If control of Neospora caninum infection is technically feasible does it make economic sense?
Post on 02-Jul-2016
ariecDepartment of Medical and Biomolecular Sciences, University of Technology Sydney,
f industry, with only minor changes expected in the relationships of
Veterinary Parasitology 142 (Keywords: Cattle; Neospora caninum; Abortions; Costs; Decision tree; Economics; Control
Neospora caninum is a protozoan parasite, which
has been shown to occur world-wide (Dubey, 1999) in
many countries including Australasia (Reichel, 2000).
The parasite causes disease in dogs (neonatal death,
hindleg paralysis) while in cattle it causes abortions,
which imposes significant economic loss on farmers.
Up to 50% of abortions that occur on a farm might be
due to N. caninum (Anderson et al., 1995; Boulton
et al., 1995; Thornton, 1996). This is especially so on
farms that experience abortion storms, which affect a
large proportion of the pregnant herd (Thornton et al.,
1994). Other N. caninum-infected herds may experi-
ence sporadic abortions (Davison et al., 1999), thought
to occur when cattle are chronically infected (pre-
sumably via the congenital route) (Hall et al., 2005a).
While the epidemiology of the disease is still poorly* Corresponding author.
E-mail address: email@example.com (M.P. Reichel).
0304-4017/$ see front matter # 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.vetpar.2006.06.027# 2006 Elsevier B.V. All rights reserved.approach is likely to be applicable to other countries and the bee
decisions versus within-herd prevalence of N. caninum infectReceived 17 March 2006; received in revised form 15 June 2006; accepted 21 June 2006
Recent work on Neospora caninum, a protozoan parasite that causes abortions in dairy cattle has focused on a number of
different control options. Modelling has suggested the most effective options for control but the present paper argues that the most
effective option might not necessarily be optimal from an economic point of view. Decision trees, using published quantitative data,
were constructed to choose between four different control strategies. The costs of these interventions, such as test and cull,
therapeutic treatment with a pharmaceutical, vaccination or doing nothing were compared, and modelled, in the first instance, on
the New Zealand and Australian dairy situation. It is argued however, that the relative costs in other countries might be similar and
that only the availability of a registered vaccine will change the decision tree outcomes, as does the within-herd prevalence of N.
caninum infection. To do nothing emerged as the optimal economic choice for N. caninum infections/abortions up to a within-
herd prevalence of 18%, when viewed over a 1-year horizon, or 21% when costs were calculated over a 5 years horizon. For a higher
(21%) within-herd prevalence of N. caninum infection vaccination provided the best (i.e. most economic) strategy. Despite beingthe most efficacious solutions, test and cull or therapeutic treatment never provided a viable economic alternative to vaccination or
doing nothing. Decision tree analysis thus provided clear outcomes in terms of economically optimal strategies. The sameP.O. Box 123, Broadway, New South Wales 2007, AustraliaIf control of Neospora can
feasible does it ma
Michael P. ReicheaGribbles Veterinary Pathology, P.O.
bMacquarie Graduate School of Management, Macqum infection is technically
economic sense?,*, John T. Ellis c
36, Palmerston North, New Zealand
University, Sydney, New South Wales 2109, Australia
the costs of various control options and, via a decision
tree analysis (Fig. 1),1 to determine which option might
be economically optimal. The general outcomes
however appear to be easily transferable to the situation
in other countries where N. caninum infection is also
known to be a cause of abortions and an issue of
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233424
Fig. 1. Decision tree analysis.
