Effects of Psoroptes ovis infection and its control with an ivermectin controlled-release capsule on growing sheep. 2. Evaluation of wool production and leather value

Treatment strategies for sheep scab: An economic model of farmer behaviour

Ovine psoroptic mange (sheep scab) is a debilitating and damaging condition caused by a hypersensitivity reaction to the faecal material of the parasitic mite Psoroptes ovis. Farmers incur costs from the use of prophylactic acaricides and, if their sheep become infected, they incur the costs of therapeutic treatment plus the economic loss from reduced stock growth, lower reproductive rate, wool loss and hide damage. The unwillingness of farmers to use routine prophylactic treatment has been cited as a primary cause of the growing incidence of sheep scab in the United Kingdom (UK) since the disease was deregulated in 1992. However, if farmers behave rationally from an economic perspective, the optimum strategy that they should adopt will depend on the risk of infection and the relative costs of prophylactic versus therapeutic treatment, plus potential losses. This calculation is also complicated by the fact that the risk of infection is increased if neighbours have scab and reduced if neighbours treat prophylactically. Hence, for any farmer, the risk of infection and optimum approach to treatment is also contingent on the behaviour of neighbours, particularly when common grazing is used. Here, the relative economic costs of different prophylactic treatment strategies are calculated for upland and lowland farmers and a game theory model is used to evaluate the relative costs for a farmer and his/her neighbour under different risk scenarios. The analysis shows that prophylaxis with organophosphate (OP) dipping is a cost effective strategy, but only for upland farmers where the risk of infection is high. In all other circumstances prophylaxis is not cost effective relative to reliance on reactive (therapeutic) treatment. Hence, farmers adopting a reactive treatment policy only, are behaving in an economically rational manner. Prophylaxis and cooperation only become economically rational if the risk of scab infection is considerably higher than the current national average, or the cost of treatment is lower. Should policy makers wish to reduce the national prevalence of scab, economic incentives such as subsidising the cost of acaricides or rigorously applied financial penalties, would be required to make prophylactic treatment economically appealing to individual farmers. However, such options incur their own infrastructure and implementation costs for central government.

A method for sheep scab control by applying selective treatment based on flock serology

Sheep scab, caused by Psoroptes ovis, is a severe and debilitating disease that can be treated and controlled by the use of acaricidal dips or the use of broad-spectrum avermectins. In Switzerland, control measures are state regulated. In particular, sheep should be routinely treated with ectocide dips or avermectin injections before they are moved onto common alpine pasture in late spring. However, a substantial part of the sheep population remains untreated and represents a potential reservoir for the mite population. Untreated sheep that are not moved to alpine pasture may infest treated sheep when flocks are reassembled in autumn. In an attempt to identify infested sheep, all flocks in the Canton of Schwyz (Switzerland) were serologically tested in 2001 and in 2002 (587 and 565 flocks, respectively). In 2003, a representative number (182 of the 531 flocks) was again investigated. Seropositive flocks were treated with doramectin (0.3 mg kg(-1) body weight, intramuscularly) from 2001 to 2003. In spring 2002, no chemo-methaphylaxis was given to seronegative flocks before movement onto common alpine pastures. Of the 587 flocks surveyed in spring 2001, 34 were seropositive (5.8%). These consisted of 21 infested with P. ovis, 1 with P. cuniculi, 4 with Chorioptes spp. and 8 of seropositivity of unknown origin; there was a decrease of seropositive flocks in spring 2002 (4.4%) with 15, 0, 2 and 8, respectively. Of the 182 flocks surveyed in spring 2003, just 4 flocks (2.2%) were seropositive. All the seropositive reactions in these flocks were the result of Chorioptes spp. infestations. There was a corresponding decrease in the proportion of seropositive animals from 6.3% in spring 2001 to 2.1% in spring 2003. These results corroborate the concept that it may be possible to target chemo-metaphylaxis and hence decrease the use of endectocides as well as of ectocides to control sheep scab. This would be of great benefit in reducing the likelihood of development of anthelmintic resistance against avermectins, decreasing the extent of environmental and human contamination with potentially toxic products and diminishing potential drug residues in meat and milk.

