Microsporidia are widespread organisms in nature, but they have been considered for a long time to have a low impact on animal or human health. With the advent of HIV and AIDS however, significant numbers of human cases of microsporidiosis have been reported and the zoonotic impact of this infection has recently been confirmed.
Microsporidia are small, unicellular parasites. Their transmission between infected and healthy organisms occurs by means of highly resistant spores that contain the infective material called "sporoplasm". Currently about 1000 species have been described but it is generally accepted that many others are present in nature (Canning, 1993).
Some aspects that remain largely obscure with respect to the biology of these parasites are: the epidemiological patterns, the susceptibility of different vertebrate organisms to the parasites and the routes of transmission between susceptible hosts.
Recent studies (van Gool et al., 1997) reported that a high proportion of immunocompromised persons harbour antibodies against Encephalitozoon spp. compared with healthy individuals. These findings can account for the wide distribution of microsporidian in the environment. They also suggest that without an impairment of the immune system they do not represent a serious health risk because they are unable to induce infection.
Human beings are likely to be infected by means of ingestion of contaminated food or water. The zoonotic significance of a microsporidian is clearly evidenced by the fact that Pleistophora-like species are commonly found in muscle tissues of fish and crustaceans (Canning, 1993). E. cuniculi is able to infect many species of mammals and E. hellem has been isolated in cage birds (Black et al., 1997; Pulparampil et al., 1998), and E. bieneusi, the most common microsporidian in humans, has been isolated in pigs (Deplazes et al., 1996).
Recent epidemiological surveys, based on the identification of parasites by means of polymerase chain reaction (PCR) and sequencing technology, showed that E. bieneusi is present also in cattle (Fayer, Santin and Trout, 2003) and in wildlife (Sulaiman et al., 2003). Data presented in these surveys also suggest that other genotypes, which are more host-adapted, can occur in animals that do not have major public health significance.
In AIDS patients the most common microsporidia are Enterocytozoon bieneusi and Encephalitozoon intestinalis, which are associated mainly with chronic diarrhoea and weight loss. These two species are normally restricted to the small intestine but more rarely, are able to spread to the gall bladder, to the pancreatic and bile ducts and even to the respiratory system (Weber et al., 1994).
Less frequently, other species of Microsporia have been isolated in human beings. Encephalitozoon cuniculi, Encephalitozoon hellem and Nosema corneae have been known to infect corneal and conjunctival epithelia. Pleistophora spp. and Brachiola vesicularum have been reported as the cause of weakness and muscular pain.
PREVENTION OF INFECTION
Due to a lack of information about the routes of transmission and the degrees of susceptibility of different hosts to infection, it is difficult to outline the correct behaviour to minimize the risk of infection. Nevertheless, it is important to emphasize the importance of the aforementioned high hygienic standards, with particular emphasis on the following rules:
avoid eating raw meat or fish;
drink only bottled, filtered or sterilized water;
avoid direct contact with faeces or litter of caged, domestic or farm animals.