are not infectious immediately after excretion because they are not yet "ripe". Therefore, having someone else who is not pregnant clean the litter box every day significantly reduces the risk (7).
Danger to pets when traveling
"When someone goes on a trip," they bring souvenirs with them - sometimes parasites too. Since many animal owners take their four-legged friends with them on their travels, international tourism increases the risk of ecto- and endoparasites spreading to previously non-endemic areas. Typical travel and import diseases in dogs are Ehrlichiosis, Dirofilariasis, Hepatozoonosis, Babesiosis and Leishmaniasis. The pathogens are transmitted by ectoparasitic insects or ticks (10).
For example, for reasons of animal welfare, dogs are currently being imported to Germany, for example from Mediterranean countries such as Italy, Spain, Portugal and Greece, but increasingly also from Southeastern and Eastern Europe. Of the so-called imported dogs, around 10 to 24 percent have antibodies against Babesia canis, 12 percent against Leishmania and 10 percent against Ehrlichia canis (11, 12). It is not uncommon for latently infected puppies to be imported from the Mediterranean region, which only develop canine leishmaniasis years later (13). Heartworm infections were also found in more than 5 percent.
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