If control of Neospora caninum infection is technically feasible does it make economic sense?

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    l a,b

    Box 5

    ariecDepartment of Medical and Biomolecular Sciences, University of Technology Sydney,

    f industry, with only minor changes expected in the relationships of

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    Veterinary Parasitology 142 (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,

    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: michael.reichel@gribbles.com.au (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

    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 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

    www.elsevier.com/locate/vetpar

    2006) 2334

  • 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.

    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

    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

    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).

    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) 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

    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 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.

    2.4.2. Test-and-cull

    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%.

    2.4.3. Treatment

    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

    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

    M.P. Reichel, J.T. Ellis / Veterinary Parasitology 142 (2006) 233426

    and daingredient: Toltrazuril) was included as an alternative

    treatment in the...

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