structure upon a decision process otherwise not seen, and gives the
process transparency that allows the decision process to be easily
challenged and ultimately changed and improved upon. The nodes of
the tree represent either a decision point or a chance/probability event;
decision nodes are marked out as squares, chance nodes by circles. At
the end of the branches one inputs the outcomes (costs or gains):
terminal values or pay-offs of the decisions or probabilities. Rolling-
back the outcomes (from right to left) to the beginning of the tree,
one multiplies the outcomes by their probabilities (on branches
emanating from chance nodes) and sums up the results for each
branch emanating from the same node. The strategy that has the
largest benefit (or least cost) is chosen at the decision point/node. For
more information on decision trees, see: http://www.psychwww.com/
mtsite/dectree.html or Smith and Slenning (2000).understood (it was only first described in 1984, Bjerkas
et al., 1984), in recent years advances have been made
in the diagnosis of the disease and serological tests have
been developed for the accurate identification of
infected animals (Pare et al., 1995a, 1995b). The
sensitivity and specificity of those, in particular the
ELISA tests, have been well described over the past few
years (von Blumroder et al., 2004). With the use of
those diagnostic tools, epidemiological studies have
determined a very high efficiency of the (vertical)
transmission of the parasite from dam to daughter (Pare
et al., 1994) and have given rise, recently, to test-and-
cull attempts of eradication of the infection from a herd
(Hall et al., 2005a). Sero-prevalence data for New
Zealand (Reichel, 1998) and Australia (Hall et al.,
2005b) range from 6.75% to 22% of cattle, respectively.
Other control options (Reichel and Ellis, 2002) that
have been discussed and developed, are vaccination
(Andrianarivo et al., 1999) and chemotherapy (Kritzner
et al., 2002). Vaccination with a killed tachyzoite
formulation, while reported to be highly efficacious in
rodents (Liddell et al., 1999) has not had the same
success rate in cattle (Romero et al., 2004) and is
estimated to be only 50% efficacious in that species.
This strategy of vaccination also appears to be
reasonably expensive and labour-intensive, requiring
two vaccinations per annum initially, and each year
thereafter. Chemical treatment, while reported to be
highly efficacious (>90%) (Kritzner et al., 2002) is alsoexpensive and can be expected to present residue
problems in food producing animals such as cattle.
Herds with persistent N. caninum infection will
continue to incur costs of abortion (loss of the calf, loss
of milk, veterinary costs), yet there are also reports of
improved neonatal mortality in infected calves (Pare
et al., 1996) and reports of the effects of N. caninum
infection on milk production are mixed (Hall et al.,
2005a; Hobson et al., 2002; Pfeiffer et al., 2002).
While previous studies have either modelled (French
et al., 1999) or practically focused on the technical
feasibility and efficacy of N. caninum eradication from
a herd (Hall et al., 2005a), the question also should be
asked whether these control options make economic
sense. The present paper developed a model (using
decision tree analysis) for determining the relative cost/
benefit relationship of various control options of N.
caninum infection in a dairy herd based on currently
available (and published) information from the relevant
literature, modelled in the first instance on the
Australasian dairy situation. The New Zealand situa-
tion, in particular, where a vaccine for N. caninum is
now available, affords a unique opportunity to model2. Assumptions for the construction of the
The probability of a dairy herd being infected with N.
caninum was assumed to be 30% (Otranto et al., 2003).
National surveys for New Zealand have suggested a
national prevalence in individual dairy cattle of up to
9% (Reichel, 1998). Other reports of within-herd
prevalence (Schares et al., 1999) and recent survey
work in Australia has suggested a state-prevalence for
1 This refers to a particular technique in decision theory for analys-
ing and evaluating problems that contain a degree of uncertainty or
probability through visualisation of the alternatives in a hierarchical,
tree-like structure. They are particularly useful where decisions are
made at discreet points (in time) and in a sequential order. Their
particular value not only lies in the outcomes, but in the clear layout of
the decision-making process, which enforces a certain degree of
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 2334 25NSW of 22% (Hall et al., 2005b). In herds experiencing
abortion storms in Australasia, the within-herd pre-
valence appears to be higher (but remarkably con-
sistent) at around 30% (Atkinson et al., 2000; Thornton
et al., 1994). Overseas, authors are also reporting
higher and lower within-herd prevalences of N.
caninum infection (Barling et al., 2001; Schares
et al., 2003).
