Peste des petits ruminants (PPR) is a viral disease, caused by a morbillivirus closely related to rinderpest virus, which mainly affects goats, sheep, and some wild relatives of domesticated small ruminants, as well as camels. Wild ruminants may play an important epidemiological role as a virus source for domestic small ruminants [1
]. PPR was first reported in Ivory Coast in 1942. Today, more than 70 countries have confirmed PPR within their borders, and many countries are at risk of the disease being introduced. PPR is characterized by high morbidity and mortality rates of up to 90% [2
]. Affected animals present with high fever, depression, along with eye and nose discharges, severe pneumonia and diarrhea. The PPR virus (PPRV) does not cross from animals to infect humans [3
]. The availability of effective and safe live attenuated PPR vaccines in sheep and goats has boosted the control program in some developing countries, and currently a total of 59 countries are recognized by the OIE as being free from the disease (Figure 1
). Other developing countries are currently unable to develop and apply an effective strategy to control and eradicate the PPR virus (Figure 1
The Food and Agriculture Organization (FAO) reported that PPR affects 30 million animals across 70 countries around the world. Sixty percent of these countries are in the African continent, and the other 40% are in the Middle East and Asia. (FAO, OIE Report [3
]). The disease causes annual economic losses of up to USD 2.1 billion each year. Looking beyond this figure, 300 million families are at risk of losing their livelihoods, food security, and employment opportunities. The inability of families, communities, and institutions to anticipate, absorb, or recover from PPR can compromise national and regional development efforts, and turn back the clock on decades of progress.
In the United Arab Emirates (UAE), PPR was first reported in wildlife in 1986. To date, a total of fifty outbreaks in both wildlife and small ruminants have been reported to the OIE. After implementing the UAE national animal health plan in 2016, which adopted a mass vaccination strategy to control and eradicate PPR, outbreaks have sharply decreased to two to three outbreaks per annum. The UAE is currently at stage 2 of the five stages of the progressive step-wise approach for the prevention and control of PPR. Despite the progress achieved in controlling PPR in the country, no simulations or trials to assess its spread and the effectiveness of different control strategies have been carried out [3
], Figure 2
and Figure 3
Few studies have been reported in developing mathematical models to simulate and assess the effectiveness of various PPR control strategies. Mitchell et al. (2017) [6
] noticed a lack of empirical studies on PPR. Such models provide policy-makers with a tool to develop powerful containment strategies for handling out-breaks of PPR by understanding how it spreads through herds, the uncertainty of the disease parameters, and the impact of each herd’s configuration on the disease’s spread. Therefore, the mathematical model, which can be used to configure any infectious disease, led to the finding that lowering the amount of time from the first identification of PPR in a herd to vaccination will significantly reduce the number of deaths that result from PPR [7
Lyons et al. (2019) [8
] estimated various cost components in pastoral and mixed-crop livestock systems in four PPR vaccination campaigns in Ethiopia. The cost of the overall vaccination process for mixed-crop livestock systems is approximately double the price in pastoral areas. Due to the lack of knowledge about the transmission potential of the PPRV, Fournié et al. (2017) [9
] used a dynamic model of transmission and elimination of PPR in Ethiopia. The outcome was an estimation for the vaccination coverage required for elimination and the level of viral transmission in an endemic setting.
Thus, this study aimed to develop a model for PPR spread in the UAE, simulate possible control and eradication strategies, and to evaluate the effectiveness and direct government cost of such strategies. To achieve this, three scenarios of PPR spread in UAE with a corresponding different control strategy were simulated and discussed using The North American Animal Disease Spread Model (NAADSM) [10
8. Peste des Petits Ruminants (PPR) Simulation Results and Discussion
Simulation results are illustrated in Figure 6
and Figure 7
, and in Table 2
below. The results indicate that the outbreak durations are 171, 148, and 73 days for scenarios A, B, and C, respectively. The total cumulative number of infected animals was reported to be 1505, 1481, and 327 animals for the three scenarios A, B, and C, respectively.
The number of susceptible animals was reported to be 2,954,213, 2,954,213, and 2,954,213 animals for the three scenarios A, B, and C, respectively, while the number of latent animals was reported to be 1327, 1316, and 315 animals in the three scenarios A, B, and C, respectively. The number of animals showing subclinical signs was reported to be 887, 889, and 271 animals for the three scenarios A, B, and C, respectively. Moreover, the number of animals showing clinical signs was reported to be 690, 697, and 255 animals for the three scenarios A, B, and C, respectively.
