Disease Control in animals to benefit PH Flashcards
What is R nought (R0)
describes how many people each infected person will infect on average, assuming that there is no pre-existing immunity in the community.
Often estimated using 3 factors:
a) duration of contagiousness after a person becomes infected
b) likelihood of infection in each contact between a susceptible person and an infectious person or vector
c) frequency of contact
Vet Med controlling infecction:
• Based on R0/Epidemic Theory infection can realistically be controlled by interventions that act in two ways
a) Prevent or Reduce Transmission
b) Reduce susceptible population
Controlling infection - how to prevent/ reduce transmission?
Prevent contact between infected and susceptible individuals
e. g movement restriction, exclusion zones, biosecurity - Removal of infected individuals e.g. culling, treatment, isolation - Reduce ability to transmit e.g. vaccination (herd immunity), therapy
Controlling infection - how to reduce the sysceptible population?
- Improve immunity e.g. vaccination, genetic resistance
- Make individual harder to infect e.g. probiotic/competitive exclusion
- Remove a susceptible population e.g. contiguous culling (non exposed but
3 key food borne pathogens
- salmonella enterica
- Campylobacter
- E coli
2 types of salmonella enterica
- Salmonella enterica serovar Typhimurium
2. Salmonella enterica serovar enteritidis
Why did salmonella fall in the UK after endemic?
- Underpinning legislation for control
- Implementation of improved hygiene & biosecurity of hatcheries (used to be horrible places – send out chickens that are already infected)
- Improved farm biosecurity
- Introduction of vaccines
- Heat treatment of feed – reduced carriage of salmonella in feed stuff.
- Targeted approach to reduce serovars of greatest public health significance
The lion mark and egg salmonella
a voluntary scheme of hygiene & biosecurity standards coupled to routine surveillance – first time routinely look at salmonella in flocks.
Current salmonella surveillance in UK
- Under NCP (National Control Plan) requirements
- Broilers - Sampling of 2 boot swabs/socks per flock/house within 3 weeks of slaughter
- Layers more intensive - Chicks-Chick boxes and any dead chicks sampled
Pullets- 2boot swabs 2 weeks before placement
Hens-2 boot swabs or pooled faeces at 22-26 weeks of age, then every 15 weeks in production - positive test means eggs can’t be sold as class A table eggs. Meaning they must be pasturised, become worthless, chickens get culled as cheaper to do this than sell these eggs
How are salmonella vacciens given
- live attenuated in drinking water = easier and mroe effective vaccines than previous
NCP rule on salmonella vaccines
- All layers and breeding birds (layers & broilers) now vaccinated under NCP
- Production broilers not vaccinated – killed at such a young age, we can’t initiate sufficient immune response in that time.
- Regarded as single largest contributory factor to reduction of salmonellosis in UK
Farm biosecurity
- Largely indoor
- major control barriers: entery, vermin proof, vechile wheel wash, concrete/ gravel around buildings, ventilation
- All in all out system = disinfection between flocks and broiler crops
- no sharing equipment between farms
Biosecurity failure works well for salmonella byt what about campylobacter?
- C is highly transmissible
- small breakdown in biosecurity = positive flocks
- fomite on boots, flies, clothing
- Most likely breaksdown due to “thinning” where catchers remove chickens ready for slaughter, leaving those that aren’t to grow = spread by catchers
Intensive production and the spread of campylobacter
- infected during production, mainly caecal but liver and muscle infection inc common
- Feed withdrawal and transport = stress and inc shedding from infected birds
- at abbatoir macheines cross contam e.g. automated plucking
- rapidly growing broiler breeds = relatively poor immune systems
Contact, Exclusion zones and movement restrictions
- Less useful in enteric bacterial infections in livestock, more for viral e.g. avian influenza
- Include bans on movement, gatherings (markets, shows etc.) and exclusion zones around infected premises
Culling
- Culling removes can remove an infected population reducing ability to transmit
- S. Enteritidis positive flocks are often culled as not economically viable as eggs cannot be sold as table eggs
How can we reduce host suseptibility to salmonella/ campylobacter?
- Better welfare and housing reduce stress and susceptibility-stress hormones increase faecal shedding in cattle and chickens
- Poor house design can increase spread in herds-common in pigs where housing is variable (e.g. faecal material is chaneled through pens in some farms) and plays a role is Salmonella transmission
Selective breeding good?
breeding for resistance
1. Chickens immune system is less polymorphic than mammals so stronger association between genome and disease resistance.
Competitive exclusion, probiotics and prebiotics:
idea of manipulating the microbiome to increase resistance to pathogens is long-established in poultry
• Competitive exclusion (CE) involves giving a microflora to animals that excludes or competes (physically occupy space) with pathogens-occupying niches for colonization or producing metabolites that promote gut health and inhibit pathogens (e.g.butyrate - )
• Probiotics are (usually) single bacterial species (e.g. Lactobacillus) that promote gut health/inhibit pathogens
• Both CE and probiotics may promote immunological development in the
gut, but CE are only given at intervals, probiotics need regular
consumption
• Prebiotics are feed additives that promote a ‘good’ microbiome
-High firmicutes that produce butyrate, lower enterobacteriaciae
• No current product gives complete protection
Slaughter and disease control
- Care in and post slaughter
- foodborne pathogens are gut/faecal associated processes in slaughter and processing should avoid gut/faecal contamination
- Hides or ruminants should be free of gross faecal contamination
- Care should be taken in removing the intestinal tract to avoid spillage & cross contamination
- However this is challenging for poultry lines at around 200 birds per minute
- Treatment of carcass - pathogen reduction treatment or gamma irradiation
- widely used in US not so much here
chlorination chicken in US to reduce salmonella and campylo
Summary
- Salmonella in poultry is a good example of how to control a foodborne pathogen
- Key factors in Salmonella control including vaccination & biosecurity are not or cannot be incorporated into Campylobacter in broiler chickens or EHEC in cattle so are harder to control
- Effective control is based around combinations of measures-single measures alone are more likely to fail-e.g. vaccination not not be used to compensate for poor husbandry & biosecurity