Abortions due to N. caninum were assumed to occur
in New Zealand and Australian dairy cattle at a
probability of three times greater in N. caninum-
infected than in the uninfected cattle population (Moen
et al., 1998; Thurmond et al., 1997; Wouda et al.,
1998), which has recently been reported to be
experiencing about 6.9% foetal loss (McDougall
et al., 2005). As some of these (6.9%) losses are
caused by N. caninum, a background figure for other
abortions of 5% was assumed for those farms where
sporadic abortions were experienced. Therefore, a 15%
sporadic abortion risk was assumed for N. caninum-
infected cattle. Sporadic abortions were assumed to
constitute the majority (P = 0.9) of N. caninum abor-
tions, with abortion storms far less likely (P = 0.1)
(Anderson et al., 2000).
In abortion storms of epidemic proportions, 50% (i.e.
10 times greater risk) ofN. caninum-infected cattle were
assumed to abort (Atkinson et al., 2000; Cox et al.,
1998; Lopez-Gatius et al., 2004; Schares et al., 1999;
Wouda et al., 1999). Once a farm had experienced an
abortion storm (in year 1), it was assumed that only
sporadic N. caninum-induced abortions (with the three-
fold increased risk) would occur in subsequent years
(Innes et al., 2000).
2.3. Costs/losses incurred due to N. caninum
In the event of an abortion occurring, the total cost of
abortion was calculated as the cost of a replacement in-
calf heifer (NZ$ 1400.00) minus the meat (cull)
value of the aborting cow (NZ$ 500.00), giving a total
cost/loss for an abortion event of NZ$ 900.00
2.3.2. Veterinary costs
The initial veterinary investigation of an abortion
case (of either, the sporadic or of the epidemic storm-
type) was assumed not to exceed NZ$ 400.00 (Hill,
personal communication).2.3.3. Testing
Serological testing of cows (as a precursor to further
intervention in the case of the test-and-cull policy) was
assumed to cost NZ$ 10.00 a sample, assuming a
volume discount (based on the assumption of a whole
2.3.4. Other cost assumptions
Other serological testing, for instance to establish the
within-herd prevalence of N. caninum infection per se,
have been treated as a sunk cost (i.e. as a cost one
would have incurred in any case, regardless of the
outcomes and these are excluded from the calculations
of alternatives). Similarly, the cost of abortions which
are not caused by N. caninum, have been disregarded
from all options (as they are assumed to have been
incurred by all alternatives) and thus only the
incremental, N. caninum-related costs have been
included. Therefore, the total cost for the non-infected
70% of herds is set at zero.
Effects of N. caninum infection on milk production
are mentioned in the literature, with varying, i.e.
positive (Hall et al., 2005a; Pfeiffer et al., 2002), as well
as negative (Hernandez et al., 2001; Thurmond and
Hietala, 1997b) impacts recorded. These have also been
excluded from the decision trees (even though some
authors mention this factor as one of the most important
cost drivers, Chi et al., 2002). Equally, the reported
possible positive effects of N. caninum on neonatal
mortality in calves have been excluded (Pare et al.,
1996), as have increased costs of veterinary treatments
in infected cows or effects on weight gain that have been
reported in the literature (Barling et al., 2000).
2.4. Treatment optionscosts
2.4.1. No intervention (do nothing)
The probability of abortion storms and sporadic
abortions were assumed to be as discussed in Section
2.2 on abortions (see above) with however only the
likelihood of one abortion storm occurring in a herd
over the observation period, being contemplated. N.
caninum repeat abortions are generally regarded to be
rare (Cox et al., 1998; Thurmond and Hietala, 1997a)
(although others disagree, Obendorf et al., 1995;
Thornton et al., 1991) and hence abortion storms were
only assumed to occur once (in the first year) and
sporadic abortions assumed in subsequent years.
Test-and-cull was assumed to be preceded (as
discussed above) by an all-herd serological test,
assuming the national mean size of a dairy herd in New
Zealand of 300 milking cows (Anon, 2005a), with an
equal number of young (replacement) stock (150 heifer
calves, 150 heifers).
Culling was assumed to occur in one (the first) year
(presenting a high present cost) calculated as the cost of
the replacement (NZ$ 900.00) of any infected cows
times their number.