Tracing had also been addressed as number of animals exposed to any infected herd were reported to be 2454, 2476, 227 animals of the three scenarios A, B, and C, respectively. While number of animals directly exposed that could possibly have been traced forward were reported to be 1833, 1837, 71 animals of the three scenarios A, B, and C, respectively. Total number of animals in units successfully identified by tracing (either forward or back) after direct contact 1638, 1644, 63 animals of the three scenarios A, B, and C, respectively. Number of animals in units successfully identified by tracing (either forward or back) after contact (either direct or indirect) were reported to be 2220, 2248, 208 animals of the three scenarios A, B, and C, respectively.
Regarding diagnostic testing, data showed that the number of animals subjected to diagnostic testing after a trace forward or trace back after direct contact was reported to be 1444, 1461, and 56 animals for the three scenarios A, B, and C, respectively. Meanwhile, the number of animals subjected to diagnostic testing after a trace forward or trace back after (either direct or indirect) contact was reported to be 1962, 2007, and 421 animals for the three scenarios A, B, and C, respectively. Additionally, the number of animals in the tested units with a true negative diagnostic test result was reported to be 1046, 1111, and 1 animals in the three scenarios A, B, and C, respectively.
The number of animals that are destroyed was reported to be 1384, 1370, and 321 animals in the three scenarios A, B, and C, respectively.
Niu et al. (2017) [17
], based on the global effort to combat PPRV, presented a global online prediction system by adopting correlational analysis, based on collected data from 2977 cases from 2009 to 2018, and showed that PPR has a severe impact on people depending on the livestock production system as a means to generate income to reduce poverty.
Controlling PPR is essential for poverty alleviation, especially in Africa, the Middle East, and South Asia. Additionally, the model results show that the outbreaks were concentrated in the continents of Asia and Africa, and widely spreading in the Middle East region [18
Cameron (2019) [19
] found that a more sustainable option for PPR eradication could be adopting guerrilla tactics, where the primary weapon is information and understanding PPR. This tactic can be divided into four main phases—the foundation, planning, implementation and demonstration of global freedom. The author asserted that continuous real-time information in the form of guerilla tactics should be the primary tool for disease eradication, not long-term vaccination. This will also optimize the use of available sources and minimize the disruption related to managing the movement of animals from infected to uninfected areas. We herein developed a model for the disease spread of PPR, simulated possible control and eradication strategies, and evaluated the effectiveness and direct government cost of such strategies.
These results show that control strategies that intensify vaccination compared to no vaccination and with no strict and intensive animal movement controls moderately reduce the outbreak duration by 57% and the spread of PPR by 78% when compared to vaccination with stamping out (B) or ceased vaccination (A) strategies. However, an integrated and targeted eradication and control strategy that applies all possible effective measures of both triggered ring vaccination and animal movement controls reduced the disease’s outbreak duration in scenario C by 57% and the total cumulative number of infected animals by 78% compared to the initial eradication scenario A (without vaccination and lesser restriction of animals’ movement).
Scenario B address the impact of vaccination. Efficient PPR vaccines are available and can induce life-long protective immunity in vaccinated animals.
The total number of susceptible animals is nearly 3 million animals in the UAE. The simulation scenarios detailed the control strategies that can be applied against PPR, and we discuss the outcomes and perform a comparison between the three scenarios’ outcomes, as illustrated in Table 2
below. The comparison between scenario A and scenario B shows the changes due to the introduction of the triggered vaccination as the disease control strategy. Due to vaccination, the outbreak duration on average of the model’s 1000 iterations reduced from 171 days to 141 days, or by −13%. The number of infected animals slightly reduced from 1103 to 1092, or by −1%. Similarly, all other parameters regarding disease spread and the impact of the vaccination strategy showed changes that varied from 0% (no change) to −1%. This indicates that a reliance on vaccination measures alone (e.g., not accompanied by restrictions on animals’ movement) would not contribute enough to achieve PPR eradication. However, the most effective scenario, scenario C, showed a reduction in the disease’s spread or the outcome of the control strategy in the range from 57% for the disease duration in days to as high as 96% for the number of animals directly exposed that could have been traced forward. The number of animals depopulated reduced from 1384 animals to only 324 animals, or by 77% in scenario C compared to scenario A. The number of vaccinated animals was also reduced from 35,061 to 2704. These results, compared to the progress the country has achieved (reaching stage 2 of PPR eradication) as of 2021, applying PPR national eradication strategy, indicate that a successful eradication strategy may apply a very intensive and strict movement control strategy integrated with vaccination and depopulation only when such measures are triggered. Such observations are in line with the global PPR control and eradication strategy, where it is stated that prevention and control measures are a combination of different tools, which can include vaccination, improved biosecurity, animal identification, movement control, quarantine and stamping out. These individual tools are likely to be applied at different levels of intensity as an individual country moves along the pathway. (FAO OIE Global Strategy for the Control and Eradication of PPR. Paris: OIE and FAO; (2015).