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by Prof. M. C. L. de Alwis

Over the years, a large number of diseases which are transmissible from animals to man have been discovered. Such diseases are termed Zoonoses or Zoonotic diseases.

Presently, the World Health Organisation has listed over a hundred diseases, which are zoonotic. These diseases may be caused by parasites, bacteria, viruses or other agents.

The close association between animals (both agricultural and companion) and man and the use of animal products for human food has caused a considerable concern on the impact of zoonotic diseases on human health.A few diseases, which have either been of current interest and much concern, or are considered as emerging diseases, may yet be underestimated.

Mad Cow Disease

The technical name for what is now considered, as mad cow disease is Bovine Spongiform Encephalopathy (BSF). This is one of a group of diseases called the Transmissible Spongiform Encephalopathies (TSE). Other animal diseases in the group include a disease known as Scrapie in sheep and goats, feline spongiform encephalopathy, milk spongiform encephalopathy and a chronic wasting disease in deer.

The diseases described as Creutzfeldt-Jakob disease (CJD) and Kuru in man also belong to this group.

BSE is a non-inflammatory, degenerative disease of the central nervous system of cattle, and is invariably fatal. It was first established as a specific disease entity in the United Kingdom in 1986.

Although the disease is clearly shown to be transmissible, the nature of the causative agent has not yet been elucidated. It is not one of the conventional infectious agents such as bacteria or viruses and the agent is provisionally described as a "Prion."Affected cattle will show a variety of nervous signs - behaviourial changes, abnormalities in posture and git, extreme sensitivity to sound and touch, and aggressiveness, particularly when handled eg. attempting to kick during milking.

Many of these signs are similar to those of rabies - hence the name mad cow disease.

Other than by the observation of clinical signs, in the affected, living animal there is no diagnostic test by which the disease can be confirmed. It is only after death, that the disease can be confirmed by observing the characteristic pathological changes in the brain. There is no evidence, whatever, that BSE can be transmitted from cattle to cattle, or from cattle to man by contact. Nor is there any evidence of transmission from the cow to its calf.

There is no evidence of transmission to man through consumption of milk. Transmission is only known to occur both to man and animals by the consumption of contaminated parts of infected cattle.

Those organs of infected cattle that are most likely to contain the agent are the brain and spinal cord, and lymphoid tissues such as the spleen. Present information confirms that the chemical extraction process in the preparation of gelatin destroys the agent. There is no test by which one could determine whether a sample of meat contains the agent.It is however significant that the agent can withstand temperatures even higher than what is used for sterilisation, and is also not destroyed by commonly used chemical disinfectants.

The Human Link

The importance of the disease rose to extreme heights when a possible link between BSE and the human CJD was discovered. In the UK during the mid 1990s, when the BSE outbreak in cattle was raging, several cases of an atypical human CJD syndrome was reported. This syndrome differed from the classical CJD syndrome in the younger age group affected and in the distribution of the lesions in the brain.

From available scientific evidence, the UK advisory committee on BSE postulated that what was described as a variant CJD syndrome in humans was caused by the BSE agent.

It was believed that this situation resulted from the consumption of contaminated meat from BSE infected cattle by humans.

BSE in the UK

In the UK up to 1987, 400-500 cases had been reported. From 1988 onwards, the number of cases per year showed a steep rise reaching a peak of over 35,000 cases per year in 1992/93. Thereafter it began to decline, reaching 1537 cases reported in the year 2000. Upto July 2000, 76 cases of human BSE (or variant CJD) have been recorded in the UK, and one case each in Ireland and France.

BSE in Sri Lanka?

The vital question in the minds of all Lankans today is, what chances are there of BSE appearing in Sri Lanka. How safe is Sri Lankan beef and beef products?

Having laid the background information on BSE, I shall now strive to answer these questions.

First, let us consider the conditions that are necessary for BSE to occur in Sri Lanka. These are:

1. We should have imported into Sri Lanka bovines incubating the disease.

2. We should have imported into Sri Lanka and fed to ruminants, cattle feed or feed ingredients contaminated with BSE.

3. Any infective material described above should enter the food chain.

Let us now examine the status of each of these facts.

Import of Animals

Amongst other statutory regulations, the Animal Diseases Act No. 59 of 1992 governs the import of animals into Sri Lanka. No import of animals is permitted without the approval of the Director General, Animal Production and Health.

It must be noted, however, that prior to the implementation of the special regulations, Sri Lanka has imported feed ingredients of ruminant origin (meat and bone meal) from the United Kingdom. In making an assessment of the risk of entry of BSE through these imports, a number of factors need to be considered. In the year 1989, the Untied Kingdom imposed, what is described as the Specified Offal Ban (SOB).

The use of such offal that is more likely to contain the prion agent in the production of meat and bone meal was banned.

Thus, the period between 1989 and 1996 (when a total ban was imposed on UK imports), constituted a 'low risk' period for the entry of BSE through imported feed, and the period prior to 1989 may be considered as a 'high risk' period.