With the rapid replacement of infected cows in year
1, no further abortion events were assumed until year 5,
when the probability of infection within the herd
(derived from post-natal infection at the rate of 0.01/
year, Hall et al., 2005a; Pare et al., 1996) was assumed
to have reached a level of 5%.
While thus far only used in a research setting
(Kritzner et al., 2002), treatment with BayCox1 (active
communication), with two doses required in the first and
subsequent years (Romero et al., 2004). Vaccination
was assumed each year for both the adult cow herd and
the, also at risk, replacement (in-calf) heifer cohort. The
efficacy of the vaccine was assumed to be 50% (Romero
et al., 2004), thus allowing abortion storms and
sporadic abortions to continue to occur at half the
assumed rate. However, as discussed above, N. caninum
repeat abortions are generally regarded to be rare so
abortion storms were only assumed to occur once (in the
first year) and sporadic abortions assumed in subsequent
2.4.5. Sensitivity analysis
The resultant costs of each control option were
calculated for individual scenarios by varying within-
herd prevalences of N. caninum infection. Reported
within-herd prevalences vary from less than 10% to in
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233426
and daingredient: Toltrazuril) was included as an alternative
treatment in the decision tree. Treatment was costed
over a 6-day period (at a cost of NZ$ 568.80 per average
500 kg cow), with a projected additional loss of the
average milk production per day (17 l at 30 cents a litre)
for a fortnight.
Assuming the high efficacy of 90% for the treatment
reported in the literature (Kritzner et al., 2002), only
10% of the remaining N. caninum-infected cattle were
assumed to be at risk of either abortion storms or
sporadic N. caninum-induced abortions.
Vaccination (Bovilis Neoguard1, Intervet, NZ) was
assumed to cost NZ$ 5.00 per dose (Wylie, personal
Fig. 2. Cost (NZ$) of four treatment options in an average New Zeal
prevalence of Neospora caninum infection of 15%.excess of 90% (Frossling et al., 2005; Mainar Jaime et al.,
1999; Pare et al., 1998; Thurmond et al., 1997; Wouda
et al., 1999). They were calculated in Microsoft Excel and
entered into the decision tree developed (Fig. 2). The
resultant total costs were analysed for two scenarios; for a
period of 1 year of observation, but also over 5 years, with
the costs in years 25 discounted at a rate of r = 0.1, and
the present values entered into the decision tree (Frino
et al., 2004). The costs of the various treatment options
were calculated on the basis of an average sized dairy
herd in New Zealand with 300 milking cows (Anon,
2005a) and 200 cows in Australia (Anon, 2005b).
In order to address the limitations that come with
point estimates only, lowest (labelled best) and
highest (labelled worst) (in terms of their effect on
iry herd (n = 300 cows) over a 1-year period at an assumed within-herd
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 2334 27
ioncosts) assumptions were also modelled in addition to the
assumptions above (labelled average); in the case of
sporadic abortion risks of 10% and 20% were modelled
(in addition to the average 15%), for abortion storms
25% (i.e. five times increased risk) and also 75% of
cows aborting were modelled (average 50%), as well
as a sporadic abortions to storms split of 8095%
(sporadic) to 520% (storms).
2.4.6. Decision tree
Decision trees were built using software (TreeAge
Pro Suite) available from TreeAge Software Inc. (http://
3. Results obtained from decision tree analyses
Decision tree analysis arrived at a number of optimal
solutions, depending on the within-herd prevalence of
N. caninum infection, the type of scenario (best,
worst or average) and the length of the observa-
tion period. Up to a within-herd prevalence of 18% (and
considering the costs/benefits only over a 1-year period)
the do nothing option was calculated to be the
cheapest (Table 1a; Fig. 2). However, in the worst
case scenario (highest cost of abortions), that threshold
was reached earlier (at a within-herd prevalence of
10.7%) while in the best scenario (lowest cost of
abortions) the threshold was not reached until the
within-herd prevalence went beyond 31%. For within-
herd prevalences equal to and greater than 18% (range
10.831.1%), the vaccination option was increasingly
the best option for the farmer when costs were only
considered over a 1-year period.
