Trichophyton verrucosum
Trichophyton verrucosum is a clonal (Kano et al. 2014), slow-growing species with global distribution. It is typically found in cattle and other ruminants (Fig. 3.6), but it can easily spread to humans and animals, including horses, donkeys, camels, rabbits, dogs, cats, pigs, and even birds (Baudet 1932; Blank 1955; Georg 1960; Dvorak et al.
1965; Ali-Shtayeh et al. 1988; Khosravi and Mahmoudi 2003; Chermette et al. 2008). Modern intensive battery farms are the main reservoir of T. verrucosum in developed countries as conditions favor its proliferation. It is mainly transmitted through direct contact with infected animals or contaminated environment, and therefore high prevalence levels often occur in overcrowded stables where the fungus can spread easily among subjects confined in small areas. In cattle, ringworm is usually more widespread in young animals because of their lack of specific immunity against the fungus. The infection is often clearly evident, with alopecic areas covered with thin farinaceous desquamations or with thick crusty lamellar scales difficult to pull off the skin. Lesions are mainly distributed on the head and neck (Fig. 3.6), but in most severe cases, the whole body can be affected. Although frequently considered as a benign self-healing infection, ringworm in cattle may be responsible for economic losses due to the negative impact on milk and meat production. Ringworm also leads to impairments in the hide and skin industries, as lesion scars are evident on leather following tawing and tanning (Chermette et al. 2008; Bond 2010; Hameed et al. 2017).Trichophyton verrucosum infection can be considered as a cosmopolitan disease as, over time, it has been reported in livestock and people in different countries from all continents. Infection rates in cattle appear variable depending primarily on the geographical and social context.
For example, low infection rates (around 2%) were detected in rural areas of Pakistan (Hameed et al. 2017) and Iran (around 5%) (Aghamirian and Ghiasian 2011). In such contexts the fungus probably does not find the conditions which are known to promote its spread, such as overcrowding of animals and high humidity, which are more typically encountered in intensive breeding. Indeed, some surveys, performed instead in intensive and semi-intensive farms in Central Italy, detected much higher infection rates, i.e., 19, 60, and 88% (Moretti et al. 1998; Papini et al. 2009; Agnetti et al. 2014). In particular, the survey which reported the highest prevalence (Papini et al. 2009) was based on the analysis of only young animals living in crowded environments; the authors found high prevalence even in asymptomatic animals (80.4%). In addition, the investigation was carried out during winter months when skin lesions are in general more common because stabled animals are in close contact. A high infection rate (31%) was also found in a study in Jordan which took into account ten large dairy farms (Al-Ani et al. 2002). Other studies reporting high infection rates describe actually outbreak episodes on a limited scale, e.g., study of Dalis et al. (2014) from Nigeria. Likewise, the infection rate reported in a Chinese study (20%) reflects the prevalence within an outbreak in a single farm, with 200 animals infected out of a total of 1000 (Ming et al. 2006). The infections are less frequent in small ruminants (Chermette et al. 2008; Bond 2010), perhaps due to a stronger inherited immune response against the fungus compared with that of cattle or to other factors linked to the breeding systems. An increasing prevalence of the disease and the existence of extensive outbreaks were documented in sheep (Fig. 3.6) in the UK, USA, and Morocco (Pandey et al. 1979; Power and Malone 1987; Sargison et al. 2002). It cannot be excluded that dermatophytosis in sheep and goats is actually an under-diagnosed disease.The incidence of infections in cattle was decreased in many regions by specific fighting measures, especially by vaccination programs or changes in the agricultural systems, such as reduction of the number of cattle in breeding units; the infections in humans decreases proportionally (Seebacher et al. 2008; Lund et al. 2014). Lack of prophylaxis with T. verrucosum vaccination accounts for the high infection rates in Italy (Moretti et al. 2013); in contrast cattle ringworm due to T. verrucosum was eradicated in Norway (Lund et al. 2014).
Trichophyton verrucosum is characterized by a high zoonotic potential. People at higher risk of infection are farmers and their families, and veterinaries and technicians involved in animal management. Several week-long sick leaves of employees further increase financial costs (Moretti et al. 2013). Human patients usually develop aggressive inflammatory skin lesions (usually on extremities and head), which may be accompanied by constitutional symptoms, such as fever and lymphadenopathy (Silver et al. 2008; Courtellemont et al. 2017). Tinea barbae and capitis are relatively common clinical forms which can result in irreversible scarring and alopecia. The incidence of human infection among other dermatophyte species is very high in some regions of Africa and Middle East (up to dozen percent) and relatively low in European countries and the USA (usually 0-2%, but up to 4%) (Havlickova et al. 2008; Seebacher et al. 2008; Moretti et al. 2013; Courtellemont et al. 2017).
Zoophilic variety T. verrucosum var. autotrophicum (described from Karakul sheep, goat, and cattle) and T. immergens (from ruminants) have morphology resembling that of T. verrucosum, but based on molecular genetic data, they are closely related or identical to T. mentagrophytes and T. tonsurans, respectively (Graser et al. 1999b; de Hoog et al. 2017). Infections in horses and donkeys due to T. verrucosum-like strains are relatively rare but occur worldwide (Lyskova et al.
2015). It is probable that at least part of these infections is caused by phenotypically similar T. bullosum that has been confirmed by molecular data from North Africa, from Middle East, and recently from Europe (Sitterle et al. 2012; Lyskova et al. 2015; Sabou et al. 2018). Large-scale and comprehensive studies have not been performed worldwide on the epidemiology of dermatophytosis in cattle, other ruminants, horses, and donkeys using DNA-based methods for identification of dermatophytes. Trichophyton verrucosum was confirmed by DNA sequencing as an agent of infection in recent studies examining T. verrucosum-like isolates from cattle and patients infected by cattle from Japan, Czech Republic, and Tunisia (Hubka et al. 2014b; Kano et al. 2014; Neji et al. 2016). In contrast, T. bullosum has been confirmed only from horses, donkey and a patient who was likely infected from a donkey. Host specificity of T. bullosum, but also the real etiology of infections in uncommon hosts, should be verified in future studies.3.7.4