Transcript

If control of Neospora caninum infection is technically

feasible does it make economic sense?

Michael P. Reichel a,b,*, John T. Ellis c

a Gribbles Veterinary Pathology, P.O. Box 536, Palmerston North, New Zealandb Macquarie Graduate School of Management, Macquarie University, Sydney, New South Wales 2109, Australia

c Department of Medical and Biomolecular Sciences, University of Technology Sydney,

P.O. Box 123, Broadway, New South Wales 2007, Australia

Received 17 March 2006; received in revised form 15 June 2006; accepted 21 June 2006

www.elsevier.com/locate/vetpar

Veterinary Parasitology 142 (2006) 23–34

Abstract

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 being

the 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 same

approach is likely to be applicable to other countries and the beef industry, with only minor changes expected in the relationships of

decisions versus within-herd prevalence of N. caninum infection.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Cattle; Neospora caninum; Abortions; Costs; Decision tree; Economics; Control

1. Introduction

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,

* Corresponding author.

E-mail address: [email protected] (M.P. Reichel).

0304-4017/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.vetpar.2006.06.027

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

M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 23–3424

Fig. 1. Decision tree analysis.

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

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 also

expensive 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 model

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

economic concern.

2. Assumptions for the construction of the

decision tree

2.1. Infection

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

M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 23–34 25

NSW 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).

2.2. Abortions

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) of N. 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

2.3.1. Cows

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

(Deverson, 2005).

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

herd test).

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 options—costs

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.

2.4.2. Test-and-cull

Test-and-cull was assumed to be preceded (as

discussed above) by an all-herd serological test,

M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 23–3426

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%.

2.4.3. Treatment

While thus far only used in a research setting

(Kritzner et al., 2002), treatment with BayCox1 (active

ingredient: 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.

2.4.4. Vaccination

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 Zealand da

prevalence of Neospora caninum infection of 15%.

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

years.

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

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 2–5 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) 23–34 27

Tab

le1

Op

tim

ald

ecis

ion

sre

gar

din

g‘‘

trea

tmen

t’’

op

tio

n(a

nd

cost

)in

anav

erag

e(3

00

mil

kin

gco

w)

Neo

spo

raca

nin

um

-infe

cted

New

Zea

land

dai

ryher

dm

odel

led

atvar

yin

gle

vel

sof

N.ca

nin

um

infe

ctio

n

(a)

Dec

isio

nth

resh

old

sfo

rsw

itch

from

‘‘do

noth

ing’’

to‘‘

vac

cinat

ion’’

for

aw

ithin

-her

dpre

val

ence

of

N.

canin

um

infe

ctio

n(i

n%

)

Sce

nar

ios

‘‘W

ors

t’’

‘‘A

ver

age’

’‘‘

Bes

t’’

One

yea

r10.7

18.0

31.0

Fiv

eyea

rs14.7

21.0

32.7

(b)

Cost

s(N

Z$)

of

and

opti

mal

contr

ol

opti

ons

atvar

yin

gra

tes

of

wit

hin

-her

dpre

val

ence

of

N.

canin

um

infe

ctio

n

Wit

hin

-her

d

pre

val

ence

(%)

Opti

mal

solu

tion

per

scen

ario

(1-y

ear

hori

zon)

Opti

mal

solu

tion

per

scen

ario

(5-y

ear

hori

zon)

‘‘W

ors

t’’

‘‘A

ver

age’

’‘‘

Bes

t’’

‘‘W

ors

t’’

‘‘A

ver

age’

’‘‘

Bes

t’’

