Agriculture Reference
In-Depth Information
4. Soil contamination with infective stages of zoonotic helminths
T. canis
is transmitted to humans mainly by incidental ingestion of embryonated eggs
present in the soil or soil-contaminated food (Acha & Szyfres, 2003). Since adult female
worms produce large number of eggs and nearly all puppies are infected prior to birth, dog
populations excrete a huge number of
Toxocara
eggs into the environment (Barriga, 1988).
Under favorable conditions (absence of direct sunlight exposure and appropriate
temperature, humidity and oxygenation), particularly in tropical countries,
Toxocara
eggs
can survive in the soil for several years. However, heavy environmental contamination has
also been found in countries with temperate climate, such as Germany (Düwell, 1984) and
Japan (Shimizu, 1993). Most (51-95%) eggs recovered from the soil of temperate countries
were fully embryonated and, therefore, infective (Holland et al., 1991; Jarosz et al., 2010).
Humans can also be infected with
Toxocara
by ingestion of raw infected tissues of other
paratenic hosts, such as cows, sheep or chicken, containing encapsulated larvae (Finsterer et
al., 2010; Nakagura et al., 1989; Salem & Schantz, 1992). Food-borne transmission appears to
be relatively common in East Asia (Akao & Ohta, 2007). Larval development progresses no
further, but parasites can remain viable for up to seven years after infection (Smith et al.,
2009). Although direct contact with infected puppies and kittens is not classically considered
a risk factor for human toxocariasis, since the eggs shed by these animals must embryonate
in the soil before becoming infective, these pet animals may carry embryonated eggs within
their fur (Wolfe &Wright, 2003), in a small numbers (Overgaauw et al., 2009).
The species most commonly involved in human CLM is
Anc. braziliense
. Eggs shed within
the feces of infected hosts hatch in the soil and develop into third-stage larvae in the
environment. Human infection occurs through contact with contaminated soil of beaches,
parks and schools (Bowman et al., 2010).
Eggs of
Toxocara
and zoonotic hookworm larva are found in soils worldwide, especially in
public parks, playgrounds, sandpits, and beaches. Reports of soil contamination with infective
stages of
Toxocara
and hookworm in public areas are available for several countries (Table 1).
Environmental and technical factors, such as soil type, pre-processing sieving, washing, and
re-suspension of sediment, solution employed for washing and flotation, and the specific
density of flotation solutions are all presumed to influence the recovery of ascarid eggs
(Coelho et al., 2001; Nunes et al., 1994; Oge & Oge, 2000; Ruiz de Ybáñez et al., 2000;
Santarém et al., 2009; Santarém et al., 2010).
Other variables, such as climatic conditions (temperature, rainfall, sunlight, etc.) or the
amount of herbage and the presence of animals, number amongst other important factors
contributing to soil contamination that may influence recovery of eggs.
The presence of dogs and cats may also play an important role on soil contamination by
agents of larva migrans. Cassenote et al. (2011) observed that the number of dogs
frequenting parks had an impact on soil contamination in public spaces.
The lack of standardisation of techniques as well as the wide range of factors influencing the
process of egg recovery can lead to false-negative results and underestimation of the
occurrence of contamination, hampering comparison of findings of different reports, and the
assessment of their implications for public health (Coelho et al., 2001).
Fahrion et al. (2010) observed that the mean sizes of
T. cati
(62.3 by 72.7 µm) and
T. canis
(74.8
by 86.0 µm) eggs recovered from feces differed statistically. According to Fogt-Wyrwas et al.
(2007), the differentiation of
Toxocara
spp. eggs from soil by ocular microscopy is extremely
difficult due to the similarity in morphological characteristics of
T. canis
and
T. cati
eggs. As a
consequence, studies have been carried out in an effort to provide molecular techniques for
amplification of
Toxocara
spp. DNA that can be applied in routine examinations.