If costs were viewed over a longer-term period, such
as 5 years (and costs in future years discounted at a rate
of 10% to give the present value of those costs in todays
dollars, taking into account the best alternative rate of
financial return on the money invested, say, in the share
market), then the breakpoint for a switch between do
nothing and vaccination was reached at prevalences
equal to and greater than 21% (Table 1a; Fig. 3), but
ranging from 32.7% (best) to 14.7% (worst),
depending on scenario.
The incremental benefits of vaccination (compared
to the next best (costlier) alternative option), viewed
over a 1-year period, increase from NZ$ 495.00 (at herd
level) at a 20% level of N. caninum infection to NZ$
7987.00 if the within-herd prevalence of N. caninum
infection was assumed to be 50% (Table 2). This
represents a return on investment (ROI) of up to 177.5%
(Table 2) at existing prices for the vaccine option. In the
worst case scenario, with assumptions leading to the
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233428
and dahighest costs of abortion, at 15% prevalence, the ROI of
vaccination is 39.5%, while in the best case scenario,
there is a ROI of 29% for vaccination but only at 40%
A calculation of costs, via decision trees, of the do
nothing option at herd and industry level was
performed for a 1- and 5-year period at varying
within-herd prevalences. Costs of N. caninum abortions
to the national dairy industry rise from a 5% average
within-herd prevalence level of N. caninum infection at
NZ$ 10.4 million per annum to NZ$ 91.4 million if the
within-herd prevalence in infected herds was assumed
to be 50%, to NZ$ 152.1 million if the costs of the
worst case scenario were calculated (Table 3).
Fig. 3. Cost (NZ$) of four treatment options in an average New Zeal
prevalence of N. caninum infection of 20%.Similar calculations were made for Australia where
average dairy herds are smaller at 200 cows (Anon,
2005b), but other costs, such as replacement heifers are
similar. The decision tree approach arrives here at an
Cost benefit (NZ$) of vaccination and return on investment (ROI) (%, in pa
infection in a herd over a 1- or 5-year period)
Optimal solution per scenario (1-year horizon) (ROI, %)
Worst Average Best
5 N/Aa N/Aa N/Aa
10 N/Aa N/Aa N/Aa
15 $ 1777.50 (39.5) N/Aa N/Aa
20 $ 3870.00 (86.0) $ 495.00 (11.0) N/Aa
30 $ 8055.00 (179.0) $ 2992.00 (66.0) N/Aa
40 $ 12,240.00 (272.0) $ 5490.00 (122.0) $ 1305.00 (2
50 $ 16,425.00 (365.0) $ 7987.00 (177.5) $ 2756.25 (6
a Not applicable as do nothing option is the least costly.annual cost to the Australian dairy industry of AU$
21.2 million (assuming 10,000 herds, Anon, 2005b)and AU$ 7060.00 for the N. caninum-infected herd with
an assumed prevalence of N. caninum infection of 20%
(Hall et al., 2005b).
4. Ramifications of the decision tree approach
Control options for N. caninum infections in dairy
cattle have been discussed in recent years (Reichel and
Ellis, 2002), and some authors have also recently
embarked on control efforts, based on the test and cull
strategy, with good success (Hall et al., 2005a). These
efforts, and other reports in the literature, have provided
iry herd (n = 300 cows) over a 5-year period at an assumed within-herdvaluable data on which to model (by decision tree
analysis) the cost and benefits of various control
methods. Others have previously modelled the costs of
N. caninum abortions to herd and industry in New
rentheses) over the do nothing option (i.e. the cost of N. caninum
Optimal solution per scenario (5-year horizon)
Worst Average Best
N/Aa N/Aa N/Aa
N/Aa N/Aa N/Aa
$ 351.06 (1.9) N/Aa N/Aa
$ 6722.88 (35.8) N/Aa N/Aa
$ 19,466.52 (103.7) $ 7985.04 (42.6) N/Aa
9.0) $ 32,210.15 (171.7) $ 16,901.52 (90.1) $ 4157.88 (22.2)
1.3) $ 44,953.79 (239.6) $ 25,817.99 (137.6) $ 9888.45 (52.7)
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 2334 29
25,42Zealand (Pfeiffer et al., 1997), but those efforts failed to
identify the costbenefits to the farm entity or the dairy
industry as a whole of treatment/eradication efforts as
they present themselves now. The New Zealand
situation, where a vaccine for N. caninum is now
available, affords a unique opportunity to model the
costs of various control options and, via a decision tree
analysis, to determine which option might be the
optimal one. In another recent paper (Larson et al.,
2004), testing for N. caninum infection and excluding
female offspring from breeding was considered to be
the best economic decision in beef herds in the US.
