M.C. Agrawal
The author is National Fellow and Professor at the Indian Council of Agriculture Research, Department of Parasitology, College of Veterinary Science and Animal Health, Jabalpur 482 001, India.
En médecine humaine, la bilharziose ou schistosomiase arrive juste après le paludisme en ce qui concerne le nombre de personnes infectées et ses incidences. Du fait de l'importance de la maladie, de sa présence dans 70 pays où elle touche 200 millions de personnes, l'Organisation mondiale de la santé (OMS) a lancé un Programme spécial de recherche concernant les maladies tropicales (TDR) pour lutter contre cette affection et contre cinq autres maladies. La schistosomiase humaine est principalement causée par Schistosoma haematobium, S. mansoni et S. japonicum.
En la medicina humana, la bilharziasis o esquistosomiasis sólo está por detrás del paludismo en cuanto al número de personas infestadas y sus consecuencias. Debido a la importancia de la enfermedad y su prevalencia en 70 países con 200 millones de personas afectadas, la Organización Mundial de la Salud, puso en marcha un Programa especial de investigaciones sobre enfermedades tropicales, para combatir esta enfermedad junto con otras cinco. La esquistosomiasis en el ser humano está producida sobre todo por Schistosoma haematobium, S. mansoni y S. japonicum.
In human medicine, bilharzosis or schistosomosis is second only to malaria with regard to the number of people infected and its implications. Because of the importance of the disease, which is prevalent in 70 countries and affects a total of 200 million people, the World Health Organization (WHO) launched a Special Programme for Tropical Disease Research (TDR) to control this and five other diseases.
Schistosomosis in humans is caused mainly by Schistosoma haematobium, S. mansoni and S. japonicum. In addition to these, there are 17 species of schistosome affecting all types of animals.
So far, however, livestock and other animals have been considered merely as reservoirs or transmitters of schistosome species to humans, and animal schistosomosis has only been studied to a limited degree for its pathogenic effect and the production losses endured by specific countries. This is reflected by the fact that the infection has not been included in the FAO/OIE/WHO Animal Health Year Book. Viral and bacterial diseases continue to predominate in veterinary services.
Why has schistosomosis been underestimated? Research in Southeast Asian countries has shown the existence of Schistosoma spindale, S. indicum, S. incognitum, S. nasale, S. japonicum, S. haematobium, S. mekongi, Orientobil-harzia turkestanicum, O. dattai, O. bomfordi, O. harinasutai and Bivitellobilharzia nairi.
If this research is extended by using modern techniques, more species, subspecies or strains with differences in host susceptibility and patho-genecity may emerge such as S. malayansis. Work performed in research institutes has shown the existence of one or more schistosome species in all types of animals; the prevalence rate varied from 0.25 to 80 percent, depending on host species, age, area and, most important, sensitivity of the diagnostic test employed. Yet the disease has failed to attract the attention of veterinary organizations and an accurate assessment of its harmful effects is still elusive.
A prerequisite for assessing the significance of any infection is a specific and sensitive diagnosis. Scientifically, it is necessary to isolate schistosomes or their eggs in animals prior to attributing a pathological condition to the infection, a task that is difficult to perform in the field.
Veterinary diagnostic laboratories at the divisional or district level examine faecal samples by using the direct smear methods or, in some cases, the salt flotation method. Both methods have proven ineffective in detecting schistosome eggs. On the other hand, results have shown that the hatching technique is a simple test; it does not require the use of chemicals and it is more sensitive than faecal egg counts. However, no field diagnostic laboratory is using this test in faecal examinations. Moreover, only selected faecal samples with a history of diarrhoea or dysentery are dispatched to the laboratories for microscopic examinations.
Additionally, there are no facilities to conduct routine post mortem examinations in the veterinary hospitals and only in legal cases are these carried out. In such cases the carcass is examined only for poisoning, trauma and some gross lesions. Likewise, no attempt is made in slaughterhouses by veterinarians to search the mesentery of any animal for the presence of schistosomes except where scientists are engaged in work on schistosomosis. Neither liver nor intestine is digested for schistosome eggs. Hence, it appears that no attempts are being made in the field to diagnose schistosomosis. At the most, a liver showing gross lesions is cut and liver flukes are often recovered in fair numbers.