Productivity effects of bovine mange and control with ivermectin

A randomised block design study was conducted to evaluate the effects of mange on cattle. Twenty-four Simmentaler Fleckvieh bulls were formed into eight replicates of three bulls based on Day -56 body weight (288-414 kg). Within replicates bulls were randomly allocated to groups G1: uninfested control, G2: infested control or G3: infested, treated with 0.2mg ivermectin/kg (1% ivermectin injection; IVOMEC, Merial) on Day 0. The G2 and G3 bulls were infested with Sarcoptes/Chorioptes mites on Days -56 and -49. Feed consumption was recorded daily throughout the study (Days -56 to 56). Body weights were measured and serum samples collected. Mites were counted at bi-weekly intervals from Day -14 on. The carcasses of the bulls and the leather produced from their hides were evaluated. Differences between variables were declared significant if P</=0.05. All G2 and G3 bulls became infested. No Sarcoptes or Chorioptes mites were detected on the G3 bulls after Day 14 or Day 28, respectively, whereas G2 bulls maintained infestation throughout the study. From Days -42 to 0, anti-Sarcoptes antibody levels in the two infested groups increased linearly, while levels in the uninfested G1 remained near zero. From Day 14 to Day 56, antibody levels in G2 continued to increase and those in G3 decreased (P</=0.05). From Days -56 to 0, G1 had significantly greater mean weight gain (94.0 kg) than the infested G2 and G3 (76.1 and 75.9 kg). During Days 0-56, G3 gained significantly more weight (90.4 kg) than G2 (58.8 kg), while G1 gained 76.0 kg, not significantly different from either G2 or G3. During Days -56 to 0, the feed conversion efficiency (kg gain/kg feed) of the infested G2 and G3 was significantly lower than that of the uninfested G1; during the 56 days following treatment, feed conversion efficiency of G3 was significantly higher than that of G2, while the uninfested G1 was intermediate. Carcass weight of G2 was significantly lower than those of G1 and G3, which did not differ significantly. Carcass yield, rib eye area and weight of kidney fat of the uninfested G1 were significantly greater than those of G2 and G3. Weights of the adrenal glands, prescapular and precrural lymph nodes were significantly higher for the infested G2 than for the other two groups. Significantly less of leather area from the infested G2 was of usable quality than of the leathers from the uninfested G1 or the infested, treated G3, and the leathers from G2 showed significantly more severe gouging or etching than leathers from the two other groups.

Serological cross-sectional survey of psoroptic sheep scab in Switzerland

Sheep scab, caused by the mite Psoroptes ovis, is a notifiable disease in Switzerland. Mandatory prophylactic treatments are carried out in spring for all sheep grazing on common alpine pastures. Despite these strict control measures the number of outbreaks has been increasing in recent years, raising questions on the spread of this epizootic disease. To improve the traditional microscopic diagnosis, we developed an ELISA for the specific determination of antibodies against psoroptic antigens with sensitivity and specificity of 93.7 and 96.5%, respectively. Using this test, a serum bank, representative of the sheep population in Switzerland (approximately 15% of registered sheep flocks, randomly chosen, collected between February and May 1998), was screened. The prevalence using 16,404 sera was 10.4, and 11.9% of the 2083 flocks were diagnosed as positive. Additionally, there was a significant increase found for mean seropositivity relative to the flock size (P<0.001, linear regression). Therefore, larger flocks had a higher risk of sheep scab, which may be caused by more frequent animal movement. Spatial analysis of the results revealed a region with significantly higher risk (relative risk = 3.5, P<0.001) of seropositivity, where no cases have officially been notified in the period between 1997 and 1999. The next goal is to establish an efficient control strategy based on serological monitoring of the total sheep population.