Discussion

4.1 State of knowledge and distribution

Although three databases were used, this work could have some bias due to the exclusion of grey literature, like technical reports, congress abstracts or thesis man­uscripts, so some sites or negative data may be excluded in the analysis [109].

Although sample size is the only factor that significantly affected richness, other factors to consider could be the analytical methods used and whether the sample was fixed or not. Sample size affects the results, generating deviations in the num­ber of species and in their prevalence, especially in places where the sample size was too low. On the other hand, a lack of methodological specifications can be observed in the techniques used. This could imply potential biases in the reporting and/or interpretation of data. In order to obtain data of higher quality, a general consensus should be reached on the techniques to be applied. It is also desirable to apply molecular techniques that allow parasite identification to species level, thus solving

records identified to family level, such as "Ancylostomatidae ” or “Strongylids”, or the recording of species outside their natural range of distribution, like Dibothrio- cephalus in the northeast of Argentina.

The presence of a greater number of species, most of which have zoonotic potential, in urban areas than rural ones is probably due to the fact that dogs can roam freely. Dogs spread the parasite eggs, thereby these areas will function as contagion points for both other dogs and humans. A further problem is that deworming in these countries is insufficient [21]. A similar situation has been detected in parks in the United States, where it has been suggested that dogs are at risk of infection with parasites at these sites, and it has been recommended that preventive strategies be considered [30,110]. Some parasitic infections could become increasingly urbanised, and an estimation for 2050 indicates that up to two- thirds of the global population will live in megacities. The slums of these megacities would concentrate high levels of intestinal helminth. Toxocariasis and other urban soil-transmitted helminths are important, yet little studied, health issues in the cities of the Americas [111].

The zoonotic broad tapeworm, Dibothriocephalus sp., is found in dogs from the endemic zone of the disease, the Andean Patagonia of Argentina and Chile [93, 104]. The records from the northeastern region of Argentina require revision, as there are no molecular studies confirming the identity of these parasites, and there are no records of fish infected by plerocercoids in this zone. Although Dibothriocephalus sp., is not transmitted to humans by dogs, they can act as dissem­inators of the disease and are often used as sentinel species for the spread of the disease in some areas. Ascaris sp. in dogs is distributed mainly in subtropical regions of Argentina, where this parasite is most prevalent in humans [107]. Some parasites are distributed throughout all the latitudes regardless of the type of climate, like T. canis., T. vulpis, and Ancylostomatids, as observed in other parts of the world [112-114]. Echinococcus sp. is distributed across almost all rural areas of the three countries, although has recently also been registered in cities [35, 47, 64,115].

4.2 Zoonoses and human cases reported

The high percentage of parasites with zoonotic potential reinforces the need to establish effective prevention measures, not only with regard to parasitosis in ani­mals but also to transmission to humans. This situation highlights the need for better integration between specialists in animal and human health [74]. A few diseases transmitted by dogs have surveillance mechanisms in humans, but there are many other important zoonoses worldwide, with numerous human cases, which are not kept watch on. Some of these have been recorded in Argentina and Chile, such as those caused by T. canis, Ancylostoma sp., A. caninum, Uncinaria sp., and Strongyloides sp. [30]. Of the main zoonoses recorded in dogs in the three countries, cystic echinococcosis is the only one which has to be reported to the health author­ities, since it is of major sanitary importance [115]. The others, like toxocariasis, hookworm and strongyloidiasis are not reported, and records of human cases in these countries are scarce. The status of these zoonoses in humans from southern South America is analysed below.

4.2.1 Cystic echinococcosis

Cystic echinococcosis or hydatidosis, produced by Echinococcus granulosus sensu lato, is a highly endemic parasitic zoonosis in South American countries, especially in Argentina, Chile, Uruguay and Brazil. It is associated with rural areas dedicated mainly to goat and sheep breeding, and causes significant economic losses [47, 69,116-118]. From 2009 to 2014, a total of 29,559 new human cases of cystic echinococcosis were registered in these countries. The average fatality rate across the three countries was 2.9%, suggesting that the disease causes approximately 880 deaths annually. The most affected are children < 15 years of age, which is indicative of a persistent environmen­tal risk leading to new cases [69,115]. In the countries analysed, Government Control Programmes have been addressed, and surveillance of the disease from a holistic perspective based on Primary Health Care has been implemented [64, 69,115,117].

4.2.2 Toxocariasis

Toxocariasis is an infection that has a worldwide distribution and is a very important zoonosis due to its frequent occurrence in humans [119]. The estimate of the overall worldwide prevalence of T. canis in dogs of 11.1% represents 100 million dogs, which should alert Public Health experts and policy makers to the need for effective intervention programs [114, 120]. This parasite species has high biotic potential since its eggs contaminate water, soil, grass, and pet fur [51]. The results presented here regarding T. canis in dogs of southern South America show higher prevalence values (around 13%) than the overall prevalence registered worldwide. Also, the risk of infection is similar in urban and rural areas, as suggested in Chile [105]. In Argentina, numerous studies that analysed the seroprevalence of toxocariasis in both children and adults from urban and rural areas reported results varying between 28% and 80% [51, 121,122]. In Chile, the seroprevalence of this parasitosis varies between 1.3% and 25.4% [105]. Although in Uruguay there are no published records of seroprevalence in humans [123], a recently published work reported that from 2014 to 2018, 20 children had been treated in the public health system for ocular and visceral larva migrans syndrome [123].

4.2.3 Ancylostomiasis

Dog hookworms are Ancylostoma caninum, Ancylostoma braziliense, and Uncinaria stenocephala, and their eggs can be found in faeces. The larvae of these parasites can cause cutaneous larva migrans in humans [124]. The main causal agent of larva migrans worldwide is A. braziliense; however, the causative agents vary among geographical areas, even within a single country. This disease is mainly endemic to tropical and subtropical developing countries with high average annual temperatures and humid climates, predominating in America from the southern United States, through Mexico, Central, and reaching South America.

4.2.4 Strongyloidiasis

Strongyloidiasis is prevalent in remote socioeconomically disadvantaged com­munities around the world, and dogs can act as reservoirs of human strongyloidiasis [128]. This parasitosis is registered in the north of Argentina, with similar infection values in both rural and urban populations and an overall seroprevalence of 19.6%

[129,130]. In Chile, the seroprevalence is much lower (0.25%) in blood donors from Arica and La Union. Human infections by S. Stercoralis in this country are therefore endemic, with very low frequency in apparently healthy individuals [131].

5.