This is substantiated by the detection of liver fluke eggs in diarrhoeal faecal samples. If serious diagnostic attempts are made, schistosomosis may even surpass fasciolosis in prevalence and pathology. It would be rational to study the effects of multiple infections including schistosomosis, since animals usually harbour multiple infections in nature rather than a single one.
The other factor, linked with laboratory diagnosis, is the timing of faecal examinations for schistosomosis. Obviously, faeces are subjected to investigation in cases of diarrhoea or dysentery. Our limited observations in experimental and laboratory animals have shown the absence of watery diarrhoea in schistosomosis (one- to three-month infection) although blood was detected in mouse faeces by the Benzedrine test. It is difficult to observe gross diarrhoea or dysentery in this or any other host species (pigs, sheep, rabbits, guinea pigs). However, dysentery has been reported in dogs in this way.
Therefore, there are virtually no occasions in the field that may prompt a veterinarian to examine an animal for schistosomosis. There are only a few reports in the literature of animals that were suffering from diarrhoea harbouring blood fluke eggs. It is worth investigating the duration and intensity of the infection, the immunological status and the host species when schistosomosis is associated with diarrhoea or dysentery.
Obviously, the question arises as to what harm schistosomes may cause in animals that are not suffering from diarrhoea or dysentery? In fact, this aspect needs to be thoroughly investigated, along with bacterial, viral and other parasitic infections that may expose animals to blood flukes. Previously, some scientists considered the infection to be pathogenic, while others did not, since the animals did not appear to be affected by the infection (there were no signs of diarrhoea, dysentery or fever). Histopathology examinations have shown the existence of granulomas in the liver and intestines but, after observation of the tissue area covered by the granuloma, they were not considered too harmful.
The limitations of such studies are that they reflect the pathology at a particular time whereas pathogenesis is a dynamic phenomenon. They also ignore immunological and biochemical reactions, which are the basis of pathogenesis in schistosomosis.
When biochemical methods were employed to assess pathology in our laboratory, the infection proved pathogenic even during the prepatent period (one-month-old infection) affecting the liver and intestines of the animals. The most important fact that came to light was weight loss or no weight gain in infected animals in comparison with non-infected control animals, although there was no apparent diarrhoea or dysentery in the infected animals. Moreover, some of the experimentally infected piglets (1 500 cercariae of S. incognitum) died one day after the infection, which continued for about 20 days, killing six of 22 infected piglets. Their death appeared to be due to the immunological reactions because of the parasite, and conventional methods failed to establish parasitic harm to the animals. Interestingly, only male flukes were recovered from the dead piglets.
Would this suggest that immunological reaction provoked the death of the female flukes and that of the host? In another experiment, a kid infected with S. incognitum died suddenly after 80 days of infection. Another sheep, infected with 900 cercariae of S. nasale also died suddenly 105 days after infection, even though there were no signs of worms or eggs in the nasal cavity, although S. nasale male flukes were recovered from the liver. In both cases, the non-infected control animals survived for more than a year.
These facts strongly suggest that schistosomosis does not only influence the health of the animals by reducing body weight, but may also cause death even prior to the excretion of eggs in faeces. However, in all these cases, there were no specific symptoms; nor was there prolonged sickness, as death was sudden. To complicate the scenario, faecal and post mortem examinations were rarely done in the field as recommended. Thus, it is apparent that schistosomosis is underestimated in the field.
In our opinion, egg detection methods should be replaced by the hatching test. During the post mortem, the liver and part of the mesentery should be separated, chopped and stored in a normal saline solution for four hours. The saline solution should then be searched for blood flukes. These two simple methods will be of great help in performing a proper diagnosis and fair assessment of schistosomosis in the field.
Moreover, the notion that death occurs only in bacterial and viral diseases, while helminths influence, if at all, only the general health of the animal, must be changed. This notion is a great impediment to the adoption of new approaches to helminth diseases and the correct assessment of losses caused by them. The whole subject clearly requires thorough investigation, especially given the vast distribution of schistosomosis influencing all types of animals and in view of the animal husbandry practices being followed in South Asian countries.