Since 1996, all veterinarians have been alerted on the risk of BSE, but no cases have yet been reported in Sri Lanka, even after a lapse of nearly 12 years, beyond the high risk period, and thus goes beyond the incubation period of BSE. The likelihood of BSE appearing in the next five years or so, resulting from imports made during the 'low risk' period will therefore be negligible.

Brucellosis

This is a disease of cattle (also buffaloes and goats) which is transmissible to man. It was introduced to Sri Lanka through cattle imported from India during the Second World War. The first cases were detected in 1948, and were confirmed by laboratory tests in 1953.

It is a disease that can infect a herd and spread insidiously without obvious signs, since clinically the disease will manifest itself only in the form of abortion in pregnant animals in advanced stages. Infertility, birth of weak calves and retention of placenta may also result.

The causative agent, a bacterium (Brucella abortus) is excreted in abundance with the aborted foetus and vaginal secretions. It is also excreted in milk. In Sri Lanka, the overall prevalence of Brucellosis in cattle and buffaloes around 4%, as evidence by serological tests.

It is noted, however, that the prevalence is considerably high in certain populations of animals, particularly in the North Central, North Western and Eastern Provinces.

Among humans, the most vulnerable population is veterinarians, livestock farm and abattoir workers and others closely associated with susceptible animals. Infection usually occurs through skin abrasions or through the conjunctiva. Infection through milk is virtually non-existent since the consumption of raw, unprocessed milk is not a practice in Sri Lanka.

Although human brucellosis in Sri Lanka is not common, the disease was recognised in Sri Lanka, the early 1960s in studies of unknown fevers (Pyrexia's of unknown origin -PUO) - when cases of human brucellosis has been detected. The acute fever in humans may pass on to a more chronic form with involvement of the musculoskeletal system and other organs, including orchitis (inflammation of the testicles) in males.

Currently, the recommended method of control of brucellosis in animals in Sri Lanka consists of two approaches.

Firstly, animals in high prevalence populations are vaccinated. In low prevalence areas, herds are tested and the positive animals are slaughtered. For many practical reasons, this policy is implemented only in the government livestock farms. A few elite farmers do seek veterinary assistance in keeping their herds free from brucellosis but these constitute only a minute fraction of the total cattle and buffalo population.

Leptospirosis

Leptospirosis is a contagious disease of animals and man, caused by a bacterium belonging to the group - Spirochetes. In animals serological evidence of the disease has been found as early as the 1960s in cattle, goats, dogs and humans. In dogs, it has been recognised clinical entity, and anti-leptospiral vaccines has been regularly used in dogs for many decades. Human leptospirosis also came to be known since the early 1960s, particularly in the investigations into unknown fever (- PUO).

Historically, human leptospirosis has been associated with rats and wild rodents, who are known to serve as vectors and they disseminate the leptospirae through their urine thereby contaminating food and water. In one study of leptospirosis in humans in the Ragama area, it has been suggested that the roaming scavenging pigs - a common sight in this area may be a source of infection. But no scientific evidence is available. In fact, the human disease is so closely linked to rats, that in Sinhalese, the media refer to leptospirosis as 'Mee Una' (Rat fever).

This evidence, strongly suggest the possible role of the buffalo (and perhaps cattle), in the transmission of leptospirosis to human.

Farmers who work in the muddy paddyfields bare footed could very well get infected through skin abrasions in their feet. Another interesting possibility is that farmers who walk behind the cattle or buffalo in the ploughing operation may be exposed to a spray of urine depending on the wind direction, and infection may occur by inhalation of infective aerosols.

There is no doubt that the role of agricultural animals as well as domestic pets in the spread of leptospirosis in humans in Sri Lanka needs further investigation.

Listeriosis

This is an infection caused in man and animals by a bacterium Listeria monocytogenes. The aspect that concerns us most is that the organism is widespread, and occurs as a food borne pathogen. A study carried out at the Kelaniya University in Sri Lanka some years ago has shown that 34% of samples of chicken (raw and frozen), 33-65% of samples of varieties of fresh vegetables and 25-33% of samples of milk and dairy products examined contained the organism.

Its presence in chicken is not surprising due to the asymptomatic nature of listeriosis in poultry. Its overall presence in food items is certainly a matter for concern.

However in Sri Lanka despite the widespread distribution of the organism, no human infections have been recorded. Even in other countries, listeriosis has been found to cause clinical disease only in children, the elderly, during pregnancy and in immuno-compromised individuals. A few sporadic cases of listeriosis have been recorded in Sri Lanka, in buffaloes, goats and geese.

The key to prevention of outbreaks arising from contaminated food appears to be proper cooking or processing.

Those in the food industry should take care to ensure adequate cooking and post processing contamination, especially of "ready to eat" fast foods, particularly those containing chicken.

(From a paper presented at the 50th Technical Program of the OPA in association with the SLVA).

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