Veterinary Epidemiology and Zoonosis

EXERCISE-7: FIELD SURVEY ON ZOONOTIC DISEASES

Introduction

• Zoonotic diseases can be transmitted from animals to humans. Four-fifths (around 80 per cent) of all human diseases are zoonotic diseases. In recent years, zoonoses and communicable diseases common to man and animals have gained increasing attention worldwide.
• As a result of dynamic of human population, change of life style, ecological encroachment for agricultural activities and subsequent exposure to little known flora and fauna, animal migration and trade, etc. have increased the possibility transmission and pose a threat to human population.
• To survive, a biological pathogen has to be a chronic infection, stay alive in the host for long periods, or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, for many 'human' diseases, the human is actually an accidental victim and a dead-end host (Examples: Rabies, Anthrax, Tularemia and many others).
• For understanding of zoonotic animal diseases in terms of their epidemiology, mechanism of transmission to man, diagnosis, prevention and control, field survey of zoonotic diseases is playing vital role.

While carrying out field survey the following questions have always to be remembered.

• What are the common zoonotic diseases prevalent in the area?
• What are the types of animal present?
• What is the main occupation of the people in the same area?
• How the zoonotic diseases are transmitted?
• Regarding the spectrum of host specificity as primary host, reservoir host and accidental host.
• Regarding morbidity and mortality rate of the zoonotic diseases prevalent in that area.
• Regarding socio-economic condition of the human population.

General guidelines to be remembered during filed survey

• · The quality of a clinical microbial report is functionally dependant on the quality of specimen submitted.
• This in turn is influenced by the nature of the specimen, timing and mode of collection of specimen.
• The preservative and transport media used, and transit time and information furnished along with sample are also important.
• A complete case history including tentative diagnosis should be submitted to the laboratory.
• This information will enable the diagnostician to select the most appropriate procedure and to furnish meaningful interpretation of result.

What is the need of collection of samples and how to collect?

• Samples are collected for three purposes, viz., direct examination of clinical material, isolation and identification of infectious pathogen, and serological investigation.
• Samples should be taken from living or recently dead animals.
• Samples should be taken from affected site as early as possible following onset of clinical symptoms.
• Samples should be collected from clinical cases and contact animals.
• Samples should be obtained from edge of the lesion and include macroscopically normal tissues.
• Samples should be collected as aseptically as possible.
• Specimen should be collected before the treatment.
• Specimen of pus, wound exudates or tissues should be sent in sterile containers.
• For bacteriological diagnosis, culturing from swabs can be done directly on solid or liquid media. If culturing is delayed, swabs should be placed in transport media to prevent desiccation, and refrigerated.
• For viral diagnosis, swabs should be sent in viral transport medium on ice (If transport medium is not available, normal saline or phosphate buffered saline or sterile water may be used and submitted immediately to the laboratory on ice). Sample should be stored at -20°C if tests are to be conducted after 24 hours of collection.
• For mycological diagnosis, sufficient amount of clinical materials can be collected in nutrient broth or sterile water. Clinical material should reach the laboratory with in minimum possible time.
• For chlamydial diagnosis, swabs should be immediately placed in transport medium with antibiotics and foetal calf serum. Swabs can be stored at 4°C, if it is inoculated within 24 hours of collection or it should be stored at -60°C.
• Labeling must be clearly given to the sample container.
• Sample should be packed in a leak proof container and submitted to the laboratory.
• Blood/serum and blood smear: Whole blood with anticoagulant should be collected and despatched on ice. It should not be frozen. Clotted blood should be collected for separation of serum. Serum can be inactivated at 56°C for 30 minutes or preserved with merthiolate 1:1000 solution. No preservative should be added to the serum, if it is used for virus isolation or ELISA. Serum should be stored at -20°C. Blood smear should be taken from the peripheral veins at peak of temperature for the identification of blood parasites or blood protozoan.
• Urine: Mid stream urine should be collected for urinalysis or culture and identification pathogen or viable bacterial count. Urine can be collected by cystocentesis or catheterization.
• Vaginal or uterine discharges: A 3-way catheter can be used for the collection of uterine contents and washings. Uterine biopsy catheter can be used for the collection of uterine mucous membrane.
• Faeces: Faeces should be collected directly from the rectum or immediately after defecation. Faeces should be processed immediately, or it can be stored at refrigeration temperature for 24 hours, but it should not be frozen.
• Skin scrapings: Skin scrapings should be collected at the active periphery region of the lesion.
• Ectoparasites: Lice and ticks can be collected manually, and mites through skin scrapings. It should be preserved in 70% ethyl alcohol or 10% formalin.
• Collection of samples at necropsy: If the dead carcass is suspected for anthrax, blood smear must be taken from ear vein and immediately stained with polychrome methylene blue stain for the detection of anthrax bacilli. If it is positive for anthrax, the carcass should not be opened. The carcass should be buried with lime powder and/or 3% peracetic acid, or incinerated.
• Heart blood swab in a sterile container.
• Heart blood smear and impression smear from various organs accordingly.
• In general, organ of choice for the disease suspected should be collected in 50% glycerol saline for virological examination and 10% formalin for histopathological examination.