These authors however looked at the effect of endemic
abortions only (not abortions) and did not include the
option of vaccination, nor to the do nothing in their
three options for comparison.
The current analysis demonstrates that N. caninum
infection is costly to the individual average-sized herd,
as well as to the dairy industry in New Zealand as a
whole, with on-farm costs on infected farms rising (with
increasing levels of prevalence of infection) from NZ$
2897.50 (at 5% prevalence) to NZ$ 25,375.00 in herds
when 50% of cattle are infected. With national
prevalence surveys putting the prevalence at between
Overall average cost (NZ$) of N. caninum infection at the herd and da
over a 1- and 5-year horizon
One-year period Five-year perio
5 $ 869.25 ($ 555.381375.50) $ 3175.33 ($ 2
10 $ 1618.50 ($ 990.752631.00 $ 5850.27 ($ 5
15 $ 2367.75 ($ 1426.133886.50) $ 8525.21 ($ 7
20 $ 3117.00 ($ 1861.505142.00) $ 11,200.16 ($
30 $ 4615.50 ($ 2732.257653.00) $ 16,550.04 ($
40 $ 6114.00 ($ 3603.0010,164.00) $ 21,899.93 ($
50 $ 7612.50 ($ 4473.7512,675.000 $ 27,249.82 ($10% and 20% (Hall et al., 2005b; Reichel, 1998) in
Australasia, the likely cost of N. caninum infection to
the New Zealand dairy industry can be estimated by the
present model at NZ$ 28.4 million (at 15% prevalence)
(ranging from $ 17.1 to 46.6 million), a figure
remarkably similar to the one (NZ$ 24 million)
modelled by others earlier (Pfeiffer et al., 1998).
Pfeiffer et al. (1998), however modelled their national
costs on a within-herd prevalence of 35% and a risk of
N. caninum abortions of only 5%, with no differentia-
tion between the risk of abortion storms versus the risk
of sporadic abortions. More recent data from Austra-
lasia that were not available to those earlier workers
have flown into the present model, which assumes agreater risk of abortion for infected cows (310 times
higher) and models both, sporadic and epidemic
(abortion storm-like) abortions. It is thus not
surprising that the present model arrives at the same
national cost with a considerably lower within-herd
prevalence of N. caninum infection.
Previously the annual cost to the Australian dairy
industry had been estimated to be around AU$
85 million (Ellis, 1997). At that time state or national
data for N. caninum infections were not available, thus
the estimate of the current paper, while considerably
lower than those previously published, appear to give a
more accurate assessment of the total cost of N.
caninum infection to the Australian dairy industry.
The present model suggests that up to a within-herd
prevalence of 18% (in the average scenario) the do
nothing option is the optimal economic decision to the
farmer. If, however, the within-herd prevalence of N.
