Johnes Disease in Dairy Herds Flashcards
1
Q
What is Johnes disease
- other name
- granulomas
- initial signs
A
- aka Paratuberculosis
- enteritis (inflamation of small intestine) of ruminant animals
- granulomas: form when immune system attempts to wall off foreign substances but cant eliminate = thickening of intestine, inhibits nutrient absorption
- initial signs subtle:
~ decreased milk yield
~ reduced BCS
~ low fertility
~ roughening of hair/coat - many animals removed from herds before diagnosed
2
Q
Johnes disease - clinical characteristics
A
- loose manure
- weight loss
- chronic and progressive
- as progress lead to ~ intermandibular odema (bottle jaw) = as a result of protein loss from bloodstream into digestive tract
- no cure = dehydration, cachexia and death
- causative bac found on 70% of US dairy farms
- UK = >50%
3
Q
What is the bacteria that causes johnes disease and important things about it
A
= Mycobacterium Avium subsp. Paratuberculosis (MAP)
- difficult to treat e.g mycobacterium bovis
- genus slow growing, resilient
- resistant to acids, alkalis and detergents
- encased by thick layer of mycolic acids = fungus like growth pattern on culture media
4
Q
Why should we care about johnes disease
A
- characterised by long incubation periods
~ infected between 2 and 10 years before CS apear
~ subclinical animals shed MAP into manure and milk, spread througout enviro w/o farmers knowing - calves most susceptable (esp via faecal-oral route)
- MAP often goes undetected - lengthy incubation, intermittent shedding, delay before antibody production (evade diagnostic test detection), variety of other explanations for consequences of subclinical infections
5
Q
What happens when a stage 3 cow enters farm
A
- cow has clinical johnes disease
- other subclinical cows in herd (stage 2) infected and shedding but no CS
- more cows stage one = infected
6
Q
Hidden cost of johnes disease
A
- susceptability to other diseases
- lose money from
~ breeding problems
~ decreasd milk production
~ loss of investment in infected younstock
~ vet costs
~ cull rate
~ relacement costs
7
Q
How does MAP enter bulk tank milk supply
A
- internal route (direct) shed into milk by infected animals
- enviro route (indirect) faecal contamination during milking
- MAP can survive pasturation if present at high threshold = >10^3 CFU/L (colony forming units
- calves sometimes fed bulk milk
- MAP and potential cause of human chrons disease - important to keep contamination level below threshold
8
Q
Diagnostic tests for MAP
A
- ELIZA, PCR, Culture
- ELIZA = detects MAP antibodies
- PCR and culture = detect causal organism
- diagnostic matrices for ante-mortem testing are: serum, milk, feaces (when cow alive)
9
Q
Limitations of diagnostic tests for map
A
- culture = takes 16 weeks and alot can go wrong in that time, lacks sensitivity (map fussy about what it requires to grow)
- ELIZA = lacks sensitivity (no ab production in early stages)
- PCR = some of targets arnt specific to MAP , found in other mycobacterial species (may give =ve result but actually got diff mycobacterial species), can be bad at differentiating between live and dead bac
10
Q
Longitudinal relationship between diagnostic tests for MAP (study Beaver) about, what tests used, lack of research
A
- 14 MAP infected cows on 2 low prevalence dairy herds for 180 days
- repeated milk and faecal sample measuremens overtime
- used ELISA, milk and faecal qPCR and faecal culture
- weekly bulk milk qPCR and bulk milk ELISA
- qPCR alow to know what bacterial load was rather than just +/-
- lack of research between tests and majority focus at single point in time across large no. herds and focused on agrement of 2 dichotamus outcomes (+/-) and no adjustment for individual cow/herd characteristics
11
Q
Problem of making result of diagnostic test dichotemus
A
- ELIZA ab level
- scale for all possible eliza values
- cut off level. after = +ve result
- 2 ab levels can be very similar but either side of cut off point
- ab level is continuous scale and should be seen as liklyhood than animal is infected
- For PCR: sample that has one colony forming unit of MAP = +ve sample
~ what use for famrer? inform whether to cull cow depending on level of sheding
12
Q
Longitudinal relationship between diagnostic tests for MAP (study Beaver) conclusions
A
- strong associations between milk ELISA, faecal culture and faecal qPCR
- temporal relationships between faecal shedding and subsequent high ELISA
~ spikes in shedding may predict ELISA results taken up to 2 months later - bulk tank milk supply never positive (not a main concern for farms with low prevalence of infection
13
Q
Mathematical Modelling
A
= simplified representation of a complex phenomenon
- SIR model (susceptible, infected, recovered)
- provides an abstraction to reduce a problem to its essential characteristics
~ biology and catagories of infection, catagories of population itself
~ relavent time period
~ main research question
14
Q
Types of mathmatical models
A
- Deteminstic:
~ parameter values fixed
~ describes what happens on average in population - Stochastic
~ randomness present
~ certain perameters vary around given distribution
15
Q
Benefits of mathematical modeling to study MAP
A
- (not suitable for real world implementation) But adding years may correspond to a fraction of a second in computation time
- altering value of a model parameter vs decades of herd monitoring to observe the true impact of control strategy (dont waste time in real world with strategies with no potential)
- mathematical models may be continuously refined to incorporate new insights and mimic real-word scenarios