5$

4585.0

0do

noth

ing

$2897.5

0do

noth

ing

$1851.2

5do

noth

ing

$14,4

11.5

8do

noth

ing

$10,5

84.4

2do

noth

ing

$7398.5

1do

noth

ing

10

$8770.0

0do

noth

ing

$5395.0

0do

noth

ing

$3302.5

0do

noth

ing

$27,1

55.2

2do

noth

ing

$19,5

00.9

0do

noth

ing

$13,1

29.0

8do

noth

ing

15

$12,9

55.0

0vac

cinat

ion

$7892.5

0do

noth

ing

$4753.7

5do

noth

ing

$39,8

98.8

6vac

cinat

ion

$28,4

17.3

8do

noth

ing

$18,8

59.6

5do

noth

ing

20

$17,1

40.0

0vac

cinat

ion

$9895.0

0vac

cinat

ion

$6205.0

0do

noth

ing

$52,6

42.4

9vac

cinat

ion

$37,3

33.8

6do

noth

ing

$24,5

90.2

2do

noth

ing

30

$25,5

10.0

0vac

cinat

ion

$12,3

92.5

0vac

cinat

ion

$9,1

07.5

0do

noth

ing

$78,1

29.7

7vac

cinat

ion

$47,1

81.7

7vac

cinat

ion

$36,0

51.3

6do

noth

ing

40

$33,8

80.0

0vac

cinat

ion

$14,8

90.0

0vac

cinat

ion

$12,0

10.0

0vac

cinat

ion

$103,6

17.0

4vac

cinat

ion

$56,0

98.2

5vac

cinat

ion

$47,5

12.4

9vac

cinat

ion

50

$42,2

50.0

0vac

cinat

ion

$17,3

87.5

0vac

cinat

ion

$14,9

12.5

0vac

cinat

ion

$129,1

04.3

1vac

cinat

ion

$65,0

14.7

3vac

cinat

ion

$58,9

73.6

3V

acci

nat

ion

costs) 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 80–95%

(sporadic) to 5–20% (‘‘storms’’).

2.4.6. Decision tree

Decision trees were built using software (TreeAge

Pro Suite) available from TreeAge Software Inc. (http://

www.treeage.com).

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

10.8–31.1%), 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 today’s

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) 23–3428

Fig. 3. Cost (NZ$) of four ‘‘treatment’’ options in an average New Zealand dairy herd (n = 300 cows) over a 5-year period at an assumed within-herd

prevalence of N. caninum infection of 20%.

highest 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%

prevalence.

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).

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

Table 2

Cost benefit (NZ$) of vaccination and return on investment (ROI) (%, in pa

infection in a herd over a 1- or 5-year period)

Within-herd

prevalence (%)

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

valuable 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) 23–34 29

Table 3

Overall average cost (NZ$) of N. caninum infection at the herd and dairy industry level in New Zealand (range ‘‘best’’ to ‘‘worst’’ in parentheses)

over a 1- and 5-year horizon

Within-herd

prevalence (%)

Herd Industry (million $)

One-year period Five-year period One-year period Five-year period

5 $ 869.25 ($ 555.38–1375.50) $ 3175.33 ($ 2993.08–3357.58) $ 10.4 ($ 6.7–16.5) $ 38.1 ($ 35.9–40.3)

10 $ 1618.50 ($ 990.75–2631.00 $ 5850.27 ($ 5485.77–6214.77) $ 19.4 ($ 11.9–31.6) $ 70.2 ($ 65.8–74.6)

15 $ 2367.75 ($ 1426.13–3886.50) $ 8525.21 ($ 7978.46–9071.96) $ 28.4 ($ 17.1–46.6) $ 102.3 ($ 95.7–108.9)

20 $ 3117.00 ($ 1861.50–5142.00) $ 11,200.16 ($ 10,471.16–11,929.16) $ 37.4 ($ 22.3–61.7) $ 134.4 ($ 125.7–143.1)

30 $ 4615.50 ($ 2732.25–7653.00) $ 16,550.04 ($ 15,456.54–17,643.54) $ 55.4 ($ 32.8–91.8) $ 198.6 ($ 185.5–211.7)

40 $ 6114.00 ($ 3603.00–10,164.00) $ 21,899.93 ($ 20,441.93–23,357.93) $ 73.4 ($ 43.2–122.0) $ 262.8 ($ 245.3–280.3)

50 $ 7612.50 ($ 4473.75–12,675.000 $ 27,249.82 ($ 25,427.32–29,072.32) $ 91.4 ($ 53.7–152.1) $ 327.0 ($ 305.1–348.9)

Zealand (Pfeiffer et al., 1997), but those efforts failed to

identify the cost–benefits 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

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 a

greater risk of abortion for infected cows (3–10 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

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

50%).

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

herd.

M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 23–3430

Table 4

Decision thresholds of within-herd prevalences (%), where a highly (100%) efficacious vaccine for N. caninum, applied annually or once in a cow’s

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 4.5

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

every 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 (as

Fig. 4. Cost (NZ$) of various options for N. can

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

years.

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

reduce 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 decision

inum control viewed over a 1-year horizon.

M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 23–34 31

Fig. 5. Costs (NZ$) of various control options for N. caninum control viewed over a 5 years horizon.

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 cow’s 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

horizon (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

(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) 23–3432

each 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.

Acknowledgements

Dr. Fraser Hill, of Gribbles Veterinary Pathology,

Palmerston North, New Zealand and Dr. David Morrison,

of the National Veterinary Institute, Uppsala, Sweden are

thanked for their critical comments on the manuscript.

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