caninum-infection exceeds 18%, then vaccination has
clear economic benefits (at the present cost of NZ$ 5.00
a dose) with returns on investments (ROI) rapidly
increasing, proportional with increasing prevalence,
from 11.0% (20% prevalence of infection) to 177.5%
(50%) viewed over the short-term. Viewed over the
ustry level in New Zealand (range best to worst in parentheses)
Industry (million $)
One-year period Five-year period
83357.58) $ 10.4 ($ 6.716.5) $ 38.1 ($ 35.940.3)
76214.77) $ 19.4 ($ 11.931.6) $ 70.2 ($ 65.874.6)
69071.96) $ 28.4 ($ 17.146.6) $ 102.3 ($ 95.7108.9)
1.1611,929.16) $ 37.4 ($ 22.361.7) $ 134.4 ($ 125.7143.1)
6.5417,643.54) $ 55.4 ($ 32.891.8) $ 198.6 ($ 185.5211.7)
1.9323,357.93) $ 73.4 ($ 43.2122.0) $ 262.8 ($ 245.3280.3)
7.3229,072.32) $ 91.4 ($ 53.7152.1) $ 327.0 ($ 305.1348.9)longer term (5 years) the returns on investment from
vaccination at prevalences of infection exceeding 20%
range from 42.6% (at 30% prevalence) to 137.6% (at
The best and worst possible case scenarios clearly
show (Table 1a) how these decision thresholds move
with changes to some of the probabilities, with the
worst case (resulting in the highest cost of abortions)
making vaccination the economically optimal decision
from a within-herd prevalence of N. caninum infection
of greater than 10.7%, while with assumptions in the
best case scenario, this threshold was not reached
until a 31% prevalence of N. caninum infection in the
Viewed over 1 year only, the major cost and decision
driver is the cost of an abortion storm. It contributes
10% only of the risk/cost in the decision tree, but if the
gap between the cost of losing a large number of calves
and having to replace cows versus paying insurance by
means of vaccination is large enough, it tilts the balance
in favour of vaccination. That happens at 18% within-
herd prevalence of infection.
Over the 5 years time frame, we assume in our model
only one abortion storm event and the literature also
suggests that abortion storms are not going to occur
they are highly efficacious control options) (Hall et al.,
2005a; Kritzner et al., 2002). This is due to their much
higher up-front costs (incurred in the first year) in
eliminating the infection (testing and culling, treatment
costs). Our modelling however suggests, that the gap to
the more economical control options continues to
persist, even if costs were spread out over a number of
It is important to note that, more efficacious
vaccines, such as the live vaccines which have recently
been described (Guy et al., 2005) could significantly
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233430
Decision thresholds of within-herd prevalences (%), where a highly (100%) efficacious vaccine for N. caninum, applied annually or once in a cows
lifetime becomes the most economical choice
Evaluation time frame Scenarios if applied annually Scenarios if applied once
Worst Average Best Worst Average Best
One year 4.8 8.2 14.1 4.8 8.2 14.1
Five years 6.7 9.5 14.9 2.0 2.8 4.5every year. Sporadic abortions only are expected/
assumed for the other 4 years. Thus the cost drivers are,
both, the increasing gap between the cost of losing
calves (and having to replace cows) in an abortion storm
and the cost of the vaccine (as over 1 year) but, in
addition, the cumulative cost of sporadic abortions,
which continue at half the rate despite vaccination.
It is interesting to observe that test-and-cull, or
treatment strategies never get close to becoming an
economically viable option, despite the fact that they
may appear (on technical grounds) to be preferable (asFig. 4. Cost (NZ$) of various options for N. canreduce or completely remove the risk of any cattle in the
herd aborting (as had to be assumed, despite vaccina-
tion, for 50% of the cattle receiving the Intervet
vaccine). This reduces N. caninum control to the cost of
vaccination alone, providing a viable alternative to
doing nothing even at very low within-herd
prevalences of infection with N. caninum. However,
whether or not this vaccine needs to be applied annually
or just once in the life time of a cow influences the
decision on when vaccination becomes the economic
optimal one. Over a 1-year time frame, the decisioninum control viewed over a 1-year horizon.
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 2334 31
r N.thresholds range from 14.1% (best) to 4.8%
(worst), with the average scenario threshold at
8.2% (Table 4; Fig. 4).
However, even with best or worst case
assumptions, the threshold prevalences between doing
nothing or vaccination with such a highly efficacious
vaccine (applied only once in a cows life time) would
reduce to a range of only 2.5% points (from 2%
(worst) to 4.5% (best) prevalence, with 2.9%
being the average), when viewed over a 5 years
Fig. 5. Costs (NZ$) of various control options fohorizon (Table 4; Fig. 5). This would reduce the
uncertainty for the farmer considerably, meaning that at
any prevalence over 5%, vaccination (in any scenario)
would be the most economical decision to make.
Bovilis Neoguard1 (Intervet) is currently not
registered in Australia. In all combinations of
Fig. 6. Costs (AU$) of control options in an average Australian dairy herd (n
of N. caninum infection of 20%.within-herd prevalence versus period of observation
for Australia, therefore, the do nothing option
appears as the economically optimal one, since the
other two treatment strategies (treatment or test and
cull) have very high up-front costs (Fig. 6). This
situation could be expected to be similar in most other
countries around the globe without a registered
vaccine. This would seem to provide an ideal
opportunity for a vaccine manufacturer (such as
Intervet in the first instance), to enter the Australian
caninum control viewed over a 5 years horizon.(or any other) market, with what is essentially a well-
priced alternative to living with the disease, providing
good returns on investment, especially in herds with
higher (than 15%) within-herd probabilities of infec-
tion with N. caninum. Similarly, calculations and
decision trees as outlined here could be constructed for
= 200 cows) over a 1-year period at an assumed within-herd prevalence
M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233432each and every country where N. caninum is known to
cause abortions, and it would not appear unreasonable
to assume that the decision would not significantly
change as the replacement value of dairy cattle appears
to be remarkably similar in a number of major dairy
producing countries at around the level (ranging from
US$ 700.00 to 1425.00, Grohn et al., 2003; Gunn et al.,
2005; Ott and Johnson, 2003; Santarossa et al., 2005;
Stott et al., 2005) as used in the present assumptions.
The threshold value (of infection, or vaccine price)
where vaccination would become the economically
preferred option might vary slightly (in the US where the
Neoguard vaccine sells at US$ 3.50 a dose, it might vary
downwards), the general trend, however, would remain
the same: at very low within-herd prevalence of N.
caninum to do nothing would be most economical,
while at a certain (somewhat higher) prevalence the
option of vaccination should become the preferred course
of action. As the threshold value where the decision
changes may well vary between countries, it seems
prudent to construct a local decision tree before
embarking on control (options) in other countries.
The decision tree approach, which has been used and
suggested in veterinary circumstances, such as for
diagnostic testing (Smith and Slenning, 2000) or disease
control decision analysis (Tomassen et al., 2002),
demonstrates in the present example that control
options that are technically sound and achievable do
not necessarily present the most economic solution to a
dairy farmer. When faced with N. caninum infection in a
herd, a dairy farmer presently would need to choose
control options that are currently less efficacious (such
as vaccination) or do nothing since they represent the
soundest economic decision to make. However, the
future availability of more efficacious vaccines for the
prevention of N. caninum abortions/infections would
suggest vaccination will become the most economical
control option to a farmer, where the within-herd
prevalence of N. caninum infection exceeds 4.5%. By
choosing the opportunity costs incurred in the event of
abortions (the replacement present value of an in-calf
heifer) the model presented here was simple without
compromising the value of the outputs and is quickly
applied to the situation in other countries or industries
where N. caninum infection prevails.
Dr. Fraser Hill, of Gribbles Veterinary Pathology,
Palmerston North, New Zealand and Dr. David Morrison,
of the National Veterinary Institute, Uppsala, Sweden are
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M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233434
If control of Neospora caninum infection is technically feasible does it make economic sense?IntroductionAssumptions for the construction of the decision treeInfectionAbortionsCosts/losses incurred due to N. caninumCowsVeterinary costsTestingOther cost assumptions
Treatment options-costsNo intervention (do nothing)Test-and-cullTreatmentVaccinationSensitivity analysisDecision tree
Results obtained from decision tree analysesRamifications of the decision tree approachAcknowledgementsReferences